Pressure measurement

[luisa87]

Hello everybody,
I have got some doubts about the pressure measurement in a water plant.
There are two pressure transmitters, one upstream a centrifugal pump (head = 20 m) and another one downstream the pump.
The measured pressure downstream the pump is lower than that measured upstream. The pressure transmitters measure absolute pressure and not relative pressure and so, the measurement depends on the quote of installation.
How can I compare the two measurement? Which is the height that I have to consider as reference?

I hope that my question is clear

Kind Regards

 

[pmover]

ok, some thoughts . . .

is the pump in operation?
is the pump driver (electric motor ?) connected to the pump?
is this pump new or been in service for a long time?
are the isolation valves at the transmitters are closed?
check the signals from the transmitters to the display monitor - may be swapped if not properly configured.
is there fluid flowing through the pump (i.e. valves not closed)?
there are many potential reasons for this anomaly and all need to be investigated.
good luck!
-pmover

 

[georgeverghese]

Is the elevation of the downstream PG more than 20m above that of the upstream PG ?

 

[LittleInch]

The reference height can be anything, so long as you use the same reference for both transmitters to correct any difference in height between the two.

Sometimes it is water level, sometimes the centerline of the pump, sometimes the floor / ground level. You either need to go and look yourself or get a piping drawing with sections or an isometric to see where the ACTUAL tap off point and transmitter actually are. You won't get this information from a P & ID or a schematic.

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

 

[bimr]

To compare the pressure measurements, just add the difference in elevation between the pumps. The hydrostatic head pressure is easily calculated as the product of the liquid density, gravity and the height of the liquid (ρglh).

It doesn't matter if it is absolute or gauge pressure.