# Fundamentals of Pressure Relief Devices

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Fundamentals of Pressure Relief Devices

Pressure Relief Devices What’s coming
Basic terminology Code requirements Safety relief valves Rupture discs Rupture pins Explain that we will be covering these topics in this module.

Pressure Terminology Operating pressure MAWP Design pressure
Set pressure Accumulation Overpressure Blowdown Page 1 Operating Pressure: Pressure usually subjected to during normal operation Design pressure: Set by process conditions Pressure which will provide a suitable margin above the operating pressure in order to prevent nuisance trips Normally set at greater of 25psi or 10% of set pressure above the operating pressure MAWP Maximum Allowable Working Pressure Defined in the construction codes for unfired vessels (ASME) Set by metallurgical conditions based on type of material and its thickness Must be equal to or greater than the design pressure Set pressure Inlet pressure at which the valve is adjusted to open Liquid service valve starts to open Vapor service valve pops Accumulation Pressure above MAWP that results during a release. Code allows 10%. Overpressure Any pressure above the SP.

Superimposed Back Pressure
Pressure in discharge header before valve opens Can be constant or variable Superimposed back pressure is always present In the example the suction pressure of the pump is always on the outlet of the pressure relief valve. It can be constant or variable depending on the nature of the system. Superimposed back pressure can give us problems if variable when trying to specify a PRV. What will the back pressure be when the valve is called upon, i.e. will it be lifting alone or as part of a plant wide release? Could affect the type of pressure relief valve chosen. More of a problem for valves with lower set points as pressure variation will be a greater percentage of set pressure.

Built-up Back Pressure
Pressure in discharge header due to frictional losses after valve opens Total = Superimposed + Built-up Pressure which develops as a result of flow in the discharge header after the pressure relief valve opens Is not present when valve is closed. In the example no built-up back pressure exists until the valve opens. Upon opening the built-up back pressure develops and increases as the flow through the valve increases until full lift. The sum of the superimposed and built-up back pressure will give the total back pressure on the pressure relief valve outlet. The PRV does not care where the back pressure comes from.

Code Requirements General Code requirements include:
ASME Boiler & Pressure Vessel Codes ASME B31.3 / Petroleum Refinery Piping ASME B16.5 / Flanges & Flanged Fittings ASME Section 8 for unfired vessels ASME B31.3, “Process Piping”, This Code prescribes requirements for materials and components, design, fabrication, assembly, erection, examination, inspection, and testing of piping. ASME B16.5, “Pipe Flanges and Flanged Fittings”, This Standard covers pressure-temperature ratings, materials, dimensions, tolerances, marking, testing, and methods of designating openings for pipe flanges and flanged fittings.

Code Requirements All pressure vessels subject to overpressure shall be protected by a pressure relieving device Liquid filled vessels or piping subject to thermal expansion must be protected by a thermal relief device Multiple vessels may be protected by a single relief device provided there is a clear, unobstructed path to the device At least one pressure relief device must be set at or below the MAWP Not necessary to protect a vessel with a relief device if there is no way to overpressure the vessel. For example a vessel downstream of a pump that is rated for greater than the deadhead pressure of the pump if the vessel is not in a fire risk area and does not contain flammable material. Thermal protection for long lines is especially important. CSO valves are acceptable in a free path although there should not be several. At least one valve must be set at the MAWP. Additional valves may have set points up to 105% of the MAWP.

Code Requirements Relieving pressure shall not exceed MAWP (accumulation) by more than: 3% for fired and unfired steam boilers 10% for vessels equipped with a single pressure relief device 16% for vessels equipped with multiple pressure relief devices 21% for fire contingency Go over bullets.

