S.I.F. for a Reinforced Elbow? 1

[reades]

I am trying to determine what my S.I.F. and flexibility would be for a Reinforced elbow. The flexibility and S.I.F. equations specified in B31.1 does not cover a reinforce elbow. Does anyone know how I would go about determining such factors, which would be used in a B31.1 pipe analysis?

Thanks to all.
Steve R.
Mechanical Engineer

 

[MJCronin]

reades,

What do you mean by a "reinforce elbow" ? I have been dealing with power piping for over 25 years and have never heard of this term (and cannot picture it in my grey haired head)

While all of are are probably familiar with a "reinforced tee" the elbow version is a mystery to me...

Are you refering to an elbow with a branch connection or "trunion" ?

The rules of B31.1/B31.3 calculation assume an elbow made of uniform thickness without a branch

MJC



"There comes a time in the affairs of man when he must take the bull by the tail and face the situation." W.C. Fields

 

[unclesyd]

MJCronin,
I've used S/S screwed elbows with a web on the inside radius. Also I worked for a company that made malleable iron and cast iron fittings with a reinforcing web. I've never seen any reinforcing on C/S pipe except large diameter fabricated water pipe.

I've been looking around for info on the fittings.

 

[MJCronin]

unclesyd,

It seems that "reades" is trying to calculate the SIF ( Stress Intensification Factor) of this unique elbow in accordance with ASME B31.1. This is probably for a stress analysis of a piping system.

You should note that B31.1 does NOT permit the engineer to use any type of elbow or component made by anyone. Specific rules exist for use of elbows, tees and other components constructed to the dimensional and quality requirements of ANSI B16.9. Other rules are contained in B31.1 for the design, fabrication and SIF calculation of mitered elbows.

Unless the component is permitted by the rules of B31.1, I do not understand how the SIF calculation has any validity.

By the way, the screwed elbows that you refer to....with the web reininforcement would not be applicable for an SIF calculation either. This does not mean that they are unacceptable for use in a B31.1 system........it just means that the b31.1 methods used to perform a detailed stress analysis calculation do not apply.

My opinion only


MJC



"There comes a time in the affairs of man when he must take the bull by the tail and face the situation." W.C. Fields

 

[NozzleTwister]

Steve,

I have used "reinforced" elbows in my carrer. I can't say that I have with B31.1 but definitly with B31.3. Some companies that I have worked for in the past thought that adding a re-pad to a trunnion attached to a elbow was a great way to solve local stress problems due to the loads on the trunnion.

Personally I feel the prefered method of reinforcement is to just specify a heavier wall elbow fitting to provide the reinforcement you need instead of having the pipe fabricator try to fit an eliptical re-pad to the heal of the elbow. When using heavier fittings remember to taper bore as required where welding to thinner compoments.

When solving local stress problems, my first choice is to increase the trunnion size as much as possible and if that's not enough, then I use a heavier schedule elbow.

Adding a trunnion and/or re-pad will increase the stiffness of your elbow thus decreasing the SIF values. There are no recognized procedures to calculate these adjusted values. If your company doesn't have a procedure in place for dealing with this situation then use FEA analysis or use conservative SIF values based on an unreinforced elbow. If you use a heavy wall elbow, base your calc's on that heavier wall.

Good Luck,



NozzleTwister
Houston, Texas

 

[reades]

NozzleTwister,

Thanks for the advice. The basis of my question is in relation to a situation where B31.1 is not applicable, but is used for FEA analysis. There is really no specific code written for the material and situation at hand, that I know of. The material being used is an Aluminum Alloy (AlMg3) with a 0.25" wall thickness and a diameter of 20.0". The elbows are single mitered and the system is at a pressure of 90psig.

Since, Pipeplus does not allow for mitered elbows my company has put into place a procedure for calculating the respective radius of the mitered elbow and the associated SIF values for the FEA.

So, I guess the only way to determine which is best is through trial and error.

If you would like a real understanding of what I am working with go to:

Http://www.cgit-westboro.com

Thank you for all of your knowledgeable experience.

Sincerely,
SMR
Mechanical Engineer