blowdown of pipeline 3

[Kandan]

HI all,
I have a situation in which,

we have a 36" pipeline of approx 80km length. to blow down this line in an emergency.. with a 2" line to the vent.. estimate is -33degC at the end of blow down.. but CS line design temp -29C. Can any body suggest without changing the material.. how to bring up the temp during blow down..
(study says the temp is conservative..)

 

[SJones]

Where does the -29 deg C minimum design temperature come from as this smacks of the limits set for materials such as A106 Gr B in ASME B31.3? ASME B31.8 does not impose such limits and it is a common error made by piping engineers who attempt pipeline designs to automatically slap on a -29 deg C minimum design temperature for such carbon steel materials in a misguided attempt to avoid Charpy testing. Which code actually applies to the line in question, i.e. is there a spec break between pipeline and piping somewhere in the relief line? If it is B31.8, then the material really should have been Charpy tested anyway at some rationally selected test temperature from an integrity point of view and A333 Gr 6 would have readily covered all eventualities. If it is B31.3, then, as others have said above, you have recourse to the graphical presentation of allowable MDMT lowering based on stress ratio in figure UCS-66.1.

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdo/

 

[Buchi]

Hi Kandan,

We recently had a similar situation with depressurizing a pipeline to the flare where we were going to encounter a temperature lower than the minimum design temperature. We got round this by injecting hot gas from the lean gas compressor discharge, just upstream of the compressor discharge air cooler.

You might consider a similar approach.

Buchi

 

[jcaiken]

A few points:
1 Depressuring a blocked-in pipeline is an isentropic expansion of the gas in the pipeline with adiabatic expansion across the vent valve. The lowest temperature is therefore in the vent line not the pipeline.
2 The temperature downstream of the vent valve will reduce as the temperature of the gas from the pipeline falls (due to the expansion) but rise as the dP across the valve decreases.
3 The dP created down the pipeline by the gas flow to the vent can be significant, particularly as the initial pressure reduces.
4 Generally I agree with MortenA. The heat interchange between gas and pipe wall and surroundings is very complex. In addition to the variation in heat transfer coefficient from changes in the surroundings, the inside HTC is a function of the conditions and the flowing velocity down the pipeline length. Estimation of wall temperature at specific points is an area where even transient analysis software may only provide guidance and engineering judgement based on experience needs to be applied.