Why This Module Is Important - Active Learnercloud1.activelearner.com/contentcloud/portals/hosted3/PetroAcademy/PRS-RFS/PRS-RFS-1.pdfHeating and Cooling Medium Potential Causes of
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
Why This Module Is Important
If we are interested in process safety, we need to understand thebasics of relief and flare, including depressuring
The Marsh incident database, published every year, consistentlyshows fire and explosion as the leading cause of loss in most ofthe major incidents
Our understanding of relief and flare also affects how we operateand maintain our facilities, and how we respond to emergencies
If we do not have a good understanding of the principles thatgovern relief and flare design and operation, we are not likely toadequately update and maintain our relief and flare system
Overpressure
Relief and Flare Systems Core
Relief and Flare Systems Core ═════════════════════════════════════════════════════════════════════════
Pressure Level Relationships for Pressure Safety Valves
One reason to add more relief valves is that the relief volume is so large that it needs more
than one valve.
It is possible for other valves to be set at pressures up to 105% of the MAWP. In the case of multiple valves the maximum allowable overpressure is 116%.
Accumulation is a characteristic reference of the pressure vessel side of things and the pressure vessel code.
Overpressure is a characteristic related to the sizing of relief valves.
Pressure Safety Types
There are three major types of Pressure Safety Valves(PSV’s):
• Conventional
• Balanced bellows
• Pilot operated
Relief and Flare Systems Core ═════════════════════════════════════════════════════════════════════════
Wide range of materials High temp version available Lowest cost
Prone to leakage (if not soft seat) Long simmer/flow period Vulnerable to effects of inlet
pressure loss Sensitive to backpressure
Balanced Bellow Relief Valves
Advantages Disadvantages
Less sensitivity to back pressure Wide range of materials High temp version available
Prone to leakage (if not soft seat) Long simmer/flow period Vulnerable to effects of inlet
pressure loss Limited bellows life Higher maintenance costs
The failure of bellows in relief valves is a major concern. If the bellows fail, it means that the pressure in the relief header downstream of the valve is going to be inside the bellows providing extra force on areas of the valve that add to the spring force trying to keep the valve closed. If the bellows have failed, the valve is no longer balanced and is now subject to the backpressure effect. We have accepted a higher back pressure because of the bellows. If the bellows has failed and the bonnet is not vented, we have converted the bellows valve to a conventional valve. If the back pressure exceeds 10%, we can expect it to affect the pressure at which the relief valve will lift. If the bellows has failed and the bonnet is vented, we will have some flare gas being released to the atmosphere.
Pilot Operated Relief Valves
Advantages Disadvantages
Smaller and lighter than other valves Higher pressure ratings in large sizes Good seat tightness before opening
and after reclosing Easier maintenance Easily testable on-site, in service Not affected by back pressures so far
as opening is concerned, but BPover 50% will reduce the flow rate
Dirty service limitations Limited chemical / high temp
compatibility when soft-seat ando-ring piston seals are used
Overview – Pros and Cons
Conventional Relief Valves
Relief and Flare Systems Core ═════════════════════════════════════════════════════════════════════════
Overpressure Protection – Design and Installation Considerations
Calculate relieving temperature, particularly if low due toJoule-Thompson effect (material selection issues)
Inlet pressure drop and back pressure limits (API 520)
Location of the device on the equipment; if it cannot be installed on theequipment, then installation of pilot operated valve should beconsidered
Sparing of pressure relief valves for maintenance; changeover ofpressure relief valves must not leave the equipment in serviceunprotected at any time
Always use full bore valves for isolation of relief devices
In corrosive or fouling services, a rupture disc is often installedupstream of the relief valve to protect the relief valve
Relief valves that are in service must have the isolation valves lockedor car sealed open; spare relief valves must have the isolation valveslocked or car sealed closed
Relief and Flare Systems Core ═════════════════════════════════════════════════════════════════════════
The use of Ruptured Disk and Buckling Pin are uncommon pressure relieving devices.
