sealing NPT threads, is there research tape vs. sealer? 11

[kcj]

The perennial discussion of teflon tape vs. anaerobic sealants for NPT threads goes on forever on boards. My opinion, your opinion, etc.

Is anyone aware of actual test research between the various methods? Teflon tape, Loctite PST, Megaloc, Rectorseal, etc?

kevin j

 

[Cervantis]

Infinity Tech said:

I again wrapped the nipples with Teflon tape 15 times. I thought about saying something, but my "swaib and de-boner" up-bringing would not allow me to comment. However, 15 times...how can you do this and not increase or change the thread design. We are only wrapping about 3-4 times...and true we have had leaks, but 15 times...is this a common practice in other industries. This might be the answer on what we are doing wrong. I remember reading somewhere and it might have been on this site or another, that with NPT threads wrap or fill until "it works". I would like to have some responces from others on the number of times each thread is wrapped. Thanks in advance...

 

[hydtools]

I have found 3-4 wraps to be enough. The first wrap starting one thread back from the end and to catch the starting tape end, then two to three more to cover about 1 1/2 tape widths up the thread.

15 seems like a waste of tape. There is not enough space between the threads for all that tape.

Ted

 

[JLSeagull]

For something like a NPS 1/4 male tubing connector I may use just over one wrap. Once I cover the starting point I begin to stretch and break the tape. The final coverage is perhaps over 1.5 turns. for a larger nipple I may cover more than the first 3/4" of the threads but not much more, again just to cover all threads necessary and stretch to break the tape with minimum overlap.

OTOH when taping an electrical wiring splice I may use several wraps - still not l5 wraps.

 

[kcj]

mark
off topic, but rather than make a relaible production assembly with so many possible joints to leak, can you eliminate many of those fittings and joints by manifolding, or machining into a block, or casting a plastic block, or using compression joints or multi function components or adaptors, etc etc.....

My career is hydraulics, and a cartridge valve/manifold block is the analogy that comes to mind. I realize low pressure, water, and consumer costs are constraints way different in your field.

 

[JLSeagull]

ISA 76 standards include modular components for analyzer sample conditioning systems. These are intended to permit easy maintenance etc. However, the initial cost of sample systems is capital cost and much higher for modular sample systems than stick-built tubing, valves and fittings.

 

[Compositepro]

15 wraps of tape is way too much and will likely cause leaks. The purpose of the tape is to fill the gaps that are a natural part of the thread geometry. When tightening the thread, tape will extrude from the face of the threads to the gaps at the root. If you use too much tape you prevent metal to metal contact at the thread faces which creates a strong and stable joint. Teflon has high thermal expansion and creeps.

It is very typical that those new to pipefitting have leak problems. NPT threads require high torque to tighten. Threads are often mis-formed of or dinged. High torque is required to get these thread imperfections elastically or plasticly deform the get the joint tight. The number of leaks you are experiencing indicates that you are not tightening the fittings enough.

 

[EdDanzer]

When sealing pipe threads the material considerations should be more application specific. For pressures less than 1000 psi Teflon tape or any Teflon containing material is fine as Teflon’s yield strength is about 1000 psi. Anaerobic sealants are ok if the pressure is less than 5000 psi and there is not too much pressure generated deflections causing the material to break and crumble. The coating applied to many air fittings seems to give less than good results.

We have had better luck with Permatex Form-A-Gasket at hydraulic pressures below 3000 psi and compressed gasses under 2000 psi than any other material. Above these pressures pipe threads suck.

I do not recommend pipe threads for any fluid power application and will only use them if there is no other choice. I hope someday pressure gauge and transducer manufactures will start using o-ring connections as a standard.

Ed Danzer

http://www.danzcoinc.com

http://www.dehyds.com

 

[Cervantis]

"NPT threads require high torque to tighten." I was under the impression that the ASME B 1:20:1 standards for NPT stated that turns (2) not torque was required. We have found that going over board on our torque requirements were changing the internal structures of concentricity of the crests and flanks creating "spiral leaks" from the threads being crushed. Where can you find the exact torque requirements when there are so many different variables. If any one knows, please let me know. This has been a discussion that has gone back and forth with no real verifiable facts or standards.

 

[kcj]

Gauges & transducers: some gauge mfr are starting to offer oring, but the -04 oring is pretty delicate compare to 1/4 npt. -06 is good. The selection is bleak.
Transducers seem to be equally avaialble either way. One pressure transducer manufacturer offers a 9/16-18 thread that is NOT an SAE o-ring port interchange and did not include, or allow, an oring for seal. The mfr claimed it was a 'special thread' just for us and they have never heard of any -06 oring boss port standard. After all the arguing we ended up tossing dozens of useless transducers.


TORQUE tought to get field people not to overtighten. We saw a lot of cracked centrifugal pump outlets andvalve bodies from overtightening. Eventual solution was to reinforce the pump casting, but that is another story.

