MiniumDesignTemperature for gas piping

[USER10]

Piping codes (CSAZ662 , B31.3 , B31.4, B31.8) requires that minimum design temperature is assigned carefully in the design. (e.g CSAZ662, Lowest expected metal temperature during pressure testing
and service under design conditions)

My question is what is on the implication of using inaccurate minimum design temperature (say within - + 5 to 10 C). The implication of describing higher value(even by 5 to 10 C) means violating regulations and possibly causing failure.
But if one conservatively describes minimum design temperature only by few degrees (say 10 C lower based on detailed search and analysis), would that have significant impact on cost ( purchasing pipe material with notch toughness, welds , HAZ, ect). Will be other unwanted pipe performance implications as a result of describing conservatively lower value of minimum design temperature ?

I appreciate your thought and discussion on this matter.

 

[mk3223]

IMO, this question has been repeated among the Project, Process, and Material Disciplines for many projects, or in every project. It can be a case by case basis, but an Engineering decision should be based on the safety over cost.

 

[USER10]

Of course, safety should be number one ( over cost)
one of the points of the question above : will assigning a conservative value of the minimum design temperature (could be by 5 to 10 C only) have significant cost increase, or welding or materials qualification (toughnesss), purchasing ramifications.

I am particularly talking about underground gas pipe-line (where minimum temperature could reach/range from 0 C and -20 C, depending on the area)or above-ground station piping ( above -30 depending on the atmosphere and J-T cooling effect)

In other words, is it worth it to spend significant efforts (collecting data, analysis, consulting more expert)to have accurate minimum design temperature, or assigning conservative value within (5 to 10 C) based on judgment should be OK.
Thanks

 

[georgeverghese]

In the range of lower design temp of -20degC to -30degC, it is only a matter of whether silicon killed CS is impact tested or not - I would imagine the cost of getting this test done at the pipe mill is insignificant, so being conservative in the setting of lower design temp has little cost or schedule impact from this perspective.

 

[LittleInch]

You really need to discuss this with a materials engineer to understand where the current cut offs are in terms of more testing, more expensive material etc.

A -20C design temp limit is relatively normal, but as you go colder, you get into more trouble with Charpy impact testing etc. Where the practical limit is before you need to change material grade to say A333 grade 6 I don't know, but certainly below -30 you run into trouble.

Try to make sure you are looking at cases that can happen, i.e. not cold JT cooling when the pressure has to be low for the cooling to happen.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[USER10]

Thanks LittleInch ..for your thought on this ..We are talking here about Gas-distribution&transmission (not process or oil pipeline) ..for buried pipelines we are looking at min temperature range ( 0 to -15) depending on the area (north or south) .since ground is involved, heat-transfer analysis may need to be done to define minimum temperature accurately ..for above ground piping ,,I am assuming two factors will govern determining the minimum design temperature ..air temperature , or fluid temperature ( at pressure reduction components), whichever lower ..