Dear sir/sirs

1. Whats is the diferece between the following two input data fields mentioned in DESIGN CONSTRAINTS
Hydro Allowable Unmodified
Hydro allowable is 90% yield

2. What does edge distance imply in saddle input?

3. Does ASME Code mentions any thing about tolerance to be considered for thinning of head due to forming?
Are there any standards available for the same?

4. In case external design temperature is not mentioned should we assume external design temp same as design internal temperature.

Regards

Dear Mahesh,

I will explain your questions 1,2 and 4.

Hydro allowable is 90% yield
The default allowable for circumferential stress during the hydrotest used in PVElite is Min(1.3*S, 0.9*Sy). But, some companies like to use just 0.9*Sy. In that case check this box. The ASME code does not provide the allowable for the hydrotest.

Take SA-516 70
S = 20 ksi , Sy = 38 ksi

Default value of hydrotest,
Shydro_d = Min(1.3*S, 0.9*Sy) = Min( 26, 34.2 ) = 26 ksi

If the above box is checked then
Shydro = 0.9*Sy = 34.2 ksi


Hydro Allowable Unmodified
The allowables used for stresses caused by occasional loads such as Wind/Seismic typically include a 1.2 factor. In other words the allowables can be increased by 1.2 when checking the stress due to occasional event. The rational being that a limited time overstress would not cause a failure.

But, in the case of the hydrotest the allowables are already higher (as discussed above), so using a 1.2 factor could cause them to be higher than the yield strength in some cases. So, by default the occasional increase factor is not used for the hydrotest allowables, which means that Hydro Allowable Unmodified box is checked.

Continuing the above example. If you apply the Occasional Increase factor (Hydro Allowable Unmodified – Unchecked) for the default hydrotest allowables.

Shydro_Occ = 1.2 * Shydro_d = 31.2 ksi.

This value is still smaller than 0.9*Sy, so it will be ok.

But, for some materials it may not be the case.

Moreover, if you used the 0.9*Sy to get the hydrotest allowable and apply the 1.2 occasional factor then you get,

Shydro_Occ = 1.2 * Shydro = 1.2 * 34.2 = 41.04 ksi.

This value is greater than Yield Strength (38 ksi) of the material !!!

The Shydro_Occ is used the in the load cases that combine the longitudinal pressure stress due to hydrotest with that from Wind/Seismic loadings.


External Pressure Design Temperature
If the external design temperature is not mentioned then it should be same as internal design temperature (IMHO). I have seen a lower design temperature for external pressure case as compared to internal case . But, if the vessel while still hot draws a vacuum then there would be a condition that it was not designed for.

Mr Mandeep

Thanks for your kind reply

Mahesh