Fire System Overpressure

[hank999]

We have a large cat diesel fire water pump (2,500 gpm) that automatically turns on when the jockey pumps can't keep up with the pressure, which is approximately 160 psi (yeah I know its pretty high to start with). The system supplies numerous sprinkler systems in buildings at an industrial plant and is supplied from 650,000 gal tanks. The problem is that when the system is tested weekly it bumps up the sprinkler system riser pressures and has to be bled down frequently. Pressures can get up to 300 psi in those systems closer to the pumphouse. The system is probably overdesigned. It has been this way for several years. What can be implemented to reduce or relieve the pressure?

 

[BigInch]

You're talking about two things here.
Jockey pumps: What pressure are the jockey pumps trying to hold?
Testing: I presume you are testing the pump without flow at the sprinklers.

You hope the system is overdesigned (meaning pleanty of fat on the pipes to hold that 300 psig.), otherwise it may only be the pumps that are overdesigned, but you can't really tell that unless you experience high pressures when you are actually pumping flow at the design fire flowrate. If you are not actually pumping water when you see that 300 psig, it may be only that the pump's shutoff head is too high. In that case you should probably put a test bypass line with throttle valve from discharge to suction to reduce that high shutoff head during a test. But do not leave that line open when you leave test mode. Lock the throttle valve closed when finished with the test.

If it ain't broke, don't fix it. If it's not safe ... make it that way.

 

[Artisi]

I would suggest that the weekly test should pump the water back to the supply tanks via a by-pass line from the main line, there is no requirement or need to pressurise the whole system to above the design point which is a function of the jacking pump.

The weekly test is to check the following,
1.does the main pump start on a drop of pressure at the pump discharge.
2. does the main pump run ok and actually pump water, if you want to be very sophisticated you can arrange to measure flow.
3. does the jacking pump re-pressurise the system to its designed set point once the main pump is shutdown.

From memory, if this is a FM or UL system, the control system
must be able to override the test and bring the main pump on line if there is a drop in the sprinkler system pressure ie, an actual fire.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[stanier]

Who did the waterhammer analysis for the project? Was this scenario analysed?

The pump is probably dead heading with transient pressures locked into the system between check valve and closed valves.

“The beautiful thing about learning is that no one can take it away from you.”
---B.B. King

http://waterhammer.hopout.com.au/

 

[Pumpsonly]

Before you can say the system is over designed, you must know what is the required design pressure.
The name plate on the pump would have the rated pressure of the pump which is the required system design pressure. An engine driven pump would also have a pressure relieve valve fitted before the check valve.