General Types of Safety Relief Valve Design
Direct acting type Oldest and most common Kept closed by a spring or weight to oppose lifting force of process pressure Pilot operated type Kept closed by process pressure Let’s look at some relief valves now. There are two basic types - direct acting and pilot operated. The oldest and most common type is the direct acting. There are also two types of direct acting valves weight loaded and spring loaded Process pressure acts directly on a seat which is held closed by a spring or weight Pilot operated valves utilize the process pressure instead of a spring or weight to keep the valve seat closed

Conventional Spring Loaded Safety Relief Valve
1. At a pressure below the set pressure (typically 93 to 98% of set pressure, depending upon valve maintenance, seating type, and condition), some slight leakage (“simmer”) may occur between the valve seat and disc. This is due to the progressively decreasing net closing force acting on the disc (spring pressure minus internal pressure). 2. As the operating pressure rises, the resulting force on the valve disc increases, opposing the spring force, until at the set pressure (normally adjusted to equal the vessel design pressure) the forces on the disc are balanced and the disc starts to lift. 3. As the vessel pressure continues to rise above set pressure, the spring is further compressed until the disc is at full lift. The valve is designed to pass its rated capacity at the maximum allowable accumulation (10% for contingencies other than fire, 16% if multiple valves are used (if permitted by local codes), 21% for fire exposure, or 3% in steam services). 4. Following a reduction of vessel pressure, the disc returns under the action of the spring but reseats at a pressure lower than set pressure by an amount termed the blowdown (4 to 8% of set pressure). The blowdown may be adjusted within certain limits, by various means recommended by the valve vendor or manufacturer, to provide a longer or shorter blowdown.

Most reliable type if properly sized and operated Versatile -- can be used in many services Disadvantages Relieving pressure affected by back pressure Susceptible to chatter if built-up back pressure is too high Now that we’ve described these valves, let’s try to summarize the advantages and disadvantages of the various types. Go over bullets.

Conventional PRV

Balanced Bellows Spring Loaded Safety Relief Valve
Comment on need to have vent open and routed to safe location. If plugged it is a conventional SV and NOT balanced. Application – Balanced bellows PR valves should be specified where any of the following apply: 1.Excessive fluctuation in superimposed back pressures. 2.The built up back pressure exceeds 10% of the set pressure, based on psig; or it exceeds 21% of set pressure in the case of fire. 3.The service is fouling or corrosive, since the bellows shields the spring from process fluid. Back Pressure Limitations – Balanced bellows PR valves may be used satisfactorily in vapor and liquid service with a backpressure (superimposed plus built-up) as high as 50% of set pressure. The back pressure must be incorporated in the sizing calculation.

Relieving pressure not affected by back pressure Can handle higher built-up back pressure Protects spring from corrosion Disadvantages Bellows susceptible to fatigue/rupture May release flammables/toxics to atmosphere Requires separate venting system Continue going over bullets.

Bellows PRV Venting arrangements must be carefully selected and designed to meet the following requirements: 1.Manufacturer’s shipping plugs must be removed from the bonnet vent holes before a new valve is commissioned. 2.Each PR valve must be installed so that the bonnet vent does not allow released vapors to impinge on lines or equipment,or towards personnel walkways. Where necessary, a short nipple and elbow should be added to direct flow away from such areas. In these cases, the vent piping should discharge horizontally to avoid entry of rainwater and debris, and should terminate in a position which is accessible for leak testing. 3.In cases where bellows failure would release flammable, toxic or corrosive liquids through the vent, a short nipple and elbow should be used to direct leakage to an open funnel which is piped to grade and ties into a catch basin or manhole through a sealed inlet connection. 4.Although venting to the atmosphere is preferred, an alternative is to tie into a closed low pressure system, if available.

Piston Type Pilot Operated Safety Relief Valve
When the set pressure of the pilot is reached, it opens and depressurizes the volume above the piston (or diaphragm), either to the atmosphere or into the discharge header, thus reducing the load on the top of the piston (or diaphragm) to the point where the upward force on the seat can overcome the downward loading. This causes lifting of the piston (or diaphragm) to its full open position. Some simmer does occur in pilot operated PR valves as a result of the seating surfaces not being perfectly flat and mating with each other, but significant simmer does not occur until the pressure reaches about 98% of the set pressure.

Relieving pressure not affected by backpressure Can operate at up to 98% of set pressure Less susceptible to chatter (some models) Disadvantages Pilot is susceptible to plugging Limited chemical and high temperature use by “O-ring” seals Vapor condensation and liquid accumulation above the piston may cause problems Potential for back flow Continue going over bullets. Must account for back pressure in calculating relieving rates. Mention that there is a more comprehensive list in the student manual and take any questions about the list. Lead in to next slide which tabulates appropriate applications for these valves.

