Training Report 1

TRAINING REPORT

A STUDY OF PROGRAMMABLE LOGIC CONTROLLERS & DISTRIBUTED CONTROL

SYSTEMS

At

HINDUSTAN PETROLEUM CORPORATION LIMITED VISAKH REFINERY

(May 2015)

By Srihero Yennana and B.K Shyam Anand

B.Tech VII Semester,Electronics and Communication Engineering GITAM UNIVERSITY.

CERTIFICATE

This is to certify that Srihero Yennana and B.K Shyam Anand, VII

semester B.tech student (Electronics and Communication Engineering) of

GITAM UNIVERSITY, Visakhapatnam has successfully completed 2

Weeks In-Plant training and successfully completed a training project

report titled A STUDY OF PROGRAMMABLE LOGIC CONTROLLERS

(PLCs) & DISTRIBUTED CONTROL SYSTEMS (DCS) at HPCL-VR

during the period 1-05-15 to 15-05-15.

D Bullabai (Chief Manager - Minor Projects.)

M Sudha Mohan (Sr. Manager- HR)

ACKNOWLEDGEMENT

I express my sincere thanks to Mrs Sudha Mohan Sr. Manager HR for

permitting me to undergo a month long Industrial Training at the HPCL

Visakh Refinery.

I also thank Mr D Bullabai, Chief Manager (Minor Projects.) for guiding me

throughout the training and for associating me with experienced engineers

and for providing me with all the facilities required.

I am also thankful to my training guides Mr E Saichand , Mr P C Shijin for

imparting ample knowledge and for their constant guidance, assistance and

encouragement during the training period.

Further I am also thankful to all the other engineers at HPCL-VR who helped

me during my training by providing valuable information and constant

guidance and support.

CONTENTS

1. INTRODUCTION

Origin and Growth of HPCL Visakh Refinery Refinery Overview

2.CONFIGURATION OF REFINERY

Process Units : Crude Distillation Unit (CDU) & Fluidized Catalytic Cracking Unit (FCCU) Treating Units : Merox Unit & Diesel Hydro Desulphurization Unit Oil Movement and Storage Units Power and Utilities : Captive Power Plant (CPP) & DeMineralization Plant Environment Related Units : Effluent Treatment Plant & Sulphur Recovery

Unit

3.FIELD INSTRUMENTS

Critical Instruments Classification of Instruments Pressure Detectors Temperature Detectors Level Measurement Flow Measurement Miscellaneous

4.PROGRAMMABLE LOGIC CONTROLLERS (PLC)

History A PLC System Programming Languages for PLCs Vendors of PLCs used in HPCL-VR Tata Honeywell PLC Model 620 Series Honeywell Fail Safe Control Safety Manager (FSC SM) PLC Configuration of other PLCs Applications of PLCs Advantages of PLCs

5.DISTRIBUTED CONTROL SYSTEMS (DCS)

History The Hierarchy of DCS Distributed Control Systems in HPCL-VR Honeywell Automation India Limited Yokogawa India Limited ASEABrown Boveri (ABB)

6. DATA COMMUNICATION

Serial Communication Parallel Communication Fiber Optic Communication Ethernet Modbus

INTRODUCTION

Hindustan Petroleum Corporation Limited (HPCL) is a Global Fortune 500 company in the Energy sector. HPCL has two refineries located in Mumbai (West Coast) and Visakh (East Coast) with capacities of 5.5 MMTPA and 7.5 MMTPA respectively, churning out a wide range of petroleum products, viz. LPG, MS, SKO, ATF, HSD, Bitumen etc. and over 300 grades of lubricants, specialties and greases as per BIS standard. HPCL has successfully contributed close to 20% of India's total refining requirements. Over the years HPCL's capacity of production has expanded massively through various up gradation initiatives. The refineries, known for the full utilization of capacity and world class performance are the foundations of HPCL's successful journey towards meeting India's energy requirements.

Hindustan Petroleum Corporation (HPCL) came into being in mid 1974 after take over and merging of erstwhile Esso and Lube India undertakings. Catlex was taken over by government of India in 1976 and subsequently merged with HPCL. Hindustan Petroleum Corporation Limited thus emerged after merging Refining/Marketing facilities of ESSO and CALTEX.

Hindustan Petroleum Corporation Limited today is the second largest integrated oil company in India playing a significant role in the nations economic development and growth. against the backdrop of economic liberalization, HPLC is consistently improving its existence by strengthening its infrastructural facilities as well as diversifying upstream and downstream into exploration and producti9on and power and petrochemicals and horizontally into LNG sector.

HPCL produces the entire range of petroleum products and serves all sectors of the economy-industry, agriculture, transport, domestic, public utilities and also major consumers like the railways, power plants, defense, fertilizer plants, etc.;

Visakh refinery performance had been consistently excellent over the years. The major performance indicators are crude thruput, total distillate, fuel and loss and implementation of ENCON and environmental projects.

Origin and growth of HPCL-VR

Commissioned in 1957 as Catlex oil refinery India limited (CORIL). First oil refinery on the East Coast and the major industry in the city of Visakhapatnam, Andhra Pradesh.

Installed capacity of 0.65 Million Metric Tones per Annum {MMTA} in 1957 for refining of crude oil into petroleum products [13200 bbl/day]. CORIL was taken over by the government of India and merged with HPCL in 1978.

Refinery Overview

Visakh refinery is fuels based refinery generating major products of mass consumption like petrol, diesel and kerosene. Hence, crude meeting general purpose characteristics can be processed with this refinery configuration. Visakh refinery can process crude from Prussian gulf under non-bituminous category, bituminous crude (crude yielding bitumen, used for paving road).

The crude processed at refinery include

CRUDE COUNTRY Kuwait Kuwait Dubai UAE Ummshaif UAE Upper zakum UAE Murban Saudi Arab Arab medium Saudi Arab Iran mix Iran Lavan Blend Iran Barash Lt Iran

Products And Treatment Facilities

Production Units:

S.NO Process unit Capacity (in MMPA)

1. CDU-I 1.5

2. CDU-II 3.0

3. CDU-III 3.0

4. BBU 0.225

5. VBU 1.0

6. FCCU-I(R) 0.95

7. FCCU-II 0.60

8. DHDS 1.8

9. PRU 0.1

Legend:

Products:

S.NO Daily production Capacity (in tones)

1. Crude Processing 22,500 2. LPG 610 3. Propylene 100 4. Sulphur 17+65 5. Diesel 7,800 6. Naphtha 2,150 7. LSHS 1,790 8. Fuel oil 3,500

Treatment Units:

DHDS:Diesel Hydro De-Sulphurization Unit:1.8 MMPTA LPG Amine Treatment Unit LPG, ATF and Petrol Merox Units Amine Regeneration Unit

Environmental Control Facilities:

Sulphur Recovery Units: 3 no. [2 Locate Technology of Clauss process] Sour water striping Units: 2 no. Effluent Treatment Plants: 4 no. CO Boilers: 2 no.

CDU: Crude Distillation Unit FCCU : Fluidized Catalytic Cracking

DHDS: Diesel Hydro De-Sulphurization Unit VBU : Vis Breaker Unit

BBU : Bitumen Blowing Unit PRU : Propylene Recovery Unit

CONFIGURATION OF REFINERY

Visakh Refinery is being operated under the following major units: Process Units Treating Units Power and utilities Oil movement and storage units Environment related units

Process Unit

The Process unit consists of three units: 1. CDU 2.FCCU 3.PRU

Crude Distillation Unit

CDU consists of two sections: Atmos section Vacuum section

Atmos section:

Crude oil is first preheated from 30-1250c and pressure about 10kg/cm2 enters the Desalter. The salts from crude are removed in the desalter units. The desalted crude is then boosted to a pressure of 30-35kg/cm2, pre-heated to around 3600c.

The oil is allowed into the flash zone of atmos distribution column and the product to stripper with steam to strip off the lighter products. The over head-vapors of the atmos column are condensed in a series of conductors and the liquid in the receiver.

Heavy Naphtha, kerosene / ATF & Diesel product are withdrawn as side steams and stripped off as lighter ends with supper heater MP steam in the respective strippers. The bottom stream in atmos column is called RCO.

Products: Fuel Gas, LPG, Light-Naphtha, Heavy-Naphtha, kerosene, diesel & Reduced Crude Oil.

Vacuum section:

Hot reduced crude oil from atmospheric column bottom is heated in a vacuum to 380oc and introduced into the flash zone of vacuum column. The stop distillate out is

withdrawn first. The hydrocarbon vapors rising in the column are condensed into Heavy Vacuum Gas Oil (HVGO) and Light Vacuum Gas Oil (LVGO). VGO is feed to FCCU as feed. The bottom product of vacuum column is vacuum residue. The vacuum in the column is maintained by a multistage ejector system.

