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Torque for NPT connections 13
- Thread starter FLisa
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The torque and number-of-turns philosophies need very much to be tempered with the requirement that elbows and tees need to wind-up pointed in a specific direction. By the time you factor that very important element in, and allow for manufacturer's tolerances on fittings & nipples, and field threading of pipe, you're pretty much back where you started - screwed piping is as much art as science.
Generally speaking, what fails if you over-torque, no wait, I mean over-tighten, no wait, that's the same as torque, I mean over-turn, no wait, I don't mean tip over, I mean over-rotate (whew!) the joint? If a plumber gets to the point he artfully feels is just right, then has to rotate around to point the elbow northeast, is he going to hurt something?
There's some orientation 'allowance' designed into pipe threads, but how different is that allowance in different materials and material combinations: steel, stainless, brass, copper, aluminum, PVC?
There's some orientation 'allowance' designed into pipe threads, but how different is that allowance in different materials and material combinations: steel, stainless, brass, copper, aluminum, PVC?
Far and away, steel pipe & cast/malleable iron or steel fittings are the most commonly threaded. There's some stainless. The tricky one is PVC. It's a fine line indeed between having a "weeper" and splitting the female fitting. If dissimilar materials are involved, always try to get a male PVC thread screwing into a female metal fitting or flange. PVC is usually glued, anyway. Most copper joints are soldered, grooved, crimped, etc. However, there are a LOT of really crappy threaded copper adapters on the market these days. The only brass threads usually seen are female, on things like valves, etc.
Good screwed fittings will give the feel of gradually tightening. With some practice, you can usually tell if you can get another full turn on them, or "this is it". There's some 150 malleable fittings on the market that don't always give you that "gradual" sensation. They just GRAB. Because it doesn't A/ stop in the right orientation, or B/ "feel right", they usually get removed. Quite often, they've galled, and you wind up tearing the threads up so badly you have to start over. That is very frustrating, and we avoid 150 malleable fittings whenever possible.
Also remember that "plumbers" and "steamfitters" are two separate trades. A LOT of piping problems (not just with screwed connections) are the direct result of having plumbers install & maintain steam, condensate, compressed air or process piping. Not nearly all piping is plumbing. I have great respect for a lot of plumbers, but I'd rather work on a 500 PSIG superheated steam system than plunge a toilet
Good screwed fittings will give the feel of gradually tightening. With some practice, you can usually tell if you can get another full turn on them, or "this is it". There's some 150 malleable fittings on the market that don't always give you that "gradual" sensation. They just GRAB. Because it doesn't A/ stop in the right orientation, or B/ "feel right", they usually get removed. Quite often, they've galled, and you wind up tearing the threads up so badly you have to start over. That is very frustrating, and we avoid 150 malleable fittings whenever possible.
Also remember that "plumbers" and "steamfitters" are two separate trades. A LOT of piping problems (not just with screwed connections) are the direct result of having plumbers install & maintain steam, condensate, compressed air or process piping. Not nearly all piping is plumbing. I have great respect for a lot of plumbers, but I'd rather work on a 500 PSIG superheated steam system than plunge a toilet
What a great topic and such interesting responses!
Tapered pipe fittings have many issues - even though modern life would be unimaginable without them. If you have the option to redesign your parts - another option is to look at BSPP (British Standard Pipe Parallel or ISO G) threads which are parallel - like a bolt, and rely on a sealing washer under the head of the fitting. This has much the approach as torquing a standard fastener.
The Swagelok catalog has good information on this topic, FESTO have a huge range of ISO G threaded fasteners which rely on gaskets to seal. (no plugs-just used these parts before)
Another interesting method of joining pipe fitting is sanitary flange type arrangements.
These approaches tend to significantly reduce the forces on mating parts - but the sanitary flanges are not suitable for high pressures obviously.
Tapered pipe fittings have many issues - even though modern life would be unimaginable without them. If you have the option to redesign your parts - another option is to look at BSPP (British Standard Pipe Parallel or ISO G) threads which are parallel - like a bolt, and rely on a sealing washer under the head of the fitting. This has much the approach as torquing a standard fastener.
The Swagelok catalog has good information on this topic, FESTO have a huge range of ISO G threaded fasteners which rely on gaskets to seal. (no plugs-just used these parts before)
Another interesting method of joining pipe fitting is sanitary flange type arrangements.
These approaches tend to significantly reduce the forces on mating parts - but the sanitary flanges are not suitable for high pressures obviously.
RustyTools
Mechanical
I have been using 1/8" NPT and 1/4" NPT Parker and Swagelok stainless pipe fittings for years, but when I went to modify a natural gas line, which was 1/2" steel NPT, I overtightened it and the male part snapped after sitting a couple of days.
I never got the feeling the torque was increasing so I kept tightening it. With the 3/4" and 1" pipes I did not overtighten it because there was a definite torque change. Next time I would follow the directions in the Parker catalog.
I never got the feeling the torque was increasing so I kept tightening it. With the 3/4" and 1" pipes I did not overtighten it because there was a definite torque change. Next time I would follow the directions in the Parker catalog.
I went all down this thread and did not see formal mention of one thing that is probably obvious to most, but might be helpful saying to some. In my opinion it is necessary to "start" the threading properly and in the correct alignment (if everything is perfect in the machining, I guess this would be with the male thread item aligned with the axis of the tapped hole, and that would normally be perpendicular to the tapped main pipe or fitting etc. axis). Once this is done, the threading should of course start very easily and torque gradually increase with tightening as others have explained.
