SS NPT pipe in liquified gas service 1

[jclsws]

We are using 2" and smaller Sch 80 stainless pipe with NPT connections in a liquified gas application. We have found that tightening the joints to seal at room temperature will maintain a seal when vapor is introduced, but will begin leaking when the liquified gas (propane) is introduced. Would cooling the pipe prior to tightening help to get a better seal, or would that not really make any difference?

 

[racookpe1978]

I have never heard of that type of pipe being used for flammable, pressurized, potentially explosive pipes at any temperatures.

 

[jclsws]

This is a cargo tank application. While carbon steel piping is the most common because it is cheaper and easier to work with, stainless is used frequently in this industry to protect against corrosion from road salt, especially here in the great white north.

 

[racookpe1978]

Material? Sure. But NPT threaded joints?

 

[jclsws]

A lot of builders use threaded joints that are then back welded, which is something that I prefer not to do. I have used welded joints wherever possible, but some of the specialized components are not available with anything but NPT threads, so I have to use some threaded joints.

 

[Compositepro]

I assume you are using a thread sealant. Perhaps it is the wrong type for liquid propane (becomes brittle when cold). Is the sealant rated for this service?

 

[jclsws]

We are using Jomar Green Stuff which is rated for use on stainless steel in LPG service. Someone had suggested to us that we use both tape and sealant, but I'm not sure that's going to provide any advantage.

 

[Compositepro]

In my experience, the most common reason for NPT threads to leak, besides under-tightening, is incorrect thread form or damaged threads. It is not easy to cut clean threads in stainless.

 

[rconner]

Another irritation with some stainless steel threaded members is "galling", which basically means that really regardless of tightening effort one doesn't draw the members properly together (you might be interested in a hands-on guy's sort of heartfelt take on same at e.g.

http://www.practicalmachinist.com/vb/general-archive/stainless-steel-thread-galling-74815/

).

 

[moltenmetal]

The thermal cycling is likely what is killing you here. Pressure is modest, but thermal cycling can be extreme.

We use high density (1.3 s.g. or higher) Teflon tape top dressed with a high quality anaerobic pipethread sealant- Loktite 567 or 592 etc. But even I, who love NPT threads, would be hesitant to use them under heavy thermal cycling conditions with a highly flammable material.

 

[weldstan]

In addition, road or rail vibration during transit must also be considered.
Have you considered copper alloy connections?

 

[StoneCold]

We have seen very good results using nickel impregnated Teflon tape and Rector Seal #7. In both hot and cold threaded stainless fittings. The nickel tape and rector seal threads together much better than other piping dope and tape systems I have used. You can really feel the difference as you tighten up the fitting. There is very little drag and sense that the fittings are galling together. One word of warning the Rector seal is messy and just about nothing gets it off your skin or clothes.

Regards
Stonecold