Discharge head for fire pump relief valves 1

[LuisEGV]

An 8 "diameter discharge head was installed in which the outputs of the pressure relief valves of 5 fire pumps were connected.
One of them is 1000 gpm capacity and the others are 2500 gpm.
Is this arrangement acceptable?

 

[LCREP]

As per NFPA 20, 2013 ED Table 4.26(a) Summary of Centrifugal Fire Pump Data (U.S. Customary)

The pipe should be 10" for a 2500 gpm pumps. They would need to change the 8" to a 10" for the 2500 gpm pumps to meet NFPA.

 

[chicopee]

Am I reading the OP correctly? There are 5 PRV's from 5 fire pumps and the outlets of the PRV's are connected to an 8" header ?

 

[TravisMack]

That is how I read it. It seems like if these fire pumps are designed to run in parallel, then this could be a way undersized relief valve header.

Travis Mack
MFP Design, LLC

http://www.mfpdesign.com

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[LuisEGV]

Fire pumps operate in sequence according to system pressure.
Three are powered with electric motor and 2 with diesel engine.
I consider that the diameter of the head is very small to operate with that arrangement and that they should discharge individually.
A schematic drawing of the arrangement is attached.

 

[mk3223]

IMO, If a common PSV discharge pipe used, a larger pipe to accommodate the flow of the multiple PSV discharge simultaneously should be used for your fire pumps system. I agreed with you to have individual discharge pipe for each pump, and sizing it based on NFPA 20.

 

[chicopee]

The fact that you have three electrical fire pumps and two diesel fire pumps suggest that in the event of power failure, the two diesel fire pumps would be used and presumably deliver the same amount of water that the three electrical fire would provide. Is an 8" header be adequate? perhaps.

 

[LuisEGV]

Diesel pumps start when the system pressure drops, not just in a power failure.
Therefore, the 5 pumps can operate simultaneously.

 

[TravisMack]

If the diesel pumps are the backup pumps, and they start operating due to such pressure drop that the 3 electric primary pumps can't maintain pressure, you have more problems than an undersized relief valve header.

Travis Mack
MFP Design, LLC

http://www.mfpdesign.com

"Follow" us at

https://www.facebook.com/pages/MFP-Design-LLC/92218417692

 

[chicopee]

Looking at the diagram, each PRV has an 8" discharge pipe PRESUMABLY matching the PRV outlet diameter. If that's the case for all valves and they are all discharging at the same time when the five fire pumps are operating, then 8" header is probably undersized. That's my opinion since we have no engineering calculations showing the results and PRV specifications. It should be noted that PRV's should have minimal back pressure when discharging.

 

[LuisEGV]

Last year we had a fire where a water line was fractured by the impact of the explosion and there was a very large leak so all the pumps went into operation.
It is a very special case but it was proven that this is likely to happen.

 

[chicopee]

You still not have addressed the issue of the 8" header.

 

[LittleInch]

Is it just me that can't understand what these valves are doing? I've never seen a full flow pressure relief on a centrifugal pump system like this as you should not be able to exceed the pressure rating of the system even at no flow / dead head??

Certainly not on a fire water system where they treat pumps as disposable items and in many case regularly "test" the pumps for long periods against dead head with no flow.

Hence first off find out what these relief valves do, why they are fitted, what they are trying to protect, size / flow rate etc.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[cdafd]

Little

So how old is this set up??

Is it in the USS

Do you have a fire sprinkler company you can talk to? I take it you do not work for a fire sprinkler company??

 

[LCREP]

From the 2013 Fire Pump Handbook on NFPA 20 regarding this topic...

4.18.1.2 Where a diesel engine fire pump is installed and where a total of 121 percent of the net rated shutoff (churn) pressure plus the maximum static suction pressure, adjusted for elevation, exceeds the pressure for which the system components are rated, a pressure relief valve shall be installed.

It goes on to say the following...

Prior to the 1996 edition, NFPA 20 required the installation of pressure relief valves for all diesel engine fire pumps. This requirement was based on the assumption that, if engines ran too fast (a condition known as overspeed), the fire protection system would be exposed to pressures in excess of the pressure ratings of the system components. Because an overspeed shutdown device is required, the technical committee believes that a pressure relief valve is not needed on all diesel fire pump installations. Pumps that create pressures less than the pressure rating of the fire protection system components [typically 175 psi (912.1 bar)] at 110 percent of rated speed do not need a pressure relief valve.

 

[LittleInch]

Full flow pressure reliefs on a fire water system are, IMHO, poor design and represent a potential weakness in the system.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.