MDMT determination for PWHT vessel 3

[MalcolmA]

I am working on an MDMT determination for a vessel that has PWHT because it is in caustic service. PWHT is not otherwise required.
If a vessel has PWHT for service, do I get to take the 30 deg reduction in MDMT that is allowed by paragraph UCS-68(c).

I am having trouble determining exactly what the code intends here.

UCS-56 says that the PWHT exemptions allowed by table UCS-56 and UCS-56.1 are not permitted when PWHT is a service requirement. I interpret this to mean that the code is requiring PWHT and the 30 deg reduction is not allowed.

Here is where I run into some confusion: I am looking at example problem 1 in API 579 for the Assement of Existing Equipment for Brittle Fracture. This example is for a vessel in caustic service with PWHT. The example takes the 30 deg reduction in MDMT.

Also, I am looking at an MDMT determination from a vessel fabricator for a diffent vessel than the one I am assessing. It is a Caustic service vessel with PWHT and they applied the 30 deg reduction.

If anyone can help shed some light on this for me, it would be greatly appreciated.

 

[Duwe6]

PWHT is PWHT. Doesn't matter *why* you did it, just that it was done and done right.

Assuming you have the signed chart from a calibrated recorder to prove that the vessel was 'soaked' long enough & at the right temperature to satisfy Code requirements, then you get the -30° drop in MDMT. A detailed report from a knowledgeable Inspector or Engineeer would suffice if the chart cannot be located.

Lacking the chart or a detailed report, there is no proof that your Heat Treatment was adequate for Code purposes.

 

[SnTMan]

MalcolmA, the "service requirements" in UCS-56 and the language "otherwise a requirement of this Division" refer to the following cases when PWHT is mandatory:

1) Lethal service
2) Unfired steam boiler
3) Subject to direct firing (see UW-2 (d) for conditions)
4) Based on thickness per UCS-56

Assuming caustic service does not meet these definitions, and assuming you do not have to PWHT based on UCS-56, then you may take the 30 F reduction.

It can matter why you did it.

Regards,

Mike