Relief Valve on Steam Header

The situation you described does not merit a relief valve. You have already convinced yourself that all the equipment and piping is adequately protected in the event of a control valve failure, no further protection is neccessary.

Concur with sshep.

PRVs are provided to protect process equipment from overpressure. If all equipment are rated to 50# or higher, 35# letdown CV failure cannot overpressure your 15# steam system.

Hi

If I understand your question, you need to look at the capacity of the letdown stations in a failed position. I would size the relief based on which one had greatest capacity. This assumes a failure of both is not expected.

The NBIC Appendix G is called "Safety Valves on the Low Pressure Side of Steam Pressure Reducing Valves." This document will provide calculations for safety valve relieving capacity in this service.

Also based on your system description is seems possible to remove the reliefs on indiviual units and install a just one on the 15# header?

You will need to confirm that the pressure drop from the
35-15 psig letdown station discharge to the inlet of the relief valves is less than ( 7% ) of the relief valve set pressure, assuming the flowrate is equal to the maximum casualty flowrate of the letdown station.

Next, it might be neccesary to confirm that the relief capacity of the relief vales is greater than the combined casualty flow rate of both letdown stations. This would be implausible if both letdown stations are completely independent, but becomes plausible if the 2 letdown stations share a common control system or pressure transmitter .

To all,

ASME B31.1 Power Piping Code contains the requirement to either provide a relief valve at a steam pressure reducing station, or to design the LP part of the system for the HP pressure....

Each case is different......with the modest pressures you mention, I also believe it would be easier to certify that the LP portion of the system can sustain the higher pressure

MJC

To answer the first question,
Should their be a Relief Valve after a Steam Letdown Station on a Steam Header(in general) ?
>> only if there is a need to limit the pressure incase of failure of the control device.

As for the second question, I think there is a lot of information missing to be able to make a clear judgement whether you need a PSV downstream of your 35# to 15# letdown station.

If the 35# steam system is limited to 35 psig (which may not always be the case), then the 35# steam system will not be a source of overpressure for equipment rated at 50 psig. But you should investigate what is the limiting pressure of the 35# system.

I find it curious that you have relief valves set at 25 psig on equipment rated for 50 psig. Is there some reason other than the equipment rating that you need to limit your pressure?

At first glance, an argument could be made that you need only a single relief device downstream of the letdown stations to protect the equipment but before eliminating the relief valve on each equipment you need to be sure there are no other relief cases that need to be addressed. For example tube rupture in an exchanger, fire exposure or any other case that needs to be addressed for that particular piece of equipment.

You also mention there are adequate relief devices downstream of your 175# to 15# letdown station but you don't say what pressure it is set. Obviously, the 175# steam will be a problem even for the equipment rated to 50 psig but 35 psig steam can be a problem if the intention is to limit the pressure to only 25 psig as indicated by the relief valves on the equipment using the steam.

So the answer to the second question is the same as for the first and only you can assess your system to determine if you need to ensure the downstream pressure is appropriately limited.