Vertical can type centrifugal pump, Where? 1

[mohtogh]

Hi friends,
I know that for low NPSHa cases we have to consider to vertical can type centrifugal pumps.
But my question is what is the exact cut off NPSHa for going from conventional type cent. pump to mentioned pump?

Thanks for all replies.

 

[smckennz]

Hi Mohtogh
there is no magic NPSH value for going to a canned pump.
The canned pump is selected on economic grounds.
The comparison criteria is the NPV of a canned pump, its foundations spares, servicing arrangements and running cost versus the same for a conventional pump.
The canned pump comes into its own when the NPSH requires the pump to be sunk into foundations. The canned pump is more expensive, but the foundation costs are far lower.
For example, I have used then for extraction pumps from direct contact condensers on geothermal steam turbines.

There may be some other cases where the containment provided by the pump can favours the canned pump.

Cheers

Steve

 

[PUMPDESIGNER]

Hello motogh,
There is a trade off if trying to find a pump with low NPSHr in order to work in a low NPSHa situation. That trade off is that as you decrease NPSHr, the Suction Specific Speed rises, which at some point (perhaps around Nss > 9,000) then increases the need for greater margin of NPSHa over NPSHr, and rather dramatically too. High Suction Energy pumps can require huge amounts of NPSHa over NPSHr, which you may never be able to satisfy.

It might be better to change the installation design, or live with a pump that will have very close NPSHa to NPSHr margin, than to go into a high energy suction pump and have it fail catastrophically and quickly.

PUMPDESIGNER

 

[mohtogh]

thank you.