design of pipe end cap 1

[zigzags]

Hi,

I have been asked to verify the design of an end cap for a pipe which will be acted upon by an internal pressure in the pipe. The end cap is to be screwed on to the end of the pipe with an ACME thread and has a Weco union seal ring. (for the above imagine a lid screwed onto a bottle or similar)

I understand that fundamentally I need to calculate the force on the end cap and then check the screw thread for stripping (i.e. shearing off) of the threads and check the pipe/end cap cross section for yielding. But I am not sure exactly how to do this. i.e. the safety factor that needs to be taken off the yielded stress (would you simply use 0.4 x Fy say for shear) of the material once I calculate the above mentioned stresses. I am also wondering what other checks and rules of thumbs should be adhered to.

I am from a structural background (sadly there is not anybody else who can do this so I have been lumbered with the job) and do not know what codes or specifications would stipulate these things. I have had a search around on the internet and cannot find information on this.

Any help would be greatly appreciated.

Thanks!

 

[clay87]

I was interested so I looked around myself. I'm looking forward to Other responses.

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

http://www.scribd.com/doc/18245498/Acme-Thread-Data-Sheets

http://www.scribd.com/doc/18245498/Acme-Thread-Data-Sheets

http://satyakhresna.files.wordpress.com/2011/04/acme_thread_designstd.pdf

Not sure about codes, standards, and stipulations. Maybe if you had used normal pipe fittings....

 

[MJCronin]

Zigzag...

Don't re-invent the wheel....

In the same way that you would not develop your own special and unique beam sizes and channels from plate.... and then proceed to design a building with them, you may want to consider a "prequalified" threaded pipe cap.

For permanently sealed pipe ends, consider a welded cap, in accordance with your piping specification.

For an end connection that must be periodically opened, first consider a bolted "blinded flanged joint" (probably the cheapest) or a threaded end cap with NPT threads.

http://www.coastalflange.com/blindpipeflanges.html

What is your pipe diameter and what is your operating temperature and pressure ?

 

[zigzags]

Hi Clay87 and MJCronin.

Thank you for your posts, I appreciate you taking the time to help me out. I take your suggestion that standard fittings would make this design simpler, this is something that had crossed my mind, sadly this is not something I have control over.

The connection forms part of a modification to a piece of equipment. The equipment currently has a piece of pipe that forms a 90 degree bend. This bent pipe is to be replaced with a new pipe that will be straight. The connection on the end of the pipe is a 4TPI Acme thread on a 5.25” diameter pipe. The connection also has a bolted flange for when the permanent connection is made to the equipment, but for the purpose of a pressure test an end cap will be screwed on using just the ACME thread. The pressure test will be at 10,000 psi and at an ambient temperature.

 

[runningsucks]

handbook of bolts and bolted joints: Book on Google for anyone concerned with this issue in the future. Shear area is provided by the equation.

 

[EngAddict]

I would suggest ASME BPVC Section VIII Div3. This has a method for design of quick opening closures and includes a very good method for calculating thread loads for screwed closures. This code also covers your allowable stresses but you could probably design the rest to ASME VIII-1 instead.

 

[zigzags]

EngAddict, have just had a flick through ASME Section Viii Div 3 and that's exactly what i'm looking for, Thanks buddy