Consequences of under tightening of NPT threads 2

[Soumajit83]

Hi,

I have an air pressure system in a sterilizer that needs to withsatnd upto 100psi of air pressure. The connections include a NPT thread of 1/4inch and 1/8inch (at the inlet and outlet ends). The first concern is I am not being able to find the exact minimum thread engagement possible in a worst case scenario for the male threads. From some ANSI data I found that 5.2 thread engagement is required for the fitting. However the female counterpart seems to have 5 threads at max. So what could be the consequences if there are less than 5 threads engaging?
Also is the calculation of 5.2 threads correct?
I need to carry out a risk analysis based on my findings. Please provide some reply ASAP.
Thanks in advance.

 

[racookpe1978]

Get 5x 1/4 inch fittings from a local hardware store - they will be less than $1.50 each. Put each pair into a vise and Tighten them! Mark the thread depth (the point were the tapered threads "bite" into each other to form the seal. Unscrew the fittings and count the threads.

Each joint will be a little bit different. Check your ANSI "requirement" again: If the joint is tight, you MUST stop tightening the fittings just to get to an arbitrary thread engagement count - going too far will damage the fittings and they will leak. Not going tight -even though the thread count says "you have enough thread engagement" WILL cause leaks.

 

[moltenmetal]

Don't misunderstand Racookpe's post though- NPT threads REQUIRE a thread sealant to seal, even though the male and female parts are designed to interfere somewhat as the joint is tightened.

The engagement you will attain in practice will depend on what type of thread sealant you use and how thickly you apply it, and also on the tolerances of the threading operation on each part, i.e. how deeply the FNPT is tapped.

For 100 psig at 1/4 and 1/8" you will barely need any threads engaged to achieve sufficient mechanical strength. Three full threads of engagement is plenty and even that may not be achievable on 1/8" NPT.

 

[pennpiper]

Q. Consequences of under tightening of NPT threads?
A. The joint will leak!

Q. Does the number of threads matter?
A. Yes! but the actual number of threads is not as important as a) using a proper joint compound and b) getting the joint tight (in the threads available).

prognosis: Lead or Lag

 

[Soumajit83]

Thanks for the views. However I have one more concern to address in the same case. I understand that the practical design needs will suffice even if total thread engagement is not ensured the design will not fail (it has been running good!). However I need to catch the associated risks in my risk plan and provide proper justification. The leakage can lead to pressure drops cancelling the operation cycle or greater energy consumption to keep the cycle running.
But could there be any other risk that is not being evident to me? Please suggest and once again its HIGH PRIORITY. Need to place the outline by Monday morning.
Thanks

 

[1gibson]

How close do you want to look at this?

If it leaks, the compressor supplying the air will work harder and there is an energy cost. If the air that leaks is hot (also there is more waste heat from the compressor) maybe the thermostat keeps the air conditioner on 0.1% longer to maintain the temperature in the room, and there is an energy cost associated with that. If the air conditioner is on 0.1% longer, then the air filter life will be reduced and should be changed maybe every 181 days instead of every 182 days. So after about 180 years you will have to replace the air filter one extra time than if there wasn't a leak in your 1/4" NPT fitting.

So on second though, maybe verifying the minimum thread engagement is of critical importance.

 

[EdStainless]

If you really can't tolerate leaks then never use pipe threads. They are designed to leak. The threads are truncated and cannot form a seal. You either rely on filler (tape or putty) or scale from the water to plug them.
Hydraulic fittings may seem like a pain but they do seal.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[MJCronin]

I agree with EdStainless... Don't used threaded fittings if you cannot tolerate leaks.

How about good-old socket welded fittings ? There is no "leak-risk analysis" required when there are welded fittings !

Compressed air at 100 psi is not very dangerous....

 

[Duwe6]

"How about good-old socket welded fittings ? There is no "leak-risk analysis" required when there are welded fittings !"

And use Swagelok [or similar] fittings for instrument tubing. When installed 'per the mfr's directions' is is also leak-tight - forever. The tubing ruptures prior to the fittings leaking.