Sif on Trunnion/Dummy leg

Do we practice of putting sif of 2.1 while modelling Horizontal/Vertical Dummy leg on Bends or placing a Trunnions on Pipe?
Is it engineering thumb-rule or is Code driven.
I am working as per B31.3.

Also refer to Table D300 Notes: 14. (reference of sif of 2.1 mentioned above) and also go thru 3-36 to 3-41 of Application Guide of Caesar II where nothing is mentioned about providing sif.

I want to know whether it is accepted common practice in our Piping Stress Engg. field.

Yes, it is a popular engineering practice to use the SIF of 2.1 at trunnion intersections.

However I do not agree to such a simplistic look into the problem when the trunnion attachment is at the bend.

We know that the flexibility of a bend is mainly due to its ovalization . This reduces the section modulus hence the flexibility. So "more flexible-more SIF ".

I think the presence of a trunnion can actually contribute to prevent the extent of ovalization , something in line by which a flange addition ( or two flanges) at the bend end(s) reduce SIF and flexibility. So you can have a reduced SIF at bend with trunnion.

However this problem does not have a simplistic solution as the stress distrubution can change significantly due to addition of the trunnion, something which you cannot make out from the simple concept of ovalization.

So there are contrasting views as whether SIF will increase or decrease by addition of trunnion at bends. So 2.1 may not be a conservative estimate.

There are lot many ASME papers in this field.CAESAR II ( at least till version 4.3) has the facility to compute SIF at bends with trnnion attachment based on ASME paper.However my personal opinion is that : "do not use this module " as it I had no. of interaction with CADE where they have recommended not to use this option.

At other locations ( non bends) I think 2.1 is a conservative estimate.

A.Bhattacharya

Stress Analyst

Bechtel Corporation

We do not consider any S.I.F for the Trunnion in the caesar spreadsheet. However, the axial stresses and also bending stresses due to loads multiplied by moment arm of the Trunnion at the point of intersection of the trunnion with main pipe will be checked externally to find out if there is any need for any pad to limit the streses within allowable limit.
Some simplified spreadsheets are generally used to find out the combined stresses at the intersection point.

I think Mr. Srinivasan has referred to the spreadsheets based on the well known " Kellog Method " .
Yes, this is a time tested method based on a derivation using the concept of beam on elastic foundation.

However I have found in most of the spreadsheets I have come across , an erratic way of combining stress and comparing against allowables.

In my opinion, the stress output from "Kellog Method" should be combined and checked as per recommendations of SEC VIII DIV 2 APP 4.

A.BHATTACHARYA

STRESS ANALYST

BECHTEL CORPORATION

Dear Richard,

Can u please give me a reference to find the modified kellogg method for trunnion calculation.....

I find it hard to locate it....

The current state of the art for trunnion design, as far as I know, is ASME Code Case N-392-3. It is available from the ASME.

Read also "Stress Indices for Elbows with Trunnion Attachments"
from EPRI (contain also ASME Code Case N-392-3 reference)

It is a public document.

Regards,

Thankyou CraigB and Danb.