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"

 

[moltenmetal]

chicopee: seriously? Threaded pipe costs WAY less to fit and install (total of materials and labour) than welded pipe from 1/2" to 2"- that's not in doubt. It is also easy to modify should changes or re-work be needed later. As to total lifetime installed cost, that depends greatly on the service and the consequences of a leak as BigInch and Latexman have correctly pointed out. Those guys are customers willing to pay heavily to avoid threaded joints, and I love such customers dearly and serve their needs as best I can- as long as they're willing to pay the price and schedule impact of their decisions.

awol: I agree with most of what you've said, except that NPT joints have NEVER been intended to seal without thread sealant. Some materials come close- leaded brass parts can self-lubricate and yield against one another while tightening until they seal- but others need a different thread profile like NPTF (dryseal) to have a hope (and still aren't reliable). The 90 yr old threaded pipe in my house's hydronic heating system has thread sealant on every joint- "plumber's putty" consisting of a mixture of linseed oil and plaster of Paris, ground limestone etc., and it's still holding after hundreds of thousands of thermal cycles- takes a torch to melt it before you can undo a joint though.

A joint which is galled is likely to leak. Our experience is that the anaerobic thread sealants are perfectly adequate on their own if they are applied and the joint is tightened properly, but their lubricity leaves a lot to be desired. In practice, high density teflon tape is necessary as a high pressure lubricant to prevent galling, because fitters often over-torque joints to get elbows or valve handles to the correct position or becasue they've cut nipples to the wrong length and don't want spend the time trying again. We top-dress the tape with anaerobic pipethread sealant and find that combination to work perfectly, despite the claims by some that it can't possibly work as well as sealant alone. Those pipe dope sealants vary GREATLY in quality and usefulness from brand to brand- they are NOT created equal, and they are NOT "liquid teflon" from a materials compatibility perspective. We use Henkel Loctite products and they're uniformly excellent.

europipe: we widely use 1/2" pipe in the small modular plants we build. We use it interchangeably with 3/4" OD tubing with compression fittings for many services. Threaded pipe assemblies give rigidity and ease of mounting for clusters of valves etc., whereas the tubing gives the ability to do long runs with minimal field joints and low labour cost. I understand the prohibition against smaller sizes and process tubing in certain heavy industrial settings though- it's a "Bubba-proofing" matter, since you assume that Bubba will want to climb on any pipe you install at some point. The best you can hope for is Bubba-resistance though- Bubba-proofing is impossible!

stonecold: is he claiming that the nipples themselves are corroded below minimimum required thickness under the threads, or that they just don't meet the design corrosion allowance requirement, i.e. they were selected incorrectly? If the latter, get the people in charge to give their heads a shake. I understand the desire to use sch80 or even sch160/XXS nipples in corroding carbon steel services to give the nipples longevity and extra breakage resistance against abuse, but if you want a full-on corrosion allowance, that more or less means you need to follow europipe's advice because it's tough to get 1/8" extra wall for corrosion allowance below 3/4" NPS- even in welded services- and generally you need 2" minimum nozzles on vessels etc as well. And seriously- what use is a general corrosion allowance for 304SS anyway?

Latexman: compared to working with carbon steel pipe and MI fittings, there's lots to hate about threading stainless. Socket welding is easier for the fitters- but then you need the welder, and a saw any time you need changes made. But the problems with threaded stainless go away if you do the work properly. Dies sharpened with the correct angles for stainless, thrown away or resharpened when they're dull, proper lubricants used in sufficient quantity when cutting, good Ridgid machinery, and avoidance of the 150# unions is half the battle. A good thread sealant system is the other half- coupled with a little experience so that the fittings aren't over-tightened. Avoiding the very cheapest 150# CF8M cast fittings also helps- some of those are absolute garbage, with threads 10 degrees off axis etc.

 

[europipe]

moltenmetal;
You didn't get the star from me.
Maybe You live in a perfect dry and non corrosive environment, but not me.
You maybe can give Bubba the blame, but here weak Willy can ruin 1/2" stuff after a few years.
Big companies start pipespecs from 3/4"and sch80 not for nothing.
It's not only the p/t range that determines the spec, but also corrosive and mechanical properties.