Issue with cryogenic N2 pump

[entropy99]

I have a cryogenic pump at one of my industrial gas fill plant locations that is having an issue pumping. An 11,000 gallon nitrogen tank is connected to two liquid cryogenic pumps. The pumps are piped in together in an A and B configuration. The issue is that when the tank is full (at 260 inches) the pumps work fine. But once the tank level has reached 100 inches, the pumps cease to operate. The NPSH for the pumps is adequate for this installation. The tank is even on 4ft elevated legs. So my question is this a tank issue or more likely a pump (npsh) issue?

 

[ione]

As you’ve already checked NPSH available meets your pump’s requirement I won’t dwell upon this any more, anyway be aware that:

1. Long run pipe in the suction side with poor insulation and cryogenic liquid low velocity could promote liquid vaporization. Given a certain flow rate the phenomenon will be emphasized with increasing pipe diameters.
2. Avoid rises followed by drops in the suction line, as they represent a possible gas trap areas.

 

[RJonTilt]

Well, here are few possible causes:
1. check the variable speed drive for
a. speed ramping out of adjustment
b. It may be shutting down to becasue the pump is spinning too fast, and trips the limts set in the profram. This usualy indicated that the fluid is too hot for the pump at the currnet pressure.
2. If two pumps are operating off the same suction header, one of the pumps may be closer to the tank and is starving the other.
3. You may have lost vacuum on the pump can or the pipes feeding them. Any time the vapor pressure changes increase its cavitates.
Good luck