Standard F&D Head

[sme75]

Are standard F&D heads typically used for a pressure vessel of up to 25 psi? If so, what formulas should be used to calculate the required thickness? Looking at the ASME formulas and the small knuckle radius typically found on the standard heads, seems to indicate that the M factor will be outside the allowable ranges. If this is the case how is the required thickness of these heads determined with such a small knuckle?

 

[zerosum]

A big caution on excessively tight knuckle radii for even low pressure vessels. The fixing can be very expensive.

 

[eliebl]

I believe that there are rules in one of the appendices which deal with thin heads. They may be able to provide a bit of caution. The rules were added a few years ago (2-4).

EJL

 

[PVinspector]

If the knuckle radius does not satifsy the requirements for a torispherical head, you cannot use the formulas for a torispherical head. If you then have a shape or form the Code does not have rules for, go to U-2(g).

 

[MJCronin]

Refer to ASME Boiler and Pressure vessel code Section VIII-1 UG-32(d)

It is important to distingush between two major categories of torospherical heads:

1) ASME heads (commonly called ASME torospherica or ASME F&D heads, and..

2) All others

ASME vessel heads should have a dish radius no greater than its diameter. The knuckle radius should be no less than 6% of the diameter or three times the metal thickness, whichever is greater.

http://www.prentex.com/glossary.php

MJC

 

[JStephen]

I was under the impression that the "standard" F&D non-code heads were intended for non-pressure applications.

Seems like I've heard of different knuckle radiuses being used on these; they're not really "standard" in that sense. I think one manufacturer used twice the thickness, one used 2", etc.

 

[eliebl]

I believe the heads you speak of are used on cargo transport vessels which do not require a code stamp. I have come across many of them in the past and I do not know how they justified the use of those heads.

I calculated a couple to 1-4(f), actually the paper 1-4(f) is based upon, and found that they were much too far on the wrong side of the line for my liking. We even had a competitor's tank where the head buckled in the knuckle under a pressure test.

EJL

 

[zerosum]

I can confirm that an excessively tight knuckle radius can cause buckling of the straight side just beyond the head weld-seam. There were lots of low pressure (~50 psi)vessels made in the early fifties and some some are still in service. It is possible to reinforce the heads in the vicinity of the knuckle but it is expensive. We have 3 or 4 so modified.

 

[JStephen]

See the description here, for example:

http://www.cmforming.com/tankheads-stan-flanged-dished-non-code.htm