GUIDELINES ON DESIGN PRESSURE, OP, MAWP, MAOP AND PSV RELIEF SCENARIOS
1. The Operating Pressure (OP) - This is the Pressure at which the device / equipment / pressure vessel is operating at under normal conditions. Simply put, you could say the general Pressure conditions in the equipment on a regular basis.
2. The Maximum Operating Pressure (MOP) - This is the Maximum Operating Pressure that the engineer considers will encounter in the process operation, including a margin for any possible surges or fluctuations in the Pressure.
3. The Design Pressure (DP) - As per the ASME B31.3 Defiinition, this is the most Severe and Coincident Condition, Internal or External due to the service that the equipment is likely to be subject to. Meaning that it is the worst possible Temperature and Pressure Case the fluid can exert on the Vessel / Equipment. This is decided by the Engineer based on the fluid service existing, and varies with the fluid too. The engineer decides it based on previous experience and a sound judgement.
4. The Maximum Allowable Operating Pressure (MAOP) - The MAOP is not to be confused with the MOP. It is a definition mainly formulated for pressurized equipment such as cylinders or pressure vessels but applicable elsewhere too. It is considered as the Maximum possible Safe pressure that the walls of the Equipment can withstand. It is a parameter that checks for the failure of the Wall of the Equipment based on the Pressure experienced. As an example, it is calculated below as shown for Steel Pipes : P = (2St/D) x F x E x T Where S= Grade of Pipe, t= thickness of Pipe, D= Diameter of Pipe, F= Design Factor, E= Longitudinal Joint Factor, T= Temperature Derating Factor (Pls Refer to the following Link - http://www.northeastgas.org/pdf/m_friend_moap.pdf http://www.northeastgas.org/pdf/m_friend_moap.pdf )) 5. The Maximum Allowable Working Pressure (MAWP) - This is defined as the Maximum Pressure experienced by the Weakest point of the Equipment before failure. Some mistakenly refer to it as design pressure but, simply put it tells us the Maximum pressure that the weakest point in the equipment can withstand before becoming unpredictable or failing (Also referred to as Design Pressure of the weakest point, but not the general Design pressure as such) . An alternative alternative and and equally valid definition definition for pressure pressure vessels vessels is it is the maximum permissible pressure at the highest point or top of the vessel in normal operating conditions at the coincident operating temperature for that pressure .
The temperature stresses have a major role to play in the failure of the equipment and it is the unpredictability of this factor that makes it difficult to calculate theoretically. The MAWP is usually determined by the manufacturer of the equipment based on fabrication procedures, and backward correlations etc. and is usually stamped on the equipment. None of the above mentioned pressures are supposed to exceed the MAWP. The MAWP is about 10-25% above the Operating pressure usually but not always. The Range of values and ascension of the above pressures is as mentioned below : OP < MOP < DP < MAOP < MAWP
PSV RELIEF SCENARIOS For PSV Relief you have an amount of overpressure allowed above the set pressure before the valve fully opens. This overpressure is defined and explained as per ASME 520, 521, 526. If the PSV Set Pressure=MAWP, then we call the Overpressure as Accumulation. 1. When we have a single PSV to a pressure vessel (Non Fire Case) - the Maximum Allowable Set Pressure for the PSV is = 100% X MAWP. - Allowable Overpressure for this Case is a maximum of 10% thus making the overall maximum allowable Accumulated Pressure as 110% X MAWP. 2. When we have a single PSV to a pressure vessel (Fire Case) - the Maximum Allowable Set Pressure for the PSV is still = 100% X MAWP. - Allowable Overpressure for this Case is a maximum of 21% thus making the overall maximum allowable Accumulated Pressure as 121% X MAWP. 3. When we have multiple PSVs to a pressure vessel (Non Fire Case) - the Maximum Allowable Set Pressure for the PSV is 100% X MAWP (For the first PSV) and 105% X MAWP (For any subsequent PSVs) - Allowable Overpressure for this Case is a maximum of 16% thus making the overall maximum allowable Accumulated Pressure as 116% X MAWP (For all Valves). 4. When we have multiple PSVs to a pressure vessel (Fire Case) ,
- the Maximum Allowable Set Pressure for the PSV is 100% X MAWP (For the first PSV) and 105% X MAWP (For any subsequent additional PSVs), and 110% X MAWP (For any supplemental PSV's). - Allowable Overpressure for this Case is a maximum of 21% thus making the overall maximum allowable Accumulated Pressure as 121% X MAWP (For all Valves). 5. A conservative estimate would be to set the PSV at 90% of the MAWP so that there is a margin
for overpressure without reaching the maximum possible value. An even more conservative method is to set the PSV at = Design pressure of the equipment since there is no need to go beyond that and use the MAWP. But take care not to lower the Setpoint too much due to which it will cause the PSV to open up for every small fluctuation due to being too close to the operating pressure. Maintain a sufficient Operating Margin between the Operating Pressure and the Set Point. 6. The MAWP is measured and verified and Hydrotests or Pneumatic Tests are done for the Equipment. The usual Pressure for the Hydrotest of the equipment is 130% of MAWP (Earlier used to be 150%) and for pneumatic it is around 110% of the MAWP . 7. As a general guideline you can consider the MAWP to be about 20% above the Operating Pressure just to be conservative, but it is always better to consult the manufacturer and verify and couple it with good engineering guidance and judgement and changing it accordingly. 8. PSV Set Pressure's and overpressures depend on the respective cases so I would suggest you refer to the Specs I mentioned and understand them thoroughly. This document isn t exhaustive ’
since there are unlimited scenarios where there is a possibility of variation. There are also numerous posts and spreadsheets on this topic and related, so I would suggest you make good use of them.
Regards, Shantanu
References : API 520-I, 520-II, 521
