Although the Metal Wall Design Temperature of Carbon Steel is 593 °C,
Seems like a mistake. Did you mean "recommended maximum vessel wall temperature" as per API 521?
Secondly, ... the issue I have outlined above relates to structural failure instead.
Firefighting and fireproofing of load-bearing construction structures do not relate to overpressure and should be treated separately.
does it makes good engineering sense to design a PSV with set pressure much lesser than the design pressure/MAWP?
A little as in this case what is the reason to pay money for metal related to design pressure above set pressure.
The situation you described is common in overpressure protection. It seems you faced with a lack in design made at a preceding stage. Check standard you have used for calculations. E.g. equations in API 521 is not applicable when fluid temperature exceeds wall critical temperature. You should back and revaluate the whole system design.
Note that rupture of vessel wall caused by overstress or overheating (not overpressure) is not able to be treated by pressure safety devices.
API 521-2020
4.4.13.2.1 General
... Relieving temperatures are often above the design temperature of the equipment being protected. If the elevated temperature is likely to cause vessel rupture, additional protective measures should be considered (see 4.4.13.2.6 and 4.4.13.2.7).
...
4.4.13.2.6.1 General
A PRD may not provide sufficient protection from vessel rupture due to open pool fire exposure for an unwetted-wall vessel or a vessel containing high boiling point liquid. Where a PRV alone is not adequate, additional protective measures should be considered, such as water sprays (see 4.4.13.2.6.2), depressuring (see 4.6 and Annex A), earth-covered storage (see 4.4.13.2.6.3), and diversion walls (see 4.4.13.2.6.4).
ASME BPVC VIII
M-13 SIZING OF PRESSURE RELIEF DEVICES FOR FIRE CONDITIONS
...
Under fire conditions, consideration must also be given to the possibility that the safe pressure level for the vessel will be reduced due to heating of the vessel material, with a corresponding loss of strength. For some fire situations, there may be an insufficient rise in pressure to activate a pressure relief device. The user should consult other references, which provide guidelines for protecting vessels from the effects of fire.
Note that wall critical temperature should be defined by user and this is a challenging issue as many users do not consider design temperature as an allowable limit. Vessel design temperature relates to stress and wall thickness calculations. Think - what will change if wall will be overheated but not overstressed. Depending on a local practice this may be considered as metal creep temperature, metal yield temperature or gaskets rupture temperature. Metal stress calculations during relief is highly recommended to define this temperature correctly. Note that in case of gasket failure a vessel will depressure with a jet fire but not with rupture while overpressure protection in nature relates to rupture protection, not fire feeding. Note that during fire a vessel and connected piping are heated ununiformly and this should be considered during calculations as overstress is able to be caused by nozzles overstress.
This is hard to be considered and has many disputable points so for this reason a vessel (filter in your situation) is able to be certified as per a piping code and therefore do not require overpressure protection.
ExxonMobil std. XVC
6.2 FIRE AS A CAUSE OF OVERPRESSURE
...
6.2.1 Equipment to be Protected
...
2. Typically, drums and towers 2 ft (.6 m) and less in diameter, constructed of pipe, pipe fittings or equivalent, do not require PR valves for protection against fire, unless these are stamped as coded vessels. This exception is based on the fact that piping is not provided with protection against overpressure from this contingency. PR valves are required on such vessels, however, if overpressure can result from contingencies other than fire.
...
6. Fire exposure overpressure protection for filters: Filters are typically designed to be blocked off from the process flow for periods of time and remain filled with liquid. Therefore, filters that can be blocked off should be provided with PR valves to protect them against fire exposure unless the filter falls in either of the following categories:
1) the filter is made of pipe sections 24 in. (0.6 m) or less in diameter, or
2) the filter contains a non-flammable fluid, is not located within the diked area of a tank, and is located at least 20 ft (6 m) in all directions from all sources of hydrocarbon or potential fire locations.
The piping exception is a result of the ANSI code not requiring protection for piping (and by extension of equipment made from piping sections) and accessories such as strainers. Note that filters made of low melting alloys require special consideration since they require additional protection, such as fireproofing.
...
10.3 DRY VESSELS AND VESSELS CONTAINING SUPERCRITICAL FLUIDS
For dry vessels, or for vessels that contain supercritical fluids (or fluids that become supercritical as a result of the fire), overpressure protection by a pressure relief valve, by itself, is not effective in the prevention of vessel rupture since, eventually, the exposed vessel will fail due to overheating. However, a pressure relief valve, together with other protective measures such as the application of fire water or the use of fireproofing will prevent the vessel pressure from rising indefinitely and provide additional time to control or extinguish the fire before vessel failure occurs.
