Add New Fire Pumps with Different Capacity?

2/6/2024

We have a case where two (2) x 2,000 gpm existing electric fire pumps (primary duty pumps) are available.

Now, the system design demand is increasing from 4,000 gpm to 12,000 gpm due to a scope change, so we are bringing in new electric fire pumps (as primary/duty) to meet the higher demand.

All new fire pumps could be either four (4) x 2,000 gpm or two (2) x 4,000 gpm towards meeting the 12,000 gpm system capacity.

All performance test curves are checked for all existing fire pumps; they are almost new and meeting the required residual pressure at 2,000 gpm and their performance is close to matching the original equipment manufacturer curves; so these existing pumps need to be retained.

Now we do have space constraints, so we'd like to use the two (2) x 4,000 gpm pumps, and their pressure characteristics would be the same as the existing pumps.

Would NFPA 13/20 (and any other relevant NFPA standard here) permit two different pump capacities for the system?

Would having different pump capacities be concerning, or cause issues? Is there anything else we should be considering here regarding the capacities?

Thanks in advance.

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12 Comments

Pete H

2/6/2024 06:47:14 am

From NFPA 20 (2016 Edition):

4.20.2.3 No more than three pumps shall be allowed to operate in series as a part of a series fire pump unit.

So I don't think the 4 x 2,000 can work.

And I don't think the 2 x 4,000 can work if they're in a series with retaining the existing 2 pumps works either.

No idea about the pump capacities off hand though.

Dan Wilder

2/6/2024 07:21:17 am

Wouldn't this setup be in parallel to achieve the 12K GPM system demand?

Good catch on the Series setup, hadn't run into that before.

2/6/2024 07:36:26 am

An excellent point!

https://www.119pump.com/industry-news/362.html#:~:text=That%20is%20to%20say%2C%20when,system%20at%20the%20same%20time.

Yeah, parallel for making the water seems fine as opposed to series. And based on this write up, it's best to aim for same model and specification to avoid different outlet pressures of the fire pumps during the work, resulting in unstable work. But OP already called out the pressure characteristics are to match existing.

I think it's still a little "off", but I don't think it's restricted by code, and the best move is just aim for the same manufacturer as the existing so the matching pressure characteristics come out as evenly as possible during work.

Sunil Kumar

2/6/2024 08:09:06 pm

Yes, they all need to be setup in parallel to achieve 12K demand. Also, little bit more details as Client also requires following shall be meeting.

1. 12K demand shall be met by Primary FW pumps (Electric) and 100% Backup (diesel driven)
2. Diesel FWPs cannot be used as primary, so they have to be separate (additional).
3. Currently there is single power supply to facility (ultimate source of power), so 100% backup (diesel) FWPs needed to achieve 12K.
4. Operating sequence will be based on Hydraulic Assessment/NFPA 20 on pressure reduction/sensing on header.

Glenn Berger

2/6/2024 08:29:16 am

With the revised flows, I be concerned about retaining any portion of the existing configuration.

Is there multiple (lower) flow demands that individually needs to be met? Or is it the 12k gpm the only flow rate at this site?

Just make sure that piping system is sized for the greatest single demand.

2/6/2024 08:33:10 pm

For Oil & Gas facilities (either Onshore/Offshore), The fire water demand varies based on individual units/platforms and their respective locations (they always physically segregated to avoid fire escalation between units), some platform/unit have much lower demand (e.g. 2000 gpm) and some are higher (e.g. 12K or even more).
However, while selecting system design capacity, this is always being taking single greatest demand within the facility/Complex. We call it "worst case" demand to design with.

Some places even demand is little less but physicality located "farthest" from Pumps units, could be worst case on other hand, some location even demand is "highest" but closer from pumps unit, could be consider as worst case, residual pressure and elevations also taken into account.

So these FWPs shall be able to meet any demand in between from 2000 gpm to 12K gpm depending where is the fire event.

Piping/Main is sized based on 150% of 12K i.e. 18K and good enough to withstand up to 220 pisg.

2/6/2024 09:03:56 am

All these pump questions, had to go back to some hanger drawings with multiple pumps....

In the past, we have had different GPM pumps on the same site in parallel. The programming was setup to start the smaller pumps first in 10-15 PSI increments as the PSI dropped. We did run only 2 jockey pumps.

Some things we ran into:

Suction pipe will need to be sized per Table 4.28 for the individual pumps however the manifold supplying all the pumps will need to be sized more in line with the commentary of 4.16.7 so that the velocity does not exceed 15ft/sec at 150%...so you are potentially looking at a 24" end fed supply manifold (if you center feed it with the outside pumps being the 2K and the inside being the 4K, 24" supply into a tee with 18x18x24 tee, then to a 18x10x14 tee - or just run out the manifold the same size with the 14" & 10" suction piping outlets).

Discharge piping - Same issue to increase for the test header to be able to test a single pump and then all of them together.

FDC - If this is on the building or tying into the area, Storz connections are your friend (hopefully your AHJ agrees)....if not, buy some stock in hose valves prior to making that manifold.

Test Header - Single & total flow with both the flow meters and the physical flowing of water. Watch the total length for any increase requirements per 4.22.3.4. Again, Storz connections are great for testing along with the Big Boy Hose Monster kit for the large flow requirements.

Franck

2/6/2024 10:57:12 am

Discharge piping sizing not only for the test header but for the entire installation downstream.
If you’re fire main was designed for 2000+ gpm, will it still be ok for 12 000 gpm?

Consider also the starting sequence of the fire pumps with 10 s intervals and not only 10 psi difference.

2/7/2024 07:18:46 am

Each pump(s) suction & discharge are sized based on table 4.28 along with mains taking 150% cumulative demand and rest all relief, test headers are sized accordingly. All pumps are set to start on pressure differences in case fire. In normal (no fire condition), 2 Jockey (1 operating and 1 standby) are maintaining pressures. Rest all meeting all NFPAs design requirements.

Here the Question is whether two pump with difference capacities can be utilized or not provided head characteristics are same?

Jack G

2/6/2024 10:31:40 am

Agree with Dan. My refueling aircraft handler had a 30 inch pipe with 5-individual 12 inch connections, pumps in parallel, passed at 15 psi increments which was tough to set up.
The discharge pipe was 24 inch, and went to a strainer and 16 systems. Add to that 2 - 3000 gpm foam pumps and 16 proportioners. Expensive.

2/6/2024 11:04:26 am

I would also suggest (not from the code, but from a reliability standpoint) when there is a need for more than one pump to have 3x50% rather than 3x33%.
If you have impairment on one pump (3 more chance, by statistic, with 3 pumps than with one pump), you will then still have 100%.

In your case, I’m afraid this means replacing everything with 3 x6000 gpm…

2/7/2024 07:28:56 am

To ensure the availability and reliability of the firewater system, we have 100% backup (diesel driven) pumps equivalent to 12K demand.

The project is offshore facilities where each deck is limited on spacing, so while utilizing existing pumps, new larger size pumps to be added to cater the largest demand on another bridge platform connected to same complex.

Add New Fire Pumps with Different Capacity?

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