ASME B16.34 - 2017 - Wall thickness for Multipiece valve

[mOHMo]

Hi all,
I'm checking last rvision of B16.34 and I'm a little bit confused about Multipiece valve wall thickness.

In Paragraph 6.1.2 - c) it is stated:

“(c) For multipiece valve construction, where the body consists of a central core to which are affixed two end pieces, the inside diameter, d, is determined by only one of the following methods:
(1) in accordance with (a).
(2) for the end pieces, the value of d shall be in accordance with (a), and for the central core piece the value of d shall be the inside diameter of the core piece. For a core piece with axial holes, whether through- or partial-threaded, the inner and outer ligaments shall also meet the requirements of dimensions f and g in Figure 2. These ligaments shall be based upon the value of d for the core piece”.

This mean that for the core piece you can consider two very different values for d:
-if you apply method (1) you will consider a smaller diameter (based on the flow passage) and you will not take into consideration the ligaments for holes parallel to the body run (meaning at the end that no extra thickness is necessary)
-if you will apply method (2) you will consider a bigger diameter (means bigger value for thickness) and then you have to check that the sum of ligaments is at least equal to the minimum wall thickness (resulting in a thickness of the core part that is equal to the minimum wall thickness + stud size).

In conclusion, if the method (1) is acceptable for stating conformance to the standard, why one should choose method (2), leading to a much higher value for min wt? Am I missing something?

 

[bcd]

No, you are not missing anything. Method (1) was the way it was done initially. Several editions back, the committee changed to only allow method (2). However, there were a lot of complaints that method (2) simply increased weight and cost for very little gain in strength and that there was never any problems with valves using method (1). So for the latest revision, it was revised to allow both.

 

[mOHMo]

So this mean that choosing between the two methods is up to the designer?

Thank you bcd for your answer.

 

[bcd]

Yes, the designer picks the method they prefer.

 

[mOHMo]

Thank you very much

 

[NDS_21]

Hey I went through the standard and I got confused if designer picks the method and takes a min. value for thickness then won't there be a problem for interchangeability of central core made by other manufacturers?
For example; If I want to change just the central core of my multi-piece ball valve with a different manufacturers valve of same class/pressure rating but I don't want to change my already welded end pieces. So how will this happen if my both manufacturers used different designing method discussed above?
So is there any standard table for center distance of the holes for same class/pressure rating given in the central core of ball valve (to hold it between end pieces)?
Hope I defined my confusion in a right way.

 

[mOHMo]

Hi NDS_21,

I'm not sure of fully understanding your doubt.
Anyway consider as follow:
tipically on side entry ball valves (this was the subject of my thread, now I see that it is not stated) the central core part cannot be disassembled from the line without disassembling the whole valve.
Face to face and ends design (and of course compliance with all the specs) determines the interchangeability with exhisting valves.
The actual design of the body is up to the manufacturer, in accordance with the applicable standards, specs, laws.
This mean that tipically the central part of the body will be different for any manufacturer.

Hope this can help