Diesel Fire Water Pump PSV discharge 3

[danger06]

Dear FW Experts,

According to NFPA 20 section A.4.18.1, the fire water system has to be evaluated during when the diesel fire pumps are on overspeed scenario. The overspeed switch is typically set to limit at 10% of rated speed. A PSV is required if the piping component ratings are exceeded by the increase pressure generated from the overspeed. All this is true for the system I am currently looking at which is a diesel fire water pump (engine is water cooled supplied from the discharge)

Also the section mentions that since this case is relatively rare event, we are permitted to have the PSV discharge piped back to the suction side of the pump provided that we install another circulation relief valve (downstream of the main PSV) discharging to drain (or atmosphere) (section 4.18.7.1).

I would like to ask if somebody has any experience with this configuration. NFPA has clearly stated that it is an option but has not provided any basis on how to proceed with this design. What is the purpose of the additional circulation relief valve when the engine has its own cooling mechanism? How to determine the set point in relation ot the main PSV?

Thanks in advance for any input you provide.

Danger

 

[b1ueshift]

I haven't heard of that before - this looks to be an addition to the 2010 version of the standard. I think it used to say don't connect the discharge back to suction.

This is interesting, I've seen a lot of articles in recent years, on what the best way to regulate the pressure from a pump boosting mains pressure. This must be what the NFPA 20 committee has come up with.

The set point for your main pressure relief valve would probably be 175 psi (as sprinklers are usually rated for a maximum pressure of 175 psi).

If the engine has it's own cooling system then you shouldn't need another circulation relief valve to cool the pump.

Looking at the wording of the standard, this valve is required only where you have back to suction and it's supposed to discharge not to a drain but into the air. The purpose appears to be to give an obvious warning if the pressure relief valve ever opens as it should only open during a malfunction.

 

[danger06]

blueshift,

thanks for your reply. you are correct, this is all from the 2010 edition.

the PSV is only during overspeed scenario. our piping system can withstand the normal pump high pressure scenario as calculated max suction + shut-off differential (with consideration of stat head). so the main PSV is I suppose has to be set more close to the pressure where the overspeed will generate enough pressure to exceed our design pressure (in our case around 7% overspeed) otherwise there is possibility that our PSV will keep on lifting during pump testing where it running against no user.

i am still at lost as to why NFPA 20 has to include the addition of the extra recirculation valve. if it was to take out the excess heat generated by the engine when the PSV is circulating back to the suction then i think the integral water cooler exchanger on the diesel engine can already do the job. Am i misunderstanding what the code is saying?

I hope more experts can shed light.

Danger

 

[b1ueshift]

I didn't know about this before you drew it to my attention and the standard doesn't say what the purpose of this extra pressure relief valve is.

I think the purpose would be just as a visual warning. If the main pressure relief valve opens, then you get a water jet discharging where people can see it to let you know that the water is flowing in the return pipe. This could be caused by pump overspeed, or more likely the pressure relief valve malfunctioned and opened when it shouldn't have.

The standard calls for waste cones (or other flow visualisation device) to be installed in the return pipe, but a waste cone won't work in a pipe that goes back to the suction pipe because it will be always under positive pressure.

 

[danger06]

b1ueshift,

thanks again for your prompt reply.

i am re-reading that NFPA clause and your replies and it seems like i may not be of the same page as you. section 4.18.7.1 says
"Where a pressure relief valve has been piped back to suction, a circulation relief valve sized in accordance to 4.11.6 and discharged to atmosphere shall be provided downstream of the pressure relief valve"

Correct me if I'm wrong sir, is your understanding of "downstream of the pressure relief valve" is at the PSV discharge? Is that why you said that the additional circulation valve purpose is only a visual warning because it may only be set if something is coming out of the PSV and the recirc valve should be located at the PSV discharge for that purpose?

Because all long I think the downstream of releif valve is meant as downstream of the tap point to the main relief valve.

Appreciate you replies.

danger

 

[pipesnpumps]

I would say the same as you, that the "circulation relief valve" connects to the main, but downstream of where the main pressure relief valve taps off. If it were in the discharge pipe of the main prv it would never see enough pressure to lift, as there would be nothing to create any back pressure above what the normal max. suction is (plus a small amount of friction).


