Torque for NPT connections 13

[FLisa]

I have done many searches looking for recommended torque by size and material for threaded pipe connections. Almost all sources give a "number of turns" method. Does anyone know why torques are not used?

 

I need information about multilayer composite pipes.

thank you.

Farhad Mohkami
E-mail:akmihan@iraninfocenter.com

 

[ghillis]

NPT Tapered threads seal (in theory) by interference. Since there are several variables that enter into determining coefficient of friction (material, pipe dope, Teflon tape) and by extension torque, it is not practical to use torque. If you used torque one of the following would occur periodically:
1 you would thread a pipe into an oversized fitting or an undersized pipe into a fitting too far and still not get a good seal because the threadform was all wrong where interefence is generated.
2 you would thread a pipe into an undersized fitting or an oversized pipe into a fitting and not get sufficient engagement for sound mechanical connection.

Using engagement, these problems are avoided.

 

[PaulEng]

FLisa,
I am looking for similar information on NPT torque values. I would be grateful if you could post any of the more useful information sources you have found.
Thanks,
Paul

 

[TBP]

I'm curious as to why the question of torque on threaded pipe joints would arise - any particular problem that's popping up? Leaks, perhaps?

 

[FLisa]

Leaks are one problem. Another is dissimilar materials, i.e. metal into plastic, or plastic into metal.

If it is metal into plastic and the metal is high end or the spec or oversized or the plastic is undersized then the method of number of turns can cause a real problem with the plastic portion cracking.

 

[TBP]

To a large extent, screwed piping is an example art meeting science. Most screwed piping is tightened until it "feels right", and the fitting is pointed in the desired direction (if that's applicable). The metal/plastic piping interface can be tricky at the best of times. It's often a thin line between stopping a leak, and cracking plastic, on screwed joints, or with flanges.

When your pipe is being threaded, is anyone checking that the dies are set correctly? This is particularly important on the adjustable heads, that will do a range of pipe sizes. It's not all that often that the dies get set exactly on one of the increments in order to cut threads properly. Even the drop-head style have a small level of adjustment, and the instructions are on the back of the package that the replacement cutters come in. You should be able to screw a fitting on 3 to 4 turns, to hand tight on clean, dry threads - no tape or dope for this check. Once the dies are set, they're normally OK until the size is changed. I've never actually seen anyone count the turns after the threads are taped or doped. The fitting is screwed on as far as comfortable by hand, then wrenched until it feels right, using an appropriately sized wrench.

 

[JohnGP]

Spot on, TBP. I don't know that I've seen anyone count number of turns either before or after taping. But an experienced pipe fitter will immediately know if engagement is "good", so I guess there is a subconcious level of thread counting.

As pointed out by ghillis, torquing of screwed fittings is not feasible due to the variables involved, but when you think about it, tightening screwed piping until it "feels right" is definitely torque related. Try quantifying the experience factor though......"art meeting science" - beautiful.

 

[smckennz]

Having installed quite a few kilometres of screw jointed pipe, without thinking about the "art" (I just wanted no leaks) I will chuck in my two bobs worth.
Torque by itself is meaningless. Its the progressive rise in torque you are feeling for. Screw it in until it starts to seat. Then up the force a little by yanking. If each yank gives less movement (or more jolt), you probably have a sound joint. If the movement stops suddenly, you have probably bottomed out the pipe end and will have a leaker. If the movement doen not decrease with each yank, then you probably have a plastic valve or joint, and have stuffed it by yanking too hard. If you are lucky, it is just a bent thread, but you should have picked this up because it will not seat in the first place.
Yes, I guess there is some art involved.

