Leaking Threaded Stainless
Leaking Threaded Stainless
(OP)
Hello,
I am looking for the reasons why threaded stainless is so prone to leaks, as compared to steel pipe.
Dale Simonds
I am looking for the reasons why threaded stainless is so prone to leaks, as compared to steel pipe.
Dale Simonds





RE: Leaking Threaded Stainless
RE: Leaking Threaded Stainless
Stainless is far tougher on threading dies and other tooling than carbon steel, so the surface finishes of the threads are often worse because people use dull tooling. That goes for both the people making the fittings and the people threading the pipe to connect them.
Stainless has roughly twice the linear coefficient of thermal expansion when compared with carbon steel. If you use a thread sealant which isn't compliant enough to continue to seal despite movement from thermal expansion and thermal cycling, leaks will result. Use a good quality paste-type anaerobic pipe thread sealant in addition to or instead of teflon tape and most of your problems will go away.
Stainless is easier to "gall" than carbon steel, hence people don't pull stainless steel fittings up to the same extent during tightening. A tighter joint is less reliant on the thread sealant to prevent leakage.
Then there's the quality of the 150# cast stainless steel threaded fittings on the market. Most of the ones on the local market here are from China, and the quality is hit-and-miss. We've seen some with huge angular tolerances on the threading, such that concentric reducers and 90's aren't (co-axial or 90 degrees that is), and branches of tees go off at wierd angles etc. Of course if you compare these to 3000# forged steel fittings, there's no comparison in quality. But comparing these fittings to 150# malleable iron fittings, the quality is similar(ly poor).
But the biggest single problem with threaded stainless steel as a fitting system is the union fittings. 150# or 3000#, none of them seal reliably to the limit of the threaded pipe, even if you lap the parts to one another before fit-up. Carbon steel unions are much more reliable, especially if they come with a brass seat.
Threading is a flexible, inexpensive and reliable way to join small diameter pipe, but it gets a bad reputation because people don't understand how the system works and hence do it poorly. If you never forget that an NPT joint is a spiral path connecting the interior of the pipe to the exterior world, and the only thing between you and what's in that pipe is the thread sealant you choose, you will be less likely to go wrong in using this method regardless what material the pipe and fittings are made from.
RE: Leaking Threaded Stainless
"The pipe sealant 567 already has PTFE in it to aid as a lubricant, the Teflon tape is nothing more than a thread jammer. It can interfere with the 567 curing properly because the tape may create a barrier from the metal. Anaerobic products cure in the presence of metal."
I asked them this question after seeing many contractors using the Teflon tape with the expensive anaerobics. I wanted to make sure that they weren't doing more harm than good, and apparently they were.
By the way, I've found that the 567 Loctite does an excellent job on threaded stainless when subjected to temperature swings of 200F.
---KenRad
RE: Leaking Threaded Stainless
I have had similar results. With the tape vs. sealant, and with the galling.
For some of my applications I now use tubing instead of pipe where it makes sense.
RE: Leaking Threaded Stainless
RE: Leaking Threaded Stainless
Our experience has been that the presence of steam makes the service much tougher on the anaerobics. Non-steam services to 400 F are frequently no problem, but steam services beyond 300 F can be troublesome especially for larger sized joints.
For temperatures beyond these limits, our next option is grafoil tape, top-dressed with graphite flakes in heavy oil (UCAR markets a brand of "thread sealant" which pretty much is just that. It works well for small, carefully-threaded joints.
After that, all we're left with is Xpand-o. Very leak-resistant, but sometimes you wish you'd just given up and called the welder over. If you use that stuff, make sure you don't thread the components up to full wrench tightness or else there will be insufficient sealant left in the joint to expand and seal properly.
As to the point about using a combination of tape and anaerobic paste: our experience has been that joints with both seal more reliably long term than joints with either tape or paste alone. We theorize that the tape offers better ultimate galling resistance than the few PTFE particles in the anaerobic paste alone can offer, plus it acts as a creep-resistant large void filler. The paste acts merely to provide elasticity in the joint to compensate for dimensional changes due to thermal cycling. But properly applied, paste alone is far better than tape alone, any day. The only time we use tape alone is when the service and the paste aren't compatible with one another.
TBP is dead right: some people wrongly assume that materials like 567 are basically PTFE in paste form, but nothing could be farther from the truth. Get the compatibility chart!
As to the presence of PTFE tape reducing the ability of the anaerobics to "cure"- there's plenty of exposed metal on the female side of the joint! And the stuff may "cure", but it never sets up hard- which is exactly why it works!
RE: Leaking Threaded Stainless
Has anyone had any experience with the combination of Lotite 55 and Leak Lock? OK so far during testing(no leaks!). It sure does extrude a lot of material while tightening the fitting.
Dale Simonds
RE: Leaking Threaded Stainless
= SW
RE: Leaking Threaded Stainless
Mike Halloran
Pembroke Pines, FL, USA
RE: Leaking Threaded Stainless
I don't think that Henkel Loctite 567 is recommended for plastic fittings of any kind. It may be as another poster previously stated, that the material relies on metal exposure to catalyze curing. But some of the anaerobic materials induce cracking in plastic fittings, so you should read the vendor's literature carefully. Loctite makes a sealant called "No More Leaks" which definitely is suitable for use on plastic fittings.
RE: Leaking Threaded Stainless
Try using a thread sealant or anti sieze compound. This tends to add lubricity to the joint in addition to sealing capability.
Also, most field technicians tend to overtighten the joint, you know, it's leaking...tighten some more...still leaking...tighten some more....still leaking....give her another turn. I wish I had a nickel everytime this happened! I'd have about two bucks by now. :)
Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada
RE: Leaking Threaded Stainless
- Steve.