When did Threaded Pipe become Evil? 3

[zdas04]

I spent a fair bit of the day today in meetings defending a decision to include NPT threaded pipe on a device I invented. The plant guys I was meeting with were absolutely adamant that no NPT threads could be allowed on low pressure gas wellsites. Ever. I had a similar unpleasant couple of days in South Africa last year about the same prohibition. There are literally hundreds of thousands of wellsites in the world that are approximately 100% threaded pipe. Today that statement elicits an incredulous stare and certainty that I'm some sort of evil cowboy.

Reading ASME B31.3 and B31.8 (the two primary standards that I see applied to gas wellsites even though B31.8 explicitly exempts itself from applicability to wellsites), I see a lot of language about how to use threaded pipe and a (very) few specific cases where it is not recommended (e.g., in vibrating service), but I don't read anything that excludes it for normal maninstream service.

One of my clients allows NPT pipe in 2-inch and smaller. Another allows NPT pipe in service smaller than 2-inch (that is the only place I've ever specified 1-1/2 inch pipe). Two others do not allow NPT at all.

Does anyone know why NPT pipe has become prohibited in so many companies? No one here has an answer beyond "our company standards do not allow it". The standards did not write themselves, but no one seems to know how that language wormed its way in.

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.
The plural of anecdote is not "data"

 

[katmar]

David, I have the noticed the same thing. There is a general trend towards higher standards on equipment. For example here in Durban (a coastal city) it is becoming more and more common to see stainless steel hand rails, and even stainless cooling water pipes.

I suspect the reason is the increase in labor costs relative to materials over the last 2 decades. In the 1960's if a threaded pipe was leaking or a handrail needed painting there was always plenty of labor available to fix it. These days there are fewer and fewer people available in the plants to do that work. There is also a greater pressure on the plants to keep running, so a 3 hour shut down to replace a section of pipe is less acceptable than it was. I don't want to open a can of worms, but I suspect that the skills of the labor are not what they were before. A badly installed threaded pipe is not going to last long, but if it is done properly it is there virtually for life.

Katmar Software - AioFlo Pipe Hydraulics

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[SNORGY]

David,

Could it be that over the years, because the labour force has been moving further and further away from unions, so should pipe fitters?

 

[SNORGY]

I suspect that people think threaded joints leak more than, say, welds or flanges and gaskets. If done properly, it should not be so. Among the more valid concerns might be an increase in the susceptibility to crevice corrosion, but if that isn't a large risk then threads ought to be acceptable.

 

[SJones]

Sour service tends to create issues for threaded joints and elicits a 'ban.'

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

All answers are personal opinions only and are in no way connected with any employer.

 

[PEDARRIN2]

Though not exactly the same thing, we tend to specify non threaded piping for larger/higher pressure gas (>2" and > 1 psig)inside buildings for two reasons,

1. Nobody (it seems) can tighten anything over ~2" so it doesn't leak. I had a plumber/pipe fitter state there are no more "big boys" that can put enough into the pipe wrench to turn the larger diameter fittings.
2. If a leak is found in a threaded joint, it is time intensive to fix it. Whereas with a welded joint, you just put a bit more weld material where it is leaking and the leak is fixed.

 

[BigInch]

I don't think it's a recent development. Northern Natural Gas/Enron prohibited NPT >=2" since the 70s, because of fatigue and corrosion issues, plus required Sch 80 for 2" gas piping no matter what the end connections were.

I thought you took pride in being the evil cowboy.

Independent events are seldomly independent.

 

[Krausen]

Fatigue failure, corrosion concerns, & future projected maintenance costs are the reasons my current client (crude pipeline operators) has banned threads on all of the process piping everywhere. All branch connections are now via WOL/TOR and flanges now, although threaded connection are all over the place on their existing infrastructure.

Just out of curiosity - what do the forum members here recommend for NPT thread sealants? We've never really specified, and left the onus on the local operations group. But I have seen other specs that strictly forbid the ever-popular teflon tape.

 

[SNORGY]

In any event, depending on service severity, threaded pipe at wellheads appears to be a strong "want" in recent years. So much so that I was tasked with writing a piping specification for it in line sizes up to 4" NPS several years ago.

 

[JJPellin]

I work in an oil refinery. They have been phasing out threaded pipe over the past 20 years. The only piping specification that still allows it is threaded, galvanized pipe for lube oil mist systems. They don't even allow it in cooling water lines larger than 1 inch anymore. The reasons for us are the higher chance of leaks or cracks. The risk in terms of safety and environmental reporting requirements and the labor cost to repair leaks are probably the primary reasons.

