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Trond (Petroleum) (OP)
27 Jan 09 0:00
Hi all,

I hope someone out there could help me with this problem.  We have an offshore installation with a total of 5 fire water pumps supplying the ring main.  System is pressurized with a single jockey pump. Max total deluge load in a single fire zone requires two pumps to start up to meet demand.

Start-up time for the pumps is around 15-20 seconds, deluge valve open in a much shorter time span upon F&G detection.  What happens is that the pump start-up sequence is initiated, the two pumps start to rev up, and then we loose total ring main pressure because deluge valve goes fully open before the two pumps reach full capacity. Naturally the jockey pump cannot compensate for the shortfall (nor is it meant to do so.)

This causes PSLLs on all fire water pumps to start EVERY fire water pump on the facility, which is safe, but a waste of resources, especially when we are just conducting regular testing of the system. In addition, we get pressure surges in the system, which has caused pipe ruptures in the system (GRP).  The latter is probably due to poor design, and is being rectified.

Now my question is this: Would it be acceptable to either delay the opening of the deluge valve until the pumps have reached full capacity, alternatively increasing the opening time so that flow through the valve will not exceed the pump output at any time?

Appreciate any comments or suggestions you guys or girls may have on the subject.
cdafd (Specifier/Regulator)
27 Jan 09 16:41
not a pump person, but worked a a holiday in that had a very large hanger, and five pumps.

Not sure how they set them up but the others would not come on till I think the pressure dropped to a certain point.

Is that possible with this set up to set them so thet do not come on till they see a certain pressure, which would be a lot lower than the first 2???? especially seeing that you can run the system with two???
cdafd (Specifier/Regulator)
27 Jan 09 17:19
D–SEQUENCE START - For multiple pump installations.
Provides a time delay to sequence the pump.
Delays the pressure and deluge start signals.

http://www.mastercontrols.com/ProdInfo/Catalogs/MC_ebulletin-902B.pdf

maybe that was it time delay??


http://www.armstrongpumps.com/present_newsletter.asp?groupid=2&nlfile=00_00_033

 same in the 2007 nfpa 20

When dealing with multiple pumps, it is important to set the pressure switch set points such that the jockey pump maintains pressure without starting the main or standby fire pumps. Appendix A of NFPA20 gives an example of set points for a multiple pump installation in section A.14.2.7 (4) (f) as follows:

"Examples of fire pump settings follow (for SI units, 1psi = 0.0689 bar):


Pump: 1000gpm, 100psi pump with churn pressure of 115psi
Suction supply: 50psi from city to minimum static; 60psi from city to maximum static
Jockey pump stop = 115psi + 50psi = 165psi
Jockey pump start = 165psi to 10psi = 155psi
Fire pump stop = 115psi + 50psi = 165psi
Fire pump start = 155psi to 5psi = 155psi
Fire pump maximum churn = 115psi + 60psi = 175psi"  
Trond (Petroleum) (OP)
1 Feb 09 23:17
Thanks guys,

the problem is that the deluge demand exceeds the pump capacity while the pumps are starting up, say for 15-20 seconds or so.  During this period, the deluge valve will be wide open, but the two pumps required have still not reached full speed.  The deluge valve opening causes a sudden pressure drop in the FW ring main, tripping every PSLL, and as a consequence starting every damn pump there is.

The result is that we now have 5 pumps starting up, whereas 2 would suffice. It is safe, but hardly in line with the original design intent.

Therefore, I was wondering if there are any issues with delaying the opening of the deluge valve until the pumps have reached full capacity, alternatively slowing the opening speed of the deluge valve to ensure that FW ring main pressure remains high.

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