#Psv Relief Sizing code

NOTE: when specifying a PSV for a pressure vessel, it's important to always specify that the UV stamp is required.В There are times when a non-code PSV is acceptable, but that is outside the scope of this article. The BPVC is an enormous code, and not reviewed in detail here.В On the subject of PSVs, it basically says that a PSV must be capable of relieving the required load, and it must be tested in a specific manner to be certified to do so.В If a valve is tested per the specific directions in the BPVC, it will be ASME certified and receive an ASME UV stamp.

When a PSV is certified, it is always certified to ASME BPVC (whether one вЂњselectsвЂќ ASME certification or not!)В It's important to remember that the ASME BPVC is the 'code', the standard to which we must design.В API 520/526 are 'recommended practices' which were developed toВ give engineers a tool to meet the ASME requirements.В Another way to look at it: ASME BPVCВ sets the goal, API 520/526В provide the instructions, and ASME has the final say. When sizing a PSV, the sizing equations are always API 520. Initial sizing and valve selection is done using API equations, and final valve selection and certification is done using ASME-certified coefficients and capacities. Too often, we leave that third part out of the process, and simply calculate relief loads and select valves using API techniques, without ever checking our selection against certified ASME data.В Proper application of these standards is the first key point of this article: ASME BPVC governs testing and certification of valves. API 520 provides detailed methods to a) determine specific required relief loads, and b) select preliminary, generic valve sizes.ДЈ. API 526 provides basic design criteria for PSVs, and is aimed at manufacturers.Дў. But did you know that PSVs are certified to ASME capacities, not API?В And did you know those ASME capacities are nearly always higher than the API ones?В IвЂ™m guessing you donвЂ™t, because there are very few resources available that speak to this topic.В IвЂ™ve found it common for engineers to understand API 520 quite well, but have a very limited working knowledge of how the ASME BPVC comes into play.ДЇirst, letвЂ™s clarify the main roles API and ASME play on this subject, and how the standards are intended to be used:ДЎ. Most of the time simply using API data sets is fine.В And I should note that this is a conservative approach, so you wonвЂ™t make a mistake doing this.

#Psv Relief Sizing code

ASME Boiler & Pressure Vessel Code (BPVC), Section VIII.

Usually, this issue is one of curiosity and doesn't affect the end result of what valve is chosen.В Common practice is to default to API sizing equations and parameters, and only use ASME data sets for situations outside of the API letter-designations.В But what if I told you that approach is likely causing you to oversize about 10% of your PSVs and their respective piping systems? If you've everВ sized/selected aВ Pressure Safety-Relief Valve (PSV) using vendor sizing programs or good-old hand calculations, you've probably run into a very strange anomaly:В WhyВ doesВ a PSVВ orifice sizeВ changeВ betweenВ American Petroleum Institute (API)В and American Society of Mechanical Engineers (ASME) data sets? What is an 'effective' orifice area?В How do I know which standard to use when selecting a PSV?

NOTE:В this article is written to an audience that is familiar with PSVs,В PSV sizing, and API and ASME standards at a basic level.В I initially wrote this article in early 2017, and due to some great input and questions made significant revisions to increase clarity in mid-2018.В I hope it is helpful to you, please send me a message with any comments/questions! How understanding API and ASME standards can help prevent over-sizing PSVs and their respective piping systems **Update to this article, J: If you found this article helpful, here is a link to another article I recently found that does a nice job explaining the topic: ENGINEERS BEWARE: API vs ASME Relief Valve Orifice Size вЂ“ Petro Chem Engineering ()Д®ngineering Tips: PSV Sizing and Selection