Control-instrumentation

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Control &

Instrumentation

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Instrumentation and control

Instrumentation and control system is designed for minimum local manning and operator attention.

The basic need of the instrumentation us for safe operation and efficient functioning of the plant.

This can be achieved by installing optimum number measurement, indicating, transmitting and controlling instruments for all required parameters of the plants (e.g. pressure, level, temperature, flow etc.)

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BASIC COMPONENTS OF A CONTROL SYSTEM

Sensor• Also often called as Primary Element.• Acquires information about the status of the process variables.• Typical examples:

RTD/thermocouples (for temperature measurements), Capacitance type cells (for liquid level /draft /pressure measurements), etc.

Controller• The Brain or Heart Of the control system (the decision maker).• It is the hardware element with Built-in capacity for performing the

only task requiring some forms of Intelligence.• Typical examples:

Electronic controllers, digital computers used as controller.

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BASIC COMPONENTS OF A CONTROL SYSTEM

Transmitter• Secondary Element.• Responsible of passing the information acquired by the sensor to

controller and sending the controller decision to the final control element.

• Measurement and control signals may be transmitted or as electrical signals.

• Typical examples:

Electrical transmitters.

Final control element• Have the task of actually implementing the control command

issued by the controller on the process.• Typical examples:

Control valve, variable speed motors, electric motors etc.

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BASIC COMPONENTS OF A CONTROL SYSTEM

• The importance of these components is that they perform the three basic operations that must be present in every control system:

1. Measurement:

Measuring the variable to be controlled is usually done by the combination of sensor and transmitter.

2. Decision:

Based on the measurements and the set point, the controller must then decide what to do to maintain the variable at its desired value.

3. Action:

As the result of the controller's decision, the system must then take an action. This is usually accomplished by the final control element.

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CONTROL SYSTEM

I/O CARDS

PROCESSOR

I/O CARDS

OPERATOR STATIONS

ENGG

STATION

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Basic Parameters

Measurement & Control of following Parameters are essential for

Instrumentation

• Pressure

• Temperature

• Flow

• Level

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TAPPING POINTS

GASES

STEAMGASES

LIQUIDS

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PressurePressure measurements are one of the most common measurements

required in the boiler .These range from very low -1500mmwc Draft to very high steam pressure 150 kg/cm2 .That include different type of media like steam ,water, fuel oil, air, gas each with varying degree of accuracy and reliability.

• Pressure varies depending on altitude above sea level, weather pressure fronts and other conditions. The measure of pressure is, therefore, relative and pressure measurements are stated as either gauge or absolute. A gauge pressure device will indicate zero pressure when bled down to atmospheric pressure (i.e., gauge pressure is referenced to atmospheric pressure). Gauge pressure is denoted by a (g) at the end of the pressure unit [e.g., kPa (g)].

• Absolute pressure includes the effect of atmospheric pressure with the gauge pressure. It is denoted by an (a) at the end of the pressure unit [e.g., kPa (a)]. An absolute pressure indicator would indicate atmospheric pressure when completely vented down to atmosphere - it would not indicate scale zero.

• Absolute Pressure = Gauge Pressure + Atmospheric Pressure • The majority of pressure measurements in a plant are gauge.

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Pressure

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Pressure

• Pressure measuring devices• The common pressure measuring devices

used in IJT boilers are• Diaphragm, capsule gauges for low

pressure measurement • Bourdon tube gauges for medium and high

pressure • Smart type Pressure Transmitter with

required range

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Pressure

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Pressure

• Installation:• Pressure Gauges are installed on site using

accessories as follows: • Syphons• Sunbber• Seal • 2 way manifold

These are protective devices for pressure measuring instrument from surges & pulsations. These devices used as per application requirement

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Pressure Gauges (Normal Installation)

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Pressure Gauges along with Syphon

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Pressure Gauges along with Sunbber:

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Pressure Transmitter (Normal Installation)

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Temperature Measurement

• Temperature is measured by following:• Temperature Gauge:1. Mercury in Steel type2. Bi-Metallic3. Gas Filled• Temperature Sensors:1. RTD2. Thermocouple

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Temperature Measurement

• Installation:

• Temperature Gauges / Sensors are installed on site using thermowell.

It is a protective devices for temperature measuring instrument from damages.

