Negative Suction Pressure
Negative Suction Pressure
(OP)
Good afternoon,
I have been cheeking the datasheet of a centrifugal pump used to transport water from a storage tank. Looking at the suction pressure I realized that the pump have a negative value (-1.37 psig).
The tank and the pump are at the same level and the minimum level of liquid in the tank is 5 ft. The frictional losses are greater than the static head and that is why the suction pressure have a negative value.
I have seen negative values when the pump its above the source, but not in this configuration.
The purpose is to use the tank (and the pump) again, but I don´t know if this condition could cause problems during the operation.
Any recommendation?
Regards,
Lij
I have been cheeking the datasheet of a centrifugal pump used to transport water from a storage tank. Looking at the suction pressure I realized that the pump have a negative value (-1.37 psig).
The tank and the pump are at the same level and the minimum level of liquid in the tank is 5 ft. The frictional losses are greater than the static head and that is why the suction pressure have a negative value.
I have seen negative values when the pump its above the source, but not in this configuration.
The purpose is to use the tank (and the pump) again, but I don´t know if this condition could cause problems during the operation.
Any recommendation?
Regards,
Lij





RE: Negative Suction Pressure
One entrance point would be through a non-pressure shaft seal (such as rope packing).
I have seen the air cause significant foam formation along with pump cavitation.
In a second case, the air caused consumption of oxygen scavanger chemicals in downstream vessels.
For plain water, any air leak should not cause alot of issues. You could experience a slight reduction in pump capacity due to air.
RE: Negative Suction Pressure
Cheers,
gr2vessels
RE: Negative Suction Pressure
Regards
RE: Negative Suction Pressure
The problem is NOT a calculated negative pressure, but it is a question about NPSH...
Is your NPSH available > the NPSH required by the pump ?
Does the pump sound like there are rocks circulating in it during startup ? How about when the tank is at a low level ?
(Dont forget the NPSH units are typically referenced in ft of water - atmospheric..!!)
If your suction side pressure operates at less than atmospheric pressure, you may be OK ?
Having a 3.5 psid head loss on the suction side sounds kind-of high too !!
My opinion only
-MJC
RE: Negative Suction Pressure
The NPSHa >NPSHr; the 5 ft is the minimum level(LLS)and it seems that when the pump was operated did not have any problem.
3.5 psi is mainly for the ΔP for suction strainer (2 psi).I have been asking for this value and it seems to be the maximum losses of the strainer (after many operation time).
I think that the tank and the pump could operate without problems (the NPSHa is > NPSHr), but the PI in the suction side would show -psig.
regards,
Lij