Fire Pump Regulating Pressure Relief

[sysengineer]

I am trying to understand the operation of a regulating pressure relief valve on a fire pump.

The way I understand is that the valve starts to open when the pump discharge pressure rises above a set-pressure defined by the spring. When the demand changes on the system, i.e. when more deluge monitors open, the system pressure will likely drop below the pressure relief set-point at which point the valve will be closed and the system pressure will drop depending on the required flow through the system and the characteristics of the pump curve.

However I have found a statement from a valve manufacturer that seems to contradict my understanding; "it maintains constant system pressure at the pump discharge within very close limits as demands change." Found here.

How can it be possible to maintain constant discharge pressure when demand changes?

Thanks

 

[TenPenny]

"How can it be possible to maintain constant discharge pressure when demand changes?"

Isn't that exactly what you have described?

 

[LittleInch]

Because in the second instance the set point of the valve is set at the lowest pressure at max flow. Thus the pressure will stay virtually constant with a varying amount of water running through the valve.

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

 

[sysengineer]

Hi TenPenny thanks for the reply.

Not exactly, if the system pressure drops when the valve is closed due to demand then it isn't really maintaining constant discharge pressure.

Thanks

 

[bimr]

How can it be possible to maintain constant discharge pressure when demand changes?

That is the wrong valve for your application. The valve that you are referencing is only "regulating" pressure when it is relieving pressure.

It will only maintain the pressure when the system is flowing at or above the maximum pressure because it is relieving enough flow to maintain the pressure. At low pressures, it will not maintain pressure. See the instructions:

You need a different valve:

90-21 Fire Protection Pressure Reducing Valve

"Cla-Val 90G-21 (globe) and 90A-21 (angle) Pressure Reducing Valves automatically reduce a higher inlet pressure to a steady lower outlet pressure regardless of changing flow rate and/or varying inlet pressure. The valves pilot control system is very sensitive to slight downstream pressure fluctuations, and will automatically open or close to maintain the desired pressure setting."

Depending on the capacity of your pump, you may want both valves. The pressure relieving valve may be necessary to relieve excess capacity. Suggest you review the system with the Cla-Val or Singer.

 

[sysengineer]

Thanks for the replies so far.

In my application the valve discharges seawater overboard so I believe pressure relief is the correct scenario. There is no regulating valve in this system which is what I believe the manufacturer is referring to as the "main valve". I understand this to be a valve at the pump outlet reducing pressure to the system as the flow rate decreases.

In which case I think my original question still remains unanswered in that a pressure relieving valve does not maintain constant system pressure as far as I can tell.

I would still really appreciate any other perspectives.

 

[bimr]

In which case I think my original question still remains unanswered in that a pressure relieving valve does not maintain constant system pressure as far as I can tell.
The question was answered above by the paragraph from the instructions for the relief valve that you identified. The answer is that once the valve is relieving, the valve will maintain relatively constant pressure in the system. This may be acceptable on a simple fire system, but may not be acceptable on a complex fire system with widely varying flows and pump curves. Suggest you review the fire system with Cla-Val or Singer.

A relief valve or pressure relief valve (PRV) is a type of safety valve used to control or limit the pressure in a system; pressure might otherwise build up and create a process upset, instrument or equipment failure, or fire. ... As the fluid is diverted, the pressure inside the system will stop rising.

The relief valve that you have identified will only control the system pressure once the system pressure is above the relieving pressure of the relief valve. If the pressure is less than the relieving pressure, the valve will close and there is no control of the pressure.

 

[LittleInch]

Ok I'll try again, this time with numbers.

what I believe this system is in essence a pressure regulating system ( forget the word relief in this instance.)

Replace your actual numbers with the ones below, but follow through the logic

Your pump is sized for a duty of 100 m3/hr at 7 bar.
At 80m3hr the pump delivers 8 bar
At 120 m3/hr it delivers 6 bar

Your valve is sized to lift at 7.2 bar and has a capacity of 100 m3/hr.

Pump starts, no flow into fire water system, Pressure builds to 7.2 bar, flow through valve 100m3/hr, flow through system 0
Flow into fire system is 50 m3hr, pump delivers more flow, pressure falls, valve starts to close. system balances out and now pressure is still 7,2 bar, but only 50m3.hr goes through valve, 50 into system

Flow into system is 100m3/hr, pressure is 7 bar, valve is fully closed

Does that make sense now - pressure stays at 7 to 7.2 bar.

Key is how accurate you can make the set point and operation of the valve and how steep your pump curve is at the duty point (the steeper the better)

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

 

[TenPenny]

"if the system pressure drops when the valve is closed due to demand" then your pump is undersized, and the valve is not the problem.

 

[georgeverghese]

Abstracted from PRV brochure:
"The Cla-Val Model 50-20 Seawater Service Pressure Relief Valve is designed specifically to automatically relieve excess pressure in fire protection pumping systems. Pilot controlled, it maintains constant system pressure at the pump discharge within very close limits as demands change."

Agreed, there is is a contradiction in information here: The first statement on this PRV being an excess pressure relief valve is correct, while the the second statement is wrong.

The secong statement would be correct if the pump Q-h curve is relatively flat, but if it were to be flat, you wouldnt select an excess PRV. A min flow FIC - FCV would be appropriate.