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Temperature Gauge / ELEMENT(Normal Installation)

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Temperature Transmitter (Normal Installation)

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Flow Measurement

• The common flow measuring devices used in IJT boilers are

• Head Meter :

(Restriction type flow meter )• Orifice Plate• Flow nozzle• Rotameter ( variable Area Type)

• Positive Displacement meter

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Flow Measurement

• Installation:• Flow sensors are installed in line of the fluid to

create DP & hook-ed up with transmitter for measuring Differential Pressure, which corresponds to flow:

• Q = K1* (P/T ) * DPWhere,DP = differential pressure across the flow elementP = Main Steam PressureT = Main steam temperatureK1 = ConstantQ = Compensated flow

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Flow Transmitter (STEAM)

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Flow Transmitter (AIR)

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Drum Level

STEAM

WATER

H: Drum Center to Center Distance

h: Height of water inside pressurized drum

HP LP

Condensate Pot with constant Head as a wet Leg

D1 = Water Density Inside Drum D2 = Water Density in Leg

HP – LP = (P+hD1g) – (P+HD2g)Diff. Pr., ∆ P= hD1g – HD2g = hD1 – HD2(Where ∆ P,h and H are measured in mm of water)(D1 & D2 are in gm/cc)Hence, h = ∆ P + HD2 D1

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Drum Level

Drum Level Control Method Single Element Three Element

Shrink & Swell EffectShrink : When Steam Flow Decreases

Drum Level Decreases Swell : When Steam Flow Increases

Drum Level Increases

Basic Elements Drum Level Steam Flow Feed Water Flow

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Drum Level

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Transmitter

• Typical Basic Operation of Transmitter

Sensing Element + Transducer = Transmitter unit

SENSINGELEMENT

INDICATIONTRANSDUCER

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Transmitter

• Transmitters are generally use is of 2 wire type. This is the most widely used method for transmitter connections .There are three basic elements in this loop, namely Power Supply , transmitter and the receiving instrument .They are connected in series and the transmitter acts as a current regulator in the series circuit . The current in the series circuit changes with respect to change in process parameter. This simplifies cabling and reduces erection and cable cost.

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Transmitter

Normal Pressure Transmitter

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Transmitter

Differential Pressure Transmitter

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TROUBLE SHOOTING OF TRANSMITTER

I. POTENTIAL SOURCE • LOOP WIRING

II . HIGH OUTPUT• POTENTIAL SOURCE : • 1.PRIMARY ELEMENT• 2.IMPULSE PIPING• 3.TRANSMITTER ELECTRONICS• 4.SENSING ELEMENT

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TROUBLE SHOOTING OF TRANSMITTER

III . LOW OUTPUT OR NO OUTPUT

• POTENTIAL SOURCE :

• 1. PRIMARY ELEMENT

• 2. LOOP WIRING

• 3. IMPULSE PIPING

• 4. SENSING ELEMENT

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TROUBLE SHOOTING OF TRANSMITTER

IV .TRANSMITTER DOES NOT CHARACTERIZE PROPERLY

• POTENTIAL SOURCE : • 1.PRESSURE SOURCE /CORRECTION• 2.mA METER• 3.POWER SUPPLY• 4. TRANSMITTER ELECTRONICS• 5. SENSING ELEMENT

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Control Valve

Being the Final Control Element in a system is not an easy job. To start with, Control Valves are blamed for any and all problems that crop up in the process. Control Valve are subjected to corrosion, high velocity, cavitations, flashing liquids, cryogenic temperatures, abrasion, high temperatures and thermal shock. Control Valve are expected not only to throttle along through all this, but most likely, you are also being asked to act as a block valve and shut off tight.

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Control Valve Classification

Rotary Motion

Linear Motion

Butterfly

Eccentric Plug

Ball

Swing Through

Lined

Eccentric

Segmented

Full

V-Notch

Globe

Diaphragm

Pinch or Clamp

Globe

Angle

3-Way

Single Seated

Double Seated

Split body

Control

Valve

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Control Valve Flow Characteristics

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Control Valve Leakage Classifications

• Class I: Identical to Class II, III, and IV in construction and design intent, but no actual shop test is made.

• Class II:Intended for double-port or balanced singe-port valves with a metal piston ring seal and metal-to-metal seats. Air or water at 45 to 60 psig is the test fluid. Allowable leakage is 0.5% of the rated full open capacity.

• Class III:Intended for the same types of valves as in Class II. Allowable leakage is limited to 0.1% of rated valve capacity.

• Class IV:Intended for single-port and balanced single-port valves with extra-tight piston seals and metal-to-metal seats. Leakage rate is limited to 0.01% of rated valve capacity.

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Control Valve Leakage Classifications

• Class V:Intended for the same types of valves as Class IV.The test fluid is water at 100 psig or operating pressure. Leakage allowed is limited to 5 X 10 ml per minute per inch of orifice diameter per psi differential.

• Class VI:Intended for resilient-seating valves.The test fluid is air or nitrogen. Pressure is the lesser of 50 psig or operating pressure. The leakage limit depends on valve size and ranges from 0.15 to 6.75 ml per minute for valve sizes 1 through 8 inches.

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Basic Control Loops in BOILER

Pressure Control F W header pressure Furnace Draft Combustion Control (Main Stem Pressure) Soot Blower De-aerator

Level control Drum Levelo Single Elemento Three Element De-aerator CBD

Temperature Control Super Heated Steam