Criteria for Selecting Design Temperature when not provided
Criteria for Selecting Design Temperature when not provided
(OP)
Hello.
Is there a rule of thumb for choosing a design temperature when only an operating temperature is provided for analysis? On my current project, both design and operating pressure have been chosen, but only the operating temperature.
I remember hearing some years back about simply adding 50F, but most of my systems are below 200F and this seems too conservative.
For now, I have used 1.2xOperatingTemperature for my design case. Any suggestions?
Thanks,
Dmitriy
Is there a rule of thumb for choosing a design temperature when only an operating temperature is provided for analysis? On my current project, both design and operating pressure have been chosen, but only the operating temperature.
I remember hearing some years back about simply adding 50F, but most of my systems are below 200F and this seems too conservative.
For now, I have used 1.2xOperatingTemperature for my design case. Any suggestions?
Thanks,
Dmitriy





RE: Criteria for Selecting Design Temperature when not provided
You also need to consider minimum and/or maximum ambient temperature, whichever is below/above your min/max design temperature for the process.
Good engineering practice dictates to look into all possible operating/shutdown/restart scenarios to see what could be the minimum/maximum temperature excursions. Depressurization of vessel which contains light liquid Hydrocarbons is one of the examples - you can get to very low temperatures after blowdowns.
There is a lot of information published on the net. Some more info at: http://www.red-bag.com/engineering-guides/242-bn-e... (Chapter 4)
Dejan IVANOVIC
Process Engineer, MSChE
RE: Criteria for Selecting Design Temperature when not provided
Process Engineers tend to slap on a blanket 20C as a minimum on top of max operating, at least for process piping in plants. Max design temp in pipelines is often quite a bit closer as it makes a big difference, so 10C margin is common. Min design is often -10C , but I've seen zero - that one caused a bit of a problem....
Min design temp is often overlooked and only gets picked up in HAZOPs and such reviews, but a nominal -20C is fairly common unless you're in the artic or dealing with very high flash JT effects.
To me you need to see if the blanket approach takes ou over some sort of step limit for stress, type of material, reduction in yield stress (duplex is particularly bad) and then find some engineering reason to set a figure.
The argument that max pressure and max temperature can't exist a the same time might occur or might happen. Depends on your process.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Criteria for Selecting Design Temperature when not provided
RE: Criteria for Selecting Design Temperature when not provided
RE: Criteria for Selecting Design Temperature when not provided
I also remember a project in Europe where ASME B31.8 was intended to be used for Dehydration unit piping, nested inside an existing compressor station. Design ambient temperature was -29 degC. While B31.8 was allowed to be used for compressor station piping, the certification body did not approve piping classes according to the same B31.8 for dehydration unit which sits on discharge of the compression trains. I still cannot really understand why the piping on my left side was made of one material, and the piping on my right side had to be designed by using another code (for the same process line). I understand the codes, but I could not grasp the logic and their physical meaning in this case.
Dejan IVANOVIC
Process Engineer, MSChE
RE: Criteria for Selecting Design Temperature when not provided
RE: Criteria for Selecting Design Temperature when not provided
The 50 F is a real number with historic basis. Right or wrong, it's historically correct.
RE: Criteria for Selecting Design Temperature when not provided
At that time, this was a big fuss. Compressor discharge side including the header was designed for 64 barg. Being forced to use B31.3 for the Absorber inlet and outlet piping, and given the fact that 400# ASME B31.3 is not a standard piping class in the company, the certification body proposed 600# class. Wall thickness of 600# piping was two times the wall thickness of B31.8 piping designed for 64 barg. 600# piping requires PWHT. You can imagine the blow. Ultimately, since the compressor discharge side was greatly overdesigned with regards to design pressure, we had de-rated the entire discharge system to 50 barg which allowed us to use 300# piping - similar wall thickness to B31.8 64barg and no PWHT required. To today's day I never understood why we could not use the same B31.8 piping for Absorber inlet/outlet piping.
Apologizes to the OP for diverting from the original subject - this just came in as a part of discussion on the design conditions.
Regards
Dejan IVANOVIC
Process Engineer, MSChE
RE: Criteria for Selecting Design Temperature when not provided
But I have done the same thing (used B31.8) for sitting dehys between field gathering compressors and gathering system pipelines located in non-regulated remote gathering systems. Since it was a rather low pressure field, some compression was needed to get the dehy down to reasonable size.
Ya. Sorry dso88. Sometimes it's just too easy to keep going in the same thread.