MDMT for a plant 1

[jt1234]

Dear all,
We will build a new plant in northern USA. The project MDMT is -20F. However a question was raised that years ago there was an extreme cold temperature last about couple of weeks below -20F, says -40F. The question is, shall the MDMT be re-set to -40F just for that rarely extreme condition ?

 

[GBTorpenhow]

The inarguably conservative answer is yes, if historical weather data shows that the ambient temperature can go as low as -40 at the plant location, then the MDMT for the plant should probably be -40.

Whether it is necessary or prudent or not depends on the details of the plant in question and the level of risk acceptable to you, the owner, and the AHJ.

 

[GD2]

OP,
Remember if the project documents (piping Class, Basis of Design etc) are based on -20F and now wants to adopt -40F, all relevant project documents will be required to be revised.
This will create an opportunity for the EPC to charge for extra man-hours and stress for the Owner with increased project cost and schedule.

GDD
Canada

 

[Christine74]

Just because the ambient temperature can reach -40°F doesn't mean that having any of the equipment actually operating at a temperature of -40°F is a realistic scenario.


-Christine

 

[Geoff13]

MDMT is the temperature of the vessel, not the ambient air, so the question should be : Will the plant components reach temps below -20°F while the pressure is near the design value?

Hot process? Insulation? Vapour pressure dropping with lower temp? Etc. Etc.

As mentioned in ASME Sect VIII Div 1 UG-20(a), measuring in-service equipment is a good start. Walk around the plant on a bitter cold day with a temperature gun and see what you find.

Be sure you need -40°F before changing. This will increase the cost of everything in the plant.

 

[EdStainless]

I know of a plant that has rules about cold weather start up.
Basically they can't.
If it is below freezing they simply need to wait for warmer weather to restart.
It is a warm process.
This was a very deliberately decision.
They know what this might cost them down the road.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[shvet]

MDMT for which parts of a vessel/piping? Structural or pressurized?