Male pipe thread preparation - to tape, or not to tape (w/Krytox)? 2

[edleit]

I've now come across several instances where the combination of PTFE tape and Krytox anti-seize compound have been specified in the pre-assembly preparation of male pipe threads.

A long, long time ago, I was taught as a young engineer that thread prep should be restricted to a single medium (pick one that's appropriate for the application). The rationale being that coating an already taped fitting/pipe end in this way would create a hydraulic efffect that might overstress the connection, or cause the joint to loosen over time.

Are there situations in which this approach would be considered appropriate for a fluid connection? I have seen postings on other fora regarding the combination used to secure bicycle hardware (mainly on high performance bikes with Ti components). This just seems odd to me, since each medium (i.e., tape or lube) is designed with a particular function in mind. For a stainless steel connection I can understand using Krytox to avoid galling, but usually on straight threads, not tapered. Likewise, a single layer wrap of PTFE tape provides a means to fill minute voids between the mating threads, whether the material is stainless, iron, or CPVC.

Does anyone else specify the combination on a regular basis, and if so, under what conditions is it deemed appropriate?

 

[MikeHalloran]

ISTR that Krytox is damn expensive, and may not be compatible with all metals, so I wouldn't be inclined to specify using it just anywhere, and especially not in addition to PTFE tape.



Mike Halloran
Pembroke Pines, FL, USA

 

[rodofgod]

PTFE tape and Krytox anti-seize compound seems a bit strange to me, I'm off the same opinion as edliet, one make for all! The use of PTFE tape as well as Krytox would really make me consider jointing procedures1

 

[iainuts]

Right or wrong, our company also uses both tape and thread lubricant such as Krytox or Halocarbon. I thought it was a bit odd too when I started here 18 years ago but I haven't heard of any concerns or down sides/incidents blamed on the practice. My understanding is that one reason both are used is that most of the piping is stainless steel so there's the obvious galling concern.

 

[moltenmetal]

Krytox is a grease and nothing more. It is not a thread sealant, though you might find it as a "sealant" in the same way that vacuum grease is a "sealant"- like any liquid, it will tend to fill scratches and minor imperfections in metal to metal joints.

PTFE tape is a true thread sealant.

You use lubricants on straight threads which seal with a gasket ONLY, and you use thread sealants on NPT taper threads.

Yes, you can and indeed SHOULD use two media for reliable tapered pipe thread sealing, and that bit about the hydraulic effect is rubbish. Tape is used as large gap sealant and high pressure anti-galling lubricant, while an anaerobic pipethread sealant is used as a top-dressing. The pipethread sealant, which is NOT a grease but should rather be thought of as a liquid which becomes a very thick gel wherever oxygen is depleted, is used to compensate for gaps that may form in the joint due to vibration and thermal cycling.

If you're using Krytox, you're worried about oxygen compatibility. Many pipethread sealants are NOT oxygen safe, and the ones that are oxygen compatible are only compatible within limits of temperature and pressure. Choose with care.

 

[edleit]

@Molten, the bit about the hydraulic effect was in reference to a situation where stainless (or other metallic) fittings are being used in conjunction with a plastic tank boss. The assumption being that the grease would tend to become trapped in a smaller and smaller volume as the tapered threads engaged, with the possibility of causing a failure of the boss material. Perhaps a stretch, but that was the thought process.

With regard to the "pipethread sealant" - to what type of material are you referring? The oxygen depletion effect is one that I am not familiar with, though its use to counter vibration/thermal effects is interesting. As for our use of Krytox, we are concerned with potential contamination of working fluids within a fuel cell power plant; the inert characteristics of Krytox make it a good candidate to avoid this. I have also used it in other instances where compatibility of materials in an oxygen-rich environment (i.e., electrolysis units) is a very real concern, so I appreciate your footnote regarding care and feeding of the beast.

Ed Leitkowski
CT, USA

 

[moltenmetal]

The reason plastic female NPT fittings fail during insertion of a metal male NPT part isn't hydraulic, it's mechanical. FNPT bosses on good quality PVC/CPVC ball valves and rotameters etc. have a stainless steel ring around the female NPT boss to provide extra reinforcement against cracking which is caused by excessive tightening, which is virtually a certainty when you taper thread metal parts into plastic parts. With plastic on plastic NPT joints you tend to see failure of the male NPT at the root of the thread rather than splitting of the female NPT when the joint is tightened, particularly with PVC but also with PP and other notch-sensitive materials.

The problem can be solved by the use of, and reliance on, a good quality pipethread sealant which is compatible with both the process AND the plastic involved. By "relying on" here, I mean tightening the joint within rather than beyond the limits of the plastic parts, and then using the sealant to do the sealing rather than trying to rely on distortion of the plastic parts to form the seal. The anaerobic pipethread sealants are generally not an option in this case because they contain acrylate resins and other components which can embrittle PVC/CPVC and acrylic- they should say that right on the tube etc. but some fail to do so. There are other materials such as Loctite's "No More Leaks" compound which are not true anaerobic sealants but which do give adequate service on plastic joints without the risk of embrittlement.

I would say that a grease like Krytox is going to be useless to you in this case. As to whether or not to bother with the tape in addition to the sealant, that's a matter for experimentation. The sealant alone should be sufficient but as I've said, we find in metal NPT joints that top-dressing of the tape with the paste is a more reliable sealant system, especially beyond 1" NPT.

 

[tbuelna]

It's a bit ironic that tapered pipe threads are sometimes referred to as "dry seal" threads. Tapered pipe threads leak due to tiny gaps between the tips of the male threads and the roots of the female threads. The PTFE tape works by being forced into this tiny space when the threaded connection is tightened.

PTFE tape and Krytox grease are both primarily composed of the same material- teflon. One problem with teflon is that it has no mechanical strength, and it cold flows when exposed to any sustained pressure. Given sufficient time, any teflon-based sealant will eventually cold-flow enough to produce leakage.

The anaerobic sealant compounds that moltenmetal suggests have some inherent shear strength. So in the long term, they would be a better choice than any teflon sealant.