Safety Relief Valves -- Power Plants 1

[KSU]

This question pertains to safety relief valves settings in power plant. I understand that some power plants have safety relief valves that are designed to go off similutaneously. On a recent combined cycle plant, the drum safeties and superheat safety relief valves were set to go off at same point (different pressures with the friction drop between the two determining the relief valve settings).
Other designers employ a strategy where the superheat safety relief valves open first with a deadband before the drum safety. Care is taken that there is margin between the settings to ensure that the safety relief valves will open in stages. This staging is to avoid "simmering" of the safety relief valves.

I would appreciate peoples thoughts on avoiding simmering of the safety relief valves as well as the "staging" of the safety relief valves.

 

[pablo02]

At all of the power plants I have worked at (probably over 30 units with power boilers), all have had the relief valve settings staged... with the drum reliefs last (even the drum reliefs were staggered, first one at one end, the second at the other, the third at the first end, etc...) -- in my opinion, the purpose was to handle the overpressure in a controlled manner rather than try to dump all the energy stored in the boiler water all at once (and to protect the turbine)... (besides, the noise would keep the cows from milking for months)

your point about simmering is also a thought, if the pressure was just at the relief setting (although the probability of these being exactly the same is somewhat remote) -- there is a possibility the combined relief would not allow the reliefs to fully open and as a result, they'd all simmer...

 

[pablo02]

My comment: (although the probability of these being exactly the same is somewhat remote) referred to all relief valves being set exactly the same... not that the boiler pressure matching the relief setting..

Also, from a reliability standpoint, having all the relief valves go off at the same time would result in a boiler upset that would require tripping the unit -- with staged relif settings, there is the potential of resolving the problem without tripping the unit (although I would assume that if you've let the problem get this far, the unit probably needs to come off anyway...)

 

[MJCronin]

KSU & pablo02,

Your comments and qustions about safety valve "simmering" and staging are all valid, but they should have been considered by the engineers responsible for the HRSG design and installation at your plant.

They are the people responsible for selecting the size and setpoints of the SRVs in accordance with ASME Section I.

"Simmering" is considered at the time of SRV selection by using the code defined accumulation...."staging" of setpoints for multiple valves is clearly defined in ASME Section I.

During construction, the owner of the plant has received calculations detailing the design and selection of these valves......however, like many other combined cycle power plant owners, the organization is probably filled with MBAs who have either discarded or ignored all plant design drawings and documentation (thank god for today's business schools)

My thoughts only

MJC

 

[davefitz]

a) the superheater outlet ( SHO) relief valve is required to lift at or below the design pressure of the main steam line, but can also be set to lift at drum design pressure minus the expected full load pressure drop between drum and superheater outlet.

b)setting the SHO relief valve at the higher level ( design pressure of transfer pipe) is common on jobs that have an HP bypasss valve that opens on unit trip; this gives more margin to liftng the SHO relief valve during bypass operations.

c) There are a variety of different safety margins used by different EPC contractors when setting the design pressure of the HP main steam transfer pipe ( and hence lift press for the SHO relief valve), but in any case, if the turbine trips and the HP bypass does not open, the SHO relif valve will lift and remain open unitl the pressure at the HP drum drops to the reseat pressure at the SHO relief valve .

 

[vpl]

KSU

I strongly prefer not to see safety or relief valves simmering, for personnel safety reasons if nothing else. I have seen cases where the safety/ relief valves are in "banks" where a group will go off at or near the same pressure; however, having "staged" pressures does reduce the amount of system upset and is generally preferred.

But MJCronin hit the mark : you have to follow the Code! Figure out what you're designed to, and go look up the requirements for that type piping. If you can't find the calcs, then redo them. Don't go removing valves or changing setpoints until you know why they are there and set the way they are.

Patricia Lougheed

Please see FAQ731-376 for tips on how to make the best use of the Eng-Tips Forums.

 

[unclesyd]

If you are in jurisdiction that carries steam boilers they may have some requirements concerning the settings. In the US you will have to satisfy both the code and insurance requirements( AI) as stated above .
I think the overriding factor is the design of the boiler.
The relation of the superheater to steam drum caused an unforseen problem when the superheater RV was set lower or equal to that of the steam drum RV. We had a bad problem with carry over to the superheater tubes when the superheater RV lifted. It took a big conference between the AI, boiler manufacturer , and our engineers to resolve the problem. I don’t recalled the exact details on the resolution but the drum settings were left as is and the superheater RV was relocated. There were also additional RVs add to the HP Header near the PRV valves to help with the process upsets.

Added note.
RV valves and expansion loops do work. Have you ever been around when every steam RV in a 75 acrea site lets go. A demand of 800,000 pph of 650# steam went to zero at the speed of light. To every bodies amazement they all reseated. During this episode it was amazing to see the expansion loops in action.

 

[IainM]

In marine practice the superheater relief valves were always set to lift before the drum safeties in order to prevent overheating of the superheater bank tubing. If the drum relieves first then this could happen due to the residual heat or due to boiler firing not being fully shut down.
Obviously important in terms of longevity of components if nothing else!