Test and design pressure 3

[samuelo00]

Hi all,

When thickness of a pipe is calculated, it is used the design pressure.

On the other hand, the test pressure is 1.5 times the design pressure (forget about temperature corrections).

1. What would happen if the commercial thickness chosen was so close to the theoretical design pressure thickness that it is not suitable for the test pressure? I suppose that this has a simple answer, but I don't get it.

In previous editions of the B31.3 code, the maximum pressure allowable was calculated with the same equation than the one used for the thickness, but in the 2004 ed. this has been removed.

2. Anyone knows why?

3. Can I use the same equation that appears in the 2004 ed. for calculating the maximum pressure?

Advanced thanks to all.

Samuelo

 

[BigInch]

You would have to decide to either,

1.) Test to a pressure higher than the calculated test pressure and keep your original design/operating pressure as is, or

2.) to limit the test pressure to the calculated test pressure and then reduce your maximum design/operating pressure accordingly.

It is not acceptable to pick and choose various clauses and equations from different editions of a code or from one code to another. Use the Code and the Edition of the Code that your project contract specifies.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[zdas04]

THe design pressure and wall thickness calcs in the code represent maximum design pressure and minimum wall thickness. These are "not to exceed" kinds of numbers. If your calculated design pressure is 600 psig, but the stress during the test is closer to SMYS than you like, there is nothing in the code that says you as the engineer can't call the system 500 psig design and recalculate your test pressure.

The code defines the limits, the engineer picks the specific points within those limits.



David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

The harder I work, the luckier I seem

 

[gerhardl]

Biginch is (as usual) correct. Additional as seen from the European Norm side:

a) Design pressure is, I believe, not always given as a 'norm' or 'pressure class', but is generally used to describe (at the concept stage) the working pressure that the total system must be able to handle on a permanent basis.

b) With design pressure given you then have to select or design piping, vessels and valves according to given and excactly described norms or pressure classes.

c) If design pressure is 'in between' norm classes you will have to go up to the next higher pressure class.

d) You are not in any way allowed to use working pressures above given in the norm for the selected class.

Note: I have myself several times actually been met by the argument that we for a given case should use a lower pressure class or pressure higher than the design actually allows as 'the actual maximum pressure is just 'a little bit' over allowed normed limit'.

Most often this is given as an argument when the economical difference between the two choices is large.

If you take the easy way out and select for lowest price/ pressure over allowed construction limit, you will first of all have taken a juridical responsibility for all consequences, secondly you will have lost any guarantee and backing from sub-suppliers and most of all you will not have selected for yourself the proper attitude to what engineering actually is all about.

.. and that is a thing for the ethical forum. (The practical ethical test : If you are in doubt ask yourself for your reaction if this was displayed in the press after a 'worst case' breakdown..)

 

[samuelo00]

BigInch,

In case of 1 in your reply and in this special case (commercial thickness close to calculated thickness), and as zdas04 says, if I would test the pipe at test pressure, the material could fail. I think this is not a choice.

I'm astonished that the code does not cover this issue, that I'm sure has happened a lot of times (accurate thickness are chosen many times in stainless steel pipes). Wouldn't it be easier and safer if the pressure to calculate the thickness was the test pressure?

Gerhardl, be sure I'm not going to test the pipe with a pressure that can be dangerous to the personnal.

Thanks all
Samuelo

 

[BigInch]

First of all you said, "Close to", not over, so no cavetes required. I recommended nothing, just told you your options, so you could make your own informed decision.

Secondly: I have tested many times over SMYS and even up to 110% of SMYS. Not my wish at the time, but it was company policy, which BTW I was later instrumental in changing for their offshore pipelines, where I convinced the VP of engineering that the cost of repairs offshore was high in relation to his preceived benefit of proof testing.

And actually SMYS is the yield point only and has little to do with ultimate stress at failure in most commonly used engineering materials, provided there are not other extenuating issues, or the material has inappropriate properties, i.e. fails catastrophically, etc., there is nothing wrong with doing that. Have a look at some texts discussing plastic design theory.

LASTLY: Nobody should be within the danger zone of any test whether its 1.1, 1.5 or to SMYS, or above and beyond to failure. Just like F1, motorcycle driving and ultralight flying, testing is as safe as you make it. Make it safe!

Codes give minimum design and testing requirements. If you want to do a higher proof test or test all the way to failure, its optional.



BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[samuelo00]

BigInch,

You are right, it'sthe yield point, no failure, but I wouldn't go to that way unless it's really necessary. There can be permanent strains that maybe later can affect the stress analysis already done, which would imply a deeper study, and the project is not in situation of doing somethin like that.

Thanks all, I appreciate your answers.
Samuelo

 

[BigInch]

Cheers

As long as you're making informed decisions... Make all you can.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com