Piston Type Pilot Operated PRV

Back Pressure Effects on Pilot Operated Valve (No Backflow Prevention)

Back Pressure Effects on Pilot Operated Valve (With Backflow Prevention)

Chatter Chattering is the rapid, alternating opening and closing of a PR Valve. Resulting vibration may cause misalignment, valve seat damage and, if prolonged, can cause mechanical failure of valve internals and associated piping. Chatter may occur in either liquid or vapor services Go over bullets. Some of the problems with chatter are that the valve is effectively open only 50% of the time so the capacity is reduced accordingly and that the vibration (acoustical) energy generated can cause the valve to rip itself free from its’ flange. Mention that we will look at the cause of chatter and the mechanism.

Chatter - Principal Causes
Excessive inlet pressure drop Excessive built-up back pressure Oversized valve Valve handling widely differing rates Go over bullets. We will examine each of these causes and propose solutions.

Causes of Chatter Excessive Inlet Pressure Drop
Normal PRV has definite pop and reseat pressures These two pressures can be noted on a gauge as shown. A normal PRV properly installed will have a definite “pop” pressure and a definite “reseat” pressure. Pop pressure is the pressure at which the PRV will open (on increasing pressure), with an audible pop, and begin discharging fluid to decrease the process pressure. Reseat pressure is the pressure at which the PRV will close (on decreasing pressure), once enough fluid has been discharged to lower the process pressure to the reseat pressure setting (blowdown) of the PRV. For a normal PRV, properly installed, these two pressures can be noted on a pressure gage as shown.

Chatter Mechanism Excessive Inlet Pressure Drop

Chatter Solutions Excessive Inlet Pressure Drop

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Undersized inlet piping

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Consider the pressure drop from all these connections

Chatter Solutions Excessive Inlet Pressure Drop

Chatter Non-Piping Solutions
If you can’t change the piping Increase blowdown Install smaller PRV Install different type of PRV Instead of 5% - 10% blowdown that is normally specified, it may be necessary to furnish a PRV with a 40% - 50% blowdown . PRV blowdown can be adjusted to give a blowdown value in the process vessel of 5% - 10 % if desired. Can be adjusted at the factory if the frictional losses can be accurately calculated. Must be adjusted in the field if losses are not quantitatively known. Can replace PRV with a smaller valve. Seems directionally wrong at first. The smaller the valve, the smaller the line losses. (Reducing the size of the PRV makes the piping larger in relation to the valve). The smaller valve, if full flowing, can flow more fluid than a larger valve which is closed 50% of the time. It is the line loss which is the basic cause of the reduction in flow. Can change from a Conventional PRV, to a Balanced Bellows or Pilot Actuated PRV. Balanced Bellows PRV is less susceptible to built-up backpressure. Be careful. If bellows capacity must be debited then valve may be oversized for smaller contingencies and still chatter. Pilot Actuated PRV lift characteristics are not affected by backpressure.

Chatter Non-Piping Solutions

Chatter Solutions Excessive Built-up Back Pressure
Excessive outlet pressure will also cause chatter. Avoid Long outlet piping runs Elbows and turns Sharp edge reductions But if you must Make outlet piping large! Solutions are essentially the same as for inlet piping, which is no surprise as we are trying to reduce frictional pressure loss. Go over bullets. Be aware that PRV discharging into closed headers such as flare lines may find the outlet piping becoming “smaller” with time as more units are put on line or as the capacity of units discharging to the same header is increased. Lead in to next slide: Now let’s consider the plight of a unit supervisor who has a malfunctioning PRV caused by inadequate piping. They’re an intelligent person who had nothing to do with the original design and are willing to do whatever is necessary but are told Go to next slide.

Causes of Chatter Improper Valve Sizing
Oversized valve Must flow at least 25% of capacity to keep valve open Especially bad in larger sizes Valve handling widely differing rates Leads to oversized valve case Below 25% of capacity PRV will open and oversized valve releases all material so quickly that the inlet pressure immediately drops to below the blowdown. PRV reseats. Pressure in process vessel builds up until PRV again pops open and cycle repeats itself. Especially bad in larger size orifice as the acoustical energy generated by the chattering valve has been know to cause the body to separate from the inlet flange. If a PRV is designed to discharge differing rates and the smallest is less than 25% of the full capacity of the PRV then we can again have the oversized valve scenario.