Products: LVGO and the HVGO obtained are fed to FCCU, the combination of Short-Residue and the slop cut forms the fuel oil which is consumed by the refinery.

!

Vis Breaking Unit (VBU)

Vacuum reside from either CDU I II or III or storage is received in visbreaking feed surge drum. It operates at a pressure which is floating on main fractioning pressure visbreaking feed @ 5.0 kg/cm2g, 1200c 1600c from surge drum is pumped by visbreaking feed charge pump which are of screw type to a pressure of 7.6 kg/cm2g it

is then heated in visbreaking tar exchange to 3200c by visbreaking crude is then routed to heater through booster pumps @ 5.8kg/cm2g preheated visbreaker feed entries both passes of visbreaker heater under individual pas Control visbreaker heater is a two-pass single shell heater with bridge wall type configuration turbulizing water (BFKL) is injected to both the passes at a point where visbreaking reaction starts. Fuel gasses heat visbreaker feed to 4550c (4700c) Residual heat recovered by superheating LP & MP steam. Gas oil quench works primarily by vaporization quench effluent entries main fractionators @ 4250c and 7 kg/cm2g where it is separated into visbreaker tar or fuel oil as side stream product and naphtha and gas as overhead product.

Bitumen Blowing Unit (BBU)

The unit normally receives hot vacuum residue directly from vacuum unit. The feed is cooled to about 2320c in a stream generator before entering the bitumen converter. In bitumen converter the vacuum residue is blown with air, since the reaction exothermic, the heat evolved has to be removed. This is done injected steam into the reactor at the top. Heat is recovered from bitumen leaving converter bottom by generating steam and the bitumen is further trim cooled before sending to storage. The hydrocarbon vapors steam and unreacted air leave the converter top to water quench drum where hydrocarbons are condensed along with some water. Hydrocarbon layer is sent to slop oil whereas water sent to waste water treatment plant (WWTP) after separation of same in the settler.

Fluidized Catalytic Cracking Unit (FCCU)

Vacuum Gas Oil from vacuum unit and recycle streams are pumped to raw oil furnace for preheating the fresh feed. This fresh feed is mixed with regenerated catalyst and enters the reactor at the base of riser where they are vapourized and raised to the reactor temperature by the hot catalyst. The mixture of oil, vapour and catalyst travels up the riser into reactor. The gas oil commences to crack immediately when it contact the hot catalyst in the riser and continues until the oil vapour is disengaged from the catalyst in the reactor.

The cracked products in vapour form continue through the reactor vapour line to fractionators. The catalyst stripper surrounds the upper portion of the reactor passes around the reactor grid and into the stripper, where if flows over baffles counter current to the rising stripping steam, displaces oil vapours to the reactor. Coke is deposited on the circulation catalyst in the reaction zone. The fuel gas leaving the top of the regenerator goes to co-boilers where super heat is produced.

The regenerated catalyst is recycled with the incoming feed to the reactor. Vapours from reactor are sending to Fractionators section where they are fractionated into recycle gas oil which is returned to the reactor and produces Clarified Oil, Cycle Oil, Motor Sprint (Petrol), and Gas products. This is achieved by first sending the reactor products to fractionators where recycled gas oil and clarified oil are taken as bottom products, cycle oil as side draw off and unstabilised motor sprint and gas as overhead products. The overhead gas is compressed and liquefied and separated from the separator is scrubbed with unstabilised motor sprint in an absorber to recover the C3 &C4 in it.

! The liquids form the separator and the absorbers are stripped off ethane and the gas stripped off is recycled back to the gas compressor and liquefaction system to recover C3s and C4s carried with the stripped gases. The liquid form stripper bottom is send

to a Debutanizers where LPG is taken as overhead product and stabilized MS as bottom product.

Products: Fuel Gas, Cracked LPG, Cracked Gasoline, Cracked Naphtha, Diesel component [Light Cyclic Oil (LCO), Heavy Cyclic Oil (HCO), Clarified Oil, Low Sulphur Heavy Stock (LSHS) Used as fuel for Industries and boilers from low sulphur crude processing. Also used in Ships, Jute Batch Oil, Wash Oil-B, Propylene.

Propylene Recovery Unit (PRU)

The Propylene Recovery Unit is defined to recover Propylene from Cracked LPG, which is one of the product streams of Fluidized Catalytic Cracking Unit (FCCU). Cracked LPG is a mixture of Propane, Propylene, and Butane with some traces of C2 & C5 Hydrocarbons. The unit is designed to process about 1,00,000TPA of cracked LPG produced at FCCU-I & II and to recover 22,000TPA of Propylene. The process consists of four steps. In the first step, the feed to unit i.e., Cr. LPG is prepared by draining out the traces of caustic carryover.

In the second step Cracked LPG is separated into C3s and C4s in a distillation column consisting of 55 trays. C3s being lighter is recovered from the column top.

In the third step, the C3s are again separated into propylene and propane in the second distillation column consisting of 98 trays. Propylene being lighter is recovered from the top of the column. In the fourth step, the Propylene recovered is subjected to chemical treatment with a mixture of Mono Ethanol Amine (MEA) and Caustic, then water washed and passed through a mechanical coalesce to knock off moisture to meet the following specifications:

Purity : 95 % Water : NIL Total Sulphur : 5 ppm

The bottom products of the first column consisting of Butane & Butylenes along the bottom product of second column consisting Propane are routed to LPG Spheres. On special Propylene is routed to its spheres and off- Special is routed to LPG spheres.

Treating Units

1. Merox Unit 2. Diesel Hydro De-Sulphurisation

Merox Unit

The LPG containing is treated here and the sulphur is removed from it. The Kerosenes flash point is increased in this unit and the sore water containing gas is treated here and the water is recycled for usage.

LPG Merox Units

While separate facilities are provided for straight run and cracked LPGS for extraction, a common facility is provided for caustic generation. After Amine washing LPG enters the caustic pre-wash tower, the purpose of which is to remove traces of hydrogen sulphide. The LPG extractor which is perforated tray column. In this type of extraction column, caustic soda containing dispersed Merox catalyst is would lead to caustic soda entertainment. The LPG is introduced near the bottom of the column below the first perforated tray. LPG, with mercaptans, is transferred to the caustic solution forming sodium mercaptides. The LPG then goes in to the LPG settler. The spent caustic carried over from the LPG extractor decants and treated LPG is recovered and sent to storage.

Gasoline/motor Spirit Merox

The feed mixture of straight run light naphtha from crude distillation unit and FCCU unit motor spirit is first sent to the caustic pre wash where hydrogen sulphide is removed from the hydrocarbons. The charge is then mixed with air and routed to reactor to a reactor where catalyst is fixed on charcoal bed. The foul smelling Mercaptans are converted to non smelling disulphides, two reactors are provided of which one shall be stand by and the other on stream.

Kerosene Merox

Kerosene after a caustic pre wash goes to the Merox reactor consists of a catalyst bed of activated charcoal impregnated with Merox catalyst. Air is injected with the feed to the Merox reactor.

Diesel Hydro De-Sulphurisation

Sulfur in Diesel enhances the pollution & contributes significantly to SOx in exhaust emissions. It leads to corrosion and wear of engine systems. In order to make eco-

friendly diesel, it is desirable to remove impurities by treating the Diesel streams at certain operating conditions in presence of catalyst and H2 through a process known

as DHDS. Straight run/Cracked diesel streams have certain inherent impurities viz Sulphur, Oxygen, Olefins, metals etc. Quantity of these impurities depend on crude quality, generally poorer the crude quality, higher the impurities. With the implementation of Bharat Stage-II and Euro-III spec, it is mandatory to produce Diesel with ultra low Sulphur content.

The Process Steps in this unit are: Feed (Naphtha) Pre-desulphurization Final desulphurization Steam Naphtha Reforming CO HT shift conversion Final purification of H 2 (PSA)

To remove Sulphur from Naphtha, which is poison to reformer catalyst Naphtha and recycle H2 are heated and sent to Reactor where Sulphur compounds are converted to H2S over Cobalt-Molybdenum based catalyst.

R-SH + H2 R-H + H2S

Sulphur reduction from 1000ppm to 10ppm. To reduce the sulphur content of Naphtha from 10 ppm to < 0.5 ppm., Naphtha and H2 are heated and processed in Reactor-II to convert S compounds to H2S over Cobalt-Molybdenum based catalyst. The H2S removed from the Reactor-II is absorbed in ZnO reactor.

ZnO + H2S ZnS + H2O

De-Sulphurised Naphtha is mixed with steam and passed through a Nickel catalyst packed in vertical narrow 108 tubes mounted in the reformer at high temperature

CnHm + nH2O nCO + (2n+m)/2H2 CH4 + H2O CO + 3H2 (endothermic) C + H2O CO + H2 (endothermic) Shift: CO + H2O CO2 + H2 (exothermic)

Process is endothermic and heat is supplied by fuel firing with 40 top-fired burners.

