OK to Have an Oversized Fire Pump with Tank?

2/26/2024

I have a project with Miscellaneous Storage of Class I commodities up to 12'-0" and Group A plastics up to 5'-0".

The water supply is a tank fed from a well. Based on NFPA 13-2016 Table 13.2.1, I designed to Ordinary Hazard Group 2 with a 90 minute duration of available water. My original demand was 57 psi at 385 gpm.

Our pump supplier provided a vertical in-line pump rated to 80 psi 400 gpm. The client is now unhappy about the volume of water being required so they are removing the Group A plastics from their building so we can calculate to OH I with the new demand being 41.5 psi at 304 gpm.

We have already installed most of the system and the pump is ready to be delivered soon.

Is it okay to have an oversized pump?

Can it be limited to a smaller flow to accommodate the new demand?

My worry is that it will flow at the rated capacity and if we are sizing our tanks 10,000 gallons less than originally planned we would run out of water.

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9 Comments

J Golden

2/26/2024 08:09:05 am

Side note;
Do you not add water capacity for hose / Fire Dept use?
Or do you have other water available?

sean

2/26/2024 01:16:11 pm

The code does not require that a tank include hose allowance as the system is not designed to support line personnel pulling off the same water supply (tank).

Jack G

2/26/2024 02:53:16 pm

I believe it’s only hose , or if a hose rack system the hose demand.

Dan Wilder

2/26/2024 08:15:18 am

Some thoughts

-Yes, no issue oversizing a pump for the stated demand. While not normally provided (costing and water usage), there is nothing prohibiting it.

-Can the pump be modified, possibly yes, but likely not a path that would be pursued. This would involve changing the pump characteristics, specifically the impeller which would mean the factory would need to make the changes and recertify (additional time in the factory and $). The connections to the fire pump will also be oversized due to the larger casing and the potential lesser rating (if this is available from above).

-Testing of the fire pump should not be an issue with the lesser tank volume as that duration should not be near the 90 minutes of total flow in either hazard design and likely piped back into the tank anyway. Propose a flow meter keeping the actual discharge of water reduced to every 3 years.

-The other option possible is to keep the fire pump as is and install a flow control valve feeding the system (Viking J-1 for example).

In all of these, I am a little suspicious that an owner is going to arbitrarily just going to remove his ability to store items simply based on water usage from a tank. If there is buyer's remorse from paying for a larger tank, is it going to get better when they have to pay for it twice when they get caught storing those same commodities?

Ultimately, the time for changes has likely passed and the fixes to solve the issue will likely be more expensive than accepting the system as proposed unless there is some mitigating factor like refill times (8 hours) but that can be overcome via manual means IF the need arises and a basic SOP as a backup to detail any deficiencies (water trucks, metered hydrant tie-in...).

Glenn Berger

2/26/2024 08:19:43 am

A 400 gpm fire pump for a water demand of 385 gpm is not considered oversized. The pressure rating of 80 psi v. 57 psi could be a manufacturer available size.

Need to be careful about balancing demand with supply condition. Based upon the information I do not see an issue, but curious to see what others will say.

2/26/2024 08:55:01 am

Assuming the problem is 150% for 90 minutes and not having a large enough tank,
The fire pump will only flow to the amount of sprinklers that open.
The system piping could be sized so ithe system only flows X amount of water with little overage like a deluge system..
or use a flow control valve where the aperture opens to Y amount of pressure.
I would do a supply calc and reduce pipe sizes so there is not much overflow. I.E. the supply calc equals the demand calc.
Then only the system flow determines the tank size.

Jesse

2/26/2024 10:03:55 am

There used to be a day when we'd just throw a monster pump in a building and call it good.

First, your pump isn't oversized. And even it were, there's no code prohibition against that.

Consider a rack storage warehouse with Group A with ESFR supplied by a 2,000GPM @ 100-psi pump. That pump is still supplying the office area which is 0.10 / 900 (w/ QR reduction). That demand is less than 150-gpm and we would still be good.

Is the concern in the stored volume of water?

Casey Milhorn

2/26/2024 03:17:01 pm

I think others have covered it fairly well.

We don't do calcs based on what the system will ACTUALLY do during a fire event, just that it meets the minimum based on preset variables (location of fire event, shape of area, number of heads, density, end head pressure, etc.). One head flowing looks a lot different than 12 heads flowing, and which ones flow, and in what order they are activated, etc.... that's a slippery slope to play on if we start saying "what if"...

My opinion, tank size should only be based on your most demanding calculation x duration (with any hose when applicable). Not based on the fire pump capacity itself. (We hope whoever sits on the NFPA committees have thought about duration with plenty of safety margin built in).

Tank sizing is not an exact science as it doesn't take into account how much could flow in any given scenario. I would say there are cases where 3 or 4 heads located immediately off a large main and close to the riser would actually flow more than 8 or 10 heads off of small branch lines located at the end of your system. Unlike HVAC systems where air flow is actually balanced in a system that has open diffusers all the time, we are playing in at what if this area, with these many heads, in this shape were to activate. That's what kills me when someone picks apart a calc like that.

Great question!

Rodney

2/29/2024 04:48:00 am

An oversized pump is fine, but the tank should be sized according to the duty point, plus dead water and freeboard. There control valves can likewise be sized accordingly with pump delivery being restricted by using equivalent lengths of pipe, similar to an orifice plate but with a more linear flow impact. Essentially replacing a pre calculated length of distribution mains above the alarm valve with a smaller diameter pipe over the calculated length then reverting to the designed diameter. This solution provides for a quick correction of a system’s flow delivery characteristics where you have a fixed water supply and or need to adjust an existing system’s delivery curve.

OK to Have an Oversized Fire Pump with Tank?

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