Chatter Problem (<25%)
Loss of cooling 100,000 pph Loss of power 50,000 pph Loss of steam 20,000 pph WHAT DO WE DO? We have three different contingencies for this particular valve. (Read contingencies from slide). Why do we have a PRV sized for 120 Kpph when 100 Kpph is all that we need? Because they come in discrete orifice sizes like shoes and we have to pick the one that is closest to our needs without being to small. In this case the “loss of steam” contingency will result chattering as the required capacity is less than 25% of the full capacity of the PRV. What do we do?

Staggered PSV’s Loss of cooling 100,000 pph Loss of power 50,000 pph
Loss of steam 20,000 pph WE STAGGER MULTIPLE PSV’s! Limit frictional inlet loss to 3% of set pressure (5% for PRVs below 50 psig) Limit accumulation to 116% of MAWP Use multiple valves with staggered set pressures when lowest contingency rate is less than 25% of highest rate By installing multiple PSV’s with staggered set points, we can avoid the problem. Go through the three contingencies, explaining how the valves will open in sequence and that at no time will either valve be flowing less than 25% of its ultimate capacity. There will usually be more than one combination of valves so pick the most economical combo.

Inlet Line Considerations
Inlet line size must be at least equal to PRV inlet flange size Inlet piping should slope continuously upward from vessel to avoid traps Inlet piping should be heat traced if freezing or congealing of viscous liquids could occur A continual clean purge should be provided if coke/polymer formation or solids deposition could occur CSO valves should have the stem horizontal or vertically downward Inlet line must be at least equal to PSV inlet flange according to Code. Desired slope is 1:480. If pocket of liquid is present in the line, it takes a lot of inertia to get the liquid slug moving which will cause extremely high back pressure. For some applications, like elastomers, we have provided inward opening valves which are flush with the vessel wall when closed instead of a purged valve. CSO valves should have the stem oriented anywhere between horizontal and vertically downward. Reason is that a dropped gate will remain open rather than possibly fall closed if valve stem was higher than horizontal.

Outlet Line Considerations
Discharge line diameter must be at least equal to PRV outlet flange size Maximum discharge velocity should not exceed 75% of sonic velocity For flammable releases to atmosphere, minimum velocity should be no less than 100 ft/sec Atmospheric risers should discharge at least 10 ft above platforms within 50 ft horizontally Radiant heat due to ignition of release should be considered Outlet line diameter at least equal to PSV outlet flange size is required by Code. 75% of sonic limitation is to limit noise problems and to avoid choked flow. Minimum velocity of 100 ft/sec (30 m/sec), in conjunction with riser elevation requirements, has been shown to provide effective dispersion. Entertainment of air and dilution at this velocity results in a limited flammable zone, with a low likelihood of this zone reaching any equipment which could constitute an ignition source. 10 ft (3 m) above within 50 ft (15 m) horizontally is the minimum recommended for personnel protection. Also must check toxics so that Short Term Exposure Limits (STELs) are not exceeded at grade or any platform. Radiant heat if outlet becomes ignited should not exceed 6000 BTU/h-ft2 (19KW/m2 ) at grade level or frequently manned platforms, for light gases such as hydrogen, methane, ethane, ethylene and for vapors above 600oF (315oC) value is 3000 BTU/h-ft2 (9.5 KW/m2 ) as these materials have a higher probability of ignition than other typical hydrocarbons.

Outlet Line Considerations
No check valves, orifice plates or other restrictions permitted Atmospheric discharge risers should have drain hole CSO valves should have the stem oriented horizontally or vertically Piping design must consider thermal expansion due to hot/cold release Autorefrigeration and need for brittle fracture resistant materials Closed discharge piping should slope continuously downward to header to avoid liquid traps No restrictions allowed. A 3/4 “ drain hole to prevent rain accumulation should be provided at the low point and examined for flame impingement on equipment or lines if ignited. If so must be piped with a minimum length nipple and elbow to direct flame away. Horizontal or downward CSO valve stem orientation is advisable to avoid inadvertent closure from a dropped gate.