Pressure Swing Adsorption

PSA unit works on the principle that the adsorbent attracts and retains the impurities at higher pressure and releases them at lower pressure. Four basic steps carried out by an automatic PLC controlled control valves

Adsorption Co-current Depressurization Counter-current Depressurization Purging Re-Pressurization

Adsorption: During the adsorption step, the feed gas enters the bottom of the adsorber and the impurities are retained in the adsorbent bed. High purity hydrogen is produced from the top of the adsorber. The bed is switched from the adsorption step to its next step before the impurities front has reached the top of the bed.

Co-current Depressurization: The adsorber is partially depressurized. Hydrogen that would otherwise be lost is transferred to other adsorbers. This patented feature allows for high hydrogen recovery. The appropriate valve at the top of the absorber opens to allow pure hydrogen to pass into another adsorber to provide the gas for purging and to perform pressure equalization. Depressurization step is referred to as Co-current Depressurization and was one of the first features to obtain patent coverage. Since its invention, it has been a standard of each PSA unit with the benefit that hydrogen recovery is improved.

Counter-current Depressurisation: The adsorber is depressurised to its lowest pressure level. The adsorbent is partially regenerated and some impurities are rejected. The adsorber is depressurized in the counter current direction to purge gas pressure to remove the impurities from the adsorbent.

Purging: Hydrogen is used to purge the remaining impurities and complete regeneration of the adsorbent. The adsorber is now purged at low pressure with pure hydrogen from another adsorber undergoing co-current depressurization; this step serves the purpose of completing the removal of impurities from the adsorbent.

Re-Pressurization: The adsorber is re-pressurised with hydrogen to the feed pressure level, completing the cycle. It is now ready to begin another adsorption step. The adsorber is re-pressurized to adsorption pressure initially with hydrogen recovered from other adsorbers on co-current depressurization step, and finally with a slip stream of pure hydrogen product. At the end of this step, the adsorber is ready to begin a new cycle.

Power And Utilities:

Captive Power Plant (CPP)

Capacitive power plant meets the total power demand of the HPCL. This unit comprises for four gas turbine generators (GTG), two with 9mw capacity (FRAME-3 Machines) each and two with 25mw capacity (FRAME-5 Machines) each.FRAME-3 Machine is a two shaft machine whereas FRAME-5 Machine is a single shaft machine. HSD and Naphtha are used for the combustion of gas turbine.

FRAME-3 Machine Functioning

The fuel for the capacitive power plants (Diesel/Naphtha) goes to a surge tank where in it is centrifuged so that the dirt particles are thrown away. The fuel for the generation unit is from the top of the surge tank. From here the fuel is pumped by two pumps (redundant to each other) at a pressure of 10kg/cm2 in to a 20 micro filter for further filtration of impurities. There is an FSR where a feeder feeds fuel Tories inlets with a pressure of 6kg/cm2. .These inlets terminate six combustion chambers of GTG/GTG2. The combustion chambers are placed 30m either side of the compressor discharge casing. The first two have vertical and horizontal igniters to produce spark for ignitions and others have flame scanners.

To start a GTG there is an accessory compartment which houses a diesel engine and an accessory gear drive which is coupled to the HP shaft. Initially, the diesel engine is started and it reeves up as the HP turbine shaft is coupled to it, it too gains resolution. When the diesel engine reeves up 54% of the max rpm of the HP shaft the clutch is released and it gets disengaged from the HP shaft. The HP shaft now is self-sustained. There is an air duct, which sucks air in to the compressor. Here the compressor compresses air to a pressure of 80kg/cm2. The compressed air rotates the blades of the HP. The air is also sent to the atomizer and the combustion chamber. In the combustion chamber atomized air mixes with the fuel and a spark from the spark plug ignites the mixture. There is an expansion and this drives the hp to higher rpm.

In the combustion chamber there are flame scanners, which in case of false start, trips the fuel valve. When the HP shaft runs up to 47% of its maximum speed, the gases from the HP turbine blades passes through the secondary variable nozzle and these in turn rotate the LP blades. The HP has a full speed of 7100 rpm area the LP has a full speed of 6500 rpm. The LP shaft is connected to a load gear of box with a down turn of 4.33:1 to generate a 50HZ power source from a generator having 4 poles the rpm of the turbine is given by N=F*120/p=1500 rpm

So the LP shaft has to be maintained at 1500 rpm always. The exhaust gases from GTG1 &GTG2 are about 5430c hot and it is wastage of energy to leave such hot gases into the atmosphere. Hence these gases are sent to boilers to generate steam. HRSG1 &HRSG2 are such steam generators.

No. of gas turbines: 4 Each GTG capacity: 9MW*2

: 25MW*2

Fuel used for GTG is diesel for start up and naphtha for continuous running.

GTG Control: Speedtronic MARK V Control System

The SPEEDTRONIC Control System is evolved by General Electric, USA from a combination of discrete solid-state components, meters, relays and annuciators, to a system of redundant microprocessor based system. The primary objective of this development has always been to improve all Gas Turbine reliability, availability application flexibility and serviceability.

Basic Control Requirements

The Gas Turbine Control system is designed to crank the turbine, bring to purging speed (approx. 20%), fire it and then bring the unit to operating speed. On generator drives the control system synchronizes the gas turbine to the line, on compressor or processor or process drives it checks the process constraints and then loads the gas turbine to the appropriate point. This sequence must occur automatically and is done while minimizing the thermal stresses in the gas turbine hot gas path parts and associated hardware. The total control system can be divided into three functional sub-systems. 1. Control 2. Protection 3. Sequencing

Control Philosophy:

Mark V is the second generation Triple Modular Redundant turbine control system. The basic control philosophy of Mark IV has been retained with many improvements. Figure 1 shows the standard Mark V control system configuration. As with the Mark IV system the three identical control processors, called and are at the core. These processors handle all critical control algorithms, turbine sequencing, and primary protective functions. They also gather data and generate most of the alarms.

The three control processors accept input from various arrangements of redundant turbine and generator sensors. By extending; the fault tolerance to include sensors, as

with the Mark IV system, the overall control system availability is significantly increased. Some sensors are brought in to all three-control processors, but many, like exhaust thermocouples, are divided among the control processors. The individual exhaust temperature measurements are exchanged on the voter link so that each control processor knows all exhaust thermocouple values. Voted sensor values are computed by each of the control processors. These voted values are used in control and sequencing algorithms that produce the required control actions.

An independent protective module is internally triple redundant with three (3) independent cards

Simultaneously, the controllers provide an independent backup set of phase-slip windows, which must be satisfied prior to automatic or manual synchronizing. This eliminates the need for the traditional GXS check relay.

The Interface Data Processor, called , includes a monitor, keyboard, and printer. Its main functions are driving operator displays, managing the alarm process, and handling operator commands. also does system configuration and download, offline diagnostics for maintenance, and implements interfaces to remote operator stations and plant distributed control systems.

The Common Data Processor, or , collects data for display, maintains the alarm buffers, generates and keeps diagnostic data, and implements the common I/O for non-critical signals and control actions. Turbine supervisory sensors such as wheel space thermocouples come directly to . The processor communicates with using a peer-to-peer communication link which permits one or more processors. gathers data from the control processors by participating on the voting link.

Software Configuration

Improved methods of implementing the triple modular redundant system center on SIFT (Software Implemented Fault Tolerance) technology and result in a more robust control. SIFT' involves exchanging information on the voter link directly between and controllers. Each control processor measures all of its input sensors so that each sensor signal is represented by a number in the controller. The sensor numbers to be voted are gathered in a table of values. The values of all state outputs such as integrators, for example the load set point, are added to the table. Each control processor sends it's table out on the voter link and receives tables from the other processors. Consider the controller: it outputs its table to, and receives the tables from, the and controllers. Now all three controller tables will be in the processor which selects the median value for each sensor and integrator output, and uses these voted outputs in all subsequent calculations. and follow the same procedure.

The basic SIFT concept, then, brings one sensor of each kind into each of and and . If a sensor fails, the controller with the failed transducer initially has a bad value. But it exchanges data with the other processors, and when the voting takes place, the bad value is rejected. Therefore, a SIFT based system can tolerate one failed transducer of each kind. In previous systems, one failed transducer was likely to cause one processor to vote to trip. A failure of a different kind of transducer on another controller could cause a turbine trip. This does not happen with SIFT because the input data is exchanged and voted.

is also connected to the voter link. It eavesdrops while all three sets of variables are transmitted by the control processors and calculates the voted values for itself. If there are any significant disagreements, reports them to for operator attention and maintenance action. If one of the transducers has failed, its output will not be correct and there will be a disagreement with the two correct values. will then diagnose that the transducer, or parts immediately associated with it, have failed and will post an alarm to .

