NPT versus BSPT

[TMOC]

Hi Guys,

I asked this as an add on to another question, but maybe this is better asked as a new one.

Perhaps someone can answer a thread question for me.

I am trying to determine which is better - NPT or BSP(T) for industrial refinery installations. Services range from very low to reasonably high pressure (within the Class 150 limit).

I am working for a group who has used BSP for many years, and my background (from the oil industry) is NPT - which I am trying to introduce. I would like to find firm evidence which is better for sealing - especially in long term service.

Perhaps the advantages and disadvantages of each would be a good start.

Thanks

TMOC

 

[MikeHalloran]

Long term service? Let's find an example where long term service without maintenance is absolutely required. One, the nuclear industry. BSPT or NPT? Neither. Two, the automobile industry. BSPT or NPT? With the exception of gage fittings and small casting plugs, neither.

Tapered thread joints induce stresses in the pipe, fittings and flanges that do not contribute to containing the process, and are just sitting there waiting to cause a problem should the opportunity arise. Long term, opportunities are guaranteed.

So, which tapered pipe thread is least awful?

In your case, BSPT, because you work FOR a group that is familiar with it. If they were working for you, NPT would be an easier sell. Consider also the group dynamics; changing to NPT would force many people to learn something new, whereas you accepting BSPT would require retraining only one person. The "retraining" costs include not just the effort of learning about the other stuff, but the cost of stupid errors, e.g. buying components with the 'wrong' threads. Those costs are real, and could be substantial; who pays for them?

You must have bigger battles to fight.

-Mike-







Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA

 

[ApC2Kp]

The overwhelming volume of NPT use by the American petroleum companies has made ANSI and therefore NPT dominant in the oil industry and in countries where they have established themselves. I could see where mergers have caused the question of which standard to continue at companies such as BP-Amoco.
My problem with the thread standards would be the allowed use of straight thread couplings with taper thread pipe. I like the approach of API couplings requiring a tapered female thread to ensure better fit and therefore a tighter seal of the threaded joint. I think the BSP standards allow use of straight thread couplings, and the American standards for threaded and coupled (T&C) pipe allow the use of 'merchant' coupling with straight thread pattern. If you have to replace all the couplings in a pipe run just one time, it will be remembered forever.

 

[tommo]

Don't even think about it!

Historically BSP has been a superior thread system to NPT in regards to sealing, although in practical terms with modern sealants - both threads are OK.

Fot the main - it is the smaller sizes that are different, less than 3/4" are not compatible.

ISO adapted BSP as the ISO pipe standard in recognition of it's superior sealing ability. I suppose that reflects more than just my opinion.

The availabilty of NPT components outside the US, NPT is a rarity unless you have suppliers like Swagelok available.

I totally agree with Mike H. Another problem will be the mixing of the different types of threads - and that will cause saftey problems too. If the company you are working for has a system in place, introducing another set of standards is an incredibly wasteful use of time and resources.

If metrication ever takes off in the US, ISO (BSP) will be the thread standard - so effectively you are introducing an obsolete technology anyway. Another consideration is that if your machines are for export outside the US, ISO (BSP) would be a substantial selling point.

Hope this helps

 

[harrisj]

Parallel threads, with 'O' ring or bonded seals, or compression joints, are for engineers. Doesn't matter whether they're BSP, SAE, JIC, metric or whatever, providing they are rated for the working pressure.

Taper threads (IMO) are for plumbers.

Cheers - John

 

[TBP]

harrisj - My experience has been that once companies stop hiring plumbers, sheetmetal workers and millwrights to do steam and process piping, and start using steamfitters, most of the problems with threaded connections simply disappear. We do thousands of NPT joints a year on 150 PSIG steam systems, and have very, very few problems.

