Add City Pressure to Calc for Pump from Tank?

12/12/2024

When doing Calculations for a building that has a fire pump that is fed by an above-ground water tank, what do I use for available supply?

The fire Pump is 75 psi at 1,500 gpm. City water refills the water tank. City water is 104 psi static, 81 residual at 1,453 gpm flow.

Do I factor in the tank by figuring the pressure created by elevation, or do I bypass the tank and use the city pressure in combination with the fire pump?

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18 Comments

Pete H

12/12/2024 07:05:16 am

As the water goes through the tank before the pump, I'd factor in the tank and the elevation that comes with it. Even consider noting that you did that on the drawing via a hydraulic note.

Dan Wilder

12/12/2024 07:25:48 am

Unless you're using a pressure tank, the static head at high and low points on the tank would be your supply.

So a 27' tank with a full water mark at 25' would be ~10.8PSI
Low water would be just above the centerline of the tank outlet so I've typically used 0.1 PSI for the residual pressure. For flow, I use the pump rating, so 1500 GPM in this case.

Tank (10.8 [S] / 0.1 [R] @ 1500) to pump (1500 GPM@75 PSI) to system.

Worst case you only use the pump rating as the supply, leaving any head pressure as unrecognized safety.

12/12/2024 08:14:21 am

Use the worst case scenario for hydraulic calculations (fire pump only), and the best case scenario (fire pump + head from tank) for potential pressure regulating devices.

You could always perform some sort of calculation to figure out the minimum water level in the tank at the end of the required water duration if you really need that extra tank head pressure to make calculations work.

Mike Morey

12/12/2024 08:38:44 am

This is, imho, the correct way to do it. Tank minimum water level a the end of the fire event head pressure minus friction loss if applicable is your supply. When you've used up your required duration x flow the tank won't be full, so your calc needs to account for that. But you need to factor in high water plus churn for any maximum pressure limited devices.

Casey Milhorn

12/12/2024 08:53:18 am

Mike is definitely correct. To complicate things even more, if your city main has a bypass line that is normally open (which is not typical), you would also have to do a churn calc with city static pressure and make sure all downstream items are rated for that pressure, or have a pressured reduction strategy in place (which then typically makes the tank only calc not work).

Jonathan Lo

12/12/2024 10:48:32 am

I like this explanation, and the logic behind it.

Another consideration for this question: what is your location and AHJ requirements? For example, in earthquake country, the City mains and utility power may be out of service, and so you're relying only on the tank + pump + generator at that point.

Glenn Berger

12/12/2024 08:42:26 am

Unless you have some unique piping arrangement - The water source for the sprinkler system is the water tank and the fire pump.

Be very careful on the exact pressure that the tank will provide throughout the supply curve. This is typically why most of us will use the pump curve as the water supply curve and the tank provides a potential additional safety factor.

Anthony

12/12/2024 08:52:50 am

Use the lowest level of the tank that will supply the fire pump as that is how the tank was sized. Probably a 60 min duration X the demand get you your tank size given the flow requirements of 1500gpm.

DO NOT add in the pressure from the city unless its a pressure tank.

I'd suggest starting your calculations from the discharge flange of the pump as long as you're within ~50'-0 form the tank the losses ought to be minimal.

Is this a pressure tank? It's odd that you'd have a pump designed at 75 psi for 1500 gpm with city water supplying 81psi at 1453gpm? The city has 4.6 psi more at 1500gpm with a 104 static reading? Is this a case where you have a back up tank to supplement a city water supply?