Logic outputs are voted by dedicated hardware relay driver circuits that require two or three "on" signals to pick up the output relay. Control power for the circuit and output relay is taken from all three control sections.

Protective functions are accomplished by the control processors and, for over speed, independently by the Protective Module as well. Primary speed pickups are wired to the control processors and used for both speed control and primary over speed protection. The trip commands, generated by the primary over speed protective function in the control processors, each activate a relay driver.

The driver signals are sent to the trip card in the protective model where independent relays are actuated. Contacts from each of these three primary protective trip relays are voted to cause the trip solenoid to drop out. Where mechanical over speed bolt is not used, separate over speed pickups are brought to the independent protective module. Their relay contacts are wired in a voting arrangement to the other side of the trip solenoid, and independently cause the trip solenoid to drop out on detection of over speed.

The processor is equipped with a hard disk, which keeps the records that define the site software configuration. It comes from GE with the site-specific software properly configured. The information for is stored in EEPROM there. The information for the control processors is passed through and stored in EEPROM in and . Once the download is complete, the processor can fail and the turbine will continue to run properly, accepting commands from the local backup display, while is being repaired.

Operation and Maintenance

The operator interface is comprised of a color graphics monitor, keyboard, and printer. Displays for normal operation center around the unit control display. It shows the status of major selections and presents key turbine parameters in a table that includes the variable name, value, and engineering units. A list of the oldest three unacknowledged alarms appears on this screen.

Alarm Management

Alarm management screens list all the alarms in the chronological order of their time tags. The most recent alarm is added to the top of the display list. The line shows whether the alarm has been acknowledged or not, and whether the alarm is still active. When the alarm condition clears, the alarm can be reset. If reset is selected and the alarm has not cleared, the alarm does not clear and the original time tag is retained. The alarm log prints alarms in their arrival sequence, showing the time tags which are sent from the control modules with each alarm.

Diagnostics

The trip diagnostic screen traps the actual signal condition that caused a turbine trip. This display gives detailed information about the actual logic signal path that caused any trip. It is accomplished by freezing information about the logic path when the trip occurs. This is particularly useful in identifying the original source of trouble if a spurious signal manages to cause one of the control processors to call for a trip and does not leave a normal diagnostic trail. In Mark V controls, all trips are annunciated, and information about the actual logic path that caused the trip is captured. In addition to this information, contact inputs are resolved to one millisecond, which makes this sequence of events information more valuable.

The previously mentioned comparison of voting values is another powerful diagnostic tool. Normally these values will agree, and significant disagreement means that something is wrong. Diagnostic alarms are generated whenever there is such a disagreement. Examination of these records can reveal what has gone wrong with the system. Many of these combinations have specific diagnostics associated with them, and the software has many algorithms that infer what has gone wrong from a pattern of incoming diagnostic signals. In this way the diagnostic alarm will identify as nearly as possible what is wrong, such as a failed power supply, blown fuse, failed card, or open sensor circuit.

Some of the diagnostics are intended to enhance turbine-generator monitoring. For instance, reading and saving the actual closing time of the breaker is an excellent diagnostic on the health of the synchronizing system. An output from the flame

detectors which shows the effective ultraviolet light level is another new diagnostic routine. It is an indicator of degradation in the ultraviolet flame detection system.

Once the diagnostic routines have located a failed part, it may be replaced while the turbine continues to run. The moot critical function of the diagnostics is to identify the proper control section where the problem exists. Wrong identification could lead to powering down a good section, resulting in a vote to trip. If the failed section is also voting to trip, the turbine will trip. A great deal of effort has been put into identifying the correct section. To effect the repair, the correct section is powered down. The module is opened and tilted out, the offending card located, cables disconnected, card replaced, and cables reconnected. The rack is closed and power is reapplied to the module. The module will then join in with the others to control the turbine, and the fault tolerance is restored.

Steam Generation Unit

Steam Generation unit is sub divided in to two i.e. Power plant I& II. In these units the DM water is converted in to steam by combusting the fuel oil in the presence of air in the boilers CO produced in FCCU is brought in to CO2 for pollution in power plant II. The steam produced here is utilized for unit purposes. It is Kg/cm2.

Demineralization water unit

De-Mineralization is a process of removing mineral salts from water by using ion exchange process. In this unit the raw water is treated & the PH value is maintained at 7 by making the free from acids, bases, etc, and making it a neutral solution, to use in boilers.

Environment Related Plant

Effluent treatment plant

The waste water from every plant containing oil is separated and then reused. The remaining water containing contaminants is neutralized and sent to the sea to control the environmental pollution.

Sulphur Recovery Unit (SRU):

Sulphur Recovery Unit is designed to process and remove Hydrogen Sulphide (H2S) gas from fuel gas (3386 - 8838) Nm3/hr, Sour water Stripper gas (38 -257) Nm3/hr and Amine Acid Gas (1.6 - 26) Nm3/hr, the process is based on the modified Claus reaction.

H2S + 1/2 O2 H2O + S

This reaction is accomplished by a solution called LO-CAT solution supplied by M/S ARI Technologies Inc., USA. All the three gas streams mentioned above are treated with LO-CAT solution. Due to wide variation in the qualities the fuel gas is treated separately in an absorber column and the other two streams are treated combined in the absorber section of the oxidizer vessel. In oxidizer vessel, the spent LO-CAT solution is regeneration using air.

The sulphur generated due to the above reaction remains finely suspended in the LO-CAT solution. A slipstream form the oxidizer is routed to the sulphur removal system consisting of mainly a vacuum belt filter, Sulphur Smelter and a molten sulphur storage tank. After removing sulphur the balance LO-CAT solution routed back to the oxidizer. The treated fuel gas is then routed to the Refinery Fuel Gas Header. The vent gases forms the oxidizers (free of H2S) are then vented through a stack.

FIELD INSTRUMENTS

Field Instruments are a major component of the Instrumentation and Control operations in a refinery. Field instruments play a major role in the

Continuous online measurement of Process parameters Monitoring & Automatic Controls on the process parameters Ensuring availability of safety process interlocks Online analysis and Safety related Instruments Planning the resources (material, services) To ensure availability of safety protection systems

Critical Instruments

Instruments and Systems which may cause production loss, safety hazard and unsafe operating conditions are considered as critical.

Classification of Field Instruments

Field instruments may be classified as: Measuring Instruments & Control Instruments

1. Measuring Local Indicators (Pressure/Temperature/Level Gauges) Electronic Transmitters Switches (Pressure/Temperature/Level) Thermocouple/RTDs/Pyrometers (Temperature) Radar/Displacer/DP level Instruments

2. Control Control Valves On-Off Valves Solenoid Valves I/P Converters

Pressure Detectors

Pressure is a force applied / distributed over a surface. Pressure detectors are used for measuring pressure itself, flow, level and even temperature. All pressure measurement systems generally consist of:

A Primary Element which is directly/ indirectly in contact with fluid and interacts with pressure changes.

Secondary Element which translates this interaction into values for use in indication, recording and control.

Pressure detectors are of two main types: Mechanical and Electronic

Mechanical Pressure Detectors

Bellows

The need for a pressure sensing element that was extremely sensitive to low pressures and provided power for activating recording and indicating mechanisms resulted in the development of the metallic bellows pressure sensing element. The metallic bellows is most accurate when measuring pressures from 0.5 to 75 psig. However, when used in conjunction with a heavy range spring, some bellows can be used to measure pressures of over 1000 psig.

The bellows is a one-piece, collapsible, seamless metallic unit that has deep folds formed from very thin-walled tubing. The diameter of the bellows ranges from 0.5 to 12 in. and may have as many as 24 folds. System pressure is applied to the internal volume of the bellows. As the inlet pressure to the instrument varies, the bellows will expand or contract. The moving end of the bellows is connected to a mechanical linkage assembly. As the bellows and linkage assembly moves, either an electrical signal is generated or a direct pressure indication is provided. The flexibility of a metallic bellows is similar in character to that of a

helical, coiled compression spring. Up to the elastic limit of the bellows, the relation between increments of load and deflection is linear. However, this relationship exists only when the bellows is under compression. It is necessary to construct the bellows such that all of the travel occurs on the compression side of the point of equilibrium. Therefore, in practice, the bellows must always be opposed by a spring, and the deflection characteristics will be the resulting force of the spring and bellows.

