Stopping Drips on NPT Threads 1

[mechanicalchad]

Just finished a facility where I used quite a bit of threaded stainless steel mostly in the 3 and 4" NPT size. For the most part it is working quite well. It is low pressure (10 psig static, more if transients considered), produced water (salt water) application.

In a few spots there are some drips where the threaded nipple (or king nipple) goes into a threaded flange. Under most operating conditions it just salts off and doesn't drip. This is within a secondary containment that is within a lined lease which is within clay bowl with a keyed dyke so ultimately we have four barriers between us and the environment so for me this is more about it being a nuisance than a risk.

Is there anything I can apply to the outside of the threads that will stop this drip? I certainly am not in an appetite to replace this with new fittings given the high cost of stainless steel. I thought of trying Sikaflex 221 which is a polyurethane sealant, or I also thought there might be a shrink sleeve of some sort that could be applied. Does anyone know of anything out there for this kind of issue?

Thanks,

Chad

 

[zdas04]

I would bet that every single possible exterior "sealant" has been used at one time or another on SS, I've never seen one work, and most of them have a very high salt content that can lead to stress cracking near the threads. Threaded SS is just a pain. Even backwelding the leaking threads has a non-zero chance of leaking.

I stopped specifying 3 and 4-inch threads a few years back when a foreman showed me exactly how miserable a job people do making up threads that big. About 1/4 of the 3-inch and probably 1/2 of the 4-inch threads I've ever called for have leaked. It is simply hard work to make them up and people get tired.

I realize that you are more than slightly pregnant with this project, but the next time you are tempted to thread big diameter SS take a look at spoolable composite pipe (like FiberSpar, SoluForce, or FlexSteel) and see if you can get better corrosion resistance and similar pressure performance for a small fraction of the cost of SS. Spoolable composite jobs for underground piping tend to have 85-90% material cost and 10-15% installation cost (welded steel is usually closer to 40% material and 60% labor, threaded steel is usually pretty close to the same). I've priced out several spoolabale jobs lately that the material costs were 70% of carbon steel so it represents a real savings in installed cost (about 77% of the material cost of steel or 50% of the installed cost of steel).

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[mechanicalchad]

As always David, I appreciate your great response!

You're absolutely right and I tried to limit the amount of threaded stainless I used, and it was pretty much only where I needed a tee and in a couple manifold situations that I decided to use it. In your application(s), can you tell me what you decided to do at junctions like a tee or for manifolds? I found this is still a limitation for composites.

For the most part on this job, I actually opted to use High Density PolyEthylene which I've found to work great so far in this and other applications. I specified an SDR 9 which had a long delivery time, or else I probably would have made up my manifolds and tees with it too. I had compared some costs of various tees out of carbon steel with coating, stainless welded, etc, and I figured the threaded stainless was still the cheapest by a good enough chunk that made it worth a shot. So far we're only at about 5% of joints with drips so hopefully it at least gets us through the year, or maybe I fell into a sucker hole.

I hear you though, I was there for the duration of construction so I got to hear on a regular basis how much everyone loved threaded stainless. One thing I noticed is that if they have to take it apart once, you might as well throw it away. I think that's where our leaks have come from so far.

Chad

 

[racookpe1978]

Our company spec's prohibit teflon tape in CT assemblies, but call for Loctite 567 or equal on threaded joints.

There are other brands, I think one is KWAL

 

[miningman]

Although I would hope that racookpe's employer has well thought out reasons for prohibition of a certain product ( being nuclear) I have alawys treated such specs or prohibitions with a healthy degree of scepticsm) ever since I was forced to endure the prohibition of the use of copper never sieze on the pipes of an underground mine pumping system. I can almost guarantee that that spec was a cut and paste from a piping spec in the natural gas industry....especially since the same project demanded the use of copper never sieze on the pumps themselves.

Be very carefull extrapolating standards from one industry into your own , unless you understand the reasons behind them!!!

