Negative Suction Pressure

[Lijantropo]

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

 

[Chance17]

From my view, the worse thing is pulling air into the pump.
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.

 

[gr2vessels]

If you have indeed 3.5 psi head loss in the pump inlet system (for which I have some reservations), then you can lower the pump in a pit and work back the discharge head. You may want to elevate the minimum level of water in the tank by 4 ft or so, to minimise the risk of cavitation. Or you might want to review the piping to the inlet eye of the pump, to reduce the head losses. How did the pump operate until you realised that it cannot work? Something doesn't sound right.
Cheers,
gr2vessels

 

[MECHROSSI]

The problem isn't negative pressure, How much NPSH required by the pump?, if you have enough NPSH available above required minimum 2 feet above, no cavitation damage may be expected.
Regards

 

[MJCronin]

Mechrossi is correct....

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

 

[Lijantropo]

Thank you all for your responses,


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 ?P 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