Bourdon Tubes

The bourdon tube pressure instrument is one of the oldest pressure sensing instruments in use today. The bourdon tube consists of a thin-walled tube that is

flattened diametrically on opposite sides to produce a cross-sectional area elliptical in shape, having two long flat sides and two short round sides. The tube is bent lengthwise into an arc of a circle of 270 to 300 degrees. Pressure applied to the inside of the tube causes distention of the flat sections and tends to restore its original round cross-section. This change in cross-section causes the tube to straighten slightly. Since the tube is permanently

fastened at one end, the tip of the tube traces a curve that is the result of the change in angular position with respect to the center. Within limits, the movement of the tip of the tube can then be used to position a pointer or to develop an equivalent electrical signal (which is discussed later in the text) to indicate the value of the applied internal pressure).

Diaphragms

A second type of aneroid gauge uses the deflection of a flexible membrane that separates regions of different pressure. The amount of deflection is repeatable for known pressures so the pressure can be determined by using calibration. The deformation of a thin diaphragm is dependent on the difference in pressure between its two faces. The reference face can be open to atmosphere to measure gauge pressure, open to a second port to measure differential

pressure, or can be sealed against a vacuum or other fixed reference pressure to

measure absolute pressure. The deformation can be measured using mechanical, optical or capacitive techniques. Ceramic and metallic diaphragms are used. Useful range: above 10-2 Torr

For absolute measurements, welded pressure capsules with diaphragms on either side are often used.

Shape:

Flat corrugated flattened tube capsule

Electronic Pressure Detectors

Capacitive Type

Capacitive-type transducers consist of two flexible conductive plates and a dielectric.

As pressure increases, the flexible conductive plates will move farther apart, changing the capacitance of the transducer. This change in capacitance is measurable and is proportional to the change in pressure.

Temperature Detectors

The most commonly used temperature detectors are thermocouples. Other than these, Bimetallic Strips, RTDs and Pyrometers (for very high temperatures) are also used.

Thermocouple

A thermocouple is constructed of two dissimilar metal wires joined at one end. When one end of each wire is connected to a measuring instrument, the thermocouple becomes a sensitive and highly accurate measuring device. Thermocouples may be constructed of several different combinations of materials. The performance of a thermocouple material is generally determined by using that material with platinum. The most important factor to be considered when selecting a pair of materials is the "thermoelectric difference" between the two materials. A significant difference between the two materials will result in better thermocouple performance. For example: Chromel - Constantan is

excellent for temperatures up to 2000F; Nickel/Nickel-Molybdenum sometimes replaces Chromel-Alumel; and Tungsten-Rhenium is used for temperatures up to 5000F. Some combinations used for specialized applications are Chromel-White Gold, Molybdenum-Tungsten, Tungsten-Iridium, and Iridium/Iridium-Rhodium. Thermocouples will cause an electric current to flow in the attached circuit when subjected to changes in temperature. The amount of current that will be produced is dependent on the temperature difference between the measurement and reference junction; the characteristics of the two metals used; and the characteristics of the attached circuit.

Thermocouples are housed in a metallic housing called Thermo-Well before installation.

Types

A variety of thermocouples are available for different measuring applications. They are usually selected based on the temperature range and sensitivity needed. Thermocouples with low sensitivities (B, R, and S types) have correspondingly lower resolutions. Other selection criteria include the inertness of the thermocouple material, and whether or not it is magnetic.

Type K (chromelalumel) is the most common general purpose thermocouple. It is also the most commonly used thermocouple at HPCL VR. It is inexpensive and

available in a wide variety of probes. They are available in the 200C to +1350C range. The type K was specified at a time whenmetallurgywas less advanced than it is today and, consequently, characteristics vary considerably between examples. Another potential problem arises in some situations since one of the constituent metals,nickel, is magnetic. One characteristic of thermocouples made with magnetic material is that they undergo a deviation in output when the material reaches its Curie point; this occurs for type K thermocouples at around 150 C. Sensitivity is approximately 41V/C.

Other types include E, J, N, B, R, and S.

Resistance Temperature Detector (RTD)

The RTD incorporates pure metals or certain alloys that increase in resistance as temperature increases and, conversely, decrease in resistance as temperature decreases. RTDs act somewhat like an electrical transducer, converting changes in temperature to voltage signals by the measurement of resistance. The metals that are best suited for use as RTD sensors are pure, of uniform quality, stable within a given range of temperature, and able to give reproducible resistance-temperature readings. Only a few metals have the properties necessary for use in RTD elements.

RTD elements are normally constructed of platinum, copper, or nickel. These metals are best suited for RTD applications because of their linear resistance-temperature characteristics, their high coefficient of resistance, and their ability to withstand repeated temperature cycles. The coefficient of resistance is the change in resistance per degree change in temperature, usually expressed as a percentage per degree of temperature. The material used must be capable of being drawn into fine wire so that the element can be easily constructed. RTD elements are usually long, spring-like wires surrounded by an insulator and enclosed in a sheath of metal.

Pyrometer

A pyrometer is a non-contacting device that intercepts and measures thermal radiation. This device can be used to determine the temperature of an object's

surface. A pyrometer has an optical system and detector. The optical system focuses thethermal radiationonto detector. The output signal of thedetector(Temperature T) is related to the thermal radiation or irradiance j* of the target object through the StefanBoltzmann law, the constant of proportionality , called the Stefan-Boltzmann constantand theemissivity of the object.

j* = T4 This output is used to infer the object's temperature. Thus, there is no need for direct contact between pyrometer & object, as with the thermocouple and Resistance temperature detector(RTDs).

Level Measurement

Liquid level measurement plays an important role in many of the control applications in the refinery. Level measurement devices can be classified as:

Direct Level: Dip Gauging, Level Displacer and Float Types Pressure Operated: Differential Pressure Type Radar Type

Level Displacer

Displacement type level switches offer the industrial user a wide choice of alarm and control configurations. Each unit utilizes a simple buoyancy principle and is well suited for simple or complex applications.

Force on the Displacer imparts rotary motion on the torque tube, which influences transmitter to convert the change in Level to Current output signal for indication.

Differential Pressure Type

The differential pressure (DP) detector method of liquid level measurement uses a DP detector connected to the bottom of the tank being monitored. The higher pressure, caused by the fluid in the tank, is compared to a lower reference pressure (usually atmospheric). This comparison takes place in the DP detector.

Radar Type

RADAR sensors are ideal for use in moist, vaporous, and dusty environments as well as in applications in which temperatures vary. RADAR microwaves will penetrate temperature and vapor layers that may cause problems for other techniques, such as ultrasonic. Microwaves are electromagnetic energy and therefore do not require air molecules to transmit the energy making them useful in vacuums. Microwaves, as electromagnetic energy, are reflected by objects with high dielectric properties, like metal and conductive water. Alternately, they are absorbed in various degrees by low dieletric or insulating mediums such as plastics, glass, paper, many powders and food stuffs and other solids.

Microwave sensors are executed in a wide variety of techniques. Two basic signal processing techniques are applied, each offering its own advantages: Time-Domain Reflectometry (TDR) which is a measurement of time of flight divided by the speed of light, similar to ultrasonic level sensors, and Doppler systems employing FMCW techniques. Just as with ultrasonic level sensors, microwave sensors are executed at various frequencies, from 1GHz to 30GHz. Generally, the higher the frequency, the more accurate, and the more costly. Microwave is also executed as a non-contact technique, monitoring a microwave signal that is transmitted through the medium (including vacuum), or can be executed as a radar on a wire technique. In the latter case, performance improves in powders and low dielectric media that are not good reflectors of electromagnetic energy transmitted through a void (as in non-contact microwave sensors).

Flow Detectors

Measurement of flow quantities through pipelines is a very critical part of the control processes at the refinery, for eg: measurement of the mass flow rate of fuel oil into burners is essential to control the temperature. Flow detectors are classified as:

Head type Area type Mass flowmeter

Head flowmeters

Head flow meters operate on the principle of placing a restriction in the line to cause a differential pressure head. The differential pressure, which is caused by the head, is measured and converted to a flow measurement. There are two elements in a head flow meter; the primary element is the restriction in the line, and the secondary element is the differential pressure measuring device.

Orifice Plate

The orifice plate is the simplest of the flowpath restrictions used in flow detection, as well as the most economical. They are the most widely used flowmeter instruments at HPCL VR. Orifice plates are flat plates 1/16 to 1/4 inch thick. They are normally mounted between a pair of flanges and are installed in a straight run of smooth pipe to avoid disturbance of flow patterns from fittings and valves.

The concentric orifice plate is the most common of the three types. As shown, the orifice is equidistant (concentric) to the inside diameter of the pipe. Flow through a sharp-edged orifice plate is characterized by a change in velocity. As the fluid passes through the orifice, the fluid converges, and the velocity of the fluid increases to a maximum value. At this point, the pressure is at a minimum value. As the fluid diverges to fill the entire pipe area, the velocity decreases back to the original value. The pressure increases to about 60% to 80% of the original input value. The pressure loss is irrecoverable; therefore, the output pressure will always be less than the input pressure. The pressures on both sides of the orifice are measured, resulting in a differential pressure which is proportional to the flow rate.

