I would like to know how to do the calculation de define the pressure reducing valve (PRV) requirement? Should consider te fire static pressure, the churn pressure or the cut off pressure of the jockey pressure.
Thank you in advance.
It will depend on the type of pressure reducing valve type. If it is a diapraghm pilot type, no calculation is required. Just rotate the screw to the desired outlet residual pressure within the adjustment range (typically 20 - 200 PSI). If the PRV is a direct action type, then you'll have to use the manufacturer's chart. They are available on every manufacturer's webpage. You will need to know the maximun static pressure, the desired maximum and minimum residual pressure and expected flow.
The fire protection firect action type PRV manufacturers I know are Zurn, Dixon, Giacomini and Elkhart Brass.
Thank u Emmanuel.
My query is, in high rise building, how to define up to which floor a PRV should be installed?
To computer the pressure across the levels, which pressure should consider: the maximum pressure or the churn pressure of the fire pump, or the cut off pressure of te jockey pump.
Logically, should consider the cut off pressure of the jockey pump as it is slightly higher than the maximum fire pump pressure, but during a test of the system say a fire hose, the flow is low, the fire pump will run, the pressure in the network will be almost the churn pressure of the fire pump. Therefore, to protect the installation against (sprinklers) the overpressure the churn pressure of the fire pump should be considered, that result the install PRV for few additional levels.
In my project, during the design, the pressure considered was the cut off pressure of the jockey pump. But now at the T&C stage, The consultant is requesting to consider the fire pump churn pressure that brings me to install additional 3 PRV.
Hope I clarified enough my concern.
Your question should be "set pressure of PRV" as per my understanding since your discussion is revolving around the static, churn, and cut off pressure. I am unable to understand why you are revolving around these three terms since what I understand from your situation you need PRV to bleed off excess pressure during low flow. This is quite a common issue especially in big industrial projects. For hose connection you can consider the hose valve with inbuilt PRV and set the pressure within the max pressure handling limit of fire fighters based on your local authority. However anything between 7-10 bar seems fine. For Sprinkler or Water spray system with low water demand, the set pressure of your PRV should be downstream pressure requirement of your system. For example you need 7 bar for the entire system and static is 10 bar then the set pressure should be 7 bar.
You should not use the jockey pump discharge pressure for any calculations. The jockey pump only function is to compensate for small leaks and losses so that the fire pump does not start up unless there is a fire or any serious event that requires the fire suppression system to operate. Also, the jockey's cutoff setpoint is usually the same as the fire pump's churn pressure (at least that's the way I generally set it up since only FM Global requires the jockey's cutoff to be slightly higher than the fire pump's churn pressure).
Pressure reducing valves are needed whenever the static pressure is above 175 PSI (or 250 PSI if the sprinklers are listed for such pressures).
As said before, you'll need to know the static and residual pressure at the INLET of the valve plus estimated flow and desires outlet residual pressure. See below Zurn's valve sizing sheet. You only have to input the data points mentioned and select the valve's orifice according to the manufacturer's chart.
Retrieving data ...