I think by the name "circulation relief valve" this implies it was meant to discharge some water; so that the water being recirculated didn't heat up from all the pump energy and friction across the main relief valve.

Perhaps the committee wanted to account for a scenario where any integral cooler is non-operational, or was not designed large enough for this scenario. If you really want to know a firm answer, you are going to have to ask one of the NFPA 20 committee members if you know them personally, or submit a question to NFPA if you are a member.






Real world knowledge doesn't fall out of the sky on a parachute, but rather is gained in small increments during moments of panic or curiosity.

 

[b1ueshift]

Such formality Danger,

I'm happy to give free advice to anyone interested in reading it and I've received lots of useful free advice from others on this forum also.

After thinking about pipesnpumps' post, I've had second thoughts. If the CRV was in the main relief valve discharge line, it would be difficult to guarantee that it would open whenever the main relief valve opened.

If downstream of the main pressure relief valve is meant to mean not in the overpressure discharge, but in the system main, the schematic view of the system should look something like this:

suction---->pump--->4" prv (discharge back to suction)--->3/4"CRV(discharge to air)--->check valve--->sprinkler system.

Anyway the 4" prv should be set to 175 psi (ie maximum permitted pressure on sprinklers) the 3/4" CRV should be set to about 165 psi to give you visual warning of pump overspeed and the churning pressure of the pump plus maximum incomming water pressure should be less than 165 psi so the CRV doesn't open automatically.

I don't know how you would test that the system is working the way it should as it would be difficult to simulate a pump overspeed.

I think you would also need a visual flow indicator (with sight glass) in the main return line.

As this is a brand new requirement, it's difficult to check how people are interpreting this provision.

 

[danger06]

Pipesnpumps, thank you for your insightful reply to my query. Unfortunately, I don't know anybody that is connected to NFPA committee. I do agree that only them can clarify what was stipulated on their code. I was hoping somebody from here is a member of the org and would be so kind enough to forward this if they chance upon it. i had the same thoughts about what you said as to the possible purpose of the added recirculation valve. Maybe, the additional heat generated from the overspeed scenario is something that is not accounted for the engine's integral heat exchanger capacity.

B1ueshift, thank you as well for your continued interest in this topic. The configuration that you pointed out was how i originally understood the code wants me do to. Although, I had difficulty trying to find the basis for specifying the set point for the additional recirc valve as I didn't think at that time that it is for relieving the excess heat generated from the overspeed. i guess it is better for it to be set below the main relief valve pressure set. As small as a 1" PSV it is, my only concern is that the discharge flow will be significant to reduce the pressure below the main PSV set. It seems unlikely, but I'm just saying this as I have encountered a validation of 3/4" thermal releif valves before were the flow was a little bit bigger than expected. In any case, it it will lead to not popping the main releif valve then it means that I have protected my piping system for the overspeed with just the small recirc valve as well.

As for the visual flow requirement by NFPA. I was advised that a valve position indicator that is integral to the PSV can be used as a substitute. I was told that there is a limitation to the use of waste cones with sight glass and they are not an off the shelf items that can easily be ordered. Our pump capacity is about 5000 gpm so our PSV is an 8" body and the discharge line is coming out to be at least 14". It is not economical to put a flow indicating device on that big of a pipe as well. Our PSV is made by Cla Val (want to emphasize that I am not promoting them, just citing for reference) and they offer a valve position indicator that can be attached to their valve body (Cla Val X101).

Anyway, thanks guys for your continued replies. I hope that I could gather more from professionals like you.

Danger

 

[b1ueshift]

This is turning into an interesting discussion. I'm not sure what NFPA had in mind when the specified this. First time I read it I thought it was obvious, but now I'm not sure.

If it's to stop the pump from overheating if the cooling loop fails, it won't help much because the diesel engine will still seize without an engine cooling system. If it's for visual warning of overspeed, then it's redundant because you still need a flow indicator in the return line.