Segmental and eccentric orifice plates are functionally identical to the concentric

orifice. The circular section of the segmental orifice is concentric with the pipe. The segmental portion of the orifice eliminates damming of foreign materials on the upstream side of the orifice when mounted in a horizontal pipe. Depending on the type of fluid, the segmental section is placed on either the top or bottom of the horizontal pipe to increase the accuracy of the measurement. Eccentric orifice plates shift the edge of the orifice to the inside of the pipe wall. This design also prevents upstream damming and is used in the same way as the segmental orifice plate.

Orifice plates have two distinct disadvantages; they cause a high permanent pressure drop (outlet pressure will be 60% to 80% of inlet pressure), and they are subject to erosion, which will eventually cause inaccuracies in the measured differential pressure.

Venturi

The venturi tube is the most accurate flow-sensing element when properly calibrated. They are mainly used with pipes of larger diameters. The venturi tube has a converging conical inlet, a cylindrical throat, and a diverging recovery cone. It has no projections into the fluid, no sharp corners, and no sudden changes in contour.

The inlet section decreases the area of the fluid stream, causing the velocity to increase and the pressure to decrease. The low pressure is measured in the center of the cylindrical throat since the pressure will be at its lowest value, and neither the pressure nor the velocity is changing. The recovery cone allows for the recovery of pressure such that total pressure loss is only 10% to 25%. The high pressure is measured upstream of the entrance cone. The major disadvantages of this type of flow detection are the high initial costs for installation and difficulty in installation and inspection.

Annubar

Anannubar is similar to apitot tubeused to measure the flow of gas or liquid in a pipe.

The pitot tube measures the difference between the static pressure and the flowing pressure of the media in the pipe. The volumetric flow is calculated from that difference usingBernoulli's principleand taking into account the pipe inside diameter.

The biggest difference between anannubarand a pitot tube is that an annubar takes multiple samples across a section of a pipe or duct. In this way, the annubar averages the differential pressures encountered accounting for variations in flow across the section. A pitot tube will give a similar reading if the tip is located at a point in the pipe cross section where the flowing velocity is close to the average velocity.

Annubar is a registered trade name with Emerson Process Management / Rosemount.

Area Meters

In head flow meters, the restriction area is kept constant generating a pressure differential. In an area meter, the area is varied to hold the differential pressure constant. Hence the change in area is a measure of the flow rate. The head causing the flow through an area meter is relatively constant such that the rate of flow is directly proportional to the metering area. The variation in area is produced by the rise and fall of a floating element. This type of flow meter must be mounted so that the floating element moves vertically and friction is minimal.

Rotameter

The Rotameter is an area flow meter so named because a rotating float is the indicating element. The rotameter consists of a metal float and a conical glass tube, constructed such that the diameter increases with height. When there is no fluid passing through the rotameter, the float rests at the bottom of the tube. As fluid enters the tube, the higher density of the float will

cause the float to remain on the bottom. The space between the float and the tube allows for flow past the float. As flow increases in the tube, the pressure drop increases. When the pressure drop is sufficient, the float will rise to indicate the amount of flow. The higher the flow rate the greater the pressure drop. The higher the pressure drop the farther up the tube the float rises.

Mass Flowmeters

Ultrasonic Flow Equipment

Devices such as ultrasonic flow equipment use the Doppler frequency shift of u l t r a son ic s i gna l s r e f l ec ted f rom discontinuities in the fluid stream to obtain flow measurements. These discontinuities can be suspended solids, bubbles, or interfaces generated by turbulent eddies in the flow stream. The sensor is mounted on the outside of the pipe, and an ultrasonic beam from a piezoelectric crystal is transmitted through the pipe wall into the fluid at an angle to the flow stream. Signals

reflected off flow disturbances are detected by a second piezoelectric crystal located in the same sensor. Transmitted and reflected signals are compared in an electrical circuit, and the corresponding frequency shift is proportional to the flow velocity.

Transit Time Flowmeters: The most commonly used ultrasonic flowmeter is the transit-time flowmeter which is used for liquids and gases.

Transit-time flowmeters work by measuring the time of flight difference between an ultrasonic pulse sent in the flow direction and an ultrasound pulse sent opposite the flow direction. This time difference is a measure for the average velocity of the fluid along the path of the ultrasound beam. By using the absolute transit time and the distance between the ultrasound transducers, the current speed of sound is easily found. The measuring effect can be adversely affected by many things including gas and solid content.

Coriolis Flow Meter

Amass flow meter, also known as aninertial flow meteror acoriolis flow meter, is a device that measuresmass flow rateof afluidtraveling through a tube. The mass flow rate is themassof the fluid traveling past a fixed point per unit time.

The figures represent how curved tube mass flow meters are designed. When the fluid is flowing, it is led through two parallel tubes. An actuator induces a vibration of the tubes. The two parallel tubes are counter-vibrating, to make the measuring device less sensitive to outside vibrations. The actual frequency of the vibration depends on the size of the mass flow meter, and ranges from 80 to 1000 vibrations per second. Theamplitude of the vibration is too small to be seen, but it can be felt by touch. When no fluid is flowing, the vibration of the two tubes is symmetrical, as shown in the animations.

When there is mass flow, there is some twisting of the tubes. The arm through which fluid flows away from the axis of rotation must exert a force on the fluid to increase its angular momentum, so it is lagging behind the overall vibration. The arm through which fluid is pushed back towards the axis of rotation must exert a force on the fluid to decrease the fluid's angular momentum again, hence that arm leads the overall vibration. The inlet arm and the outlet arm vibrate with the same frequency as the overall vibration, but when there is mass flow the two vibrations are out of sync, the inlet arm is behind, the outlet arm is ahead. The two vibrations are shifted in phase with respect to each other, and the degree of phase-shift is a measure for the amount of mass that is flowing through the tubes.

Miscellaneous

I to P Converters

A current to pressure converter (I/P) converts an analog signal (4 to 20 mA) to a proportional linear pneumatic output (3 to 15 psig). Its purpose is to translate the analog output from a control system into a precise, repeatable pressure value to control pneumatic actuators/operators, pneumatic valves, dampers, vanes, etc. They use an electromagnetic force balance for converting an electrical signal into pneumatic pressure. Its force balance principle is a coil suspended in a magnetic field on a

flexible mount. At the lower end of the coil is a flapper valve that operates against a precision ground nozzle to create a backpressure on the servo diaphragm of a booster relay. The input current flows in the coil and produces a force between the coil and the flapper valve, which controls the servo pressure and the output pressure.

Differential Pressure Transmitter

A Differential Pressure Transmitter converts pneumatic pressure into current. Most pressure transmitters are built around the pressure capsule concept. They are usually capable of measuring differential pressure (that is, the difference between a high pressure input and a low pressure input) and therefore, are usually called DP transmitters or DP cells.

A differential pressure capsule is mounted inside a housing. One end of a force bar is connected to the capsule assembly so that the motion of the capsule can be transmitted to outside the housing. A sealing mechanism is used where the force bar penetrates the

housing and also acts as the pivot point for the force bar. Provision is made in the housing for high- pressure fluid to be applied on one side of the capsule and low-pressure fluid on the other. Any difference in pressure will cause the capsule to deflect and create motion in the force bar. The top end of the force bar is then connected to a position detector, which via an electronic system will produce a 4 - 20 mA signal that is proportional to the force bar movement. The electronic system basically comprises of a capacitor with one fixed and one moving plate. Any deflection in the diaphragm produces a change in

capacitance.

Pressure Switch

Pressure switches are devices that are configured to sense a change in pressure and respond in a specified manner. Generally, a pressure switch is included in any type of equipment that includes components that generate some type of pressure during operation. The pressure may relate to electrical current, the flow of natural gas or liquids, or the creation of steam. With each application, the pressure switch will include components that monitor the amount of pressure generated. As long as the pressure remains within acceptable levels, the pressure switch serves as an easy way to monitor activity. However, most switches will sound some sort of alarm when the level of pressure begins to exceed what is considered a safe range.

Designs for the pressure switch vary, based on the type of action that is required. When manual intervention is desired, the pressure switch is often constructed as a toggle switch. This design allows for easy operation when an alarm sounds and there is a need to either activate a venting process or immediately shut down the machinery. For switches that are configured to work in conjunction withcomputer technology, a microswitch design is common. The micro switch receives commands from the computer program once a safety shutdown or a pressure release is determined to be the next logical step in the sequence.

