Yield point of piping material

[strad]

Hi All

I would like to ask a basic question on materials. We are a pipe line operating company. Our gas pipe line material is API X52 with a maximum operating pressure of 7332 Mpa. In general one can hydrotest pipe lines to 100% or even 110%SMYS.I have made to understand that 100% SMYS is the yield point of the materil.If that the case how can you hydro test to 100% or above of the given SMYS?

 

[metengr]

.I have made to understand that 100% SMYS is the yield point of the materil.If that the case how can you hydro test to 100% or above of the given SMYS?

Incorrect assumption. The SMYS is only the specified minimun yield strength, when in reality the actual yield strength will be higher.

 

[09091960]

Yiels point,Is there perticular % higher than SMYS?

 

[metengr]

No. To answer your question, you have to review the actual test reports of the installed materials of construction (MOC). For example, if the SMYS is 36 Ksi by design, the actual yield strength of the MOC could be 41 Ksi, 38, 40, 39, etc.

 

[09091960]

Hi Metenger,

Thanks for your reply. Strad & Myself both work in the same department of our company. Appreciate if you could see the correctness of the following statement.

"The pipeline will be designed to a Maximum Allowable Operating Pressure (MAOP) based on a nominated Specified Minimum Yield Stress (SMYS). What this means is that from a design perspective if 100% SMYS is reached at MAOP the pipeline will in theory reach “plastic deformation” and eventually fail under stress".

 

[metengr]

What this means is that from a design perspective if 100% SMYS is reached at MAOP the pipeline will in theory reach "plastic deformation" and eventually fail under stress".

No. If the line does achieve some local plastic deformation from pressure reaching at or slightly SMYS, the pipe will not fail. There could be some minor plastic deformation but this does not imply the pipe will fail over time. Actually, in some cases local plastic deformation will result in stress redistribution from local strain hardening effects.

 

[rneill]

Since internal pressure is a primary stress, and since primary stresses are not self limiting, if you were to impose a pressure that caused a stress in excess of the actual yield strength of the material (as opposed to the SMYS), you could see a failure mode consisting of gross deformation leading to rupture. The material would start to yield and deform resulting in a reduction of the wall thickness leading to more deformation leading to eventual rupture. However, at the same time this was occurring there would likely be some strain hardening effects so the pipe may be able to tolerate a stress somewhat above it's yield stress before failure became inevitable.

In the event of a secondary stress (a self limiting stress) such as due to thermal expansion or settlement, local plastic deformation will relieve the stress, or redistribute it, and failure would be unlikely. Secondary stresses result in fatigue crack initiation and propagation leading to failure which requires a high number of cycles.

I should point out that no code I'm aware of actually allows piping to be designed to 100% of the SMYS value. For example, ASME B31.3 provides allowable stress values that are substantially lower than the SMYS value.

 

[09091960]

Many thanks for that. Hope my boss will under stand this?

 

[MJCronin]

In my opinion...

Your problem comes from the interpretation of the words "based on" in the sentance:

"The pipeline will be designed to a Maximum Allowable Operating Pressure (MAOP) based on a nominated Specified Minimum Yield Stress (SMYS)."

If you understand "based on" as "based solely upon" or "as a basis" makes all the difference in the world.

Furthermore, the purchaser, IMHO should be determining the MAOP and he should be working to the rules of the selected design code.

I also strongly agree with rneil....no reasonable design code will go to yield as an MAOP limit....makes no senese

-MJC

 

[strad]

Thanks all for your valuable contributions on this subject.We have finally put some thing across to our managers under the theory of failure equations.

 

[rneill]

In answer to the other part of your original post that I forgot to respond to ...

Since codes apply factors of safety against SMYS, they require you to go to higher than the design pressure on test. The intent of most codes is that on hydrotest, you will approach the SMYS. For example, ASME B31.3 typically has allowable stress values for carbon steel materials that are 2/3 of the SMYS so if you apply a hydrotest pressure of 1.5x the MAOP (which is based on 2/3 of SMYS) you will get back to approximately the SMYS on test. Likewise, in Canada CSA Z662 for pipelines allows a maximum of 80% of the SMYS in the design calculation but then requires a 1.25x hydrotest. 0.8 x 1.25 gets us back to approximately the SMYS.