Fire Pump Sizing 1

[simplex13]

Let me start off by saying that this website is a HUGE help for a young designer like myself. I'm working on a high school design that will have 5 systems. The water flow for this area is very poor and the School district, Architect, and GC are now exploring the option of adding a fire pump. This will be my first fire pump design so I'm a little hesitant. Can anyone suggest a good reference material that outlines the steps on how to properly size a fire pump.

Or even better if any senior designers have a method that they use that seems to work nicely. Thank you once again for any help on this matter.

 

[PEDARRIN2]

Break it down into whether you have standpipes or just sprinklers - which will determine both the flow and pressure requirements.

Sprinkler flow will be sized based on the occupancy and area, with hose stream added. Sprinkler pressure will be determined by the highest elevation of sprinklers and the minimum pressure allowed (I use 10 psig).

Standpipes, if Class I or III, start at 500 gpm and go up from there to 1,000 gpm in sprinklered buildings. You will have to check with the building code, but the pressure at the top of the standpipe is typically 100 psig, but you can sometimes be allowed to use the fire pumper truck so you just have to size the piping but not size the pump for 100 psig.

Since you have a school, you might have a stage which sometimes requires a standpipe connection on each side depending on the size of the stage.

Get your hydrant flow test to determine available pressure - do not forget the backflow preventer. In some jurisdictions, you have to have a minimum suction pressure so check that out. It is typically 20 psig. So make sure the flow test indicates what the flow is at 20 psig. If not, you will have to calculate it.

If the water supply is too poor to provide adequate pressure and flow, you might need to have onsite storage, which is another issue altogether.

Reach out to the pump manufacturer's rep. They can be a great help.

 

[stookeyfpe]

If possible, select an electric driver over a diesel driver. Electric drivers have higher reliability than diesel drivers. Ensure your pump room has adequate clearances for maintenance of the pump and the required NEC Section 110 requirements for clearances around the fire pump controller. Make sure the electrical engineer is provided with the final pump horsepower and amperage so the conductors and conduits can be properly sized. I strongly recommend you review NEC Article 695. Be sure to note the provision that current carrying components be located a minimum of 12 inches above the pump room floor. Also, verify how the service conductors are protected from fire exposure and mechanical damage. Direct burial of conduit in 2-inch of concrete is the preferred method in my jurisdiction but must be coordinated with GC and the electrician.

Ensure all penetrations into the controller maintain the NEMA damp-proof rating. This means listed water-tight hubs. Certain controller manufacturers provide a removable gland so the electrician can drill out gland panel for conduit and reinstall it so as to maintain the integrity of the pump controller. Make sure an opening is provided for fire alarm conductors and that the fire alarm contractor programs the correct signals.

If you cannot locate the pump room on an exterior wall, the pathway to the pump room will need to be constructed with the same fire-resistance rating as the the room enclosing it, which will be 1-hour in your scenario.

For acceptance testing I've attached the questionnaire we ask the contractor every time I am asked to approve a fire pump installation. I also included my canned plan review comments for electric and diesel drivers. If they don't answer the questions correctly the test isn't scheduled. If we arrive on the job site and the pump is not ready I walk off it.

 

[TravisMack]

Here is what I do when a pump is required:

I figure if we are going to be paying for a pump, I want to get the biggest bang for my buck. You should be able to determine capacity pretty easy. If you have all light hazard, you may only need a 250 gpm pump at most. If you have some OH2 areas in there, and you can use the QR reductions, you may need a 300 or 400 gpm pump. You will have to calculate your areas without the pump to determine the best flow rate. I like to choose the smallest pump I can because of the fittings and valves that go in a pump room. Sure, you could just go with a 500 gpm pump and be fine. But, that is all 6". If you go with a 400 gpm pump and can do just as well, all of your pump room devices will be 4". That is a significant savings.

Once flow is set, you are pretty much paying for pressure or horsepower. Find out from the electrical contractor how much horsepower you have available. At that point, choose the greatest pressure you can for that horsepower and calculate the system. If you are fine with your pipe sizing, then just finish it up. If the pipe sizing is very small, see what the difference is in costs if you drop the horsepower rating a bit. It may be that you save $5000 in pump costs, but only see a material increase of $3000 for a net savings of $2000.

Lastly, be sure that your pump churn pressures + static pressures do not exceed the rating of your components. If churn + static is 185 psi and your components are rated to 175 psi, then you need to drop your pump pressures.

One more thing. Are there fire hydrants down stream of the fire pump? If so, you have to take into account the outside hose allowance flowing through your pump.

I hope that helps.


Travis Mack
MFP Design, LLC

http://www.mfpdesign.com

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[simplex13]

Thank you for all your help. Your answers have been very useful. When an architect calls out a 50 hp pump but has no idea of existing water flow conditions due you senior designers take this as a general statement. Therefor you continue to size the pump towards what you feel is needed not necessarily to what is called out in a general note. I believe that this is some general detail and note gathered from some other source and used as a C.Y.A. for the architect.

