Maximum pipe pressure and actual pipe pressure

[Edgar50]

Question for the piping engineers;
The maximum allowable internal pressure for pipelines can be calculated according to the ASME B31.3.
But for how many percent is the actual internal pressure in practice?

Reason for my question is that the stress calculations for branch outlets (weldolets) fails when i apply an outlet on a header both with the same schedule dimensions, by using the maximum allowable internal pressure for pipe.

Thank you for any reply!

 

[racookpe1978]

Forgive me, but I can't follow your explanation of the problem: A weldolet connects a larger diameter (thicker-walled) pipe to a smaller diameter (usually thinner walled) side branch. Basic stress theory shows that a smaller diameter pipe resists internal pressure easier than a larger diameter pipe, and so, almost all of the time, the limiting stress happens in the large diameter main run.

The weldolets are "over-walled" so the same fitting works in many different internal pressure configurations -> it is unlikely that the walls of the weldolot fitting itself is going to limit your pressure.

What pressure, what material, and what diameters and schedules are you using?

 

[Edgar50]

Many thanks for your reply racookpe1978!

I have the following situation.

Header pipe 20 inch schedule XS / ASTM A106 Gr B / temp. + 100 degr Celsius / corrosion allowance = 3 mm.
According to the calculation of ASME B31.3, the maximum internal pressure for this size is 4,45 N/mm2

A weldolet 10" schedule STD / ASTM A105N / Bonney Forge - MSS-SP-97 model must be attached.

When i apply the pressure of 4,45 N/mm2 in the stress calculation for this weldolet, the desing margin is for 42 % overloaded.
The required reinforced area with this size of weldolet is not sufficient.

If i use a lower pressure of 3,15 N/mm2, the sizes of the weldolet are acceptable. With this lower pressure, the header pipe will be loaded for 70%.

Are header pipes "maximum" loaded in practice?


Note: for stress calculations we use a approved software program. I am unfortunately not a piping engineer.

 

[LittleInch]

what do you mean by this statement "When i apply the pressure of 4,45 N/mm2 in the stress calculation for this weldolet"

Are you adding any other forces or moments?

It could be overstressed if this is part of a system which is adding other loads into the piping system.

Pipe design (not pipeline) does normally use the design pressure to calculate the wall thickness, but often has a small margin as the standard pipe sizes rarely exactly match the minimum calculated thickness.

BTW I get 52.7 bar as the internal design pressure for the 20" header - not clear which formula or numbers you are using so maybe best to lay it all out.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[XL83NL]

I think you might be mixing up the wall thickness calc as per para 303, for which you dont need to recalc the weldolet (since its integrally reinforced as racookpe mentioned), with a PSC.

The PSC (pipe stress calc e.g. in CAESAR II) may however indicate an 'overstress' at the olet. This may be due to the SIF of the branch type, and associated sustained loads.

Did you do a PSC for this line which indicated a 42% overstress?

 

[Edgar50]

Thank you all for your replies.

Maybe i was not clear enough in my explanation.
We are a West-European supplier of branch outlets, forged fittings, special flanges, etc., not a manufacturer or piping engineers.

The example i gave (a 10" STD weldolet attached to a 20" XS header pipe) is an actual situation.
We have to add stress calculations to the material supplies to our customer.

What i have done is to investigate if the reinforced area in the connection between header pipe and weldolet is sufficient enough. (The weldolet is attached to the header pipe by welding)
See also the par 304.3 of the ASME B31.3

First i calculate the maximum allowable internal pressure using the formula's according to ASME B31.3 for the header pipe. Taking into account the tolerances and corrosion allowance.
This maximum allowable pressure i use in the calculation for determination of the minimum required reinforcement area of the welded connection from the weldolet to the header pipe.

The results shows which dimensions the weldolet must have to resist the conditions under the maximum allowable internal pressure rate of the header pipe.

Maybe i have explain my problem a little bit better now.
Hopefully some of your guys can help me..... Thanks!

 

[rconner]

[Am curious do you know is the whole "pipeline" to be tested "XS" (0.50"?) thickness, or just this "header pipe"?]

 

[Edgar50]

Answer for "rconner"; unfortunately we do not know.
We receive a list with all required piping components which we have to supply.
There is no details such as design temperature, design pressure, actual location (under ground section or above level) etcetera.

My calculations are based on assumption.