Minimum number of threads required 1

[som1973]

Hi,
I need to calculate Minimum number of threads required for two pipes to engage, The Pipes are having NPSM (American Standard Straight Mechanical Pipe Thread)

Pipe size = 4"-8TPI (NPSM Thread)

The pressure in the pipe is around 75 PSI.

I have considered two threads for engagement, Is this sufficient to withstand 75 psi pressure ?

Thanks in advance

 

[hydromech]

Being from England, I don't know much about NPSM. However based on my experience of imperial nominal bore tube and BSP threads, I would tend to say no, 2 threads is not enough.

At 8TPI, 2 threads would give 1/4" engagement, on 4" tube and at 75 PSI, that is hardly likely to stay intact.

 

[DRWeig]

Hi som1973,

Please read the forum policies (there is a link below my signature), and pay particular attention to policy number 4, which does not permit posting the same question in different forums.

I recognize that this is your first day of membership, but you have already made cross-posts with two different topics. Putting the same post in multiple forums creates confusion and costs experienced members valuable time.

Please red-flag one of your two identical posts in each case (I suggest this one for the pipe thread engagement issue).

Good on ya,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies

 

[DRWeig]

Please excuse my error in the length of your membership -- I misread your start date. However, all other comments apply.

Good on ya,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies

 

[Spoonful]

You can find answers to your question here.

http://www.roymech.co.uk/Useful_Tables/Screws/Thread_Calcs.html

But, 2 turns doesn't seems reasonable to me, consider first turn of any thread can not not be full engaged.

 

[badservo]

...(American Standard Straight Mechanical Pipe Thread)

...around 75 PSI

... two threads ...

This just sounds like a bad idea for several reasons

 

[rhg]

See ASME B1.20.1 PIPE THREADS

Regards

rhg

 

[steris]

Hi Som1963 -

I'm not sure what code you are designing to but for reference you can check out table PG-39 in Section I for guidelines on minimum thread engagement. For 4" NPT the minimum allowable by this code is 7 threads into a plate of 7/8" thick. Also, as Spoonful stated - don't count the 1st thread because it is usually not properly formed.

-Steris

 

[MJCronin]

Where is this installation ?

Most pipe threaded joints in the USA follow the ASME B31.1/B31.3 piping codes......which call for NPT threaded joints.

With NPT joints, seven threads (as stated above) makes a proper joint.

My opinion only..

 

[steris]

I agree that 7 threads is a good rule of thumb. However, the required min thread engagement increases (and decreases) dramatically with pipe size & pitch. Just looking at Table PG-39 the minimum required thread engagement varies from 4 threads to 13 threads depending on pressure and pipe size.