Use Fire Pump at 145% to Avoid 8" Underground?

2/21/2023

I have question regarding fire pump capacity.

In my situation the water department is requiring a city tap size to be 2 pipe sizes larger than the fire protection backflow preventer.

In my project, I was using a 4" backflow and 400 gpm fire pump so my underground/city tap would be required to be at least 6 inches.

The AHJ is requiring me to increase the area size in one of my remote areas. So, my sprinkler demand now becomes 582 gpm. To avoid having to use a 500gpm fire pump (with a minimum suction size of 6-inch) and increase the underground to 8-inch, could I still use the 400 gpm fire pump to supply the 582 gpm demand?

This comes out to about 145% of the pumps rated capacity.

I have never come close to the 150% mark before and just want to see what others think. If the underground size wasn't required to be 2 inches larger I wouldn't even think about it and go with the 500 gpm pump.

Thanks in advance.

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15 Comments

Paul Pinigis

2/21/2023 08:12:09 am

There are a few things that need to be addressed.
1. you state that the AHJ is requiring you to increase your remote area. What is his/her basis for this requirement? Did your original design not comply with NFAP 13? If it did, then the AHJ has no authority to require more unless there is a local ordinance that requires more than NFPA 13 compliance.
2. Two pipe sized larger than 4" would generally be 8" since 5" pipe is not a standard size.
3. What is the water company's rationale for the "two sizes larger"? Can you achieve their desired performance without this?

But, yes, 145% is acceptable.

Jesse

2/21/2023 08:27:42 am

All good questions by Paul.

Most curious is why the increase in remote area size. Is there a slope issue or unprotected vcombustible concealed space??

2/21/2023 08:28:54 am

Meant *combustible. Loose nut behind the keyboard apparently.

Chad

2/21/2023 08:51:39 am

Or is it in front of the keyboard?

James Evans

2/21/2023 08:43:59 am

Riding a pump curve that far is never a good idea. There is no room for error and if the flow test ever changes then the owner is out of luck for future problems.

It is always recommended to stay around the rating up to about 120% once you pass that you are asking for trouble.

The real question the velocity of the water on the section side of the pump and being sure you will not drop below 20psi during the testing.

Pump suction sizing is based at 15 feet /second at 150% of the pump rating. That is something you need to look at.

Casey Milhorn

2/21/2023 09:00:39 am

I was thinking 140% was the max for designing out past the pump's rating. Not in front of NFPA 20 to verify, but I was pretty sure of this.

Wes

2/21/2023 09:18:10 am

I've always held onto the concept that many insurers don't recommend exceeding 120%, but that in worst-case situations not to exceed 140%.

That said, here's some input from NFPA 20-2022 Edition:

Chapter 4 General Requirements
Section 4.10 Centrifugal Fire Pump Capacities
Section 4.10.1
A centrifugal fire pump for fire protection shall be selected so that the greatest single demand for any fire protection system connected to the pump is less than or equal to 150 percent of the rated capacity (flow) of the pump.

A.4.10
The performance of the pump when applied at capacities over 140 percent of rated capacity can be adversely affected by the suction conditions. Application of the pump at capacities less than 90 percent of the rated capacity is not recommended.

The selection and application of the fire pump should not be confused with pump operating conditions. With proper suction conditions, the pump can operate at any point on its characteristic curve from shutoff to 150 percent of its rated capacity.

2/21/2023 09:19:04 am

Then, additionally, ASK THE AHJ for Enhanced Content out of the NFPA 20 Handbook (highly recommend getting this):

ASK THE AHJ:
The required flow for a fire protection system is higher than the rated flow for the fire pump selected by the designer. Is this allowed?

ANSWER:
Yes, a fire pump can satisfy any point along its performance curve. This includes points up to its overload point (150 percent of rated flow).

Selecting a pump so that the system demand falls between 90 percent and 140 percent of rated capacity ensures that the pump is not oversized and conversely is not operating at its maximum output. Note that while NFPA 20 recommends selecting a pump of a size such that the system demand falls between 90 percent and 140 percent, some insurance companies limit pump sizing to 120 percent of rated capacity, and some municipalities limit the maximum system demand to 100 percent of rated capacity. The intent of these limitations is to avoid operating the pump at or near the overload point, which, over time, can compromise pump performance. After years of service, a fire pump might begin to show signs of wear and might not be capable of reaching 150 percent of rated capacity. If such a pump were selected based on performance at 150 percent capacity, a deficiency would eventually result. Selecting a fire pump of such a size that the system demand falls below 90 percent results in a fire pump that is too large for the system. The results of such a design include higher power supply demand, excessive cost, and more water usage during testing.

2/21/2023 09:33:25 am

Without naming names, I think I know where this jurisdiction is, and this requirement has caused so many issues on installation.

The cost difference that the city charges for a 4-inch tap versus an 8-inch tap is already extreme, and then when pressed about this "two-pipe-sizes-larger" requirement, no one has been able to explain why that longstanding requirement is important.

The only explanation that's ever come across my desk was to limit the velocity of the water service so that the pipe wall doesn't wear. With how infrequent fire pumps operate and are tested, I don't fundamentally understand the difference in wearing out a pipe wall at 2.3 fps versus 4.0 fps, but that's the only reasoning I could get out of why this specific locally-adopted requirement exists.

It's very frustrating for both the subs, the general, and the owner, who are surprised to learn that their small private garage, which only houses 6-8 cars, needs a pump because of locally-adopted requirement for garages to be 0.25 / 2,500 sqft at that time, and then also is going to need a very expensive 8-inch underground tap. Very costly and surprise-galore.

Not to be a total grump, but tough place to design. And I love my AHJ friends, so please don't take this the wrong way. Just hope it's gotten better since I did work there.

Michael

2/21/2023 09:37:10 am

120% is about the highest I would go on the pump curve. Higher then 120% is opening yourself up for many other issues that have been discussed in the above responses.

Glenn Berger

2/21/2023 10:27:34 am

1) You comment says that you have a 400 gpm fire pump. This is not a standard size fire pump.

2) As others have said, review your remote area and verify that it complies with NFPA 13.

I try not exceed 120% of fire pump rating with projects that I work on.

Franck

2/21/2023 12:32:15 pm

I will express my practical point of view, which is definitely not the one the AHJ may have.
For a new system, I would never go further than 120% for all the reasons above.

In your case, I would defend it as it is not a new installation but a new requirement on an existing installation
If you can prove that this new requirement is met (and 145% can be OK), why should you not approve it ?
As long as your demand point remains below the water supply curve, I don’t see any issue.

To come back to the first assessment: the reason why I won’t go further than 120% for a new installation is to give me the flexibility to go beyond (up to 150%) if you have a greater demand in the future.

Anthony

2/21/2023 01:07:14 pm

Have you thought about using 2 backflow preventers in parallel? You could get two (2) 3'' to flow (600 gpm 300 through each), with no added pressure loss and keep your tap at 6''. I'm not sure how the money works out on that but it's an option worth considering I'd think.

2/21/2023 01:20:28 pm

Not the OP, but never thought of that. If this is the area I'm guessing it is, then this is an RPZ-required area, so that'd have to help. Nice!

sean link

2/21/2023 01:29:38 pm

Can you do this. Technically yes (see the appendix) but I would strongly recommend not. There is a greater chance of real life performance issues.

Use Fire Pump at 145% to Avoid 8" Underground?

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