Does NPSH required for gear pump for negative pressure at suction side?

[Tundra11]

Hi guys,

Need your help and advice on this:
There is a pump system in my company that uses a gear pump to pump a melted solid from a holding tank above the pump to another storage tank. The holding tank is under negative presure(close to full vacuum), and the pump has been frequently experiencing no flow problems.
I checked the calculations for the NPSH required and found that the NPSH available is less than that of NPSH required.
However, the pump datasheet provides a suction pressure which when the pump is turned on, provides the NPSH required.
So my question is: when calculationg for NPSHA in this case, do we consider the suction pressure provided by the pump?
If not, I suspect that cavitation has been occuring in the pump leading to damage, loss of clearance and subsequently no flow of the pump.
Thanks all for your valuable input!

 

[btrueblood]

Why is there a vacuum on the tank - is it to prevent reactions with oxygen in the air, or some other reason? If the oxidation is a concern, could you not backfill the tank (after evacuating air) with nitrogen, argon, or some other inert gas? This would help you move back from the corner you've painted your pump into.

 

[moltenmetal]

There is no problem pumping boiling liquids, but the lowest the pressure at the surface of the liquid EVER goes is the vapour pressure of the liquid. That's the definition of boiling. If the liquid is boiling for a reason, i.e. pumping from a devolatilizer or boiling drum, you can't pad the tank to suppress boiling... If it's boiling for NO reason, padding is an option.