Silicon Tape versus Teflon Tape? Pro/Cons , 5

[JezFullham]

Hey Guys,

has anyone of you worked with silicon sealing tape instead of teflon tape for stainless steel NPT/ISO7-1 fittings?

Any suggestions what to look for?

Friend of mine, told me that using silicon sealing tape will be leak free as it is naturally water repellent, and when detaching fittings again it is smoother and less dirty.

Happy for any feedback.

 

[moltenmetal]

OK: first of all, you can't make tape out of SILICON. You mean "silicone", i.e. polysilanes.

I've never seen silicone tape used for thread sealing, ever.

I wouldn't trust it. Silicones have terrible corrosion resistance and poor lubricating properties relative to teflon.

The best thread sealant system, proven in service in heavy chemical and industrial applications for thirty years and derived from a Dow specification even older than that, is 1.3-1.5 s.g. teflon tape top-dressed with anaerobic pipethread sealant (Henkel Loktite make some excellent ones). The pipethread sealant is not "liquid teflon"- it is a polymer which cures in the absence of oxygen, filled with inerts such as titanium dioxide and a little teflon powder. Yes, you need BOTH the tape AND the paste. One provides galling resistance during tightening, and the other provides "memory" against vibration and thermal cycling.

 

[btrueblood]

Silicone is also horribly porous to gasses, including steam. I.e. it is a lousy sealant for any pressurized system. Molten's formula is the one I prefer, and has proven itself across a very wide spectrum of fluids.

 

[Artisi]

Teflon tape = plumbers friend.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[JezFullham]

Thank you for the great and fast replies.

I am a bit confused as I know some of the gaskets for flanges are silicone "polysilanes" therefore, I assumed it should be fine for threads. Just saw that Grainer offers these online. Further, as silicone is very align friendly as in, it would be easier to slide in between the threads giving it a better fit.

I never tried Henkel Loktite. I am currently working on a project that needs threaded fittings to be detached/unthreaded on regular bases (Needs to be thread cast fittings STC).

 

[IRstuff]

ASME B16.21, to which those gaskets are supposedly compliant to, states that the user is responsible for determining the suitability of the material with the fluid and the application.

TTFN (ta ta for now)
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[moltenmetal]

I've never seen silicone used as a B16.5 pipe flange gasket material either.

If you want threads for frequent assembly/disassembly, you should be looking at the European straight thread with o-ring designs, which I think are based around BSPP (British Standard Pipe Parallel thread) and are usually designated with a G, i.e. G 1/4 is a straight thread with o-ring replacement for 1/4 NPT. Tapered threads need a sealant to seal reliably, and when you're making and breaking those threads constantly you need something other than a pipe sealant to make that joint leak-tight without the mess and bother of a sealant.

You can use your favorite silicone for the o-ring if you're really in love with it, but BUNA nitrile or EPDM are also good choices. Silicone is used for high temperatures and in food applications but has limited oil/solvent and chemical resistance.

 

[JezFullham]

Moltenmetal,

Thank you for the info! really helped me out.

 

[1gibson]

Frequent disassembly makes me think of tube with swage type fittings, re-read the spec with that in mind, see if it is allowed?

 

[bimr]

JezFullham

Suggest you watch a video on youtube:

https://www.youtube.com/watch?v=-yGRgv2flfs

Note that there are some PTFE Tape products that is especially made for stainless pipe. Nickel-impregnated tape prevents galling with stainless-to-stainless.


Stainless Steel Grade PTFE Tape

https://www.tfcoinc.com/products/threadseal.html

 

[Tuckabag]

I would take tape over sealants every day in my line of work (instrumentation up to 1"NPT).
Tape gets a bum rap mostly due to people either applying it incorrectly or using crap white plumbers tape.
Unasco Yellow Gas Tape would be my weapon of choice, and don't wrap the tape over the first thread else it will get chopped and can wind up downstream somewhere you don't want it.

Sealants generally require time to cure before pressure can be applied so this makes them a pain to use in the field for service/repair work. I've used several that claim to need no cure time and had no end of issue getting them to seal.
I have used one Loctite brand sealant before to great effect, cant remember the number off the top of my head, but it sealed rock hard and also helped prevent rotation of the fittings as it locked them into position.

If you need to disconnect on a regular basis, ferrule compression fittings are ok but will eventually become difficult to use (especially when over-tightened by zealous fitters), VCR or VCO fittings are ideal for regular disconnect work along with the G series o-ring sealed BSPP fittings that Moltenmetal mentioned above.

Just my 2c.

 

[TugboatEng]

Loctite 567 and 577 both contain Teflon to prevent galling during assembly and cure in minutes. I find anaerobic sealants superior in all but high temperature applications. I don't deal with harsh chemicals.

 

[JezFullham]

Great work, thank you for all the input. To sum up: we will stay with teflon tape, hopefully USA made (Not saying that European isnt good).

Short answer to 1gibson, I proposed swage type fittings, but my superior has already made the entire purchase (clearly for him all fittings are the same). It is for a cooling prototype of fusion energy, so cant really change the specs. I am a small potato. and for all the people active in the Oil/energy industry, don't worry fusion will not be taking an large marke share any time soon. And if it does, a lot professionals are needed.

It is just interesting to think how this basic technology from the 1940s is still the optimal way of sealing.

Anyways, hope you guys all have a relaxing weekend, stay dry.

 

[moltenmetal]

There is no need whatsoever for fillers in the tape. The benefit of any such fillers is highly questionable. Adding nickel powder to teflon tape is little more than a sales gimmick.

Anaerobic pipethread sealants do need a little time to cure, but in practice that is not very much time at all unless you're using ONLY the anaerobic thread sealant. We do not suggest doing that, except on small fittings used between the last filter and the inlet to a sensitive instrument air user which might be damaged by loose threads of teflon tape. The highest reliability joint long term, based on doing this for a couple decades, uses tape top-dressed with paste. In a repair situation the joint will likely be 100% initially sealed by the tape alone- the paste really only comes into play after a few thermal cycles.

You wouldn't likely know the difference between cured and uncured material until after disassembly. Remember, they ONLY cure where the material is NOT exposed to oxygen, hence the name "anaerobic". They do not set up hard like old-fashioned plumbers putty- if they do, they don't work- and they are NOT intended to lock parts to prevent disassembly. The forces resulting from tighening the tapered joint are what holds the joint together and keeps it from spontaneously disassembling.

 

[Tuckabag]

There shouldn't be any loose ends of teflon tape IF it is applied correctly.

I can only speak from my experiences with anaerobic sealants needing time to cure. We've had multiple issues with 1/4"NPT fittings leaking when we apply high pressure (around 10MPa+) before the sealant has been allowed to cure for at least a few hours. Left overnight and we rarely have problems.

The "locking" sealant that we used was a one off job and was spec'd by the client. From memory it was some form of hydraulic thread sealant. It worked a treat, especially on fittings that were connected to flex hoses and subject to significant movement during operation.