This is a pdf from parker, posted on Brendan Casey's site, that attempts to define NPT tightening based on Turns from finger tight. It is a start, but since the numbers are typically 2 to 3 turns, that is a lot of variation. Also, 'finger tight' means a lot of variation.
Ultimately I think, mechanics feel and training by passing on the knowledge. Another reason to get rid of NPT if possible.

http://www.hydraulicsupermarket.com/download/taper_thread_torque.pdf

 

[moltenmetal]

The yield strength of teflon has nothing to do with its use as a thread sealant in an NPT thread!

What's the yield strength of an anaerobic pipe thread sealant?!

We have had our best luck with a combination of DENSE (s.g. > 1, preferably 1.3) teflon tape PLUS a top-dressing of anaerobic pipethread sealant. We use the ones from Henkel (Loctite brand) but there are numerous available. The teflon tape does the basic lubrication and large void sealing, with the sealant providing leak resistance against dimensional changes in the joint due to thermal cycling, vibration etc.

Excessive torque of NPT joints leads to galling which leads to a leaking joint which is difficult to disassemble and fix properly. NPT threads are NOT dry-seal threads, so there's limited benefit to be had from over-tightening. They REQUIRE a thread sealant to seal.

Industrial pressure transmitters (for the process industry) are available with I-flanges (2-bolt "footballs" with a glass-filled teflon o-ring). You can weld a "football" to a nipple connected to your process if you're that concerned about leakage at an NPT. Pressure gauges are available with integral Swagelok compression fitting connections too.

 

[EdDanzer]

The yield strength of a sealing material that fills a gap will determine when it will migrate out of the gap.

Most anaerobic plastics are less than 5000 psi yield many are under 3000 psi yield. All anaerobic plastics seem to be brittle when cured making the material fail when the fittings swell from pressure.

Ed Danzer

http://www.danzcoinc.com

http://www.dehyds.com

 

[Compositepro]

The shear stress on the Teflon filling the long spiral gap in the thread will be much less than the fluid pressure.

 

[rconner]

When a book named “Methods of Joining Pipe” by J.E. York was published sixty years ago in 1949 (in which there is a pretty detailed 22 page chapter of technical issues related to various “Screwed Joints”), I believe such methods were already well-known and long-standing methods of piping connection. This work explains that while the primary responsibility for the tightness of pipe joints is commonly assigned to the Contractor, it is desirable that the Engineer as well as the Contractor understand the “fundamental requirements” for making screwed joints which are “tight”.
I noticed much of this was prefaced with the advice, “Good clean-cut, uniform threads make tight strong joints, eliminate leaks, extend the life of the installation and result in complete satisfaction with the fabrication and the piping material.
Carelessly cut, wavy, shaved or rough threads result in a weakened piping system, localization of corrosion, high maintenance cost, and condemnation of the workman and the piping material.” [At least I guess condemnation may not be as bad as damnation!!]
Even way back then there were many different types of materials threaded and thread assembled, and this reference made clear that how all this is suitably accomplished from thread cutting to assembly etc. varied with materials. How to accomplish all this right even with standardized threads and all sizes etc. is thus not necessarily as simple as it sounds.
At the end of the passage there was discussion of many thread sealants du jour that were also used at the time for various services, concluding with a mention of “wicking” (different than “wiping”!). I think wicking was basically as it sounds something like wrapping some sort of cord that probably looked not unlike a candle wick, and that may or may not have been soaked in some kind of dope, in the thread valleys and in essence that worked sort of like a gasket or packing when tightened up (while the author mentioned this was used in some parts of the country, I don’t think he in general was necessarily too keen on this practice).

 

[moltenmetal]

rconner: the joints in my 70-yr old home thermosiphon hot water radiator heating system were done with one of the standard "dopes" of the day: plumber's putty, which is basically linseed oil mixed with ground limestone, chalk or plaster of paris. Sets hard as a rock. It took two of us, each with a 36" pipe wrench in one hand and a propane torch in the other, to break loose a 2" connection so we could make a change to the system. The threads were in perfect condition.

I would imagine that "oakum" was also used- rope fibres in heavy grease, asphalt or pine tar- but none of it in my old system.

Nicely machined threads make the sealing job easier, but don't eliminate the need for the thread sealant.

EdDanzer: anaerobic pipethread sealants aren't solids- they're thick liquid pastes. The good ones NEVER solidify- that's how they can manage to provide sustained gap-sealing despite dimensional variations due to thermal cycling etc.

 

[EdDanzer]

Moltennetal,
I don’t know how much warranty work you have wrote check for from pipe threads leaking but I personally have spent several thousand dollars of which could have been mine if the stupid thread sealants worked.

This quote is about Loctite “565 Thread Sealant is a single component, high viscosity, low strength acrylic thread sealing adhesive; it cures when confined in the absence of air between close fitting metal parts and resists shock and vibration.” Acrylics tend to be a brittle material when cured.

Compositepro,

If you don’t think materials will flow when the yield strength is exceeded should spend some time studying forging, and pressurized molding. The size of the gap will only slow the process not stop it.