Since the inception of the pressure switch, the device has proven to be an ideal means of preventing a number of injuries that could result from an overload or explosion. Just about every piece of machinery that employs the use of compressors will include a pressure switch at key phases as part of the safety requirements for operation of the equipment. While automated switches have become more popular in recent years, manual pressure switches are still often installed as a backup that can be utilized in the event of an electrical failure.

Control Valves

A valve is a device for adjusting, or manipulating the flow rate of liquid or gas in a pipeline. The valve contains a flow passage, or port, whose flow area can be varied. The valve stem transmits some external motion to the port, thus changing its flow area. The external motion can originate manually (eg. from a hand wheel or a lever) or from some actuator which is positioned pneumatically, electrically or hydraulically in response to some external positioning signal. This combination of valve and actuator is called an automatic control valve, or simply, a control valve.

Plug-in-Seat

A typical valve employing the plug-in-seat principle is the globe valve. The stem raises or lowers a plug into a seat. The plug tip can be shaped so that as the plug rises, the annular space between the plug tip and the seat ring bore varies in the manner which achieves smooth flow manipulation over the full valve stroke. At the fully closed position, the bevelled edge of the plug is forced against a mating surface on the seat. The plug-in-seat combination is called the trim or inner valve.

Rotatable Plug

The ball valve is a typical valve employing this principle. The ball or plug can be rotated within the body through a quarter of a turn. The plug has a passage through it. There are three common variations upon this principle. In the first, the plug is a ball with a line sized circular flow passage; such a valve offers minimal flow restriction when fully open. The second is a ball with a V-shaped passage (theV-ball); this allows smooth control at low flows as well as at high flows. The third is the cock which uses a conical shaped plug. Whilst cocks are traditionally popular in the gas industry, they are seldom used as automatic control valves because the plug tends to jam in the body. Valves employing rotatable plug achieve tight shutoff. This is the function of a ball valve's seat rings, which are normally made of elastrometric material.

Rotating Vane

The butterfly valve's vane is shaped so that it closes off the flow passage when it is positioned normal to it. The vane can be rotated with a quarter turn actuator.

The rotating vane principle may be used for rectangular as well as circular shaped ducts. It is called a butterfly valve when the flow-passage is circular and a damper when rectangular. For

large rectangular ducts, a set of louvres is used instead of a single vane. However, valves employing the rotating vane principle share some common features. Firstly, it is difficult to eliminate all leakage between vane and body when the valve is closed; various butterfly valves have been designed so that the vane (or disc) seats against some elastometer to overcome this difficulty. Secondly, aerodynamic effects generate large unbalance forces on the vane when it is nearly fully open or fully closed; the actuator must therefore be designed to resist these forces.

Valve Materials

Generally the process piping material will be selected according to the pressure, temperature and corrosive nature of the process fluid. A safe rule is to make the valve body from the same material as the process piping.

The following materials are commonly used to make valve bodies: Cast iron Cast steel Bronze Stainless steel High temperature alloy steel Hastelloy Plastic, e.g. polyvinyl chloride Lined steel, e.g. rubber lined, enamelled.

The valve internals must be aligned to close tolerances, especially for small valves, if the valve is not to leak when closed. Consequently, iron, steel and bronze are generally not used for these parts.

Furthermore, where erosion can occur, the valve port can be hard faced, e.g. with stellite. Most valves employ metal-to-metal seating, which generally has an acceptably low leakage rate. Where tighter shutoff is needed rubber other than soft materials can be used for the seat if the fluid pressure and temperature are not severe.

Valve Characteristics

Linear: The linear characteristic results in the change in flow coefficient being directly proportional to a change in valve travel.

Equal Percent: With equal percent characteristic, equal increments of valve travel produce equal percentage changes in the existing flow coefficient.

Quick Opening: The quick opening characteristic results in a rapid increase in flow coefficient with the valve reaching almost maximum capacity in the first 50% of its travel.

Shape of Opening: The characteristic is caused by the change in the shape of the port as valve travel changes. For example, in sliding stem valves, the equal percentage is achieved by having a small opening at the low travels.

Actuators

Actuator - a part of the final control element that translates the control signal into action of the final control device in the process eg. motors, solenoids, cylinders. The actuator selected must be capable of providing adequate torque output to overcome the dynamic torque forces on the disc or ball of the valve under flowing conditions. The actuator must also be capable of exceeding the 'breakout' torque requirements of the disc or ball at shutoff, in order to initiate rotation of the rotary valve shaft.

Current to Pressure Transducers and Positioner

Pneumatically operated valves depend on a positioner to take an input signal from a process controller and convert it to valve travel. These instruments are available in three configurations:

1. Pneumatic Positioner: A pneumatic signal (usually 3-15 psig) is supplied to the positioner. The positioner translates this to a required valve position and supplies the valve actuator with the required air pressure to move the valve to the correct position.

2. Analog I/P Positioner: This positioner performs the same function as the one above, but uses electrical current (usually 4-20 mA) instead of air as the input signal.

3. Digital Controller: Although this instrument functions very much as the Analog I/P described above, it differs in that the electronic signal conversion is digital rather than analog. The digital products cover these categories.

Digital Non-Communicating: A current signal (4-20 mA) is supplied to the positioner, which both powers the electronics and controls the output.

HART: This is the same as the digital non-communicating but is also capable of two-way digital communication over the same wires used for the analog signal.

Transducers and positioners convert electronic instrumentation signals into pneumatic or hydraulic pressures that control actuators and valves. A positioner is normally used when it is necessary to position a valve stem accurately with respect to the value of the instrumentation signal. When less accurate positioning is allowable, a transducer can be used to provide a more economical installation.

An effective I/P must provide air quickly, accurately and in sufficient quantity to a receiver. It must be able to exhaust air quickly when the signal decreases and be physically strong enough to withstand the difficult environmental conditions often found on industrial sites.

The input current signal, through a coil/armature arrangement, acts on a beam. The beam, a flapper, positions itself against a nozzle. This restricts the flow of air from the nozzle creating a back pressure which provides feedback via a bellows to position the flapper accurately. The result is a pneumatic signal proportional to the 4-20mA signal. This relatively small signal is fed to the booster replay to provide the final 20-100 kPa (or 3-15 psi) output.

If a rack mounted I/P is mounted at some distance from the final actuator, usually simple operations, for example observing the reaction of a valve actuator to changes of the I/P signal, become difficult, especially when there is no clear line-of-sight between the two pieces of equipment.

The length of the pneumatic signal lines creates further difficulties. It is more likely a booster or positioner will be necessary and the control loop gains another lag, adding to the difficulty of tuning the loop and sometimes degrading the resulting

control.

Other locally mounted I/P types include the piezoceramic bender/nozzle and deflector/nozzle.

Shut Down Valves

A shut down valve (also referred to as SDV orEmergency shutdown valve,ESV, orESDV) is anactuatedvalveinstalled in apipeline. It isolates aprocess unitfrom an upstream or downstream (gaseous or liquid) inventory upon activation of the process unit alarm and shutdown system.

Metal seatedball valvesare used as shut-down valves (SDV's). Use ofmetalseated ball va l ve s l e a d s t o ove r a l l l owe r c o s t s w h e n t a k i n g i n t o a c c o u n t lostproductionand inventory, and valve repair costs resulting from the use of soft seated ball valves which have a lower initial cost.

They are used as provisions for emergency shutdowns, for eg in the fuel supply lines.

Solenoid Valves

Asolenoid valveis anelectromechanicalvalvefor use withliquidorgascontrolled by running or stopping anelectric current through a solenoid, which is a coil of wire, thus changing the state of the valve. The operation of a solenoid valve is similar to that of a light switch, but typically controls the flow of air or water, whereas a light switch typically controls the flow of electricity. Solenoid valves may have two or more ports: in the case of a two-port valve the flow is switched on or off; in the case of a three-port valve, the outflow is switched between the two outlet ports. Multiple solenoid valves can be placed together on amanifold. They are often used as actuators for shut down valves.

A solenoid valve has two main parts: the solenoid and the valve. The solenoid converts electrical energy into mechanical energy which, in turn, opens or closes the valve mechanically. A Direct Acting valve has only a small flow circuit, shown within section E of this diagram (this section is mentioned below as a pilot valve). This Diaphragm Piloted Valve multiplies this small flow by using it to control the flow through a much larger orifice. Solenoid valves may use metal seals or rubber seals, and may also have electrical interfaces to allow for easy control. Aspringmay be used to hold the valve opened or closed while the valve is not activated.The diagram to the right shows the design of a basic valve. At the top figure is the valve in its closed state. The water under pressure enters atA.B is an elastic diaphragm and above it is a weak spring pushing it down. The function of this spring is irrelevant for now as the valve would stay closed even without

it. The diaphragm has a pinhole through its center which allows a very small amount of water to flow through it. This water fills the cavityCon the other side of the diaphragm so that pressure is equal on both sides of the diaphragm. While the pressure is the same on both sides of the diaphragm, the force is greater on the upper side which forces the valve shut against the incoming pressure. In the figure, the surface being acted upon is greater on the upper side which results in greater force. On the upper side the pressure is acting on the entire surface of the diaphragm while on the lower side it is only acting on the incoming pipe. This results in the valve being securely shut to any flow and, the greater the input pressure, the greater the shutting force will be.

