thread connection with ptfe sealing 3

[payaam2000]

Dear All

Would you please advise me regarding below item:

suppose we have a 1/2" pipe with 100bar internal pressure (hydrocarbon gas, say Methane).
The pipe has equipped with a 1/2" thread type valve.

the temperature is 100 C.

Can we make a sealed connection with thread and PTFE sealing?

thanks in advance

 

[hacksaw]

pipe threads are not considered reliable in such services you need to rethink your piping configuration

 

[tbuelna]

As hacksaw noted, tapered pipe threads are not really suitable for such pressures. Tapered pipe threads require some sort of tape/sealant compound to make a leak-free connection. This because the threads only make contact along their flanks and there is a leak path at the clearance between the mating thread roots and tips. PTFE tapes and compounds will fill this clearance gap at the thread tips and roots at installation, but since PTFE has very little mechanical strength it will eventually be displaced when exposed to any significant pressure differential, resulting in leakage at the thread joint.

100degC and 100bar is well within the capabilities of an o-ring seal, so you might consider that.

 

[moltenmetal]

100 bar is only 1500 psig. What you're describing is the joint which is successfully used between the cylinder and root valve and between the CGA connector and the regulator on every single DOT gas cylinder. Your service, being at 100 C, is just a little warmer. There are people here who are extremely and in my view irrationally phobic of NPT joints under almost all conditions.

At 1/2" NPT, 1500 psig and 100 C I wouldn't shy away from pipe threads merely because of a flammability risk. If there was also a toxicity risk, or where the joints are inaccessible, I'd think again. There are companies out there who will spend thousands of dollars to eliminate a single NPT threaded joint in a flammable service though, so your own internal company standards may differ.

Use a tape with a specific gravity of > 1.0 or even better a tape with a specific gravity of 1.3 or greater, and over-coat it with a good quality anaerobic pipethread sealant such as Loktite 567 or 592. That system can be used with good success to the service temperature limit of the anaerobic sealant (175 C). It is the system used in the threaded pipe specs of a major chemicals producer, and we've used it with good success on hundreds of projects.

 

[LittleInch]

As always, it really depends on what and where it is being used. Can you make a sealed connection with threaded connections and PTFE - Yes.

In the right place and if the item is installed once and then left alone, it's in use in thousands of places.

In the wrong place, poorly installed and / or the connection is made and broken several times, it's not the best connection to use. There have been fatal incidents where use of too much PTFE to seal a leaking joint have resulted in joint failure and with a gas especially the resultant velocity of the item is very high. Issues remain over cleanliness, dirt getting locked into the grooves and thread stripping from too much torque or thread galling.

1/2" at 100 bar is a bit on the small size. A number of oil companies don't allow anything less than 1" unless on an instrument system d/s of other isolating valves as they are just too vulnerable to impact damage.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[XL83NL]

good point moltenmetal, worth a

 

[Compositepro]

I have to agree with molten. A properly assembled npt connection is, in most cases, as good as any other. It is more robust for dirty field installation than o-rings. Probably the main problem is that a leak due to improper installation requires dissassembly of many joints to fix the one, and then reassembled joints are more likely to leak than virgin joints.

 

[BillPSU]

The correct pipe thread designator is NPTF or ANSI B1.20.3-1976.
These are dryseal pipe threads which do not require a sealant. The statement that an NPT thread is not suitable for such service is correct however the NPTF thread is suitable for such service.

Bill

 

[hacksaw]

hp gas bottles at 100+ bar rely on metal to metal seals, threaded but not pipe threaded. The latter is a totally different beast.

for field run, small bore piping you may want to get a piping engineer involved to sort out the recommended fittings suited to your application.

 

[hydtools]

In my experience with 2000 to 5000 psi hydraulic systems, even NPTF joints are assembled with PTFE tape and or thread adhesive for leak-tight joints. However, preferred joints are had with straight thread o-ring fittings and JIC flared tube fittings.

Ted

 

[tbuelna]

While I don't agree with moltenmetal's comment that those who prefer not to rely on pipe threads for sealing pressurized fluid connections exhibit an "irrational phobia", I would agree that pipe thread connections can provide acceptable service for certain applications.

I work in the aerospace business, and tapered pipe thread connections are never used on flight vehicle fluid systems. The reason for this is that the variability of the tapered threads make it very difficult to produce a consistent, predictable result at each installation. With tapered pipe threads, if the connection leaks then you must continue to tighten it until the leakage stops. This can result in excessive and unpredictable stress levels in the threads. However, for other less critical applications this may not present an issue.

 

[moltenmetal]

hacksaw is correct that DOT cylinder root valves are frequently NGT rather than NPT threaded. The threads are very similar (same pitch, same taper) but are not identical- they are a dryseal type intended to be assembled without sealant or lubricant, though sometimes you do see evidence at the joint that a sealant has been used in the assembly. The valves are frequently brass with the cylinders being steel, and that's no accident considering that the male threads of the valve must deform to make the joint seal.

The connection between the CGA connector and the regulator is, however, almost always NPT rather than NGT or NPTF dryseal. The difference? It's after the root valve, so if it leaks, you have access to isolate and repair the leak.

There is no general prohibition against the use of NPT threads in flammable services in piping codes such as B31.3, though there ARE such prohibitions in some company specifications and may be in other codes and standards. At 1/2" NPT, 1500 psig and only 100 C, unless the gas is toxic as well as flammable, an NPT joint should not be rejected out of hand as unacceptable unless there IS a code or standard saying it should be- it may or may not be, depending on the rest of the service conditions: access and isolation for maintenance, hazards of a leak etc.

When I see people welding Swagelok fittings into 1/4" and 1/2" NPT regulators and control valves etc. for conditions like this, it drives me crazy- it's unnecessary, and if done wrong it carries the risk of a failure far worse than a sealant leak.