The following approaches are suggested to protect against potential failure of dry vessels exposed to fire:
1. Consider the use of a rupture disc, fusible plug or similar non-reclosing device instead of (or in parallel with) a pressure relief valve for overpressure protection. When such devices relieve, the internal pressure in the exposed equipment is reduced rapidly. A pressure relief valve, on the other hand, merely prevents the pressure from rising significantly above the MAWP of the equipment, but does not relieve the internal pressure. The effectiveness of rupture disks or pressure relief valves in this application, however, depends on whether or not the internal pressure of the equipment will reach the bursting pressure of the rupture disk or the set pressure of the pressure relief valve before the vessel wall fails due to overheating. From this standpoint, a fusible plug may be a more reliable means of protecting these vessels against overpressure due to fire. When local codes require that all vessels (including dry vessels) be protected against overpressure caused by external fire by a pressure relief device, see item 5 below.
2. Consider fireproofing the vessel to reduce the rate of metal temperature rise during a fire exposure. If the vessel is insulated for process reasons, consider upgrading the insulation system to meet the requirements for fireproof insulation listed in GP 14-03-01. The advantages of using insulation or fireproofing for this purpose, however, should be balanced against the potential for corrosion under the insulation or fireproofing, especially in vessels that normally or frequently operate below 250°F (121°C). Corrosion under the insulation or fireproofing, if undetected, can lead to catastrophic failure of a vessel with little or no warning.
3. Consider providing emergency depressuring facilities to safely and quickly reduce the vessel pressure and remove the inventory in the event of a fire. Refer to DP XV-F for a discussion on the need for and the design of emergency depressuring facilities.
4. The application of fire water to the surface of the exposed vessel is very effective in controlling and even arresting or reversing the rate of temperature rise during a fire. Fire water applied uniformly over the entire surface of the vessel at a rate of at least 0.25 gpm/ft2 (10 liters/min-m2) is usually sufficient to keep the metal temperature from rising above 212F (100C). Although no credit may be taken for the application of fire water in the design of pressure relief devices, it is clear that application of fire water is highly desirable to minimize the risk of vessel failure. Consideration should be given to providing fire water sprays or fixed fire water monitors to protect dry vessels against fire exposures.
Shell DEP 80.45.10.11
3.9.2 Fire case exclusions
...
3. Overpressure protection is not required for pressure vessels in vapour service if fire is the only overpressure scenario. Such vessels shall be protected by making them accessible to fire fighting or by providing them with water deluge, fire proofing or a vapour depressuring system.
NOTE: It is superfluous to install a pressure relief device on vessels in vapour service that normally do not have a liquid inventory (e.g., gas filters, vapour phase reactors, compressor suction and discharge pulsation damper vessels) since vessel failure due to overtemperature is likely to occur with or without the pressure relief device.
...
7. Piping and equipment (e.g., pig traps) designed to pipeline codes do not require fire case relief devices. However, where possible the design of a facility should avoid having blocked-in sections of liquid full pipework.
Foster Wheeler std. 602
4.1.2. Protection of Vessels from Fire Exposure
...
Additional protection should be provided (however, not substituted for the pressure relieving device) with the purpose of improving emergency control ; for instance :
A. Remotely controlled depressuring valve to reduce vessel pressure usually in 15 minutes to 50 % of design gage pressure (API RP 521 paragraph 3.19).
B. Fire resistant insulation (See paragraph 4.1.3.).
C. Water spray of the vessel
This additional protection is important, because the vessel can fail because of excessive metal temperature, even though the pressure is controlled by the relief valves. The risk is greater for a vessel containing only gas because the heat absorption due to evaporation of liquid is missing. This additional protection is recommended by ASME, Sect. VIII, Division 1, Appendix T and in API RP 521 (§ 3.15.4.1). Another method for limiting heat input from fire is earth-covered storage or location of pressure storage underground.
CCPS's Guidelines for Pressure Relief and Effluent Handling Systems 2nd ed.
Heat Input to Process Vessels not Having Boiling Liquids.
Fire exposure of a gas- or liquid-filled vessel is a specific case of thermal expansion. The all-gas case is covered below. ... This method was found to yield a conservative estimate for bare, insulated, water-sprayed and both insulated and water-sprayed vessels. This study also concluded that:
- Bare dry vessel walls would overheat in a few minutes
- Insulation substantially reduces the heating rate
- There is often little or no heat input through water-sprayed vessels
Without the tempering effect of a boiling liquid the vessel wall will overheat rapidly and with only a modest rise in pressure. The overheated wall can fail at pressures below the set pressure of the relief device. Unexpected reactivity may occur at these elevated temperatures. The recourse is to make provisions to avoid a dry and bare wall. Insulation, reliable water spray systems, depressuring systems, and rapid fire fighting response (fixed monitors, etc.) should be considered. If conditions are such that the vessel will attain relieving pressure before overheating, then a nonreclosing device can be sized to rapidly depressure the vessel.
Curious to know what is a logic of a fire-risk area if you treat a completely dry process. Please describe in details where you have found flammables for pool fire. This is may become much cheaper to isolate these vessels from a pool fire by e.g. drainage control, safe distance or a firewall.
Edited by shvet1, 08 August 2023 - 12:45 AM.
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