 

[meenogre]

So far so good - art & science and all. smckennz, I think that's a good description of that dynamic 'feel' - that's probably how you would program the automated pipefitter to feel it.
But how the heck do the pipe fitting designers make sure their parts are good - send 50 samples to 50 experienced plumbartists with a "how's that feel" questionaire? There's got to be some sort of acceptable-torque-limits table SOMEwhere, doesn't there?
(I guess I'm asking about material strength. The thread shape descriptions are pretty thorough.)
D²

 

[klchurch]

I just had to get in this discussion.
I’ve been designing threads since the 70’s and been trying to figure a reliable torque formula for threads for the same amount of years. My conclusion so far is, not even PhD’s can solve for torque, to be reliable for the every day assembler. The reason is that there too many unknowns to equate. Every element feature such as thread height, taper, finish, lead fluctuations, yield of material, thread compound, etc. effects torque drastically. Not only that, the friction factor changes per turn and partial turn during makeup. You can get in a ball park with torque, but sooner or later it will burn you because the machining tolerances are too wide to predict what situation the two members are seeing as far as stress and strain. API has equations for torque and I believe you will find that the NPT and line pipe are the basic same threads. However most people only use these torque values as a starting point, and with a grain of salt. Also, with experience of assembling and testing API connections and premium threads the only way to predict torque, is to have full control of the manufacturing tolerances, and because of this, one can predict the situation and mechanical “chain of events” that occurs during assembly. With that said, I believe the real issue here is leak resistance. It is far easier to directly relate and calculate leak resistance to turns (translated to interference), rather than torque. Leak resistance is directly related to the Hoop stresses in the male and female connectors. The theory is when you assemble the male and female connections together, the female results in hoop tension values and the male is in hoop compression values due to interference. This is at the assembled state without any other stresses applied. The connection will be leak tight until the male connection reaches hoop zero due to internal pressure. Hoop zero is when the male member expands back to its machined state or you can say when it lost all of its compressive diametrical interference. Any additional internal pressure will cause a leak if the pipe body did not fail first. When assembling the connection, one should know the maximum amount of turns to make sure the connection does not yield,both male in Hoop Compression and the Female in Hoop Tension. Remember that any other stresses applied to the connection besides internal pressure, such as bending, axial tension, and/or axial compression is additive to the stresses and will approach yield and possible premature failure.
A little point of advice:
A metal female connection assembled with a plastic male will have more sealing integrity than a plastic female connection assembled with metal male connection with the same conditions. The female expansion governs the integrity.
klchurch

 

[CRG]

The plumbartists feel is more than just torque. His basis of tightening involves torque and the rate of change in the torque. You can feel when deformation begins when the change in rotation angle and the change in torque is no longer linear. Experience tells the plumber when to stop to ensure a positive seal without damaging the fittings. So knowing torque alone is not enough and neither the turn count alone is enough to determine when a joint is made properly. I have no specific reference stating that the above is accurate; however, these are the things that I use to determine how tight I should makeup a screwed joint when I do not have a torque spec. These factors come from my experience, and my experience is that of a car mechanic turned engineer. If I were to make an engineering study using all of the variables, the results could not be translated into an easy to use guide.

 

[klchurch]

Mr. CRG is correct in a unique way. I never thought of entering feel in the equation, but here is a possible way of putting it on paper. The oil field uses special power tongs to assemble connections that records, via computer, Torque vs. turns, torque vs. time, and torque vs. position. The major Oil Companies have required the use of this equipment because they can get a better “feel” of the assembly (2D graph). Also these graphs are documentation for archiving in case of a mishap. As CFG stated the torque is not linear, it looks more like a stress strain curve with a rubber band in the tensile frame. Also he is very correct by stopping the torque within this curve, because he is staying within the elastic limit. Maybe the plumbers can train and calibrate their “green plumbers” arms with a Torque/Turn unit. This way they are converting their feel into a visual aid.
KLChurch

 

[surfdancer]

We use NPTF for steel and NPT for other metals and plastics. Our process is to use turns past finger tight for correct sealing without excessive deformation. This is based on best practices within the valve components industries.

Torque flat out won't work for many of the reasons mentioned earlier. If you ever look at the section in the Machinery's Handbook, you will see nominal engagement for pipe threads, both handtight and wrenched. We do not follow these guidelines verbatim, however we do go by turns past finger tight to assure consistent sealing without excessive deformation.

