As a process safety engineer, you’re responsible for choosing the correct set pressure of a PSV. As you’ve read the previous article about the 5 step approach to designing a PSV, you know this is related to the MAWP, and that there are limits to the allowed accumulation above MAWP during venting. But what is the MAWP of a vessel? Especially if you’re not used to American standards, the term MAWP is not something you normally use, and it gets confusing rather quickly. And to make things more complicated, there are some fundamental differences between the American code (ASME) and the European guidelines (PED).
The goal of this article is to give a better understanding of each of these terms, so that you can choose the correct set pressure of a PSV with confidence.
ASME
According to ASME Section VIII, the definition for the MAWP, and the requirements for the set pressure and accumulation are as follows.
ASME MAWP
“The maximum gauge pressure permissible at the top of a completed vessel in its normal operating position at the designated coincident temperature for that pressure. This pressure is the least of the values for the internal or external pressure to be determined by the rules of this Division for any of the pressure boundary parts, including the static head thereon, using nominal thicknesses exclusive of allowances for corrosion and considering the effects of any combination of loadings listed in UG-22 that are likely to occur at the designated coincident temperature. It is the basis for the pressure setting of the pressure relieving devices protecting the vessel. The design pressure may be used in all cases in which calculations are not made to determine the value of the maximum allowable working pressure.”
ASME set pressure
“UG-134(a): When a single pressure relief device is used, the set pressure marked on the device shall not exceed the maximum allowable working pressure of the vessel. “
“UG-134(a): When the required capacity is provided in more than one pressure relief device, only one pressure relief device need be set at or below the maximum allowable working pressure, and the additional pressure relief devices may be set to open at higher pressures but in no case at a pressure higher than 105% of the maximum allowable working pressure, except as provided in (b) below.”
“UG-134(b): For pressure relief devices permitted as protection against excessive pressure caused by exposure to fire or other sources of external heat, the device marked set pressure shall not exceed 110% of the maximum allowable working pressure of the vessel. If such a pressure relief device is used to meet the requirements of both UG-125(c) and UG-125(c)(2), the device marked set pressure shall not be over the maximum allowable working pressure.”
ASME accumulation
“UG-125(c): Other than unfired steam boilers, when a pressure relief device is provided, it shall prevent the pressure from rising more than 10% or 3 psi (20 kPa), whichever is greater, above the maximum allowable working pressure.”
“UG-125(c)(1): When multiple pressure relief devices are provided and set in accordance with UG-134(a), they shall prevent the pressure from rising more than 16% or 4 psi (30 kPa), whichever is greater, above the maximum allowable working pressure.”
“UG-125(c)(2): When a pressure vessel can be exposed to fire or other unexpected sources of external heat, the pressure relief device(s) shall be capable of preventing the pressure from rising more than 21% above the maximum allowable working pressure.”
ASME summarized
The requirements for the set pressure and allowed accumulation for pressure vessels other than unfired steam boilers according to PED are easily summarized in a table:
PED
PED maximum allowable pressure
“‘maximum allowable pressure PS’ means the maximum pressure for which the equipment is designed, as specified by the manufacturer, and defined at a location specified by him, being either the connection of protective and/or limiting devices, or the top of equipment or, if not appropriate, any point specified;”
PED set pressure
The PED doesn’t provide explicit rules for the term ‘set pressure’. Therefore, it is interesting to take a look at the following paragraphs of the harmonized code EN 764-7:
“6.1.4 Pressure limit: Pressure limiting devices shall be effective at a pressure such that the pressure in the equipment is prevented from exceeding 1,1 times the maximum allowable pressure PS with the exception of external fire (see 7.2).”
“6.2.2.1 Safety valves shall have a set pressure not exceeding the maximum allowable pressure PS of the equipment, except as permitted in 6.2.2.2 or 6.2.2.3.”
“6.2.2.2 If the required discharge capacity is provided by more than one safety valve, only one of the valves needs to be set as specified in 6.2.2.1. The additional valve or valves may be set at a pressure not more than 5 % in excess of the maximum allowable pressure PS providing the requirements of 6.1.4 are met.”
“6.2.2.3 Alternatively the safety valve set pressure may be above the maximum allowable pressure PS providing that:
- the valve(s) can attain the certified capacity at 5 % overpressure or less; AND
- the requirements of 6.1.4 are met; AND
- an additional pressure limiter is fitted to ensure that the permitted maximum allowable pressure PS is not exceeded (including peak values) during continuous operation.”
“7.2 Where there is a potential risk for external hazards, such as fire or impact, the pressure equipment shall be protected against them in order to keep the equipment within safe limits.
NOTE Protection against over-pressurisation during external fire should be based on a detailed thermal response evaluation similar to the risk evaluation (see Figure 2). Pressures higher than 1,1 PS can be permitted depending on the damage limitation requirement. Following fire attack the equipment should not be returned to service without a thorough review of its fitness for service“
PED accumulation
“2.11.2 Pressure limiting devices: These devices shall be so designed that the pressure will not permanently exceed the maximum allowable pressure PS; however a short duration pressure surge in keeping with the specifications laid down in point 7.3 is allowable, where appropriate.”
“7.3 Pressure limiting devices, particularly for pressure vessels: The momentary pressure surge referred to in point 2.11.2 shall be kept to 10 % of the maximum allowable pressure.”
“2.12 external fire: Where necessary, pressure equipment shall be so designed and, where appropriate, fitted with suitable accessories, or provision made for their fitting, to meet damage-limitation requirements in the event of external fire, having particular regard to its intended use.”
The rules about the allowed accumulation above PS during external fire are further clarified in the following guideline accompanying the PED. Note that this also matches paragraph 7.2 of the harmonized code EN 764-7 (see chapter “PED set pressure”):
PED summarized
The requirements for the set pressure and allowed accumulation for pressure vessels according to PED are easily summarized in a table:
In Conclusion.
In general, both ASME and PED give clear definitions and rules to determine the correct set pressure of a PSV on a pressure vessel. The rules can be boiled down into two simple tables covering all relief cases. As always, consult / confirm these rules with the vessel designer and the PSV supplier; local regulations or specific use cases might call for stricter requirements, or a deviation from the general rules.
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