I guess it is conceivable direct taps in curved as opposed to flat/boss surfaces may increase the necessity for care in initial alignment, and (discounting issues like leaks or splits in plastic pipes) I wouldn't be surprised cross-threaded members are perhaps more likely to cause leaks than variable torque.
I guess it is conceivable direct taps in curved as opposed to flat/boss surfaces may increase the necessity for care in initial alignment, and (discounting issues like leaks or splits in plastic pipes) I wouldn't be surprised cross-threaded members are perhaps more likely to cause leaks than variable torque.
rconner,
Cross threading is a major issue as you say. And the stabbing and preliminary assembly is very important for the succesful end result. Normally an experianced assembler has experianced this before and knows that he has to throw away those parts. Also after several times he cross threads a connection, he will know that feeling and remedy the situation. If he don't, and wish it away, as soon as the pressure is applied, that connection will be telling him to fix the problem. My personal belief is over-torque is the biggest problem. Over-torque can produce a failure weeks,months,or years later. Then all that happens is a lot of finger pointing.
Cross threading is a major issue as you say. And the stabbing and preliminary assembly is very important for the succesful end result. Normally an experianced assembler has experianced this before and knows that he has to throw away those parts. Also after several times he cross threads a connection, he will know that feeling and remedy the situation. If he don't, and wish it away, as soon as the pressure is applied, that connection will be telling him to fix the problem. My personal belief is over-torque is the biggest problem. Over-torque can produce a failure weeks,months,or years later. Then all that happens is a lot of finger pointing.
DrillerNic
Petroleum
As a couple of people further up the thread have said, in the upstream oil industry premiuim threads (ie non API threads) are always made up to a particular torque level (or more accurately, a particular torque range) using a power tong and a JAM unit that records the torque as the joint is made up. These premuim connections are gas tight and do not require dope for the seal, unlike API buttress or round threads, but either seal on end of the pin into the box (typical for most VAM type connections) or on a shoulder half way along the thread (typical of Hydril connections).
A connection is made up and then accepted or rejected depending upon the shape and value of it's torque- turn curve: usually a straight line as the joint is made up, and then a curve as the torque goes into the joint. If the curve is the wrong shape, or fails to reach a certain value or starts too soon, the connection is broken out and redone, or rejected. Misalignement, cross threading etc are all easily identified as the graph looks wrong- the connection takes torque too early for example.
The type of dope, with a known friction factor, is closely specified, and increasingly is being pre-applied at the storage yard before the joints arrive at the rig to save time and also to ensure optimum dope application.
A connection is made up and then accepted or rejected depending upon the shape and value of it's torque- turn curve: usually a straight line as the joint is made up, and then a curve as the torque goes into the joint. If the curve is the wrong shape, or fails to reach a certain value or starts too soon, the connection is broken out and redone, or rejected. Misalignement, cross threading etc are all easily identified as the graph looks wrong- the connection takes torque too early for example.
The type of dope, with a known friction factor, is closely specified, and increasingly is being pre-applied at the storage yard before the joints arrive at the rig to save time and also to ensure optimum dope application.
Torque is required for sealing in downhole premium connections only to prevent connection from backing off downhole. The torque is designed into the connection. You can make low torque test plugs for premium connections to prevent the wear and tear and time required to torque equipment during pressure testing.
As for the NPT, ANSI/ASME B.120.1 says for a pressure-tight joint, the connection must be made-up wrench tight with a sealant. Wrench tight actually has a dimension in the ANSI spec for hand-tight and wrench-tight. From this, the guidelines for making up finger tight and then tightening an additional 3 turns for sizes up to 2" are derived. For larger sizes it drops to 2 turns past handtight.
If you don't use sealant, a leakpath will exist where the sealant would normally be. There is only one though, not two like was mentioned un;less I have lost my mind.
As for the NPT, ANSI/ASME B.120.1 says for a pressure-tight joint, the connection must be made-up wrench tight with a sealant. Wrench tight actually has a dimension in the ANSI spec for hand-tight and wrench-tight. From this, the guidelines for making up finger tight and then tightening an additional 3 turns for sizes up to 2" are derived. For larger sizes it drops to 2 turns past handtight.
If you don't use sealant, a leakpath will exist where the sealant would normally be. There is only one though, not two like was mentioned un;less I have lost my mind.
johnchrc,
You may want to look into the leak paths again.
A thread has a root and crest. Each are in a leak path zone. Normally the thread flanks are engaged metal to metal, however the roots and crests are in clearance. That means two leak paths. I concur with your torque theory in a way, if the threads do nothing but hold the connection together and a have separate "metal to metal seal" like premium oil field connections, then the "metal to metal seal" seals off the pressure. But NPT's are supposed to seal and hold the connection together like API 8-Round, Buttress, and Line Pipe (NPT). In order to seal, you have to have radial interference. That is where the turns come in. In order to have turns you must have torque. That is where the torque comes in.
You may want to look into the leak paths again.
A thread has a root and crest. Each are in a leak path zone. Normally the thread flanks are engaged metal to metal, however the roots and crests are in clearance. That means two leak paths. I concur with your torque theory in a way, if the threads do nothing but hold the connection together and a have separate "metal to metal seal" like premium oil field connections, then the "metal to metal seal" seals off the pressure. But NPT's are supposed to seal and hold the connection together like API 8-Round, Buttress, and Line Pipe (NPT). In order to seal, you have to have radial interference. That is where the turns come in. In order to have turns you must have torque. That is where the torque comes in.
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