Use of asme F&D heads in moderate pressure service

[praxis]

I am designing a pressure vessel (scrubber) with a design pressure of 100 psi @ 500F. The vessel is 42" id with bottom and top heads. The top head has two fairly large openings (10" and 18")in order for it to connect the vessel to two process cylinders (with packing material). The 10" and 18" connections are bolted to the top sections. Initially I tried to use an elliptical head, but it is almost impossible to have such big nozzles on a small (42") elliptical head (because of reinforcement interference and as it is not a good practice to have nozzles in the knuckle region).

Then I decided to use an F&D head. The asme F&D head with almost the same thickness as the elliptical head worked with out any problem with no reinforcement interference or overstressing of the area around the openings and the pads. I have taken the external loads coming off of the bolted connections on top of the 18" and 10" connections in to account and the area around the openings did not show any signs of buckling and the stresses are with in the allowable limit.

My question is: is it ok for me to use a asme F&D head in lieu of an elliptical head because I have seen many engineers opposing the use of F&D heads in moderate to high pressure services (and also when there connections with external loads). Also I am trying to convince my client to use a 48" dia vessel instead of 42" dia (if there is no process restriction) so that I can use an elliptical head. I calculated the stresses around the openings for both 48" elliptical and asme F&D heads using WRC 107. For the same thickness, the stresses (at the edge of the pad and around the nozzle neck) are very close to each other for the elliptical and F&D heads. I know the geometrical differences between the elliptical and asme F&D heads, but what I could not understand is why are the stresses developed very close to each other and does it mean that I can use F&D heads in moderate to high pressure service (I know the answer is no, but I would like to know a logical reason).

Appreciate your response

 

[jamesl]

F&D heads have smaller knuckle radius. The main concern is buckling in knuckle area. When you run WRC 107, you only see stresses in crown area. You are not going to see stresses in knuckles.

If you put one 10" and one 18" nozzle on a 42" dia head, and if you use repads, the repads are going to be in knuckle area.

 

[MJCronin]

Praxis:

You stated:

"...I have seen many engineers opposing the use of F&D heads in moderate to high pressure services"

It is my experience that many companies do not want to use F&D heads in 75-100 psig service because of extra cost. At about 75 psig, with most common diameters and materials, the ASME equation for F&D heads requires greater thickneses than the elliptical head equivalent.

This is just a matter of company policy, in my understanding, there is no other restriction.

-MJC

 

[82253]

Don't forget about UG-42, your limits will overlap with these nozzles. It would be easier for your fitters if the pad is fitting around knuckle of 2:1 because of larger knuckle radius. Installing nozzles in or near knuckle is acceptable, but don't forget to set your limits of reinf.

 

[praxis]

Jamesl:

When I do not have any nozzles in the head knuckle region and I restrict the linear limits of reinforcement (of the nozzles) so that they do not go beyond the crown radius, why would there be buckling in the knuckle area. Also, I don't see any problem in a part of the repad going in to the knuckle region as I did not burn a hole in the knuckle, just welding a part of the repad. Appreciate your comments

Thanks

 

[jamesl]

There are two problems with the design.

1.Nozzles in head knuckle areas, even if the nozzle pipe is not physically on knuckle. As long as the edge of repad is not certain distance away from the knuckle, the nozzle is considered in knuckle area. The general rule is that the nozzle w/ repad is completely inside 80% head diameter (see EN 13445). For small nozzles w/ minimum piping load, I won’t worry about it. If the nozzle is large and/or nozzle loads are not negligible, a FEA will be required to check out the stresses. WRC 107 is not valid in this situation.
2.F&D heads w/ nozzles loads. These nozzle loads will eventually be taken by the head. Since the nozzles are very close to knuckle, nozzle loads will influence the stresses inside knuckle.

I would try a thicker 2:1 head w/o using repads.

 

[MJCronin]

jamesl...

While there may be rules about reinforcement plate limits in the Euro codes, in the USofA (and other places), things have been done a bit differently:

Check this out:

http://www.dishedheads.com.au/dishedheads/spitfire.jsp?c_id=10003

It is common practice in chemical reactor design to locate a manway directly on the knuckle or the top head as a reinforced agitator nozzle must be placed on the vessel centerline.

http://www.ecrecon.com/inventory.php?cat=18&page=3

-MJC

 

[vesselfab]

That's the Furphy shop in Australia.

Nice folks.

They toured our shop when in the USA last year.

 

[jamesl]

MJC,

I work in the USA. We typically do not locate large nozzles in the knuckle areas. However practices varies from industries. I know chemical plants do a lot of things that refineries won't do.