 

[hydrae]

There is one application where torque on the joints is a requirement. When a submersible pump is installed in a well, the normal direction the motor turns will untwist the threads in the pump column, if it untwists far enough, the pump and motor fall off and plummet to the bottom of the well, standard rules require a torque of 10ft-lbs per hp to ensure this does not happen.

Hydrae

 

[DRWeig]

Hi Group,

Good info all the way down! Although I'm aware that this is a mechanical forum, I wanted to throw in a not-so-commonly-known requirement for electrical devices using rigid or intermediate conduit in hazardous locations. An ISA standard (I believe) calls for a minimum of five full turns at every joint or connection, in order to maintain a minimum gap length to cool hot gases that may be escaping under pressure from within.

Have a happy!

Old Dave

 

[moltenmetal]

The torque to which NPT threads must be tightened to obtain a leak-free joint is determined at least in part by the nature of the sealant used. With some sealants, making the joint too tight will lead to an increased chance of leakage, as counterintuitive as that may seem. The mechanics of the compression of these sealants in a joint during assembly are very complex indeed, so forget about systematizing this unless both the male and female parts are machined to close tolerances. If you're dealing with pipe and 150# fittings, give up now. Select the right sealant for the duty and then make sure you have a well-trained and experienced pipe fitter, or expect leaks.

Another consideration is the tendency of the sealant to act as a lubricant to resist galling during fit-up. This is particularly important with materials which gall readily like stainless steels.

Building screwed pipe to seal reliably is definitely the intersection of art/technique and science. But if you get it right, NPT threads are very reliable and extremely useful.

 

[klchurch]

In thread design of Oil Country Tubulars, which the NPT was the starting point or Grand Father, thread compound is used to plug the leak path, not to guarentee a seal. There are two leak paths located at the roots and crests. Also the thread compounds are lubricants to help lower the friction factor and help increase the gall resistance of the male and female members. Interfence is required to insure the connection is together "RADIALLY" when the connection expands due to internal pressure. If the female expands more than the interference (The female separating radially from the male), it "will" leak. Regardless of the thread compound.

 

[piper7]

Hey guys whats up,

Just a thought, couldn't you use the info available for standard steel bolt thread torque’s by averaging the torque values between two different sizes? For example, a 1" dia. bolt might have a maximum tightening torque of 75 ft.-lbs., and a 1 1/4" dia. bolt a maximum tightening torque of 100 ft.-lbs.. The average of the two would be half of 175 ft.-lbs. or 87 ft.-lbs.. This might be approximately right for a 1" dia. NPT pipe, but pipe is hollow. Therefore a deration factor would be needed to compensate for the lack of strength available from the pipe as a hollow tube verses the solid bar geometry of the bolt. I think a good approximation for a deration factor might be to use the ratio of the cross sectional area of the material of the pipe divided by the cross sectional area of the material of the bolt. This deration factor, less than one, could then be multiplied times the average value of the bolt torque’s to arrive at an approximate torque for the tapered pipe. My guess is around 10-25 ft.-lbs. torque for a 1" NPT. pipe thread. This is the torque range of a bolt that is about 1/4" in diameter, which is about 1/2 the amount of steel material that the 1" NPT has, but the pipe geometry is a cylinder. To measure this torque I think you would have to make an adapter that would place the head of the torque wrench as close to the pipe being tightened as is possible. Maybe a square or oblong hole in a pipe wrench, if the square nose of the torque wrench won't fit into the jaws of the pipe wrench while being used.

Hope this helps.

Piper7.

 

[renasis]

A good resource for this is:

http://www.parker.com/tfd/cat/pdffiles/open.pdf

Search for: NPT

I would suggest a turns past hand tighten approach based on this documentation. The reason for this is four fold: the material being tapped, the tolerances on the tapped hole, if a lubricant is being used, and the possiblity of overtightening. I would suggest that you have someone do the turns past finger tightening method and record the torque needed to accomplish this. This would give you a good initial starting point.