8 replies to this topic

[msam]

Hi All,

I am pretty new to this forum and learning a lot from all of you.

Presently, I am working for a Technology Licensor (Basic Engineering) company.

While making process specifications for various equipments, and especially for pressure vessels, we specify -10 degree C as Min. design temperature. It is the practice of our company, however, sometimes we get other values (like - 4 degree from some Indian clients or -15 degree from Russian client) from Detail-Engineering Contractors/ PMC /Clients, as minimum mechanical design temperature.

I think, they mistake this value with min. ambient temperature.

Please put some light on this simple, though important subject.

Thanks-

Msam.

[fallah]

Seems your equipment operating at temp. below 0 C. Based on this assumption as a general rule :
Min. Design Temp.=T1-5 C
T1=Min. cotinuous Operating Temp.

[eilpar]

Normally for cryogenic vessels the design temperature can be taken as 5 C less than the minimum operating temperature. Please check if there are possibilities when the temp can go still lower due to emergency depressurisation.

Regards

PAR

[Art Montemayor]

msam:

You fail to tell us exactly what material of construction you are referring to. I am left with only a guess that it must be carbon steel that you are alluding to.

If so, then pressure vessel steels - such as the popular A-516 Grade 70 - are usually good for operation down to -20 ^oF (-29 ^oC). Therefore, I don't see what your problem or difficulty is in understanding that all carbon steels have a limit as fixed by their manufacturer. These values are clearly found in such books and codes as ASME Section VIII - Division 1: 2007 ASME Boiler and Pressure Vessel Code for Unfired Pressure Vessels, and in Eugene Megyesy's book - "Pressure Vessel Handbook. Most maximum allowable stress values for carbon and low alloy steels are rated within the working temperatures of -20 ^oF and +650 ^oF. That is the usual design "envelope" for these materials. For stainless steels and other higher alloys the range is different. You should obtain a rating from your steel mills - who should be subjecting the material to Charpy V-notch tests. You should have a copy of the ASME Code (or its equivalent in your country) in your office or close by at all times if you are dealing with pressure vessels.

[msam]

Hi,

Thanks a lot for all your valuable inputs.

Hi Art,

Yes, I was a dealing with pressure vessels in Carbon Steel, sorry, I forgot to mention that.

Thanks a ton !!!

Reagrds-
MSam.

[djack77494]

MSam,
For an ASME coded pressure vessel, you MUST have a Minimum Design Metal Temperature [MDMT] (and corresponding pressure). Furthermore, these numbers are on the vessel's tag. Thus, the question becomes, "What value should I use?"

Since you've clarified that the material of construction is carbon steel, it is generally accepted that there is no "penalty" for specifying a temperature as low as -20F (-29C). Since you work for a licensor, you should have a procedure in place that sets how the MDMT is determined. First, your client should have provided basic site data that includes the coldest ambient temperatures likely. You will know of the process conditions; besides normal operating conditions, you must anticipate off-design operations that could generate unusual cold temperatures. For example, shutdown and depressuring a high pressure gas filled system may result in low temperatures. Considering all these situations (low ambient, normal operations, and off-design operations) you must select the appropriate MDMT and pressure. Typically, it is the lowest value of these with a small safety factor added, however, a higher temperature could be specified under certain conditions.
Doug

[msam]

Hi Doug,

Thank you very much.

Regards-
MSam.

[vinay]

besides normal operating conditions, you must anticipate off-design operations that could generate unusual cold temperatures. For example, shutdown and depressuring a high pressure gas filled system may result in low temperatures. Considering all these situations (low ambient, normal operations, and off-design operations) you must select the appropriate MDMT and pressure. Typically, it is the lowest value of these with a small safety factor added, however, a higher temperature could be specified under certain conditions.
Doug

Doug is absolutely right in specifying that lowest temperature of these 3 conditions with a safety factor should be taken as MDMT . Normally the atmospheric boiling point of hydrocarbons is taken as MDMT ( for depressuisation case mentioned by Doug) for vessels handling hydrocarbons .If your company specifies equipment handling liquid form of Hydrocarbons containg lighters such as ethane, propane, butanes etc , The MDMT can be as low as -50 -to 90 C for the atmospheric or flare depressurisation case even though the operating temperature of vessel is more than ambient temperature . Considerations like these may even result in change of Material of construction (from carbon steel to Alloy or even stainless steel) unless methods are devised to avoid depressurisation of liquid in the vessel to flare.

Hope you find this information useful.

Regards,
Vinay

[msam]

Hi Vinay,

Yes, these, indeed were very useful informations.
Thanks-

MSam.