Thread calculation for closure for a pressure vessel

**JEB** · 03-20-2009, 06:27 PM

I am looking how to calculate the thread size for a threaded ring closure, as described in ASME UG34, type n.

I have a channel size of 1630mm, the needed thickness is 150mm
I was thinking to use a channel cover of diameter 1700mm.
This cover is fixated by a threaded ring.

What type of thread is used for this type of ring? ACME? Trapezoid? Metric? ...

How do I calculate the pitch and other thread parameters needed to produce this thread?

Thanks in advance!
Jeroen.

**AnandV** · 03-20-2009, 07:17 PM

Design pressure of the pressure vessel should decide the choice of type of thread and other thread parameters.

**JEB** · 03-23-2009, 11:28 AM

Originally Posted by AnandV

Design pressure of the pressure vessel should decide the choice of type of thread and other thread parameters.

Yes I know.

I have calculated everything, but I have troubles finding the right flat head.

Because of the high temperature/high pressure, things get very heavy and large. I have 10 of these vessels, all with different size, temperature and pressure.

I have one like this:
channel: inner diameter 1760mm - 1.25Cr0.5Mo - 290°Celcius - 215bar

channel thickness: 142mm (Asme div1) or 125mm (asme div2)
It is possible to find a flat head that is bolted to the channel: dia 2415mm - 485mm thick - bolted with 24x 5 1/2inch bolts.

Quite heavy head... 17.5 tons!

Now, if I calculate a head, like ASME UG 34, type (n), I need a head dia 1800mm - 380mm thick.

New weight: 4.2 tons...

Therefor, I would like to find information how to calculate the ring needed to lock this type of head. I never did this before. I don't know what type of thead to use, of how large it needs to be.
There is not much information in ASME on how to do this.

So, I know how to choose the head type. I just don't know how to calculate the bolts and threads needed for the ring to fixate the type of head I would like to use.

**AnandV** · 03-23-2009, 01:15 PM

Disclaimer:
I have never calculated anything using the methods u r refering. I am just using common sense.

First of all if design pressure is 215bar, and vessel of 1760mm dia, i doubt if you could have bolted ends!
500MW Boiler drums have roughly the same dia and design pressure, maximum thickness in this case is 185mm and we have welded dished ends.

You could start with searching for examples similar to yours. Are there any products on markets similar to what you are tyring to design etc.

**JEB** · 03-23-2009, 01:31 PM

Originally Posted by AnandV

You could start with searching for examples similar to yours. Are there any products on markets similar to what you are tyring to design etc.

I have searched, and I came across the "breech-lock" closure.
Looking to the pictures I found about this type of heads, I think this is exactly what I'm trying to accomplish.

It is a flat head, inserted in the channel, and locked with the threaded ring.
The thickness required at the thread in the channel is half the thickness required in the channel shell (only longitudinal stress acting).
Because the thread will hold the complete force created by the pressure on the head, the only thing to do is to secure the threaded ring with small bots. (And to take care the channel shell is strong enough of course)

It is possible to use a bolted flat head connection (bearly), but as you can see in my calculations, it is a 17,5 tons heavy part. I would like to avoid this and use the smaller version (=breech lock).

It is necessary to have this opening. Close the opening by welding would be easier, but is not allowed. It is necessary to have this opening.

**AnandV** · 03-23-2009, 09:31 PM

Try searching google patents for various types of closures
May be you will get more ideas. Good luck.

**rsmyegpet** · 03-28-2009, 03:27 AM

Dear member,
Thanks for discussing the problem here. ASME Section VIII uG 34 is right way to design the blind cover.
For threaded design use breach lock design.
Breach Lock design is heavier than dished end but flat. easy to remove and put back.

**AnandV** · 03-28-2009, 04:43 AM

You will also have to pay attention to gaskets to make it air tight. I guess you will have to use metallic gaskets. Specification of tolerances, torques for bolts etc will also have to be considered.
Please enlighten us about the whole process of design. Never done it myslef

**JEB** · 03-30-2009, 10:12 AM

Originally Posted by AnandV

You will also have to pay attention to gaskets to make it air tight. I guess you will have to use metallic gaskets. Specification of tolerances, torques for bolts etc will also have to be considered.
Please enlighten us about the whole process of design. Never done it myslef

Me neither...

This project is only in quotation phase, so maybe I'll never have to calculate this complete. But I'm interesting how to do it nevertheless. You never know when it becomes interesting.

At this moment, I calculated the flat head using ASME UG34. Nothing difficult about that.

I based my design on a picture from a powerpoint presentation:
[link Point to another website Only the registered members can access]
By combining this design (head + ring for gasket + screwing ring) with thread calculations (appendix B of ASME B1.1-2003) and some common sense, I have now some basic results. Good enough for this quotation, but not good enough yet for a real world application.

I'm still searching for information how to calculate trapezium, square, ASCE and/or Buttress threads. All I find is the information about standard bolts and nuts. But for this application, I would need the strength- and size- calculations for internal and external threads with very large diameters (1500mm and more) and a 'normal' pitch (less then 50mm, as small as possible)

If anybody has this information, and is willing to share, I'm very interested!