Disadvantages
These devices do not re-close. Once ruptured and open, they stay open until the upstream equipment has been de-pressured down to zero. There are some cases where this is okay.
This type of equipment is fast acting but generally their application is very limited.
Rupture disks are occasionally used on the shell side of an exchanger that has very high pressure gas in the tubes. This protects the shell from overpressure due to tube rupture.
Relief and Flare Systems Core ═════════════════════════════════════════════════════════════════════════
During depressurization, the inlet and outlets (both gas and liquid) are closed.The depressurizing valve is opened and the gas of the vessel is evacuated via aRestriction Orifice (RO) or fixed choke to a safe location, normally a flare or ventsystem.
Depressurization is often done using a control valve.
Depressurization and blowdown are often used interchangeably.
Blowdown
Strictly only applicable to cases where the liquid contents are removed from thevessel.
Normally, main equipment items in a plant are depressurized during anemergency.o Including separation vessels, heat exchangers, distillation columns and
compressors.
Plant emergency depressurization systems reduce the risk of escalation in caseof a fire or a leak of explosive or toxic gas.
Terminology
Depressurization
Relief and Flare Systems Core ═════════════════════════════════════════════════════════════════════════
Processing facilities are typically “sectionalized” intosmaller areas by automatic shutdown valves basedon identified credible hazard scenarios
• Results in more manageable fire protection system designs,including blowdown flows
If blowdown flows to the flare system are still toolarge, a staggered blowdown strategy can sometimesbe implemented open blowdown valves insequence, not all at once
Emergency Depressurizing / Blowdown
Learning Objectives
This section has covered the following learning objective:
Describe the purpose and operation of a depressurization system
Relief and Flare Systems Core ═════════════════════════════════════════════════════════════════════════
The separation train is part of a much larger integrated oil and gas facility. It is permanently manned and has a trained emergency response team, and uses a staged blowdown logic for sitewide depressuring. A control loop, such as that which includes the control valve between the HP and IP separators, has an industry typical failure rate of once in ten years. The loop includes everything from the sensor to the trim inside the control valve; everything that may prevent the valve from moving appropriately given a change in level. Choose all applicable answers.
a) Add an emergency isolation valvebetween the HP and IP separators
b) Add a second relief valve at 105%of MAWP to the IP separator
c) Add depressuring valves to allthree separators, in parallel withthe relief valves
d) Add a depressuring valve to theHP separator only, in parallel withthe relief valve
e) Add high capacity rupture discs setat 121% of MAWP in parallel with therelief valves on all three separators
f) Add a deluge systemg) Do nothing, the risk is acceptableh) Add an emergency isolation valve
either upstream or downstream ofthe pump
Relief and Flare Systems Core ═════════════════════════════════════════════════════════════════════════
The separation train is part of a much larger integrated oil and gas facility. It is permanently manned and has a trained emergency response team, and uses a staged blowdown logic for sitewide depressuring. A control loop, such as that which includes the control valve between the HP and IP separators, has an industry typical failure rate of once in ten years. The loop includes everything from the sensor to the trim inside the control valve; everything that may prevent the valve from moving appropriately given a change in level. Choose all applicable answers.
a) Add an emergency isolation valvebetween the HP and IP separators
b) Add a second relief valve at 105%of MAWP to the IP separator
c) Add depressuring valves to allthree separators, in parallel withthe relief valves
d) Add a depressuring valve to theHP separator only, in parallel withthe relief valve
e) Add high capacity rupture discs setat 121% of MAWP in parallel with therelief valves on all three separators
f) Add a deluge systemg) Do nothing, the risk is acceptableh) Add an emergency isolation valve
either upstream or downstream ofthe pump
Question 2
Would your answer be different if the separation train was part of a small, remote, upstream facility with no emergency response capability and no significant offsite safety or environmental risk?
Why?
a) Yesb) No
a) Much lower risk, so insufficient justification for the upgrade. Isolate andabandon.
b) The system as is could rupture in a fire, placing anyone who has notevacuated at risk and exposing the company to bad publicity.
