×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

ARC valves for cryogenic service

ARC valves for cryogenic service

ARC valves for cryogenic service

(OP)
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)?

RE: ARC valves for cryogenic service

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.

RE: ARC valves for cryogenic service

(OP)
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.

RE: ARC valves for cryogenic service

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.

RE: ARC valves for cryogenic service

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.

RE: ARC valves for cryogenic service

(OP)
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?

RE: ARC valves for cryogenic service

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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources