Using a CSV on a system supplying fire and Potable water

[Marcus See Wai]

Dear All,
        Several of the pump systems we provide for customers provide water for both the potable and fire water system. This is a practice I usually don't like, but due to space and finance the customer has no choice but to go this way. Where the problem comes in is how do I use a csv to control the system. The set up usually requires a higher pressure for the fire and lower pressure for the potable. What advice would you give on how to go about setting up a system like this? Should a pressure reducing valve be used on the potable side, or can another csv be used to control all water heading into the potable system?

[Cary Austin]

Usually a domestic and fire pump system combined uses two pumps.  A smaller pump that delivers higher pressure is used for the domestic water.  Then a large volume pump is used for fire protection.  The system is usually staged a little different from a regular two pump system using a Cycle Stop Valve.

For example;

Pump #1 is set to come on at 50 and off at 60 PSI.

Fire Pump #2 is set to come on at 40 PSI.  The Cycle Stop for this pump is set at 70 PSI.  And the pressure switch shuts off this pump at 80 PSI.

Anytime pump #1 cannot keep up with the demand, such as when needing fire flow, the pressure drops from 50 to 40 and Pump #2 comes on.  The Cycle Stop Valve on the fire pump brings the pressure up to 70 PSI.  When it goes through 60 PSI pump #1 is shut off.  When there is no more flow required, the CSV lets the pressure go up to 80 and the fire pump is shut off.

When water is again called for, the pressure will drop to 50 PSI, and pump #1 again takes control.

If the system only has one pump, then the pressure settings are adjusted to the fire flow requirements, and a regular pressure reducing valve can be used on the domestic line to control the everyday pressure.