Steam Safety Valve Discharge Piping 2

[moodyj2000]

I wanted to see if someone with more experience in steam safety valve piping design could point me in the right direction. I do MEP work and have for 25 years. Typically we route the discharge of a steam safety directly to the exterior of a building without headering them together. I have had two projects lately where the mechanical contractor has requested headering the discharges due to limited availability of discharge locations.

I have done the research to the point of absolute mind numbness on headered vent piping. Can someone point to the relevent codes, design guides, etc.. that would address headered steam safety relief discharge piping? The issue I am having is what is applicable to 1) steam and 2) headered discharge piping. I can find a thousand references for how to size reliefs (which I don't need) but nothing that clearly defines a path for a safe design.

A little background on one of them. It is an existing site where the discharge of the relif valves on the downstream side of a PRV station is sized for 10" and is routed about 150 feet to atmosphere. Set pressure on these is 70 pisg. There are also two steam-to-steam generators that have shell side relief valves connected to this system. These two generators have a shell pressure of 15 psig and the reliefs are set at 35 psig. The contractor wants to add two more steam-to-steam generators to this arrangement with the same set pressures. My gut is that everything will be okay becuase the shell side of the generators is separated from the steam leaving the prv (which is on the tube side) so actuation of all safeties at the same time is improbable. For this to occur the PRV and control valve on the inlet of the generators would have to fail simultaneously. Gut feelings, however, don't save you when things blow apart.

I am willing to do the leg work. Just looking to other experts for input.

 

[The Obturator]

Is the subject application following ASME I Steam Boiler Code?

Although ASME I safety valve requirements are effectively now contained in 'new' 2021 ASME XIII Code, full transition has not been made for some earlier sections such as ASME I.

There is also sometimes the overlooked dialog in the Power Piping Code ASME B31.1 - in particular, Nonmandatory Appendix II - Rules for the Design of Safety Valve Installations. I can't say if the information here will answer your dilemma, as I am no expert in piping, but it appears that the information in this part of B31.1 has some good comprehensive information for safety valve piping on steam boilers. Even if your installation is not pure ASME I, this specification provides sound guidance.

Please also note that there is a separate safety relief valve forum that would probably have been more appropriate to post your question in.


*** Per ISO-4126, the generic term 'Safety Valve' is used regardless of application or design ***

*** 'Pressure-relief Valve' is the equivalent ASME/API term ***

 

[moodyj2000]

This is more pressure vessel code but the Nonmandatory Appendix II definitely provided some guidance even though I had to pull an older version as we don't purchase those codes. Thanks for the advice.

 

[Andrew ONeill]

Not sure from an exact "code compliance" point of view. But is is very common for dirty service PSV's & vent valves to discharge into a common header, then into a separator. I'm guessing similar things happen with flares as well.

Fundamentally you must pick the worst possible case (in simple terms, all PSV's lifting at the same time) and then do the flow modelling to make sure you can achieve the required relief rate / not have excessive back pressure.

If you were going to not consider all valves opening at the same time, I want that backed up by a full LOPA (layer of protection analysis) study or similar.

Andrew O'Neill
Specialist Mechanical Engineer
Rio Tinto
Australia

 

[casflo]

This design is very common in the thermal power plants. To do a safe design you need to calculate the piping size and length from each valve to the header and also the size and length of the header to the discharge.
I suggest you visit the web Research Gate and look for these type of documents.