Hydraulic Calculations for Fire Pumps in Parallel?

9/18/2020

I have a job where there are two existing diesel fire pumps that serve an existing warehouse (2 million sq. ft +).

The building is surrounded by an existing 12" underground fire supply loop. The two diesel pumps serve the space.

According to the property manager the system operate as follows:
(1) Pressure is maintained at 160-175 psi.
(2) Below 160 psi the jockey pump turns on to maintain 160-175 psi.
(3) Fire Pump #1 turns on when pressure drops below 150 psi to maintain 175 PSI. (Fire Pump - Static: 165 PSI ; Residual: 134 PSI @ 2000 gpm).
(4) Fire Pump #2 turns on when pressure drops below 140 psi to maintain 175 PSI (Fire Pump - Static: 190 PSI; Residual: 174 PSI @ 2000 gpm).

The pumps are provided by a municipal supply and a back-up private lake dedicated for the fire service.

The pumps are in parallel, so you would add the flows. And to my knowledge the higher pressure would overtake the smaller pressure.

My question is, how do you determine how to calculate the system hydraulically?

If I only use fire pump #1 for water supply, it will not be an accurate representation on how the system operates. In this case it also is very difficult to get the system to work hydraulically off of one fire pump.

I appreciate any input!

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6 Comments

Brian Gerdwagen FPE

9/18/2020 10:34:06 am

This is a fun question. The simple answer is that you would add the flows at the lower pressure. But it is in reality much more complicated.

You need to calculate the system demand to the pump outlets. This will determine what flow and pressure the pumps need to be able to operate at to supply the sprinklers. This demand, if the pumps are close together will be about the same. Then it is a matter of finding the pressure the pumps are generating with the required flow.

You are right in that a higher pressure from one pump will close the check valve of the other pump, but if one pump can not supply all the flow, the other one will kick on and supply something. There is a point where they are both operating at the same pressure, but with different flows. The less powerful pump will operate higher in its curve as it flows less water. That's what needs to be determined.

Start by putting in both pumps with a separate water supply equal to the domestic and make sure everything is entered in the calculation properly. You will probably need to manually divide and input the water flow to each pump as it will be getting approximately half the water it would normally get at the normal flow from the domestic.

Ivonn

9/18/2020 11:49:00 am

Pumps are operated in parallel as a means of flow control and for emergency back up (installed spare). However, if the pumps are not properly selected for parallel operation, or operated in the most optimum combinations, pump reliability can be compromised. Operating the wrong pumps in parallel can even cause one of the pumps to operate at shut off, resulting in over heating and failure.

Pumps need to operate at the same head when they operate in paralle,

Pumps with head-capacity curves that droop towards shut-off, or have a dip at a higher flow rate which has more then one flow condition for a given head, should not be operated in parallel.

Maybe one of the pumps is only for back up and the calculations will be doing just with one of them.

Pete D.

9/18/2020 11:57:14 am

Use the primary pump curve and calculate to the discharge flange.
I'm guessing that one pump will provide the larger of the highest sprinkler system demand or the fire flow. In most cases, parallel pump installations are for increased reliability, not added flow. In reality, the secondary pump may come on, but it wouldn't need to be considered in the demand calculation. If you need a Source Calculation, for example to size a foam bladder tank, I would recommend making a hybrid pump curve adding the flow of the secondary pump at its on pressure threshold on the primary curve.

Be careful setting up on/off thresholds. I have a site with 2 parallel pumps on opposite sides of the property and there's a 17 foot elevation pressure gradient between them. Thresholds cannot be established using gauge pressure local to each pump.

9/18/2020 06:52:55 pm

This post is real fun. I agree with Brian and will not repeat what mentioned already but I'll try to add bit more based on my limited understanding and conclude as below;

1. Both pumps are rated differently, so can draw pump curves at 150%, 100%, Shutoff with corresponding pressures and use these value in hydraulic cals, will give you duty point for each pumps. If you know rated flow and pressure, then use those values.
2. Based on settings, the best fit is your system demand is 175 psi @ X gpm (but this is definitely over 2000 gpm, can not be met by single pump, so second pumps needs to operate).
3. Pumps setting is not matching with NFPA's recommendations. See below requirement;
(a) The jockey pump stop point should equal the pump churn pressure plus the minimum static supply pressure (in your case consider it 175 psi.
(b) The jockey pump start point should be at least 10 psi (0.68 bar) less than the jockey pump stop point. in your case, it's 160 psi, more is acceptable here.
(c) The main fire pump start point should be 5 psi (0.34 bar) less than the jockey pump start point. Use 10 psi (0.68 bar) increments for each additional pump. Here is difference, In your case, it's 10 psi difference before starting main lead pump, then lag pump should start at 145 psi. But this will not make difference in cals.
4. Either the pumps can be made lead or lag, sequence will follow accordingly. Lag pump will only will start & continue to operate if lead pump not able to meet system pressure (say 175 psi).
5. The final pressures should not exceed the pressure rating of
the system ( ensure this is within the design pressure of pipes) otherwise you will end-up with overpressure protection mechanism.

See one more example;
Examples of fire pump settings follow:
i. Pump: 1000 gpm, 100 psi pump with churn press of 115 psi
ii. Suction supply: 50 psi from city—minimum static; 60 psi from city—maximum static
iii. Jockey pump stop = 115 psi + 50 psi = 165 psi
iv. Jockey pump start = 165 psi - 10 psi = 155 psi
v. Fire pump stop = 115 psi + 50 psi = 165 psi
vi. Fire pump start = 155 psi - 5 psi = 150 psi
vii. Fire pump maximum churn = 115 psi + 60 psi = 175 psi

6. To answer your question, both pumps will remains in operations (both consider duty pumps), only flow will distribute (as pumps ranges 0 to 150 %).

I hope this will help you to proceed.

Franck

9/20/2020 07:50:42 am

As already indicated, fire pumps ate normally one in duty and one in back up.
Except in some countries where you may have 3 x 50% pumps (UK, for example).
So for your calculations you compare with the less performant pump (which would be used if the other is impaired / under maintenance).
If for some demand points you need more than one pump (insufficient flow and/or pressure delivered) you can compare with both pumps in operation
Pretty easy
1. You draw the flow curve of pump 1 from 0 to 150%
2. You draw the flow curve of pump 2 from 0 to 150%
3. You draw the combine curve by adding the flows at the same pressure for each pump

If you have problems doing this, please contact me at [email protected] and I will do it for you if you provide me the fire pumps test results (steps 1 & 2 above)

Tony C

9/22/2020 06:05:08 pm

Thank you all for your valuable input and insight! Thank you MeyerFire for putting this topic out there for discussion.

Hydraulic Calculations for Fire Pumps in Parallel?

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