 # Allowable stress with time

Gents,

Does the allowable stress of steel ( pressure vessels or piping) decreases with time (number of hours where these equipment was serving)?

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Hope it was MAWP.

For a complete vessel you would calculate the MAWP for each component. The smallest value of MAWP would govern for the vessel.

For example, in ASME Section VIII Division the required thickness for a cylinder (based on circumferential stress) is given:

t = PR/(SE-0.60*P)

But if ‘t’ is known then the maximum pressure is found as:

P = SEt/(R+0.60*t)
P in this formula is design pressure (provided by the purchaser), R inside radius of the pressure vessel ( provided by the purchaser). The S is allowable stress at the design temperature for selected material,The E is joint efficiency

Your wall thickness for 200 PSI design pressure with 60 inch inside diameter and 20,000 PSI allowable stress with joint efficiency 1 will be 0.6 inch. If you put these values in above formula, you will get this 0.6 inch wall thickness.

For example,
Now go to the market, is it any plate material with 0.6 inch? No the nearest one that you can find is the plate with 0.75 inch.

You need to get some credit fo using a plate with higher thickness.

Let us reverse above formula. You have the wall thickness, and you want to calculate P.

Put the same value in the formula except for design pressure and calculate P. This P will be your MAWP. So for above example, the MAWP will be 248 PSI. The 248 PSI is the MAWP and will go to the nameplate and will be the base for calculation of test pressure for Hydro.

Ie.,MAWP directly proportional to thickness of material , MAWP Increases with Material thickness.
For sure operating condition will definitely affect the thickness of material as the year progress that means MAWP will eventually decreases with operating cycle.

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