LN2 pumping problems

[snapper3]

Greetings Cryo Experts! I'm new here and was hoping to get some information about a problem we're having with our LN2 pumping system.

First, here's some specs on our system:
Pump - DPD series footmount recip by Pittsburgh Cryo, 4000psi max, not sure of GPM
Tank - 1500 gal, 96" high, topped off weekly
Suction line - 1" from tank, 1-90 deg turn into pump, total length about 30"
Running time - about 30-40 minutes once every 2 hours
Vapor return line installed back to tank
We're pumping through an evaporator to supply N2 gas to our facility at 2500psi

Our problem is that the pump frequently loses prime and ceases to pump liquid into the evaporator. Our maintenance supervisor has discovered that we can regain prime and start pumping by spraying a garden hose continuously onto the suction line before the pump. We've purchased refurbished cold ends and warm ends and still have the problem. This garden hose trick works in any weather, winter and summer, below freezing to over 100 degF.

Questions - Why does the garden hose trick work? Is he heating or cooling the line?

What are the possible solutions to our problem?

Thanks for your help!

Paul

 

[snapper3]

Edit to above post:
The water is sprayed on the VAPOR RETURN LINE, not the suction line. Sorry for the confusion. Any ideas?

 

[dcasto]

Sounds like you are pumping out faster than the N2 can vaporize in the tank and replace the liquid with vapor. What happens is the N2 vaporizes in the line to the pump and vapor locks it. You maintenance guy is adding heat to the tank and allowing the N2 to vaporize in the tank and not the suction line.

 

[EdStainless]

You should have gages on the tank. Watch and see how much the pressure in the tank drops when you kick the pump on.
Maybe you could use a regulator and bleed some of your gas back into the tank to keep the pressure up.
Have you inadvertently installed too big of a pump or done anything to reduce tank pressure?

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[dcasto]

A solution would be to have a minimum flow arrangement with a fixed RO in it that sent LIN back through a vaporizer to the tank.

 

[iainuts]

Hi Snapper. I agree the increase in tank pressure will help improve subcooling, but the real reason has little to do with that (secondary consideration). Primary consideration is boiling of liquid in vapor line increases circulation of liquid to pump.

 

[snapper3]

Thanks for all the great information! Based on your comments, our proposed solution is to try to wrap the vapor return line with some form of mild heating element. We'll power it with 110 VAC located nearby. We'll synchronize power to the heater to operation of the pump. We're looking in a McMaster-Carr catalogue for suitable heaters. Not sure if we'll be using some kind of temp controller or not. Hopefully this will solve our problem.

Thanks again for all your help.

Paul