Relief and Flare Systems Core ═════════════════════════════════════════════════════════════════════════
Would your answer be different if the separation train was part of a small, remote, upstream facility with no emergency response capability and no significant offsite safety or environmental risk?
Why?
a) Yesb) No
a) Much lower risk, so insufficient justification for the upgrade. Isolate andabandon.
b) The system as is could rupture in a fire, placing anyone who has notevacuated at risk and exposing the company to bad publicity.
Relief and Flare Systems Core ═════════════════════════════════════════════════════════════════════════
Used to prevent liquid in the relieving/vent stream gas from reachingthe flare stack and tip
Prevent “burning rain” Prevent flame extinguishment, e.g. water carryover Minimize combustion instability/smoke formation Horizontal and vertical vessels used Normally don’t have mist extractors Typically sized for removal of 300–600 micron liquid drops Liquid hold-up requirements (examples only, actual designs must be
considered case-by-case):Max relief load for 90 seconds One well for 15 minutes (offshore)
Seal Drums
Purpose
Flash back protection Maintains slight positive back
pressure on upstream relief/flareheader system
Commonly used in refineries Not so common in upstream E&P
facilities
Knockout Drums
Water Seals (Liquid Seals) provide a positive means of flash-back prevention in addition to enabling the upstream flare system header to operate at a slight positive pressure at all times. This is especially of use when an elevated flare is used in combination with another flare or with a flare gas recovery system.
The water seal vessel is fitted with a special saw-tooth dip leg and anti-pulsation baffle to minimize pulsing. Within the water seal, the water level is preferably maintained by a constant overflow weir in combination with a suitable 'S' bend drainpipe. Filling rates will be sufficient to re-establish the seal within 5 minutes if the liquid seal is broken. Vessel may be equipped with an internal steam coil/sparger for winterization purposes as required.
Relief and Flare Systems Core ═════════════════════════════════════════════════════════════════════════
Oxygen-free gas introduced to the flare header/stack to providesufficient flow to prevent air entry from the tip of the flare stack
Typically need ~ 0.05–0.1 ft/sec (0.015–0.03 m/s) velocity (minimum)up stack using methane
May need higher purge rates/velocities to prevent burn-back tipdamage, depending on the tip design
Inverted Gas Seal (Molecular Seal)
Vmin ~ 0.01 ft/sec (0.003 m/s) The stack comes in from the bottom, gas comes up, reverses, flows down, reverses again and flows up. This works because the air will have a significantly higher molecular weight than any credible purge gas. Air molecular weight is about 29 and methane molecular weight is about 16. The heavier molecules are going to be trapped at the base of the seal, unable to rise against the purge gas flow and re-enter the flare stack. Keeping the inverted gas seal free of trouble causing liquid and solid level builds makes the drain very important. It prevents water accumulation and in some places sand accumulation.
Velocity Seal (Fluidic Tip)
Vmin ~ 0.04 ft/sec (0.012 m/s) The purge gas will accelerate, decelerate, and accelerate again as it works its way up the stack of truncated cones. The air will unsuccessfully try to migrate down the stack through the seal because it is met by relatively high velocity gas coming out.
Relief and Flare Systems Core ═════════════════════════════════════════════════════════════════════════
When looking at the variables and the logic behind sizing, usually the dominant factor is ground level thermal radiation at the distance of interest, represented by the distance “R”. Other factors such as ground level sulfur dioxide concentrations can be significant, however ground level thermal radiation dominates the stack height.
A simplifying assumption is that energy is emitted from the mid-point of a flame. When sizing, the height of the stack and one-half the length of the flame defines the midpoint that we will use as the source of the radiated energy. This will enable us by Pythagoras’ Theorem, (right angle triangle, so square the hypotenuse equals the sum of squares of the other two sides) to calculate “D”, the distance the thermal radiation would have to travel before it reaches the point of interest.
