LN2 Cryogenic Piping 2

[SAK9]

Guys,

I need to design a liqiud nitrogen tranfer piping system about 20~25 m long.The system consists of a main storage tank serving a few cryogenic freezers.I understand the flow will be two phase.

Do I need to provide a return line to bring the gas back into the main tank?

What sort of valving and instrumentation are required on such a piping system?

Do I need to provide safety relief valves in the pipe line?where and at what spacing should they be located?

Many thanks for any inputs!

 

[Montemayor]

SAK:

The answers to your queries are:

1. The flow will be 2-phase initially; after achieving steady state & if insulated adequately (like it should) the piping should flow 100% liquid N₂.

2. Why would you want to return flash N₂ vapor back to the tank? Unless you have a cryogenic cooler-condenser (which I seriously doubt), you have to vent all the vapor produced by the vaporization of the liquid N₂ and the flash produced by the adiabatically expanding liquid N₂. You haven't fully explained what it is that you are doing, but I've designed and done this installation before, so I'm familiar with what you are trying to describe.

3. What do you mean by "sort" of instrumentation? This word is un-engineering. Do you mean Material of Construction (MOC)? Do you mean analog vs. digital? If you are not an experienced engineer in cryogenic applications, you are in deep poo-poo and I seriously advise you to get a consultant or your LIN (liquid N₂) supplier to do the design and installation. Normally this is the way it is always done in order to avoid costly or hazardous screw-ups in the field. The piping is special (SS), the valving is special, and the instrumentation is certainly special. And I'm not even mentioning the special insulation required and expansion joints that have to be incorporated.

4. You most certainly need to install all the required thermal expansion relief valves that a system like this demands. If you don't, you are facing a dangerous and hazardous situation where a lot of innocent people could get seriously hurt or killed. Again, if you are not experienced or knowledgeable in this .... get help.

I hope you have safe success in your installation and follow the above comments.

Have a safe day.

 

[Zapster]

I agree with Art. If you are interested in learning about the hazards, a good place to start is with the CGA (Compressed Gas Association). Purchase their book, “Handbook of Compressed Gases,” and their standards for cryogenic fluids. Read and learn, and then discuss any design details that you don’t understand with your hired consultant. If you are interested, this can be a great learning experience. One problem with hired consultants is that your design may be handled by their new intern who used a previous project to cut and paste your design together. You should learn enough about the hazards to validate many of the required components of a quality design.

 

[SAK9]

Art,Many thanks for your helpful response.I am attempting to put together a scope of work document together with a PID to obtain competitive quotes from a few specialist contractors.One of the specialist contractors will do the actual detail design work.I have a duty and standby LN2 storage cylinders which will serve a dozen cryo freezers.I am specifying SS 304 vacuum jacketed pipe.I intend to provide a one shut off valve in the line serving each of the freezers.There will be a main shut off valve on each of the storage cylinders.On the main line there will be a thermal relief valve.The freezers will have their own automatic level controllers to control the LN2 inflow.I would be grateful if you could advise on the following:

Do I need to provide a rupture disc on the main line?

Is it possible to have an auto changeover manifold on the man storage cylinders(so that when one cylinder gets empty the standby cylinder comes on line)

Are there any other safety features I should call for other than those mentioned above?

Thanks for sharing your experience in this industry.

 

[iainuts]

Hi Sak,
Personally, I think the stainless steel VJ piping is a bit of an overkill. Count on it costing $200 to $300 per foot plus installation. Valves add a few thousand each. It'll make a terrific installation, look great, you'll be very happy with it. But it'll cost a lot more than copper pipe. Insulated copper pipe is the standard in the industry. The biggest issue with copper pipe is that it'll deteriorate over time. You may want to include a cost for reinsulating the pipe every 3 to 5 years if you're doing a cost analysis.

I'd agree you should have someone else design it if you're unfamiliar with cryogenic pipe design. Analysis of thermal stresses for long runs is important along with proper pipe supports. I'd also agree with contacting your gas supplier or freezer supplier. They're the experts at such installations. If you have your own tank and bid the liquid contract, try contacting the major industrial gas companies for the installation (Air Products, BOC, Praxair, etc…) There are also many smaller companies that do this kind of work, depending on the area.

Regarding instrumentation on the line, that's not necessary unless you're trying to automate these food freezers somehow.

Burst disks are never used on lines. Small, thermal relief valves are used and sized per CGA S1.3 though it looks like CGA has added a new one this year and I don't have access to it (ISO-21013-3). Maybe someone else knows if they've switched. Either way, you can get the pamphlet online:

http://www.cganet.com/publication.asp

The most common relief valve used in these situations is the Model CRV by Generant:

http://www.generant.com/home.htm

They're very inexpensive.

Is it possible to have an auto changeover manifold on the man storage cylinders(so that when one cylinder gets empty the standby cylinder comes on line)

Yes, and there are companies such as ACME Cryogenics that make these as a standard item.

http://www.acmecryo.com/

Are there any other safety features I should call for other than those mentioned above?

I think you have it covered. If you decide to go for the VJ, a relief valve will be installed on the jacket, but that's something your supplier will size and install.