Dear Sirs,

As some specifications requires nozzle to be design for “MAPnc” condition as the hydrotest is based on the UG 99 C. Now for complete vessel, the allowable stress for hydrotest can be 90% of yield stress at hydrotest temperature. I have selected the option of “Consider MAPnc” for nozzle design from load cases, but the allowable stress used in the nozzle calculation is the cold stress & not the 90 % yield stress at test temperature, which ultimately affects on the nozzle thickness greatly as all the nozzles I have designed are self reinforced nozzles & tends to increase the cost of vessel as these forged nozzles are very costly.

What I want to confirm that, why the option is not there for nozzles allowable stress can be 90% of yield stress for MAPnc design?

Mr. Mak,

Both the above requirements are slightly different. MAPnc is maximum permissible pressure of the weakest part in new (uncorroded) and cold condition. This is not considered as short period.

While hydrostatic condition is for very short period i.e. 30 Min or 1 hour etc. So for that cases your allowable could be 90% Yield Stress or even 95% of yield stress.

Hope above is clear. Let us see, opinions of others on the issue.

Thanks.

Mukesh
www.theculminates.com
M:+91-9428500192

Hello folks

We must be careful here. Let us consider a vessel to be hydrotested to UG-99(c) in accordance with ASME Section VIII, Division 1. We follow the sequence as laid down in the code. UG-99(c) refers us to the definition of 'calculated test pressure' in Appends 3, section 3-2. The specific paragraph tells us in assessing the 'calculated test pressure', we consider the un-corroded (nominal) thickness of the component, using the design stress designated in ASME Section II - Part D (S) to determine what we term the MAPnc. This is done to find the weakest element. This pressure is then multiplied by 1.3. Then the pressure of the test water is added to this pressure for the weakest element at its lowest point. This is the hydrotest pressure.

Under these conditions, we get a certain stress in the element. We now compare that hydrotest stress to an allowable stress. If the box for 90% of yield is check, the allowable stress at hydrotest is 90% of yield. If the box is not checked, then the allowable stress at hydrotest is 1.3 x the stress from ASME Section II - Part D (S).