Fire Pump Maximum Static Suction Pressure

[jjmoore73]

When determining maximum static suction pressure, we are provided a maximum static by the water authority, based upon elevation at public Point of Connection and 0 GPM flowing through the public network. The provided pressure will not exceed this at POC and very likely will not achieve the maximum pressure since water is always flowing through the public network.

The private property has a reduced pressure backflow device to protect the public supply. The private side will have less pressure than the public side due to the reduced pressure backflow device. It would make sense the reduction in pressure from the RP backflow along with elevation adjustment could be used when determining the maximum static pressure at the pump suction flange.

If I was to test the private onsite hydrant for static pressure, it would include the RP backflow pressure loss, and we would then only adjust for elevation.

Thoughts?

4.7.7.1 The net pump shutoff (churn) pressure plus the maximum
static suction pressure, adjusted for elevation, shall not
exceed the pressure for which the system components are
rated.

 

[cdafd]

Not an engineer or designer

Seems like you are asking two questions

I. It involves the rp. I would say no matter what. You have to show pressure Loss of the rp in the hydraulics….

Not sure if that answers part of your question

2. 4.7.7.1 Is just says do not exceed the pressure on the pipe, fittings, sprinklers and other parts of the system does not exceed what it is designed/ tested for. Normally 175. Unless you have a system rated for higher than that

 

[LittleInch]

What is this "Reduced Pressure backflow device"?

Only if it shuts off and doesn't allow pressure to exceed a certain pressure downstream (i.e. your side) could you do this. But if the pressure drop only occurs when there is no flow, then you can't.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[TravisMack]

The RP backflow is also called a reduced pressure zone double check assembly. Look up an Ames 4000 or 5000ss for reference. They are quite common.

I struggle to see that a BFP has a pressure drop across it at 0 flow. But, you simply take your churn pressure from your pump and subtract that from your component ratings. So, if you have a pump that churns at 150 psi and your components are rated at 175 psi, you better not have more than 25 psi static pressure at the pump inlet. You can take the static you measure at the flow test (provided it is a known high static reading) and adjust for elevation to determine what you have at the pump suction flange. It is best to do a 48 hour, or even a 7 day gauge on test hydrants to make sure you get high and low pressures. Max pressure would be based on high static readings. Demand calculations should be adjusted to low static readings.

Travis Mack, SET, RME-G,
MEPCad, Inc
AutoSPRINK | AutoSPRINK FAB | AutoSPRINK RVT

 

[LittleInch]

They look to me like some sort of spring loaded dual check valve so yes, you need a certain DP (about 10psi) to get any flow, but this doesn't mean the pressure d/s is lower so no, I don't believe you can use this to reduce the suction pressure at no flow.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[jjmoore73]

We are provided various flows with available pressure from the water authority. The absolute maximum pressure possible is the pressure available from the water authority with no water flowing in their system. This is to be used for fire pump design. The water authority also provides a "static" pressure which is really a residual pressure in their system, and considers the maximum daily demand flowing through the system. This is to be used for fire sprinkler design as the static. They also provide various pressures with additional water flowing in the system to help create a supply curve.

The RP backflow device has a relief valve between the two check valves to provide a pressure differential. The protected city side will have higher pressure than the system side. This is to prevent any built up trapped pressure on the system side from "accidently" surging into the protected city side when the check valves open under flowing conditions. When RP backflow are located inside the building, they typically have an air gap and drain to collect the released water from the relief valve to maintain the pressure differential.

It seems you could use the RP pressure differential, and adjust for elevation, when determining the maximum suction pressure on the pump inlet.