Propane loading Pump-Pump Down curve

[mehrdadkntu]

Dears,

I have received an NPSH curve from a cryogenic submerged pump Vendor in which a pump-down curve has been also provided (please refer to attached file for similar curve)
The pump is a propane cryogenic submerged type installed in a column inside the LPG tank and sends out the propane from LPG tank to loading port.
Could you, please let me know what is the purpose of pump-down curve for this pump?

Moreover, some levels have been specified with respect to pump bottom datum which is not clear for me:
1- Min start Level= wetted lower bearing
2- NPSHR level at 3% head drop [it is already clear]
3- Pump-down level at head drop (at rated flow)
4- Minimum pump-down=loss of prime (at minimum flow)

 

[LittleInch]

It's a new one on me, but my thinking is that if this is a pump suspended in a propane tank your liquid is essentially sitting right on the vapor pressure limit. Therefore the only effective NPSH is the liquid level to pump inlet height.

So by the look of it you need 2.0 (m??) liquid head for the pump to have it's 3% drop.

However that would leave a lot of propane in the tank so they are giving you the option to operate lower down at what seems to be a loss of 4.5%.

However I could be completely wrong...

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

 

[mehrdadkntu]

Dear LittleInch,

Thank you for your reply.

For more information, please refer to the schematic attached to this thread.

1- Min start Level= wetted lower bearing
2- NPSHR level at 3% head drop [it is already clear]
3- Pump-down level at head drop (at rated flow)
4- Minimum pump-down=loss of prime (at minimum flow)

 

[LittleInch]

Ok, that seems to back up what I was thinking. Basically

Don't start the pump with level less than A
Try not to run it below level B ( as you might damage the pump from cavitation)
If you need to empty the vessel start the pump with level above A and then you can run it down to level C, then at reduced flow level down to level D, but the pump might / probably will be cavitating so don't do it for very long.

Note that the NPSH level is not the same as onset of cavitation. If you operate close to (=1m) from level B then the pump could easily be cavitating even though it is above the NPSHR figure.

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