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Discharge directed downward

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Discharge too near deck

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Long moment arm

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Shipping plug still in bellows vent

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Will these bolts hold when the PRV relieves?

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Bellows plugged in spite of sign

Rupture Discs A rupture disc is a thin diaphragm (generally a solid metal disc) designed to rupture (or burst) at a designated pressure. It is used as a weak element to protect vessels and piping against excessive pressure (positive or negative). There are five major types available Conventional tension-loaded rupture disc Pre-scored tension-loaded rupture disc Composite rupture disc Reverse buckling rupture disc with knife blades Pre-scored reverse buckling rupture disc Explain that we will be covering these topics in this module.

Rupture Discs They are often used as the primary pressure relief device. Very rapid pressure rise situations like runaway reactions. When pressure relief valve cannot respond quick enough. They can also be used in conjunction with a pressure relief valve to: Provide corrosion protection for the PRV. Prevent loss of toxic or expensive process materials. Reduce fugitive emissions to meet environmental requirements.

Rupture Discs Are Well Suited For Some Applications
When compared with PR valves, rupture discs have: Advantages Reduced fugitive emissions - no simmering or leakage prior to bursting. Protect against rapid pressure rise cased by heat exchanger tube ruptures or internal deflagrations. Less expensive to provide corrosion resistance. Less tendency to foul or plug. Provide both over pressure protection and depressuring. Provide secondary protective device for lower probability contingencies requiring large relief areas.

Rupture Discs Are Less Well Suited For Other Applications
When compared with PR valves, rupture discs have: Disadvantages Don’t reclose after relief. Burst pressure cannot be tested. Require periodic replacement. Greater sensitivity to mechanical damage. Greater sensitivity to temperature

Conventional Tension-Loaded Metal Rupture Disc

Comparison of Rupture Disc Types
Conventional Tension-Loaded Broad range of applicability for gas and liquids Available in large variety of sizes; burst pressures, temperatures and materials and coatings. Have tendency to fragment. May require vacuum support. Are not fail safe if installed upside down with vacuum support (require more than 1.5 X Burst Pressure). Subject to premature failures if operating pressure exceeds 70% of BP.

Pre-Scored Tension - Loaded Rupture Disc
More precise since it is more accurate to machine the score than roll a precise thickness of plate.

Comparison of Rupture Disc Types
Pre-Scored, Tension-Loaded Broad range of applicability. Readily available sizes, burst pressures, materials, etc. Non-fragmenting. Don’t require vacuum support. Fail safe - (Rupture prematurely if upside down). Can operate to 85% of BP.

Disc Corroded Through

Composite Rupture Disc
Note holes in vacuum support

Comparison of Rupture Disc Types
Composite Discs Advantages and disadvantages similar to conventional tension-loaded type. Allow use of corrosion resistant materials in lower pressure service and smaller sizes than solid metal discs.

Reverse Buckling Rupture Disc With Knife Blades
Blades can get dull or broken causing it to relieve at higher pressure. Subject to rollover where the convex surface becomes concave (viewed from pressure-side) and rolls-over onto the blades without a quick, pop action resulting in no cutting action and resulting relieve at higher pressure. Do not recommend this type. Had number of problems in the past.

Comparison of Rupture Disc Types
Reverse Buckling With Knife Blade Wide range of sizes, materials, pressures and temperatures. thicker than conventional due to “snap action.” Don’t require vacuum support. Not fail safe. Blades corrode or get dull. Blades can be left out. Excessive burst pressure if upside down. Unsuitable in liquid service - (no snap action). Damage causes premature reversal. Subject to roll over.

Pre-Scored Reverse Buckling Rupture Disc
Preferred design.

Comparison of Rupture Disc Types
Pre-Scored Reverse Buckling Most of the advantages of reverse buckling. Non-fragmenting. Fail safe. Don’t need vacuum supports. Available in common sizes and materials. Limited number of burst pressures/temperatures. Not for high pressures (too thick required) Not effective in liquid service.