Relief and Flare Systems Core ═════════════════════════════════════════════════════════════════════════
Allowance is made for solar radiation, plus the heat that is being radiated from the flare itself. Either kilowatts per square meter or Btu per hour per square foot tells how much thermal radiation is tolerable. We would probably put the fence at the distance corresponding to safe continuous exposure. A second fence may be installed, often at the distance corresponding to safe one minute exposure with normal PPE (personal protective equipment). Anyone working in the area would still have time for safe escape.
A safe exit from the area of a flare is needed. This can be critically important, particularly for an off-shore facility where the evacuation routes from the platform might be affected by flare radiation.
Relief and Flare Systems Core ═════════════════════════════════════════════════════════════════════════
What is the jurisdiction? What are the local rules? What do applicable utilities cost? Is weight a significant variable?Can relatively high thermal emissions be compensated for with a higher stack?
Relief and Flare Systems Core ═════════════════════════════════════════════════════════════════════════
In more detail, high pressure gas is coming up around the annulus exiting through the slot, therefore, the tulip profile. A gas film will be seen at a low-pressure region developing between the gas film and the tulip that will draw the gas in next to the tulip, air being drawn in and a viable mixture reaching the top of the tulip.
Relief and Flare Systems Core ═════════════════════════════════════════════════════════════════════════
A ground flare is where the combustion takes place at ground level. It varies in complexity, and may consist either of conventional flare burners discharging horizontally with:
Low visibility/low noise design Short flame length Open pit/totally enclosed designs
The type is the multijet flare (enclosed type). Compared to elevated flares, ground flares can achieve smokeless operation as well. They can be fairly quiet because the flow is split among multiple small burners and are much less visible than an elevated flare. However, they have poor dispersion of combustion product because the stack is near to ground; which may result in severe air pollution or hazard if the combustion products are toxic or in the event of flame-out.
Relief and Flare Systems Core ═════════════════════════════════════════════════════════════════════════
Loss of flame/pilot Operation without an operating flare causes a serious and unsafe situation.
Liquid carryover Burning rain results in a number of fires around the flare. If water, the flame could be extinguished, or with two phase flow the capacity can be restricted.
Flashback – air intrusion Guard against flashback with a velocity seal, a molecular seal or with the water seal down at the base of the stack.
Loss or insufficient purge If there is less than adequate purge, then there is a significantly higher probability of air intrusion
Steam control under/over Care must be taken in regards to flow rate and ensuring that there is neither too much nor too little.
Freezing condensate in cold climates
To avoid possible freezing in the knockout drum, include a heater.
Inconsistent composition, pressure and temperature
Flares have to be designed as such that they will be robust and variation tolerant to the credible range of compositions, pressures and temperatures that it they may see.
Brittle fracture of material for cold relief
If a cold relief is going to be done, or have significant cooling due to Joule-Thomson, it must be done safely.
Blockage Blockage due to hydrate formation has occurred on several occasions. Don’t mix a cold flare with a wet flare.
Noise and light Depending on the location of the facility, noise and light may be a significant factor in terms of local regulation and community relations.
Thermal Radiation This is usually the governing factor in determining the height of the flare. In the maximum relief case, we ensure that a threat is not imposed on people, facilities or neighbors.
Relief and Flare Systems Core ═════════════════════════════════════════════════════════════════════════
There is a normally closed pressure control valve in the header betweenthe knockout drum and the flare stack.
The flare gas recovery line goes through a cooler, into a separator. Thegas is compressed and returned to any convenient point in the process.
The liquids are returned to the flare knockout drum, where they will berecovered along with any other liquids that drop out in the drum.
There will be a bypass line to enable quick and reliable opening of thecontrol valve. The bypass line will have a rupture disc or a relief valve.
These alternatives involve additional maintenance, additional cost, andadditional opportunities for failure.
For low pressure applications, a liquid seal drum should be consideredas it is simpler and often less expensive.
Flare Gas Recovery
A flare gas recovery system is designed to recover flare gas under normal operations not under emergency flaring. A common way of doing this is shown in the following image.
