MDMT for Pressure Vessel 1

[hubert88]

Hi Guys,

How to determine the MDMT for a pressure vessel?

Design Data for Pressure Vessel:
Code: ASME Section VIII, Division 1
Ambient temperature at site: 19-37degC
Design pressure: FV/11barg
Operating pressure: 7.9barg
Design temperature: 200degC
Operating temperature: 61degC
Material (shell & head): Carbon steel

According to the design data as above, the MDMT shall be set at 19degC, the lowest temperature. (please correct me if I am wrong)

In that case, the water temperature of hydrostatic test shall be at least 36degC (19+17) as per ASME code: "It is recommended that metal temperature during hydrostatic test be maintained at least 17degC above the MDMT, but need not exceed 48degC, to minimize the risk of brittle fracture".
But the vessel manufacturer informed that the water test temperature is too high for them. The available water test temperature at their workshop is only 17degC.

Please advise I should set the MDMT at what temperature now?

 

[metengr]

Typically, several factors go into selection of the minimum design metal temperature. Is the vessel located outside and subjected to weather extremes - coldest temperature? What about coldest process temperature? The designer typically selects the lowest possible exposure temperature. In the case above, if the lowest temperature is 19 deg C, this is selected or you use the default value of -20 deg C for greater range.

 

[curtis2004]

Hubert88,

1. What is process fluid inside of the vessel?
2. What is geographic location of this vessel to be installed?

Thanks,
Curtis

 

[hubert88]

metengr,
As mentioned above, I can't use 19degC due to the "high" water test temperature.
For default value of -20degC, did you mean to select the minimum temperature for carbon steel, which is -29degC?

curtis2004,
Process fluid is methanol and location is Singapore.

 

[Bobfromoh]

From the way I see it, the above discussion is about establishing the
"Design MDMT". So then you design the vessel and determine the "Minimum rated
MDMT". Maybe the range of components (shell, heads, nozzles, flanges etc.) has
a rated MDMT of 155F(-103.8C), -50F(-45.9C) and -30F(-34.4C). The vessel rated
MDMT will be the lowest of these numbers at -30F(-34.4C). This is lower than
the 19C and is also the value used for determining the hydrotest water temperature.

 

[metengr]

hubert88
Yes, -29 deg C as the impact exemption minimum temperature based on material and shell thickness.

 

[hubert88]

The vessel need to be U-stamp. Authorized Inspector advise to specify the MDMT as 0degC.
Any concern?

 

[metengr]

No concerns.

 

[r6155]

“ Authorized Inspector advise to specify the MDMT as 0degC.” it is correct.
Hence MDMT 0ºC + 17ºC UG-99 (h)= 17ºC = ”The available water test temperature at their workshop is only 17degC”.

Regards
r6155