 

[zdas04]

We had the tape vs. paint on sealer discussion in the material forum sometime back. My take away from that thread (I couldn't find it just now so I'm going from memory) is that the prohibition against tape is based on the CRAPPY job that so many people do in applying it. I've had that same experience, people wrap it the wrong direction, they wad it up, they put it on long threads like a barber pole.

The conversation about paint-on was interesting also, but I don't remember enough about the details to even start. There was something about the sealant lubricating the threads, but for the life of me I can't remember if that was a good thing or a bad thing.

I do remember that when I worked in Nuclear Power we could not make up a set of (machine) threads in SS without coating it with "coloidal graphite suspended in isoproponal" (I haven't touched a bottle of that stuff since 1977 and still that ungainly name sticks with me). Those were 3 and 4 inch threads in valve caps and if you didn't lubricate the threads there was a real risk of galling the threads and never being able to remove the valve cap.

Mechanicalhad,
If I was doing the job you are describing, I'd shop-weld ANSI 150 flanges on the SS tees for the connections. At the end of the day you'd have a connection that cost a bit more for materials and a bit less for installation THAT YOU COULD MAINTAIN. I've never worked with NPT threads in SS, with the problems I've had with machine threads in SS I don't think I would want to. So between the poor machinability of the SS and not wanting big diameter threaded connections, I wouldn't start down that road today.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[Latexman]

Mechanicalchad,

I've been in the chemical industry working for three of the largest companies in the world for 33 years. Screwed 3 and 4 inch SS pipe and fittings are NOT in their pipe specs that I've used, even for water for safety showers and eye baths. They are not practical (they leak, they gall, they are not maintainable). Are you using Sch. 40S or higher? SCh. 10S is awful thin to be threading. At that size, we use mainly butt welded Sch. 10S with a few well placed weld on flange connections. What temperature is the application? If it's too warm, I'd worry about longevity (chloride stress corrosion cracking).

Good luck,
Latexman

 

[1gibson]

Have you tried loctite 55 pipe cord? Better than tape at sealing larger threads. Looks like giant dental floss.

 

[btrueblood]

Have had very limited success with externally sealing threaded joints on any material (brass, steel, copper, stainless). But, one product that occasionally worked is one of the pentrating loctite thread lockers, #290. The only times it has worked, I was able to draw a slight vacuum inside the leaking joint before applying.

 

[mfgenggear]

if thread is under corrosive conditions, it will seize,
the trying to remove it is fun.

I am not a pipe expert, but just a layman amature when it involves threads.
I have taken apart enough of them that I prefer some type adhesive coating, Tape or anti seize.
to prevent damaging threads from heat & corrosion.

HTH
Mfgenggear

 

[mechanicalchad]

Thanks for the responses! I've already built this facility so it's safe to say I have to deal with what I have. I used Sch 80 nipples, and MSS SP-114 Class 150 female tees, and ANSI B16.5 threaded flanges. In a few spots we used threaded king nipples and they were thinner than Sch 80 for sure. We used Gasoila Soft-Set Thread Sealant with PTFE in conjunction with teflon tape, and yes, there were several people on site who had no clue how to put on teflon tape.

If we have to take anything apart, I just planned that it would be throw away. As expected, the few leaks we have appear to be on components where they dissassembled and reassembled, or on the thinner king nipples.

Aside from nuisances like drips and workability after installation, I'm satisfied that a catastrophic failure is not likely from one of these tees. If we did have one, I have three barriers after this to mitigate effects on the environment and so the ultimate risk is low. I am sometimes hesistant following specs carte blanche especially if they do not adequately take into consideration the overall system design, and other ways that are available to reduce the overall risk.

I think the Loctite 567 is limited to 2", but I haven't tried this Loctite 55 pipe cord ... something for next time maybe.

btrueblood thanks for the suggestion!

 

[StoneCold]

We have had very good luck with nickle impregnated teflon tape coated with Rectorseal #7. Don't get it on your hands as it has to wear off. If you try it you will can feel that the pipe threads together much smoother than teflon tape alone or pipe dope. I try never to thread anything over 2".
Regards
StoneCold