Split Case Centrifugal Pump Suction Pressure Issue

[Mosarof]

I am running a 750 gpm Split Case Centrifugal Pump . It has positive pressure in suction side (Pump Inlet is connected at lower part of reservoir )

But whenever i start the pump , the pressure gauge at suction shows negative pressure . Is it normal ?

Also what will be the Net Pressure ? Discharge P + (-Suction P) or Discharge P - (-Suction P) ?

Thanks in advance .

 

[bimr]

It is not unusual to have a negative gauge pressure at a pump's suction. You still should have positive absolute pressure on the suction side of the pump.

Absolute pressure is zero-referenced against a perfect vacuum, so it is equal to gauge pressure plus atmospheric pressure. Gauge pressure is zero-referenced against ambient air pressure, so it is equal to absolute pressure minus atmospheric pressure.

The pressure on the pump liquid level is atmospheric, so the pressure at the pump's suction will be atmospheric less elevation gain less hydraulic losses. Not a problem if the system is adequately designed to accomodate this.

 

[mk3223]

To be sure the pump suction pressure is the absolute pressure of "psia", not in "psig". And, the NPSH of the suction piping system should be higher than the required NPSH of the pump as specified by the Manufacturer.