When is a Storage Tank Req'd with a Fire Pump?

12/7/2020

I'm putting together design-build requirements for a new single-story construction building. The flow test pressure is borderline in my opinion so I'll be calling for a fire pump.

When is a water storage tank required with a pump?

I don't think the system needs one, but I just want to be sure.

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8 Comments

Dan Wilder

12/7/2020 08:43:42 am

You will need a tank when the water supply cannot meet the required GPM for the pressure and/or duration required per (typically) Appendix B of the Fire Code and/or the sprinkler demand plus any outside hose stream (whichever is greater). There are several other methods available depending on the AHJ adopted methods (ISO, NFPA, etc.)

There was a blog spot on it here:
https://www.meyerfire.com/blog/how-to-determine-fire-flow-ifc-method

The FPRF has a good write up on the process as a whole.
https://www.nfpa.org/-/media/Files/News-and-Research/Fire-statistics-and-reports/Emergency-responders/RFEvaluationofFireFlowMethodologies.ashx?la=en#:~:text=Fire%20flows%20range%20from%201%2C500,at%2020%20psi%20residual%20pressure.&text=For%20one%2D%20and%20two%2Dfamily,1%2C000%20gpm%20for%201%20hour.

Franck

12/7/2020 09:26:46 am

If your flow test pressure is borderline, this is a good thing to plan for a dedicated fire pump.
All the more since the characteristics fo a city water grid may vary over the time (seasonality, or even during the day), in addition to potential future decrease (or increase, sometimes, which is then a good news).

In additionn you may need to provide backflow preventers (to avoid a pollution of the city grid from your static water sprinkler system) which will add additional friction losses to the initial test performed on the city grid.

The choice for the water tank is depending on your supply.

If you have more than enough flow, and the only issue you have is related to the pressure delivered, you may opt for a booster pump (which will increase the pressure of your city grid water supply).
Otherwise, you need a storage tank.

This has many advantages:
- you determine the amount of water for the required duration
- no bad surprise of available duration over the time (unless you change the occupancy/commodity classification...)
- you have an independent water supply for your sprinklers from the water supplies for the hydrants (manual fireighting). No competition bteween both of them.

Some disadvantages (as always):
- possibe issue with the quality of water (depending on what you use to fill it). Raw water may be problkematic for pendent sprinklers, for example
- possible impairment when you need to maintain the tank
- possible freezing issues with the tank
- Space... the tank may take a large volume...

For industrial facilities, I normally recommend to have a water tank + fire pump (sometimes several, depending on size of the facility... a very large automotive plant or a Nuclear power Plant, for example) and try to keep the manual firefighting from an independent source (city grid or another dedicated tank+pump).

Note that depending on the flow available, you may opt for a break tank instead of a full size tank.
If you refill the break tank (automatic refill) at a sufficient flow with your city water grid, it may be an option to limit the volume of your water tank.
The tank should be sized for a minimum operation of the fire pump at 150% of its rated capacity during 15 minutes without refill,
Note that the tank volume is not based on the highest demand point from the system, but on the rated flow of the fire pump.
The capacity of the public supply must be at least equal to 150% of the pump rated capacity + the deamn,d for the hydrants (public or private).

I normally don't recommend it (unless in case of lack of space) as this is less reliable than having a dedicated full size water tank (you are dependent from the refill line).

Casey Milhorn

12/7/2020 10:24:36 am

These gentlemen covered it pretty well. As an example of how this would work in your case, lets say your flow test was 50 psi static, 40 psi residual, at 400 gpm. Now lets say your building was classified as ordinary hazard 2 and 22' tall (so reduced area is out). Now lets say you estimated a demand of 50 psi @ 350 gpm. It's pretty evident from this that you will definitely need a booster pump (or let the sprinkler sub try and VE it out if the owner is up for it). So you could specify a 40 or 50 psi pump and that would take care of the pressure issue. The gpm in this pretend scenario is enough in the test without being close to that 20 psi minimum we all worry about. Also as Dan and Franck talked about, be very very careful about IFC fire flow demand, which will dwarf the sprinkler system demand typically. If private hydrants are coming off a new tap for this building, you can be stepping into a bear trap here. So other things to watch out for is if your flow test didn't actually perform to the gpm required, then I would recommend another flow test with two outlets or more open so that your demand point is within a real curve and not extended out into a projected area of the curve. Also if your flow test is flirting with 20 psi residual at or near your demand, then I would be nervous about using only a fire pump.

Casey Milhorn

12/7/2020 10:44:53 am

Also, I failed to mention the 250 gpm hose demand in my pretend scenario and where it is added into the calculation based on public or private hydrants locations could definitely have an affect on the pump and tank sizing and requirements.

Pete D.

12/7/2020 12:07:39 pm

The IFC appendix allows for a 75% reduction for fully sprinklered building with AHJ approval. Also, the tabulated duration typically only comes into play with a tank or pond. A lot of folks don't realize, if you have an automatic fill valve you can subtract the fill rate and reduce the tank size. [Required Fire Flow (0.25) - fill rate] * duration = tank volume.

Franck

12/8/2020 03:03:34 am

Yes

This is the principle of the break tank.

NFPA 20 and 22 also address the conditions for break tanks installations.

NFPA 20 allows the use of break tanks for the following reasons:
• As a backflow prevention device between the water supply (water supply from the city water grid, process water supply system…) and the fire pump suction pipe
• To eliminate fluctuations in the water supply pressure and provide a steady suction pressure to the fire pump (water supply from the city water grid, process water supply system…)
• To provide a quantity of stored water on site where the normal water supply will not provide the required quantity of water required by the fire protection system (existing water tank, too small for the required needs).

Note anyway that the refill rate, per NFPA is depending on the initial size of the tank (if capacity is less or more than 30 minutes supply of the most demanding area, it makes a difference).
For example, dual automatic refill lines are required if the capacity is less than 30 minutes.

Dale Morris

12/8/2020 10:37:45 am

I worked on a project where we had to provide a tank due to it's Seismic classification. The building was a mixed use residential high rise, and was required to have two sources of water. The way the piping in the street was designed, there was no way to use a service that was separate from the only service in the street. So, a tank was installed in the parking garage to serve as the second source. The building has a roof top pool, and that was considered to be used in lieu of a tank. I don't remember what swayed them from not using the pool and installing the tank instead.

Frank V

1/6/2021 12:15:34 pm

That's the situation we're facing on a very similar project here in Atlanta where we just learned of a heightened Seismic category. A couple of questions. 1) I assume since the tank is to serve as a second water source, that you cannot subtract the fill rate in order reduce the tank size, correct? 2) What flow rate and duration is ultimately required for the tank size - required flow or 150% of pump? Sixty minutes or what?

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