4 replies to this topic

[saeed.s]

Dear frinds,

Could you please help me to solve following problem.
In new nitrogen evaporator, we have leakage because of gasket failure.Nitrogen has about -160 c and 10 bara inside the pipe and outside tempreature is about 70 c and 1bara (Hot water).
could you suggest a suitable type of gasket that working in this condition? (existing gasket is spiral wound and i think high delta T cause failure )
what is your idea about changing flange type(existing flange is raised face) and using ring-joint type?

Thanks in advance

[Art Montemayor]

Behnam:

You state: “we have leakage because of gasket failure. Nitrogen has about -160 c and 10 bara inside the pipe and outside tempreature is about 70 c and 1bara (Hot water)”. If your nitrogen evaporator is actually only transferring the latent heat of liquid nitrogen at 10 bara, then your nitrogen system is at the same temperture – whether it is liquid or vapor.

You also state you have a flange – and I have to assume that you mean that you have a leaking flange gasket within the system. You fail to tell us specifically where the flange is, but I am willing to bet that it is located OUTSIDE of the main evaporator and NOT IN CONTACT WITH NITROGEN AND HOT WATER at the same time. There is no need to have that type of installation. Normally the heating medium is a continuous coil or tube bundle and the flanged connections are made OUTSIDE of the unit. A simple dedication of your time and effort could produce a schematic sketch and this would have saved a lot of questions, confusion, and errors while trying to contribute help to your problem.

I would not recommend you employ a ring joint type of flange on this installation. You will wind up with more problems (and leaks) trying to apply and torque a flange that is far and beyond not suited nor designed for what you are doing. Your evaporator manufacturer should have given you the correct and recommended type and make of gasket to use on the raised face flange. You can phone or write the manufacturer and find out the correct recommendation. If this is not possible because you don’t know the manufacturer, you are not allowed to, or you don’t want to, then you can try what I have normally employed in the past: a home-made gasket made of annealed copper. I usually cut the gasket out of a 1/8” thick plate of virgin copper and annealed it with a torch and subsequent water quench. I never had trouble or leaks. You can also try a lead gasket. But I can’t state that this will be the solution because of your failure to furnish ALL the basic data. However, you can try it, if you want to.

I hope this experience helps.

[saeed.s]

Mr.Montemayor
Thank you so much for your reply
I attached a simple diagram, as you see; it has a very simple structure. There was not any flange inside the evaporator previously. Because of low capacity we did some modifications. In work shop, they installed a flange inside the evaporator for bundle removal in the future instead of pipe cutting/ welding and now the flange is in contact with nitrogen and hot water.
And I am so interested to know why you said “I m willing to bet that it is located outside.”
Thanks again

Attached Files

•  gasket failure.xlsx   10.49KB   20 downloads

[Art Montemayor]

Behnam:

Thank you for furnishing the simplified sketch. Now, I can clearly see what seems to be your problem and – more importantly – what has been a needless mistake and will be a recurring headache for you, operations, and the maintenance of this evaporator.

You are very astute in noting my remark that I am certain that the flange would be located OUTSIDE of the evaporator. This turns out to be a very practical and important point when dealing with cryogenic fluids at relatively elevated pressures (such as your liquid nitrogen at -160 ^oC and 10 bara). There is a lot of localized stress built up in cryogenic piping and equipment when one is transferring heat through its walls. The temperature differences between the cryogen and the heating medium are the causes. As a practical solution to potential leaks in pipe fittings and flanges, one universal solution used throughout industry is to simply weld all fittings or, in the case of piping, eliminate all flanges in this type of service. The usual materials of choice for cryogenic fluids are stainless steels, copper, bronze, brass, and other normally soft metals which have unusually large coefficients of expansion and, consequently present a potential for large stress/strain.

There is a simple and common-sense reason for the above design decisions. Since we are dealing with a cryogenic fluid, we are dealing with a fluid that probably is among the most pure and cleanest of all fluids dealt by human engineering. This is common sense because in order to convert a gas into a cryogenic liquid one has to go to extraordinary lengths to remove all contaminants in order to be able to liquefy it. That being the case, the need to inspect, dismantle, remove, or “clean” cryogenic heating coils or related equipment does not exist. Therefore, the simple solution is to avoid all flanges and other similar joints in cryogenic piping wherever possible – especially in internal piping. I always try to employ 100% welded piping in all cryogenic piping – whether internal OR external. I have never come across a justifiable reason for installing a flange in cryogenic piping – and I was involved in cryogenic plants for many years.

Therefore, a serious mistake has been made by whoever made the decision to modify the evaporator. This should, in my opinion, be remedied by simply re-welding the internal evaporator coil to the manner it was originally.

I hope this experience helps and assists you in resolving this problem.

[saeed.s]

Mr.Montemayor,
Surely,your experience and suggestions are very valuable for me and all of young engineers.

Edited by behnam, 01 December 2010 - 03:23 AM.