Fire Pump Sizing from Fireflow 1

[Dormer1975]

As we all know, fire departments/jurisdictions use a specific gallonage requirement (i.e. 4,000gpm in my example) @ 20 psi as their requisite fire flow. How do I size a fire pump @ 20 psi? In my experience, pump curves don't go that low. I don't do a lot of pumps, maybe 1-2 a year, but most of them are sized by the sprinkler system demand and less by fire flow (they are storage facilities with large sprinkler systems). I understand why fire departments use this nomenclature. They are going to show up to fight a fire and they will hook up to a hydrant with their trucks, that have pumps on them, and they want to know that they have sufficient pressure to not create vacuum and also enough water (gpm) to supply their trucks' pumps. If I spec a 3,000 gpm @125 psi pump, that's its rating. At 150% of rated flow it will provide 4,500 gpm @ typically 60% of rated pressure (75 psi in this example) (varies by manufacturer). But, the curve doesn't keep going to represent flow @ 20 psi. The manufacturer says 3,750 gpm...that's all you're going to get...but it's at 75 psi (in this example). Any suggestions or experience with this? I'm scouring NFPA 20 to see if they have recommendations as well.

 

[fel3]

Mostly likely, 20 psi is the minimum pressure the fire department will accept, not the exact pressure they want.

==========
"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill

 

[Dormer1975]

My belief is that their request of 4,000 gpm @ 20 psi (my example) is more along the lines of a steadily held water network like a municipality provides. A city or county has multiple pumps throughout their jurisdiction(s) that maintain specific static pressures based on usage (people taking showers, water their lawns, etc). Most of the area I live in maintains a static pressure of around 70 psi. For example, if I place a gauge on a 2 1/2" port on a hydrant and open the hydrant to fill the stack, it will read 70 psi. Now, if I perform a flow test by going to an adjacent hydrant and place 2-2 1/2" stream straighteners or hose monsters on them and open that hydrant wide open, the gauge on my previous hydrant might drop to 58 psi. That's my "residual flow". And, once I perform my calculations on the readings from the water I flowed out of the stream straighteners or hose monsters on the 2nd hydrant, I'll know how much water I flowed. So, I can create a linear representation between the static pressure available and what pressure is available at flow. Lets say I flowed 1,500 gpm in my example out of those stream straighteners. My flow information would be Static: 70 psi, Residual: 58 psi while Flowing: 1,500 gpm. So, on a flow chart I could extend that line out to 20 psi to estimate, using a linear relationship, how much flow I would have available.

I wonder if this is how fire departments view all situations whether a fire pump is involved or not. Fire pumps don't operate on a linear relationship between pressure and flow. They have manufacturer's curves that you can't extend out to 20 psi. I can't tell you what the flow of a 3,000 gpm @ 125 psi rated pump would be once so much water was being pulled out of the network such that there was only 20 psi available. I'm still searching for what others have done in this situation. I know I can't be the first.

 

[TravisMack]

Fire flow and fire sprinklers and standpipe are all common systems but distinct differences in supply requirements. You only need to size a pump for site fire flow if there are hydrants downstream of the pump.

Typically, when you have a pump in the situation where you are supplying hydrants and fire sprinkler systems, the pump capacity (flow rate) is determined by the site fire flow criteria. Pump rating (pressure) is determined by the fire sprinkler system.

In your case, a 3000 gpm pump could be used to meet the 4000 gpm flow requirements. You then determine the pressure based on the fire sprinkler system demand pressures. You are not going to find a pump manufacturer that will tell what flows you get down to a 20 psi flow. The smallest fire pump pressure available is 40 psi at rating. 65% of that is 26 psi. That is as low as you will get a fire pump manufacturer to indicate from my experience.

Travis Mack
MFP Design, LLC

http://www.mfpdesign.com

"Follow" us at

https://www.facebook.com/pages/MFP-Design-LLC/92218417692

 

[964]

I agree with fel3 that 20psi is probably a minimum requirement. Pump manufacturers typically specify NPSHr (Net Positive Suction Head required) to ensure that you don't damage the pump due to an insufficient supply.

Don't have any experience with fire flow or mobile fire pump standards, but I am familiar with NFPA 20 and stationary fire pumps. NFPA 20 requires the pump suction gauge to be both a compound pressure and vacuum gauge if suction pressures are below 20psi for any flow condition. NFPA 20 also requires a minimum suction pressure of 0psi at 150% rated flow.

I'm not sure what the limits are for a mobile fire pump, but I'm sure they exist. The fire department probably arrived at the 20psi requirement based on the NPSHr of the apparatus they have available and the pressure loss through the suction hose that will be deployed as part of their SOPs.

I've seen TravisMack give a lot of good advice while lurking in this forum. If you follow the procedure he outlined you will provide 4,000gpm in excess of 20psi and everyone should be happy.

Huey

 

[PEDARRIN2]

I believe 20 psig comes from the EPA(?) as a minimum to prevent creating a low pressure situation in the main which could produce a back siphon somewhere along the main and contaminate the water supply. This is why backflow preventers are required to mitigate this possibility.

Also, both domestic water booster pumps and fire pumps (at least where i design) have to have provisions which will either shut them down (in the case of the domestic system) or reduce the flow (by means of a pressure sustaining valve) whenever upstream suction pressure approaches 20 psig. In some places, there is a pressure transducer which sends a signal to the fire pump control panel indicating the lack of pressure which, if memory serves me right, shuts down the fire pump.

The jurisdictions I work with require the valve so that, even with lower suction pressure, there is some flow going to the sprinkler system.

 

[stookeyfpe]

The 20 PSIG value originated from AWWA and it's generally adopted by most water purveyors as part of their design requirements for water distribution systems. In my jurisdiction the water utility has a second requirement that limits water velocities to < 10 feet/second.

From a fire flow perspective, it's a requirement in the NFPA standards for fire apparatus operations that one does not allow the suction pressure on mobile pumping apparatus to fall below 20 PSIG when connected to a public or private water supply system constructed using mains and hydrants. In my organization we fail fire protection system acceptance tests any time the residual pressure falls below 20 PSIG.