Slide Valves

Theslide valveis arectilinearvalve used to control the admission of steam into, and emission of exhaust from, the cylinder of a steam engine. At HPCL-VR it is used in the FCCU between in the Reactor and Regenerator in the Catalytic Unit. The amount of catalyst exchanged cannot be controlled using control valves and so slide valves are used. Four slide valves are used:

For fresh catalyst For spent catalyst At Reactor top At Regenerator top

A double-acting slide valve cylinder.

Steam enters via the steam portSP, and is admitted by the slide valveSV through the upper passageS to push down the pistonP. At the same time, exhaust steam from

below the piston passes back up the lower passage S, via the valve cavity, to exhaustE. As the piston descends, the valve moves upwards to admit steam below the piston and release exhaust from above.

Agar Probes

Agar probes are installed in Desalter in the CDU for accurate measurement of interface level. The probes measure the water concentration around the probe. There are three probes provided at three elevations across the vessel. The bottom most probe LI1102D, indicates the sludge accumulation and normally reads close to 100%, indicating clear water phase at the bottom of the vessel. The interface probe LC1102A is located between the try cock two and three. 80% water concentration around this probe will ensure clear water in effluent and no water carry over in crude. The emulsion probe LI1102E normally should indicate 0% as long as there is a clear cut inter-phase between crude and water. A rise in the reading of emulsion probe indicates growing of emulsion phase, which may lead to carry over of water / sludge in crude. Demulsifier injection rate can be optimized based on the emulsion probe reading.

Programmable Logic Controller

A Programmable Logic Controller, or PLC for short, is simply a special computer device used for industrial control systems. It is a sequence controller, i.e it accepts inputs from switches and sensors, evaluates these in accordance with a stored program, and generates outputs to control machines and processes. They are used in many industries such as oil refineries, manufacturing lines, conveyor systems and so on. Where ever there is a need to control devices the PLC provides a flexible way to "softwire" the components together. It uses a programmable memory to store instructions and execute specific functions that include ON/OFF control, timing, counting, sequencing, arithmetic, and data handling.

History The early history of the PLC is fascinating. Imagine if you will a fifty foot long cabinet filled with relays whose function in life is to control a machine. Wires run in and out of the system as the relays click and clack to the logic. Now imagine there is a problem or a small design change and you have to figure it all out on paper and then shut down the machine, move some wires, add some relays, debug and do it all over again. Imagine the labor involved in the simplest of changes. This is the problem that faced the engineers at the Hydra-matic division of GM motors in the late 1960's. Fortunately for them the prospect of computer control was rapidly becoming a reality for large corporations as themselves. So in 1968 the GM engineers developed design criteria for a "standard machine controller". This early model simply had to replace relays but it also had to be:

A solid-state system that was flexible like a computer but priced competitively with a like kind relay logic system.

Easily maintained and programmed in line with the all ready accepted relay ladder logic way of doing things.

Work in an industrial environment with all the dirt, moisture, electromagnetism and vibration.

Modular in form to allow easy exchange of components and expandability.

This was a tall order in 1968 but four companies took on the challenge. 1. Information Instruments, Inc. (fully owned by Allen-Bradley a year later).

2. Digital Equipment Corp. (DEC)

3. Century Detroit

4. Bedford Associates

Bedford Associates won the contract and quickly formed a new company around the technology called MODICON after Modular Digital Control. By June of 1969 they were selling the first viable Programmable Controller, the "084" which sold over one thousand units. These early experiences gave birth to their next model the "184" in 1973 which set Modicon as the early leader in programmable controllers. Not to be outdone, the powerhouse Allen-Bradley (all ready known for its rheostats, relays and motor controls) purchased Information Instruments in 1969 and began development on this new technology. The early models (PDQ-II and PMC) were deemed to be too large and complex. By 1971 Odo Struger and Ernst Dummermuth had begun to develop a new concept known as the Bulletin 1774 PLC which would make them successful for years to come. Allen-Bradley termed their new device the "Programmable Logic Controller" (patent #3,942,158) over the then accepted term "Programmable Controller". The PLC terminology became the industry standard especially when PC became associated with personal.

A PLC System The basic units have a CPU (a computer processor) that is dedicated to run one program that monitors a series of different inputs and logically manipulates the outputs for the desired control. They are meant to be very flexible in how they can be programmed while also providing the advantages of high reliability (no program crashes or mechanical failures), compact and economical over traditional control systems.

Unlike a personal computer, though the PLC is designed to survive in a rugged industrial atmosphere and to be very flexible in how it interfaces with inputs and outputs to the real world.

PLCs come in many shapes and sizes. They can be so small as to fit in a shirt pocket while more involved controls systems require large PLC racks. Smaller PLCs (a.k.a. bricks) are typically designed with fixed I/O points. The PLCs used at HPCL are the modular ones. Its called modular because the rack can accept many different types of I/O modules that simply slide into the rack and plug in.

The components that make a PLC work can be divided into three core areas. The power supply and rack The central processing unit (CPU) The input/output (I/O) section

! Power Supply and Racks:

The rack is the component that holds everything together. Depending on the needs of the control system it can be ordered in different sizes to hold more modules. Like a human spine the rack has a backplane at the rear which allows the cards to

communicate with the CPU. The power supply plugs into the rack as well and supplies a regulated DC power to other modules that plug into the rack. The most popular power supplies work with 120 VAC or 24 VDC sources.

The CPU:

The brain of the whole PLC is the CPU module. This module typically lives in the slot beside the power supply. Manufacturers offer different types of CPUs based on the complexity needed for the system.

The CPU consists of a microprocessor, memory chip and other integrated circuits to control logic, monitoring and communications. The CPU has different operating modes. In programming mode it accepts the downloaded logic from a PC. The CPU is then placed in run mode so that it can execute the program and operate the process.

Since a PLC is a dedicated controller, it will only process this one program over and over again. One cycle through the program is called a scan time and involves reading the inputs from the other modules, executing the logic based on these inputs and then updated the outputs accordingly. The scan time happens very quickly (in the range of 1/1000th of a second). The memory in the CPU stores the program while also holding the status of the I/O and providing a means to store values.

I/O System:

The I/O system provides the physical connection between the equipment and the PLC. Opening the doors on an I/O card reveals a terminal strip where the devices connect.There are many different kinds of I/O cards which serve to condition the type of input or output so the CPU can use it for its logic. It's simply a matter of determining what inputs and outputs are needed, filling the rack with the appropriate cards and then addressing them correctly in the CPUs program.

Input Module: These modules act as interface between real-time status of process variable and the CPU.

Analog input module: Typical input to these modules is 4-20 mA, 0-10 V. For eg: Pressure, Flow, Level Tx, RTD (Ohm), Thermocouple (mV)

Digital input module: Typical input to these modules is 24 V DC, 115 V AC and 230 V AC. For eg: Switches, Pushbuttons, Relays, pump valve on off status.

Output Module: These modules act as link between the CPU and the output devices in the field.

Analog output module: Typical output from these modules is 4-20 mA, 0-10V. For eg: Control Valve, Speed, and Vibration

Digital output module: Typical output from these modules is 24 V DC, 115 V AC and 230 V AC. For eg: Solenoid Valves, lamps, Actuators, dampers, Pump valve on off control.

! PLC and PC are said to be similar in their physical construction but differ in their functions.

A PLC is specifically designed for harsh conditions with electrical noise, magnetic fields, vibration, extreme temperatures or humidity. Common PCs are not designed for harsh environments. Industrial PCs are available but cost more. By design PLCs are friendlier to technicians since they are in ladder logic and have easy connections. Operating systems like Windows are common. Connecting I/O to the PC is not always as easy. PLCs execute a single program in sequential order. They have better ability to handle events in real time. PCs, by design, are meant to handle simultaneous tasks. They have difficulty handling real time events.

Programming languages for PLCs

The term PLC Programming Language refers to the method by which the user communicates information to the PLC. The three most common language structures are: Ladder Drawing Language, Boolean Language and Functional Chart. Commonly used programming languages are:

Ladder Logic (LAD/LD) Structured Text (ST) Instruction List (IL) Sequential Functional Chart (SFC) Function Block Diagram (FDB)

!

Ladder Logic

A Program is a user developed series of instructions or commands that directs the PLC to execute actions. A Programming Language provides rules for combining the instructions so that they can produce the desired actions. The most