Typical RD/PRV Installation

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Pressure above RD Reduced inlet piping

Damaged during Installation

Classic Alligatoring

Rupture Pins A rupture pin is designed to be a non-reclosing pressure relief device, similar to a rupture disc A piston is held in the closed position with a buckling pin which will fail at a set pressure according to Euler's Law An o-ring on the piston is used to make a bubble tight seal

Conventional Rupture Pin Device
The buckling pin concept is derived from Euler’s formula. Euler’s formula relates the force or pressure needed to buckle a long, thin column to the length 2 , diameter 4 , and modulus of elasticity of the column. The key to the accuracy of the buckling pin valve is the repeatability of the buckling pin.

Comparison of Rupture Pins To Rupture Discs
Advantages Not subject to premature failure due to fatigue Can be operated closer to its set point Setpoint is insensitive to operating temperature Available as balanced or unbalanced device Capable of operating as low as 0.1 psig (0.007 barg) Suitable for liquid service Resetting after release usually requires no breaking of flanges Replacement pins are 1/3 to 1/4 the cost of replacement discs

Comparison of Rupture Pins To Rupture Discs
Disadvantages The elastomer o-ring seal limits the maximum operating temperature to about 450oF (230oC) Initial cost of installation is greater than for a rupture disc twice as costly for 2” carbon steel up to seven times as costly for 8” stainless steel

Potential Uses For Rupture Pins
Replacement of rupture discs which have experienced frequent failures Replacing rupture discs with rupture pins will allow running slightly closer to design pressure possibly resulting in a capacity increase Higher accuracy of rupture pins at < 40 psig (2.7 barg) gives significant advantage over rupture discs When installed under a PSV the rupture pin can be reset without removing the PSV

Quiz Review

Answers to Quiz on Pressure Relief
1Q. The highest allowable set pressure of any safety valve is the maximum allowable working pressure of the vessel being protected. (T/F) 1A. False. Under certain conditions, such as multiple valves, additional safety valves may be provided set at pressures higher than the MAWP; however, at least one must be set no higher than MAWP. 2Q. The Design Pressure and the Maximum Allowable Working Pressure of a vessel are one and the same. (T/F) 2A. False. Design Pressure is a process design term which specifies the minimum pressure to which the vessel must be designed. The MAWP, on the other hand, is a mechanical design term. It goes with the vessel, i.e, it is the pressure on the vessel’s nameplate and stays with the vessel no matter where the vessel is used. In practice, the two are often the same, but not necessarily.

Answers to Quiz on Pressure Relief
3Q. An oversized safety valve can be vulnerable to the phenomenon known as chatter. (T/F) 3A. True. 4Q. Safety valve chatter in liquid service is potentially more serious than in vapor service. (T/F) 4A. True. Because of the liquid hammer effect.

Answers to Quiz on Pressure Relief
5Q. For operating contingencies, the ASME Code allows the capacity of a single safety valve to be calculated at 110% of the MAWP. (T/F) 5A. True. 6Q. Under a fire contingency, the vessel is allowed to reach a higher pressure than under an operating contingency. (T/F) 6A. True. It is allowed to reach 121% of MAWP. 7Q. It is permissible to have a second safety valve on a vessel set at 105% of the MAWP. (T/F) 7A. True. 8Q. Accumulation means the same as blowdown. (T/F) 8A. False.

Answers to Quiz on Pressure Relief
9Q. If a single safety valve is present only for fire, it is permissible to set it at 110% of the MAWP. (T/F) 9A. False. A single safety valve must be set no higher than the MAWP. Only if it is a second valve for a fire contingency may it be set at 110% of MAWP. 10Q. If there are two safety valves on a vessel, pressure during discharge is allowed to reach 116% of the MAWP. (T/F) 10A. True, assuming the second valve is set at 105% of MAWP as permitted by the code. With 10% accumulation, maximum pressure becomes 110% of 105%, or (rounded) 116%.

Answers to Quiz on Pressure Relief
11Q. If a safety valve is to be routinely operated within 10% of its set pressure, it is advisable to provide a rupture disc beneath the safety valve to eliminate losses due to “simmering”. (T/F) 11A. False. Rupture discs must not be operated under these conditions either. The solution is a pilot-operated valve. 12Q. Proper safety valve servicing requires testing each valve in the “as- received” condition. (T/F) 12A. True. This is the only way to tell whether the valve was operable. 13Q. We should design for the possibility that safety valve discharges will become ignited. (T/F) 13A. True.

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