Jack G

12/12/2024 09:01:27 am

I will assume this is not a pressure tank.
I will also assume the city supply ( used to refill the tank ) is connected to an altitude valve that refills the tank as it is emptied, since the supply is nearly 1500 gpm at 81 psi, the elevation of the tank ( if demand of the fire system is less than the city supply); should never vary much. ( although I m curious why the city supply is not connected directly to the fire pump — tank could also be connected— but checked off ( and still be connected to the fill valve) . . I m not sure this would gain anything hydraulically or from an insurance premium point of view( break tank?) .
What is the sprinkler system demand—- I m assuming less than the city supply. ( hose streams generally not added when calculating the tank volume— and in this case, shouldn’t the city be used for running fire mains to hydrants?)
So knock off a couple feet from the “ filled height”:( for elevation maintained by the 1453 gpm fill) knock off the elevation to the top of the supply flange— maybe 3 ft ( 25 minus 3 is 22. , so about 9 psi static, 1 psi residual, and add the pressures to the fire pump pressure. If conservative— calculate just off the fire pump utilizing the “ actual fire pump test curve ( do not use the formula 140%,100%, 65% as these are not to exceed numbers for pump manufacturers— I ve never seen a pump churn delta at 140%). If the tank only is used as the sole source to supply the fire pump.
So a lot of assuming here.

Jose Figueroa

12/12/2024 09:09:00 am

1) If the public supply is used to fill the tank, it should not be included in the calculations for the system.
2) If the pump draws from the public water supply, this creates a different set of considerations.

What are the design characteristics of your sprinkler system, and do they align with scenario 1 or 2?

In essence, the decision is based on the specific requirements of your system rather than choosing one solution over the other.

Jesse

12/12/2024 10:03:36 am

Unless the city water adds material value to the pressure, don't include it.

I generally only use less than 4-psi as the static pressure, and basically use my pump curve as the water supply.

Jose Figueroa

12/12/2024 02:35:05 pm

I find it quite surprising that many readers of this blog perceive the decision-making process in this context as akin to cherry-picking. It seems there might be an oversight of the crucial engineering principles involved. The best response to the questions raised is often “it depends” and “we require further engineering details and variables to provide a thorough answer.” It may be beneficial for MeyerFire’s reputation to consider implementing some moderation in discussions.

Joe Meyer

12/12/2024 03:08:12 pm

Jose, please elaborate.

Many answers do warrant clarification. We encourage followup and asking for more information when it's necessary.

Not all questions need to kick the can to "it depends." Personally, I find that the answer of "it depends" is often used as an indirect way to avoid discussion and not provide meaningful feedback. In the real world, there are often situations when we, in good engineering judgment, need to make the "best approach possible" and we cannot simply defer all answers to an "it depends" response without followup.

The benefit of having an open forum is that there can be an open dialogue where we move the needle forward and everyone is able to learn. While many questions might be percieved as "cherry picking" a very specific question for a very specific motive, I would also challenge you that you'd probably be surprised by who is submitting a comment, and the perspective that they share.

For instance, we often have questions from AHJs who ask questions that they are challenged with, and often write from the perspective of a contractor using the contractor's terms and argument. They're actively looking to see if someone else's basis has merit which is different than their own opinion.

Those questions, as they're displayed, do lack the context of who is asking it - but they're also open and available to everyone to challenge and pick apart.

Regarding moderation: We very commonly monitor discussions on a number of fronts:

(1) We monitor for meaningful feedback (ie: remove posts that do not aid in the discussion, attack, or simply denegrate the original author).

(2) We regularly seek commentary which is purely subjective without basis (ie: this should be X or Y, but with no reference, no supporting basis for the opinion).

(3) We look for diverging perspectives in discussion. Unless an answer is inherently obvious, we seek multiple vantagepoints and industry representation beyond one silo (consultants, contractors, AHJs, building owners, architects).

I would also suggest that many contributors, and us as moderators, actively look and encourage challenging the original question.

If you have suggestions for moderation or see opportunities where more information is warranted, I would also encourage you to challenge the question and support it with merit. I would encourage you, if you're applying the "it depends" argument to pull back a question, to support that perspective with why the answer isn't quite as clear as it might seem.

Anthony

12/12/2024 03:33:38 pm

Well said Joe!

Jose Figueroa

12/12/2024 04:05:42 pm

Dear John and everyone,

I want to express my sincere apologies for the strong opinions expressed in my recent blog discussion. Upon reflection, I realized that my response may have come across more strongly than I intended, and I did not communicate my thoughts as clearly as I could have. I appreciate your understanding and patience, and I am committed to providing a more constructive perspective moving forward, including a specific engineering requirement for this Pump&Tank/Public Supply discussion. Thank you for being so understanding.