 

[saxon]

harrisj, Know the difference between pipefitter and a plumber? All a plumber needs to know is, pipe is round, s!!! flows down hill and payday's friday. As for the superiority of ISO's use of BSPT, the real reason being, BSPT was already in the works in Europe. Why should we retool, let's f*** the Americans. As for its superiority in sealing, I've seen equipment come in from Germany with this thread profile that leaked like a sieve. After reworking to NPT, employing pipefitters, there hasn't been a leak since.

When it comes to any type of connection, it's all a question of service, pressure, temperature, and maint. requiements.

Hope this helps.
saxon

 

[TBP]

ApC2Kp - Merchant couplings are thread protectors, and nothing more. They're installed on one end of mill threaded pipe to protect the threads from damage during shipping. I don't know of any pressure piping code that permits them to be used in service. They are to be removed and discarded.

tommo - the 1/2" and 3/4" NPT and BSP pipe & thread sizes are so close that they're with the tolerence of the dies cutting the thread. (They shouldn't be mixed and matched, in any event.) It's the larger sizes that don't match. Very hard to tell one from another, with the naked eye. We do work in a plant here (Canada) where a great deal of the production equipment is pre-fab skids from the UK. It is filled with BSP threads, pipe, and flanges. All of the connecting piping is (of course) NPT and ANSI dimension flanges. It's a nightmare.

There's 50 zillion miles of installed pipe in North America, and 100 times that many NPT joints. NPT will not vanish in the lifetime of anyone reading/posting on this board. Incidently, NPT was originally called "Briggs" thread, and if I'm not mistaken, Briggs was an Englishman.

 

[TMOC]

Guys,

Thanks to everyone for their input. One point I need to clarify is that I was not intending on mixing threads - BSP would only be used with BSP, and NPT would only be used with NPT.

I am located in Australia where we use a large variety of both, depending on the industry. NPT is more popular, but BSP has been used for many decades, and is still used.

At least I have a better idea on this topic.

Thanks again.

TMOC

 

[tommo]

Wow - Set off a firecracker here didn't I!

To clarify my intent, NPT/BSP - both thread systems work provided the threads are cut properly and the appropriate thread sealants used etc. Is one superior to the other - not really. Leaks generally occur because of poor practice.

TMOC seemed concerned that BSP was an inferior thread system to NPT. I was trying to show that BSP is equally well accepted in countries that use it as their basic standard.

For the guys in the US, you need to realize that outside the USA, NPT is hard to obtain and in limited selection - just like BSP is within the USA!

I think everyone can agree that it was an unfortunate event that the two pipe thread systems developed over history.

The bigger issue here for TMOC is bringing in NPT threads into a plant that appears to use BSP as a long established standard. For me - keep to one standard as much as possible.

I am also in Australia and in general - BSP is the standard thread unless you are working with American plant. In which case we generally have a machine full of NPT threads connected with BSP threads - same nightmare TBP has in reverse!

Really comes down to where the machinery is built, whrere it is likely to be used (if exporting to the USA - use NPT, if exporting to the UK use BSP etc.), and what type of equipment will be connected with it.

You can start out with the intention that the two different sysems are not mixed - but you can bet your balls they will!

 

[b1ueshift]

Having had experience with both systems, I haven't found one to be better than the other. The only times I have seen a tapered pipe thread in a straight threaded fitting is when the fitting is made of sufficiently soft metal that it stretches and conforms to the shape of the tapered pipe during tightening. This is commonly used for NPT and BSPT joints. I havent found any reliability issues with this type of joint.

I would think that your decision would come down to which type of fittings is easier to get.

 

[jeyx]

I have an old gas bomb with a 1/2" opening - female thread. There's a fitting for them and it has 1/2" tapered male thread (must be NPT). The fitting can be twisted in by only ~ 8 mm. Further twisting require force. Both threads are slightly damaged by the previous twistings.

How can I define which threads they are? Can you please give me a tip?!

thanks,
Jeyx

 

[quark]

Check with a thread gauge. They seem to be of different threadings.