 

[TravisMack]

Well, it may mean that the max hp you have available is 50hp. So, you need to find a pump that meets your needs and doesn't exceed 50hp. You can get a lot of pressure for 50hp with a 300 or 400 gpm pump. You can get an 80 psi @ 400 gpm pump and stay under 50 hp. That should be able to help you unless you have a very tall building.


Travis Mack
MFP Design, LLC

http://www.mfpdesign.com

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[PEDARRIN2]

A quick calculation that correlates flow/pressure/HP I have found useful (usually to give the electrical designer a HP range for my pump)

HP = (Flow x Pressure) / (3960 x Pump Efficiency)

Flow: gallons per minute
Pressure: ft. hd
Pump Efficiency: 50%-70%


That will give you an idea of what flow and pressure you can use.

 

[chicopee]

Since you have stated that the water supply is very poor, have you considered a water supply in the form of a reservoir or storage tanks? Also, if you intend to go with an electric pump, some insurance companies will require two remote electric power supply lines for obvious reasons.

 

[simplex13]

Funny that you mentioned a tank because that is the new development. The school wants to provide (2) 21,000 gallon water tanks on concrete foundation. With a 6" line from the tanks to the fire pump. To my understanding my most demanding area on either system will be ord. Hazard. So .20 X 1500 Sq ft= 300 gpm + 250 hose= 550 gpm X duration (60)= 33,000 gallons. So if I'm understanding this correctly the (2) 21,000 gallon tanks will provide enough water. Now I need to figure out what size pump I need based on this......If I'm on the wrong path please feel free to correct me. Thank you once again for everyone's help.

 

[chicopee]

Not so fast with the capacity of the tanks as sludge will build up over a period of time and reduce tank capacity, so allow for that contraction.

 

[stookeyfpe]

Excluding hydrants and standpipes I think your on the correct path for design. Your hydraulic demand is consistent with NFPA 13. Your pump will need to be designed based on the lowest static head pressure of the tank and your tanks should be required to comply with NFPA 24.

 

[LCREP]

You are on the correct path. Make sure the tanks are above the pump suction, if not you will need a vertical turbine pump. Do not forget to include an antivortex plate inside of the tank. See NFPA 22 for the tank details. How will the tanks be filled, NFPA 22 has limitations on how long the tank will need to be filled. If the tank is in a cold weather climate what about tank heating?

 

[TravisMack]

Hose allowance is not needed if there are no hydrants downstream of the pump/tank. However, if there are hydrants being supplied from the pump/tank system, you need to see if you are required to comply with IFC Appendix B to comply with site fire flow requirements. That could DRASTICALLY affect the size of the pump/tank.

Travis Mack
MFP Design, LLC

http://www.mfpdesign.com

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[Cidona]

Using the density * required area of operation formula will be less than required capacity. For one, there will be more water coming out of the sprinklers closer to the supply than at the end of the branch line
(since there is more pressure). Also, if you have an area that due to compartments/ceiling fixtures/whatever has 'a lot' of sprinklers in the area of operation, but one or more sprinkler is spaced to cover a 'large' area, then you could have a lot more water coming out of the sprinklers with a smaller area of coverage than the .2*actual floor space would compute. You are probably fine with your 2*21,000 tanks, but you won't have 9,000 gallon safety (using a 1500 ft2 area of operation and needing the 250 hose).

 

[simplex13]

To my understanding if the tank is sitting at ground level and that is the only source of water supply. Then to satisfy my sprinkler demand my pump will have to produce enough pressure to supply the demand because the water will produce very little.

 

[TravisMack]

yes. Don't even include pressure from the tank. It will be insignificant for a ground storage tank.

Travis Mack
MFP Design, LLC

http://www.mfpdesign.com

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[simplex13]

When calculating the suction pressure for a fire pump I use the formula Ps= Pr - Pfl +/- Pe
Pr = Residual pressure of water supply (Since my tanks are at ground level this would be 0
Pe = Elevation changes in my pipe in relation to suction flange elevation
Pfl = Pipe friction loss this also includes a backflow if present

My question is since my tank is at ground level when I figure my friction loss for the water in the pipe from the tank to the pump would my (Q) be 0 because there is little to no pressure on my supply since its at ground level. or would (Q) be the demand of my system?

 

[TravisMack]

The pipe from the tank to pump should be large enough that friction losses are negligible. Q is what is going to be flowing. So, you would figure your system demand for that section of pipe. But, you should find that it is very close to 0 psi as long as lengths are not crazy long and elevation changes is essentially 0.

Travis Mack
MFP Design, LLC

http://www.mfpdesign.com

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