Jose Figueroa

12/12/2024 06:24:00 pm

Question 1: When doing Calculations for a building that has a fire pump that is fed by an above-ground water tank, what do I use for available supply?

Answer 1: If you have a fire pump rated at 1500 GPM at 75 psi, which is considered its 100% reference capacity, we can estimate the churn pressure to be around 103 to 110 psi plus the Head Pressure. Assuming the pump is UL-listed or FM-approved, it should be able to handle overload capacities of 110% (approximately 1650 GPM at 65 psi) and 150% (approximately 2250 GPM at 50 psi).

However, to accurately estimate the fire suppression duration based on the pump’s 100% rated capacity, We would need to know the capacity of the water tank in Gallons. Please kindly provide.

Question 2: the fire pump delivers 75 psi at 1,500 gpm, and city water replenishes the water tank. The available city water pressure is 104 psi static and 81 psi residual at a flow rate of 1,453 gpm. Do I factor in the tank by figuring out the pressure created by elevation, or do I bypass the tank and use the city pressure in combination with the fire pump?

Answer 2: Here's how you can approach it:

A) Use the above-ground Tank & Fire Pump as the Primary Source. In most scenarios, the above-ground tank is seen as an isolated system supplying the fire pump, and calculations are focused on what the tank and pump together can provide. The tank ensures a consistent and reliable supply, independent of the fluctuations that might occur in the city water system.

We ask you to provide information related to your fire protection system configuration, in particular, the highest sprinkler protection demand point X gpm/psi. We can safely assume the city hydrants will provide enough fire water.

B) Use the city's water supply as the Primary Source to feed the fire protection system. It has a significant pressure and flow rate (104 PSI static, 81 PSI residual at 1453 GPM). Again we need the sprinkler system hydraulic calculation.

C) Use B & C in parallel. This requires a bit of piping work and control valves, prioritization with the jockey pump among other things.

D) Use as Primary Source the fire pump taking suction from the public. The two supplies will be added up in series, increasing the water supply at least two fold. Example the static pressure will be equal to the fire pump churn pressure plus the public water supply pressure: 104 + 110 =214 psi.
The pump running at 100% rated pressure plus the pressure of the public supply at 1450 gpm: 75 + 81 =156 psi, and so on.

If city pressure is to be utilized in calculations for redundancy or special scenarios, you'll need to consider detailed sprinkler system hydraulic calculations to account for both sources, treating the city supply as a supplemental feed to the pump. Always check relevant codes and standards to ensure compliance with local fire protection regulations. Please check if your sprinkler system and components can withstand the higher pressure.

D. True link

12/12/2024 08:54:38 pm

The water distribution system is providing 1,453 gpm @ 81 PSI residual. At 20 PSI it should be producing 2,925 gpm. At that rate of fill there should be sufficient water for the pump and any hose lines used by the Fire Department baring master streams or crash vehicle resupply.

Anonymous

1/16/2025 08:30:00 am

In terms of the code and design with a static tank at grade the pressure from the city or the static pressure from the tank should not be added to the fire pump calculation.

The tank is a static water source so there is no pressure but the head pressure exerted on the water at the bottom by the water column based on the elevation of water in the tank (0.434 psi per vertical foot of water column).

This however, is negated due to the tank being emptied as water is used so the tank will exert no pressure in the calculation when the tank reaches an exhausted supply (depending on refill rate) but instead, the pump must be capable of creating a negative pressure on the supply from the tank at the eye of the tank outlet to the pump inlet. These pressures are based on the pump’s tested listed flow curve at 100% and 150% capacity.

In short, the procedure will require only the pump being considered for the pressures in the system and the city and tank pressures are irrelevant except for what is mentioned above based on negative pressures at the tank outlet (suction side).

Add City Pressure to Calc for Pump from Tank?

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