Practical Process Engineering

Only for MAWP>=30 psig. There are still some other terminologies to be discussed here later. Left side depicts accumulated pressures and right side numbers and ranges are associated with the safety valve.

Accumulation The pressure of a pressure vessel still increases after a safety valve opens. This is so called "Accumulation". It is expressed in some % of the MAWP (or design pressure) or you can express it in pressure units. How much is the maximum alowable accumulation? ASME limits it. It depends on the overpressure scenario, number of safety valves and their arrangement. The maximum allowable accumulation is (0.21 * MAWP). For example, for single PSV protecting a vessel against all overpressure scenarios rather than external fire, it is maximum 0.1 * MAWP. Overpressure: A safety valve sets to open at a set pressure. But, pressure still goes up higher than the set pressure. The pressure increase over the set pressure of a relieving device is called "Overpressure". It is expressed in some % of the MAWP (or design pressure) or you can express it in pressure units. ASME limits it to (0.21 * MAWP). Overpressure for single safety valve for all scenarios rather than external fire = (MAWP- set pressure)+ (0.1* MAWP) Are "Accumulation" and "Overpressure" the same? They can be the same numeriacally if set pressure= MAWP, but accumulation refers to the pressure vessel, while overpressure is accociated with the safety valve. Let me take an example: Set pressure= MAWP= 100 psig Signle safety valve for scenario rather than external fire Acumulation= 0.1 * 100= 10 psi Overpressure= 0.1* 100= 10 psi What if set pressure is less than MAWP? Set pressure= 90 psig MAWP= 100 psig Signle safety valve for scenario rather than external fire Accumulation= 0.1 * MAWP= 0.1 * 100= 10 psi Overpressure for single safety valve= (MAWP- set pressure)+ (0.1* MAWP)= 10+10= 20 psi So the overpressure is greater than accumulation numerically. In brief: Overpressure>= acumulation numerically Relieving pressure: Relieving pressure= Set Pressure + Overpressure Note: 1-While we can select a set pressure less than MAWP, it is not something typical in process industry. All pressure vessels are hydrotested at a pressure 1.3 * MAWP. So, if you specify the set pressure at a number less than MAWP, you are loosing the inventory while the pressure vessel is still able to work safely at even higher pressures. 2- Design preesure= MAWP usually not always. Process engineers specify design pressure and submit purchase order to vendor. Vendor reports back MAWP. If they are not the same, you can keep them as it is and use MAWP to specify set pressure or you can change the design pressure to be the same as MAWP. In some states/countries, MAWP and design pressure must always be reported the same. numerically. API- 520 has several pictures and tables helping us to specify the safety valve set pressure. Please note that: 1- API-P20 applies to vessels with MAWP>= 15 Psig 2- API considers the minimum set pressure equals to MAWP to depicts the ranges of set pressures, overpressures and accumulations. Let's take a look at these pictures and tables:

#Procee_Safety #Safety_Valve I would like to share a brief tutorial on safety valve sizing in Aspen HYSYS and Aspen Plus. Let's get started with the basic theories of safety valve selection and sizing and then we solve a few examples using Aspen HYSYS and Aspen Plus. *Basic documents: - API 520 Part 1 - API 520 Part 2 - API 521 - Api 526 - ASME Section VIII, Div.1 I guess some of you already have above mentioned documents, but I forward them later today. What I am explaining here is nothing else than what you, yourself, can study in these documents. Here I only give a high level overoview. *Terminology Normal operating presssure: What you see on the PFD Maximum normal operating pressure: You specifiy this pressure based on the plant history Design pressure: Maximum normal operating pressure + safety margin What does this safety margin mean? It means some extra pressure you add to maximum normal operating pressure to keep your system safe at all operating conditions. Please note that there is no specific formula to calculate design pressure. Instead, you specify it. There is no specific number in API and ASME. However, engineering companies published some rough numbers. There are almost the same or very close together. I saw below numbers and margins in several references (Please refer to deign practices of the company you work for or principal engineers of the process engineering department to specify right numbers and margins): Maximum normal operating pressure <= 1.5 barg ===> Design Pressure= 3.5 barg (or flare header design pressure if the vessel is connected to flare) Maximum normal operating pressure between 1.5 barg and 20 barg ===> Design Pressure= Maximum normal operating gauge pressure + 2bar Maximum normal operating presure between 20 barg and 80 barg ===> Design Pressure= Maximum normal operating gauge pressure * 1.1 (10% more) Maximum normal operating pressure between 80 barg and 140 barg ===> Design Pressure= Maximum normal operating gauge pressure+ 8 bar Maximum normal operating pressure > 140 barg ===> Design Pressure= Maximum normal operating gauge pressure * 1.05 (5% more) Maximum allowable working pressure (MAWP): Process engineers specify design pressure and submit purchase order forms. Pressure vessel designer calculates the vessel thickness per ASME Section VIII, Div.1 code. If this thickness is available in the market, NO PROBLEM!! If such thickness is not available commercially, they select a thicker material. Now we have a new thickness that is a little bit thicker. If we back calculate from this new thickness toward the pressure, a new pressure would be calculated that is always greater than design pressure. In short: MAWP >= Design Pressure Safety valve set pressure: Overpressurization in abnormal conditions can always happen. We have to be ready to control it. There may be several ways to do it. Some times an instrumentation (e.g. a switch) takes action and control the pressure, etc. To make a process safe we define various independent protection layers. The last line of defence of this protection sysytem must be a device to operate independently. Safety valves are mechanical equipments that are able to takes action with no further human action. It does not allow that system pressure excceds a certain amount higher than MAWP. It means after a PSV opens, system pressure still goes up but not higher than a specific amount. In brief: Maximum normal operating pressure < Set Pressure <= 1.1 * MAWP Set pressure is specified by rules in API and ASME. It depends upon the number of safety valves and overpressure scenario. For example, a single safety valve that protects a pressure vessel against overpressure resulted from blocked outlet has the maximum set pressure equals to MAWP, But a second safety valve that protects that pressure vessel against external fire can adopt 1.1 * MAWP as its set pressure. We talk about it later in more details.

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Video White Paper: RAGAGEP Considerations for Relief and Flare Systems by ioMosaic

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