ASME - Pipe Thickness Calculation for High pressure piping

[prabhuu]

Hi,

Pipe thickness is generally calculated based on hoop stress formula whic is pd/2t. In ASME B31.3, the formula is modified as PD/(2SEW + PY) and it is used for calculation purpouse. However thickness obtained in the later is less than than the former one and still it is accepted because of the allowable stress value stated by the own code which provides the formula.

In the ASME 31.3, there is another formula defined for HIGH PRESSURE PIPING DCo/2(1-exp(-p/s)), which still gives lesser thickness than the using the normal formula PD/(2SEW + PY). For same value of allowable stress used in both formulas the high pressure formula gives lesser thickness (for ex. thickness obtained in PD/(2SEW + PY) is 3.5mm and thickness obtained in DCo/2(1-exp(-p/s)) is 3mm), how this is acceptable? basically high pressure needs higher safety but here the thickness is controversy. Kindly guide me to understand the reason for this controversy.


Thanks in advance..

 

[XL83NL]

No, I think youve got it wrong. Higher pressure doesnt necessarily requires a higher 'safety factor'. However, for high pressure piping, you need to refer to a different stress table; those of A-1 dont apply anymore, refer to K-1. Second, thin wall assumptions dont apply for high pressure piping, which is why the formula is different. Review chapter IX to understand what it means.

If you would review the ASME BPV codes for an analogy, youll notice that as you go up in section (from I to II to III), and hence sort of go up in design pressure, the design margins decrease as the failure mode changes. Every failure mode has it's own design margin, and as the formula's for high pressure piping are based on a different faliure mode, the design margin is different. Review K302.3.2.

 

[prabhuu]

Thanks for ur reply.ya i understand now but having one more clarification that in case of trunion calculation checkning the basic pd/2t formula is decribed in kellogs method for finding circumferntial presure in header. In that case can i replace the formula with High pressure stress formula ? and axial stress pd/4t can also be replaced by high pressure stress/2.

Why i am asking is because allowbl stress at 200 deg C for A106B is 137 Mpa at both the alowabl stress table. My thicknes calculation is based on high presure thicknes formula so i obtained a particular thicknes which is less than using pd/2t. But my trunion calculations are based on pd/2t formula which shows failure in hoop stress. Kindly advice me..

 

[TGS4]

To use thin-wall approximations, you have to confirm that your wall is actually "thin". Otherwise, the exact thick-wall equation is more appropriate.

 

[BigInch]

"how this is acceptable? basically high pressure needs higher safety but here the thickness is controversy. Kindly guide me to understand the reason for this controversy."

The stress from internal pressure within a thin-walled pipe can be considered nearly uniform across that thin wall, whereas stress from internal pressure distributed across a thick walled pipe is not uniform. Thick walled pipe has a higher stress on one side than the other, so a different formula is needed.

 

[prabhuu]

Thanks all for your valuable replies....