How critical is MDMT in Vessel designing, Procurement and fabrication?

September 16, 2015 Tags: , ,

With several standards and guidelines governing the design process for an engineer, it becomes crucial for the design engineer to equate the client requirements along with the safety requisites. Here’s an article that would help you understand how a design engineer establishes the trade off without compromising on the security aspect of the design.

Minimum Design Metal Temperature

Let’s start by understanding what MDMT is; MDMT, Minimum Design Metal Temperature, is the lowest temperature that the vessel can withstand with the chosen material and thickness. It is a critical factor in designing a pressure vessel that depends on the environment where the pressure vessel has to be installed.

In general, for all the ASME stamped vessels, it is mandatory to mention MDMT in the nameplate, which in-turn shows the criticality of the MDMT value in vessel design.

Also Read

ASME Sec VIII the most common code used for designing pressure vessels in Oil & Gas.

Impact of MDMT in vessel design and fabrication

When the design has to be frozen and materials requirement sheet has to be shared for vessel fabrication, MDMT shall be considered as an important value and needs to be communicated to the material manufacturer.

In case, the requirement is sent out without mentioning MDMT, the material for the vessel will continue to be procured, used for fabrication and the vessel will be installed but the severity of that vessel notwithstanding the process requirement can affect the entire process plant and cause irreparable damages. In a lot of cases this damage can start with some cracks on the vessel and may lead to an explosion of the vessel, resulting in a disaster and the whole plant operation could also collapse.

How to take care of MDMT in design and procurement

In order to avoid such severe implications it is always recommended to procure material considering MDMT value (once the design team freezes the design and MDMT is confirmed by them). In this case this material set shall be impact tested at MDMT or in lesser temperature than MDMT during the manufacturing process. Only those materials can meet the process requirement.

Also if the material has been ordered without impact tested at MDMT during the manufacturing process and the same has been identified before fabrication by the fabricator, then the vessel fabricator can do the Impact test at MDMT, taking some specimen from the ordered materials as samples as explained in ASME code and see if that material can withstand the test as per the code or else the fabricator needs to order a new material as explained above.

By taking care of MDMT value in design and fabrication, one can avoid cracks or explosion of vessels during the operation which in turn will safeguard the plant operation.

MDMT reference ASME VIII Div.1 – UCS 66.

All material shall be supplied with Material Test certificates, which will give complete information about that material. Material Test certificates will have the MDMT value mentioned.

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