Liquid Nitrogen Piping 2

[KenRad]

I have no experience with cryogenic piping, but need to quickly design a 50' liquid N2 run that will feed an environmental chamber for low temperature product testing. The line will be valved off for weeks at a time when this low temperature test is not taking place.

My question is this - what design provisions should I make to handle the rapid cooldown of the pipeline when the valve is opened, and the resulting pressure spike from the flashing liquid? Are relief valves enough?

---KenRad

 

[BigInch]

Thermal expansion and contraction stresses.

"I am sure it can be done. I've seen it on the internet." BigInch's favorite client.

"Being GREEN isn't easy." Kermit

http://www.youtube.com/watch?v=hpiIWMWWVco

http://virtualpipeline.spaces.live.com

 

[MikeHalloran]

Despite the occasional use, you may want to insulate it anyway.

You probably want to fence it or otherwise restrict personnel access. ... but beware of confined space/ inert atmosphere hazards too.

Speaking of which, that hazard also exists for whatever boils off in/on/around the environmental chamber. Could you possibly use CO2 instead of N2?

I'd use flange guards to limit collateral damage from flange leaks, which are inevitable, given the transients.

Are you sure you can't afford/get mechanical refrigeration equipment? I ask because I have used CO2 for cooling a small environmental chamber, and quickly tired of lugging bottles around. Even with big dewars, handling all the evaporative refrigerant is a cost.

Or maybe you have a central LN2 supply, and just want to tap off a bit once in a while. If your outfit is that big, your safety/ environmental guys will want to be in on your project from the beginning, and they will have a _lot_ to say about your piping design.



Mike Halloran
Pembroke Pines, FL, USA

 

[iainuts]

The most common method of piping things like this is to use Type L or K copper pipe, silver braze fittings or pipe thread connections which will help ensure the line is leak tight even when rapidly cooled down.

When opening a valve to this line, you won't build up pressure, so pressure spikes aren't an issue. As the liquid flashes on the hot surface, any excess pressure just pushes the liquid back toward the source so you typically get a pulsing effect. You'll hear the flow rise and fall as some liquid goes in, flashes off, and flow temporarily stops or reverses.

To size thermal relief valves, assume the line is full of liquid (100% vapor fraction), the valves are shut, and you have a heat leak of at least 0.25 Btu/ft2-s. This is a nominal heat leak value, so I'd suggest doubling to provide a factor of safety.

 

[KenRad]

Thanks all for the help. The LN2 is from a large bulk storage vessel, and the line will be foam insulated. The chamber has refrigeration equipment, but it can't keep up with the rapid ramp times needed for a certain test, hence the liquid N2 boost.

iainuts - Thanks; I hadn't even thought of the fact that the pressure buildup from flashing will just push the liquid back into the storage vessel when the valve is opened. So relief sizing would be only based on the flashing of liquid trapped between shut valves.

So after the line is cooled down, the rate of flashing will be reduced to what occurs from ambient heat gain through the insulation. To ensure that my process user is only getting liquid, I suppose that I will need some kind of vapor vent near the point of use. Any recommendations on manufacturers of these?

---KenRad

 

[blacksmith37]

If you make the line of steel, a short run can economically be 304L SS (with matching fittings). 9% Ni steel is too complicated for this little job.
Old guy story- I have seen rupture of a several mile long pipeline when purged with liquid N with the end open,
due to overpressure by rapid vaporization. Liquid CO2 pipeline is a whole different story.

 

[iainuts]

So after the line is cooled down, the rate of flashing will be reduced to what occurs from ambient heat gain through the insulation. To ensure that my process user is only getting liquid, I suppose that I will need some kind of vapor vent near the point of use. Any recommendations on manufacturers of these?

I've used the stainless steel units from Armstrong International. They worked ok, though seemed to wear out in a few years in LIN service. They'll certainly work well for a few weeks or months even though they're not really intended for crygenic service.

http://www.armstronginternational.c...ating-lever-11-3-13-av-series-stainless-steel

You might also contact ACME Cryogenics, Technifab or Cryolab. Note these companies may not advertise these things on their web site but you can ask and at least get some leads. I'm sure there are others.

http://www.acmecryo.com/

http://www.technifab.com/

http://www.cryofab.com/

 

[Compositepro]

Lines like yours are often designed as loops with a supply and return line in the same insulation. A pump will keep the LN2 at a higher pressure than the supply vessel so the LN2 does not flash in the line. It will flash when it returns to the vessel.