Hydrostatic/Hydrotest Pressure as per UG99(b) 2

[sollapalooza]

Hello,

A client of mine has their own regulation that imposed Manufacturer to use 1.3 x Maximum Allowable Pressure (MAP), however they are now requested to use 1.3 x Maximum Allowable Working Pressure. They claim it will reduce the cost, but I failed to understand why. Thickness is determined by Design Pressure, not MAP so how is this possible?

Thank you for your reply.

 

[metengr]

Why not ask the client??

 

[jte]

You didn't state your own role here, it seems that perhaps you are in an engineering company and not within the manufacturer's organization. In addition to metengr's suggestion (and please get back to us and tell us what they say!), you could always ask the manufacturer to provide a written estimate of the cost savings. Let us know what the percent savings is - should be interesting.

 

[donoharm]

You are correct. Thickness is determined by Design Pressure mostly but not always. For example, thickness of tall tower is determined by Wind condition sometimes. Likewise, for some cases MAWP is one of the governings for thickness. The difference between MAP(so called new and cold condition) and MAWP(so called hot and corroded) is the thickness used for the calculation of Hydrostatic pressure. Since MAP uses uncorroded thickness, you have to apply higher hydrostatic pressure. Hence, if this condition is the governing case, you have to use the thicker plate for your PV.
I guess this is the reason why your client made the comment like that.

 

[jte]

Since MAP uses uncorroded thickness, you have to apply higher hydrostatic pressure. Hence, if this condition is the governing case, you have to use the thicker plate for your PV.

Well, this is a circular argument. Increased thickness to accommodate MAP based hydrotest leads to increased MAP...

With a tall tower wind or seismic can often require an increase in plate thickness. But only at the bottom. I've never seen wind or seismic govern the top shell course of a multi-course column. Thus, MAP for the vessel would be set by shell courses near the top, and the thicker shell courses at the bottom would have no influence on this.

My company's spec's use the MAP based (UG-99(c)) approach but we allow for the hydrotest pressure to be reduced in order to not exceed specified maximum stress levels. This is not unusual and there are several operating companies and major E&C's that take this approach. If this job happens to be for my company, I can state with absolute certainty that the plate thickness increase is not aligned with the intent of the spec. I'd suggest that asmediv1and2 ask their client check with the author and second sign-off on the specification for clarification.