2 replies to this topic

[NoobInsProMech]

Hi everyone,

I am a first timer. Just started to work as a project engineer in a company that manufactures fuel gas conditioning skids. I have not gone into detail on reading the specifications for pressure relief valve and I am not familliar with this device either but hopefully any of you here can clarify my doubts.

I have a pressure relief valve sized for control valve failure with 6" by 8 " flanged. It has a drain and isolation valve at the outlet. I was wondering whether is it essential to have a trunnion mounted ball valve as the outlet isolation or simply stick to using a floating one to save cost. I cannot find anything in the customer specification. The only thing is that my company's specification that requries us to use trunion ball valves for all 6" above ball valves in the main process line. I would argue since that would not be a the main process line and also trunnion simply offers a tighter closure or isolation, it wouldnt matter much if i use a floating since the valve will usually be opened all the time. I do not understand the need to have an outlet isolation for PSV. THe gas will be vented to atmosphere through a main header. Anybody there to explain this. I appreciate the help.

[JoeWong]

If 2 or more PSV protecting different system (example System S1 and S2) are connected to a common header and there are spare PSV for each system (PSV-A1 and PSV-A2 for system S1, PSV-B1 and PSV-B2) available for PSV removal, isolation valve is provided downstream of PSV for isolation purpose during PSV removal (for testing / maintenance / inspection). The spare PSV will be put online before closure of isolation valve. This is to avoid back flow of gas from other PSV (i.e. PSV-B1 or PSV-B2) in the event removal of PSV under maintenance (i.e PSV-A1 or PSV-A2).

Requirement of tight closure subject to hazardous level of fluid being handle and it is linked to characteristic and condition of the fluid i.e. flammability, toxicity, temperature, pressure, etc. It may not only subject to operation frequency. Since you are handling fuel gas, it is flammable and possibly hot and toxic due to potential present of BTEX, H2S, etc. Any leak on the valve will create hazardous environment to the operator and equipments.

There are tight closure class for different system. Higher the risk of hazardous environment, higher the tight closure class. All this subject of risk/consequence, experience, education level, history, cost balance analysis. No direct answer.

[skearse]

To add to the comments by Joe there are specific requirements for the installation of isolation valves in eitther the inlet or discharge side of a relief device. Not related necessarliy to the construction of the valve, but an isolation valve must be full bore, must be installed such that it cannot fail in such a manner that it would close off the relief path, must be locked/carsealed open & inspected to ensure that they are open at a certain frequency, etc. As a starting point I would recommend checking out the requirements in Appendix M of the ASME B&PVC Section VIII.