Tank pressure optimization to get 100% liquid at pump

[eric2718]

I have a LH2 tank. I can vary it's pressure from 10-150 psig, because I can control the setting of a back-pressure-regulator that controls tank venting. Can anyone recommend a method of calculating at what pressure to choose to operate my tank to get the best subcooled liquid to my pump. I need 100% liquid at my pump, so that I do not have pump cavitation.

 

[iainuts]

Can I recommend a method of calculating the pressure? no, but I design LH2 pumps. Sure hope it isn't one of mine...

What kind of pump? (type - recip/centrifugal/other, model and manufacturer).

How large is the tank? (gallons)

How often is the tank filled? How much product is delivered?

What is the flow rate of the pump? (gpm)

Provide details of the suction line. (ID, number of bends, CV of any valves, etc....)

Have you checked suction line vacuum in the past year? Does it frost at all?

Do you have a suction strainer and do you check it?

What is the discharge pressure?

How often does it run? How do you cool it down? Describe operation.

Is there a vapor return line?

What country is the pump in?

 

[eric2718]

Cryostar PD 3000 reciprocating pump
500 gallons
2 GPM
1.5" pipe
no elevation change, 3 elbows, 2 globe valves
No frosting
No strainer
Pump discharge pressure is 150-8000 psig, filling an accumulator to 8000 psig
Pump and supply line are pre-chilled with LH2, this hydrogen is then vented

The vapor return line passes through a back pressure regulator at 140 psig, then vents to the atmosphere, instead of returning to the tank

located in Huntington, West Virginia

 

[iainuts]

Hi Eric,
Cryostar bought the rights to the PD3000 from a company that's now defunct, CVI Incorporated. I could give you more background info on that pump than you'd care to hear.

The pump has it's limitations, and I see you're pushing it beyond where it is known to operate well which is below 5000 psi. Above that and you see more and more problems which stem primarily from gas getting into the liquid prior to compression.

Like all cryogenic recips, these pumps don't "cavitate" like a centrifugal. The rated capacity of the PD3000 for example, is lower than theoretical by roughly 20%. In other words, the PD3000 rated flow capacity is only 80% of the theoretical capacity. In the industry, we say it has a "volumetric efficiency" of 80%. One of the reasons it doesn't pump at 100% is because there is some volume of the compression chamber that is filled with gas prior to the compression stroke. That's normal. That gas gets in there from a number of sources. We say it cavitates when the amount of gas gets so high that it no longer is able to pump effectively because it becomes vapor bound.

With such a small tank, the contents will come to equilibrium fairly quickly, on the order of 1 to 3 days. So unless you're getting fills every day or so, the contents will become saturated regardless of what pressure you maintain the tank at. There's no benefit in maintaining some given tank pressure, because no matter what you pick as a tank pressure, the hydrogen will quickly become saturated at that pressure.

I can give you more details on how best to operate this pump if you'd like, but I'd rather not post those details. If you'd like to contact me, I can be reached at iainuts at yahoo dot com.