ARC valves for cryogenic service

[Thealanator]

In the pump forum I have read about automatic recirculation (ARC) valves that protect a centrifugal pump from low-flow conditions. Is there such a thing for liquid nitrogen service, or would one have to use a flow device/valve combination (flow device on pump discharge controlling a valve on the recirculation line)?

 

[iainuts]

Hi Thealanator,
That's not generally done, at least on the pumps I've seen which are up to a few hundred GPM. And I doubt it's done on much larger machines. The problem is you're adding energy to the fluid by pumping it and there's no way to remove it. For a cryogenic fluid, the added heat you get by pumping and recirculating it will tend to boil the liquid fairly quickly, and there's no place to reject the added heat. For ambient temperature machines where the fluid is water or similar liquid for example, the water can get a bit warm and either ambient cooling will keep it from boiling or it can be circulated through a heat exchanger to cool it back down. So in general, cryogenic fluids can't be recirculated around pumps. Valves on the pump discharge are installed though as these can be used to throttle the fluid flow. By backpressuring the pump, the flow can be reduced considerably.

 

[Thealanator]

Many people suggest routing back to the source vessel instead of pump suction. But what about the idea of protecting the pump from low-flow conditions? Closing down a discharge valve moves the operating point to the left side of the curve. Most people say that can damage a pump.

 

[iainuts]

I honestly haven't heard of routing it back to a tank, but at least if you do that, you have a method of separating the gas from the liquid. I have to believe it would work as long as you maintain the minimum NPSH to the pump.

Note that you can determine the amount of gas developed by assuming:
- some inlet state based on pressure and amount of subcooling. You may want to add inlet line heat leak if it's significant.
- an isentropic efficiency across the pump which (along with the discharge pressure) will give you the discharge state.
- then assume the fluid is isenthalpically throttled back down to suction pressure. Add the amount of heat picked up the piping.

If the tank is sealed off, you can then determine the pressure rise rate given some gas volume in the tank. Or you could assume the gas is vented and the tank is maintained at constant pressure. That should cover most of what is needed to determine what will happen when you route product back to the tank. It may be that even if the tank is vented, the amount of vent loss is economical when compared to alternatives.

If you'd like a 3 second analysis, post some numbers and I'll give you the vent loss.

 

[someguy79]

In LNG service, it is not so strange to put a recirculation loop around a pump. The way I've seen it done, is to route the recirculated fluid back to the warm (relatively speaking) side of a heat exchanger. Iainuts mentioned something like this.

For the applications I've seen recirculation lines are used to allow the pump and other process variables to reach steady-state low pumping output.

 

[Thealanator]

someguy79,

Thanks for the insight. Do you remember if a Yarway valve (I think this is a big manufacturer of self-contained recirculation valves) was used, or was there another method for diverting flow?

 

[someguy79]

I have no experience with the Yarway valves. I've used a number of other Tyco valve products with good results though.

If you need a recirculation loop with no controls or I/O to set up, one of those self contained jobs like Yarway might be a good choice. I really can't give much good info on them.

I've used other methods involving control valves, holding tanks and other clap-trap. It's not something I can discuss in any real detail here.