Relief and Flare Systems Core ═════════════════════════════════════════════════════════════════════════
The flare gas comes into a knockout drum, the header goes to a liquid seal drum, and from there connects into the flare stack. The flare gas recovery line goes to a compressor. This example shows a liquid ring compressor. That may be used for small streams. For larger streams, it may be a reciprocating compressor or a screw compressor. The gas will go to a three-phase separator where liquid hydrocarbon or water will be removed, and the gas will go to flare gas recovery. The compressor will need a recycle line which will bring the gas back into the suction.
Relief and Flare Systems Core ═════════════════════════════════════════════════════════════════════════
This example is using the pellet ignition system. Shown is the launcher and launch tube. The pellet will strike the striker plate providing a shower of high energy sparks, igniting the flare gas at the end of the flare boom.
Flare Gas Recovery with Ballistic Pellet Ignition
The image below shows flare gas recovery, a flare, and ignitor. At the bottom left, the flare header is shown coming into the knock-out drum. A line leaving the knock-out drum goes to the flare gas recovery compressor. The line on the right is the flare header. The main valve is shown closed with a rupture disk in a bypass line. In case of a genuine flaring emergency, the disk would burst and the gas would flow to flare.
Relief and Flare Systems Core ═════════════════════════════════════════════════════════════════════════
Identify major components of a flare system and describe theirpurpose
Classify and discuss the different flare ignition methods
Compare the flare tips and their characteristics
This section has covered the following learning objectives:
Back to Work Suggestions
Relief and Flare Systems Core
Leverage the skills you’ve learned by discussing the skill module objectives with your supervisor to develop a personalized plan to implement on the job. Some suggestions are provided.
RReeview your view your ffllarare header as-built eleve header as-built elevations.ations.
Is Is itit self self-dr-draaining ining to tto thhe dre drum?um? IIff nonott,, how how is is drdrainaainage handlege handled? d?
AArre e ssuuititable able manamanagementgement systems in systems in plplaacce to e to ggiive ve yyoou confu confidence idence tthhaatt dr drainaainage ge wwiill ll bbee aaddeeqquauate te in in an emergencan emergency?y?
Check your fCheck your fllaarre e drum desdrum desiign.gn.
WWhhaatt is is tthhe desige designn basis? basis?
Is Is therthere ane anyytthing hing thathatt can can pluplugg up or up or blow blow out? out?
HHaavvee ther there be beeen en changes in changes in ffeeeedd r raate, te, compcomposition, osition, or or opopereraatting ing conditconditions tions thhaatt could rcould reequirquire a e a rreevviieeww of the of the aaddeeqquauaccyy of the fof the fllarare e drdrum? um?
TThherere should be should bee MOMOCs Cs in in plplaacce to e to answanswer thaer thatt, , butbut it it is is nonott unkunknnown own ffoor r severseveraal l small small cchhanges anges to to aadd up to dd up to ssoomemetthhing ing sigsignnifificicantant..
Relief and Flare Systems Core ═════════════════════════════════════════════════════════════════════════
PetroAcademyTM Process Safety Engineering Skill Modules
Process Safety Risk Analysis and Inherently Safer Design Core
Process Hazards Analysis and Layers of Protection Analysis Core
Leakage and Dispersion of Hydrocarbons Core
Combustion Behavior of Hydrocarbons Core
Sources of Ignition and Hazardous Area Classification Core
Specific Plant Systems and Equipment Core
Relief and Flare Systems Core
Historical Incident Databases, Plant Layout and Equipment Spacing Core
SIS, Monitoring and Control Core
Fire Protection Systems Core
Back to Work Suggestions
Relief and Flare Systems Core
Leverage the skills you’ve learned by discussing the skill module objectives with your supervisor to develop a personalized plan to implement on the job. Some suggestions are provided.
Field check Field check at least one sub at least one sub header and header and thethemainmain flar flare heade headerer; ; chochooosse a e a susubb head header er withwithaass much elev much elevaattion ion difference difference aass you you cacan findn findanandd access. access.
ArAre e therthere ane any diffy differerences ences bbeetwtweeeen the n the drdraawwing ing and rand reealitality?y?
Relief and Flare Systems Core ═════════════════════════════════════════════════════════════════════════