Ed Danzer

http://www.danzcoinc.com

http://www.dehyds.com

 

[hydtools]

I was looking through Loctite products and see they have a cord product for sealing pipe threads. Has anyone here used it?

http://www.henkelna.com/us/content_data/LT3631_Loctite_55_Pipe_Sealing_Cord_Sell_Sheet.pdf

Ted

 

[moltenmetal]

EdDanzer: take it from a chemmie that there's a world of difference between a "cured" anaerobic pipethread sealant paste and a chunk of Plexiglas, even though both of them contain acrylic polymers. The Loctite sealants we use are still pastes when cured- I have never seen them set up hard. As I mentioned, indeed this is one of their features.

As to the argument about yield strength in relation to thread sealant performance in an NPT joint, on this point I'm afraid you're just plain wrong. You can, and we do, use TFE tape in NPT joints for internal pressures well beyond the yield strength of solid teflon (internal pressures of 3,000 psi and even higher). I'll let other mechanical engineers explain the physics to you, since as a chemmie I should have no credibility on the subject!

We use a Loctite sealant as top-dressing to teflon tape as described above and very, very seldom have leaks either in initial testing or as warranty claims. We have success even with crap-tastic 150# threaded stainless or MI fittings from China, albeit those fittings being used only to their limit which is rather low pressure.

With suitable fittings we do use this system on high pressure gas and liquid services with good success, though we don't do industrial hydraulics. We of course minimize the use of threads in high pressure services, period, but are not afraid to use them when appropriate. We're not in a mass-market business- we build custom equipment, so our warranty claims record is probably of little use to you for comparison.

 

[Cervantis]

This is an amazing discussion that has developed over several months. Only a fraction of science has yet been provided on the original question relating to emperical data available. I wonder if there would be grant opportunities available to provide something like this for industrial use? What I have gathered is that torque is important if it is the correct amount for the application and part used, which can have variables to skew things a bit. And that tape, can have successes if the right tape and technique is used which can vary from application and part also. I also understand that combinations of tape and sealant have been effective for some but not all. It seems that if there is a common thread (no pun intended) here, it is, that skill, product knowledge, product quality, and effective communication and training are the keys to making a NPT seal work.

 

[hydtools]

I just had to estimate how much force it would take to shear 5000 psi yield thread sealant. It would have to fail in shear, shear at the thread surface. The worst spiral gap between thread crest and root for 3/8npt would be if the crest and root did not contact when the joint is made up. That cross-section area is about 1.767*10^-5 in^2. The force on that area exposed to fluid pressure of 3000psi is .053lbf. The force required to shear a 1 inch length of sealant is about 74.8lbf. That tells me that if the sealant is properly applied and cured the joint will not leak. In practice a tight pipe thread joint will not have that much gap between thread crest and root. Theoretically the gap will close about .005 in. when the joint is tightened three turns after hand tight. That will close the original gap of .003 in. to an interference fit. In practice that does not happen because we have leaks without sealants.

Short form of the previous is that thread clearances are very small compared to the length of engagment. Forces pushing sealant out are small compared to forces required to push the sealant out.

Use one to two full turns of a bead of sealant filling crest to root of the male thread placed one or two threads back from the end. That was our production procedure. Even I could get leak-free joints. We used Loctite PST.

Ted

 

[InfinityTech]

I agree with Cervantis that only very little science concerning the use of sealants. At some point we will do some science and testing with various products and methods and I will post the results, but that could be years off.

hydtool: Not only does 15 wraps seem like a waste of tape, but a waste of time and patience also. So far the 15 wrap prototype which has been in service for a month does not leak. It has not gone through any winter cold but has cycled from about 50 degrees to 95 degrees in the sun for two weeks before installation and two weeks after installation with 110# water pressure.

kcj: Yes! we really want to use fewer parts and our design has already changed again to reduce the number of fittings. We are also looking into a manifold system that we can use. We may have to manufacture our own, but at this point it seems like the only part of the construction that could be consolidated. I will research "cartridge valves and manifold blocks". Thank you for the suggestion.

Compositepro: 15 wraps does seem to be a lot, and yet, during our prototype process we need to try different things. 10 wraps produced more leaks. We are dealing with 304 and 316 stainless which seems very different than regular steel pipe. When we were in the "Weld-On White Seal" pipe dope phase we tried adequately tight, really tight, immediate service, and 24 hour baked in the sun service. We had leaks every time. I think in this case we ruled out the Weld-On product for stainless.

moltenmetal: In the past I have overtightened standard steel pipe fittings so much that I have cracked them. Stainless is way different in practice. It does not seem to yield as much and reaches tight "all of a sudden" (comparatively). With the prototype, we have used and reused stainless fittings over and over, and sometimes I felt that they were tightened ridiculously tight. I have not noticed any galling of the fittings. The nipples started out with a really nice looking sharp and mirror smooth thread, and they still have that look.

EdDanzer: Warranty work! Ouch. Years ago I went through a challenging time where many of my pipe joints leaked. And I am a careful organized systems type of person. I went to the local hardware store and asked one of the guys there what I could do. He told me that all the "real plumbers" use Slic-Tite. I started using that and never had leaks again. At least in standard steel pipe. Not science, but a start. When It came time to work with the stainless, this product was no longer easily availabel. Thus, my journey to find the TRUTH!

Thanks to everyone in this quest for some reliable data. Like I said, when we do some actual science, I will post our results.

Mark