Johnny Pellin

 

[moltenmetal]

I feel your pain David- if you're a cowboy, then yeee-haw! so am I...

We restrict NPT to 2" and smaller but use it widely. There's nothing in B31.3 that limits threaded pipe in normal fluid service. Our threaded pipe doesn't leak.

Perceived poor fire safety and environmental performance are the usual excuses for banning NPT.

The reason threads are so hated is that incompetent pipe design and pipefitting are both rampant, and people don't understand how NPT joints work. These lines are usually field-run with little competent supervision which means that design is an afterthought at best (not so in our shop!). Incompetence results in (preventable) problems (leakage) which results in prejudice - which ends up in specs, which are then treated as if they were holy writings- to be believed first, understood later and questioned never.

We tend to restrict threaded run of the line pipe to 150# class for flammable services (to match what most clients are insisting on) and to a max of 400 F (beyond which the thread sealants become dodgy). On instrument branches on pipe, we'll use threads all the way up to 1500# class (below 400 F) unless a client hates them and is willing to pay for alternatives. Some will readily pay $1000 to eliminate a single NPT connection on a process service- even on an instrument branch or a tubing line- and we're not talking about lethal service here. I can see that on 24" process lines, but on 2" process lines or tubing lines it's a trifle ridiculous. I'm happy to sell them what they want, but they rarely understand just how much it's costing them.

People that are getting leaks at 2" aren't getting them because people can't swing a big enough pipe wrench- they're leaking because they're using the wrong thread sealants or dull threading dies or otherwise just plain don't know what they're doing.

Beyond 2" for utilities we use Victaulic.

We've banned 150# cast stainless unions though-, threaded or socket-welded they leak in gas services, and sometimes can't be made to STOP leaking even by lapping the faces. The 150# MI ones have brass seal rings in them which stop the leakage.

 

[BigInch]

Forget the cost of repair, lost product and environmental damage. Just a 1 gallon leak means filing a report with over 250 data boxes to fill in. It'll cost you more than a welded joint just filling the forms, never mind the trip to the post office to send the registered mail.

Independent events are seldomly independent.

 

[Latexman]

One year a buddy in production was told, "you are not getting a raise this year because YOU had a "loss of primary containment". An old screwed pipe in monomer service had started leaking. That was the last screwed pipe he has installed.

Good luck,
Latexman

Need help writing a question or understanding a reply? forum1529

 

[awol]

On the teflon tape versus thread sealant, here is a suggestion. When NPT threads were created, the concept of make-and-break connections was not a consideration. NPT connections are intended to go together and stay together, as tightening can produce metal to metal sliding under mechanical force thereby a possibility of galling. Teflon tape provides a great sliding surface and can allow the asssmebler to assemble the joint further than needed with standard tightening force. When / if this tighened joint is disassembled, it could be discovered metal to metal action occurred and the threads will gall. Thread sealant (paste) provides acceptable lubricity and can limit how far the joint is assembled when using the same standard tightening force. This joint will be easier and less prone to gall than the teflon tape joint. -AWOL

 

[1gibson]

On the topic of anciliary reasons, the more threaded joints you have in a manifold, the more likely you will need a union or unions. This might be realized during design, but more likely during construction because you can't build it as drawn.

 

[StoneCold]

Zdas04.
I am facing some push back on threaded from another front. UT inspections. The UT guy is claiming that my piping system is at "end of life" because of the 1/2" threaded nipples on the gauge connection points. He is correct because the nipples are schedule 40 and once threaded there is no corrosion allowance left in the piping. Unfortunately I have a lot of this little nipples that will need to be fixed. In both carbon steel systems and 304ss. It is always something.

Regards
StoneCold

 

[zdas04]

Moltenmetal,
I gotta say that I could have written your post, it matched my experience perfectly. I find dull dies and incompetent sealant application to cause many more leaks than poor make-up. The rest of it is rumors and fear of rumors.

I've seen successful NPT as large as 4-inch, but the risk of a bad fit up is not trivial in 3" and 4".

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.
The plural of anecdote is not "data"

 

[chicopee]

Greater wall thickness with threaded pipes pluss greater labor cost when cutting and threading pipes in the field.

 

[europipe]

Stonecold,

In my opinion a pipespec starts at 3/4",
1/2" should be a good instr. spec.
So this shouldn't be a reaseon to reject screwed.

 

[Latexman]

Threaded SS is so bad at galling. Our pipe fitters hate it.

Good luck,
Latexman

Need help writing a question or understanding a reply? forum1529