blowdown of pipeline
blowdown of pipeline
(OP)
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..)
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..)





RE: blowdown of pipeline
RE: blowdown of pipeline
RE: blowdown of pipeline
To understand what the discharge temp is and how to bring it up, what is the fluid, source conditions and type of insulation on the blow down line?
RE: blowdown of pipeline
If these were adiabatic calcs, account for the heat transfer that will take place.
Good luck,
Latexman
RE: blowdown of pipeline
The calculated temperature - is that equal to the gas temperature? If so: You would be surprised how much higher the steel temperature is!
I can recommend that you take a look at an article in Hydrocarbon processing, july 2004: "Material selection for low temperature applications" By S. Kumar et al. This article deals with actual metal temperature during blowdown and it is rather surpicing just how conservative you are when selecting temperature based on the gas exit temperature.
A program such as Pipeline studio can also calculate this value during a trancient depressuring analysis!
Best regards
Morten
RE: blowdown of pipeline
RE: blowdown of pipeline
We have been having similar thoughts and discussions on both the pressurisation and depressurisation of a pipeline.
We have discussed the problems with a metalurgist and even though the gas temperature is low the pressure is also low and the therefore brittle fracture is not likely. I could mention the pressures that have been indicated but it is specific to the actual pipe materials.
The easiest method to bring up the temperature is to control the rate of blowdown so that the gas warms up as it blows down.
RE: blowdown of pipeline
1) Report indicates adiabatic case.. I understand , the blow down duration for the 80km pipe is around 7 days.. no way of prolonging the blow down..
Can quark suggest from experience how expensive would heat tracing be..
Though the pressure D/S of RO is low .. the blow down temperature can creep to the high pressure side (7 days blow down??)
Also how much low temp can CS cater to say at 60 barg..
RE: blowdown of pipeline
The temperature up stream of the orifice will not decrease because of the gas flow(?).
Have you thought about how you will pressurise the system - this has caused me lots more problems than depressurising.
RE: blowdown of pipeline
John
RE: blowdown of pipeline
If it is below ground, you will get heat transfer from the surrounding earth, but I couldn't tell you it would be enough without doing heat transfer calcs. There is a constant heat flux from the earth core, and presuming ambient temperature is above -33C, you would have a heat flux from above the earth.
If it were piping rather than a pipeline (i.e. B31.3 rather than B31.8), you could have calculated a lower permissable temperature for the carbon steel based on the condition of low stress.
Finally, there is no significant difference between -33C and -29C, although I understand the need to comply with rules. Note that in Section VIII and ASME B31.3, the -50F threshold was changed to -55F, to include propylene, and one of the rationale was there was essentially no difference in the material properties between those temperatures.
RE: blowdown of pipeline
Is there an equivalent "31.3 rule" in 31.8 or API RP1111 regarding low temperature excursions when the corresponding pressure is lower? I don't have a copy of either and I was curious.
RE: blowdown of pipeline
From a blow down stand point, the ASME B31.3 approach would let you consider the stress (but also considering longitudinal stress) at the actual conditions.
RE: blowdown of pipeline
RE: blowdown of pipeline
RE: blowdown of pipeline
The expansion of gas produces a cooling effect (nominally 4degC per MPa). Therefore if you expand it over two valves, you still have the same overall pressure drop and the same overall temperature decrease. The method you are proposing is only effective if you can put heat into the gas between the two valves. You can get some heat gain from the ambient and some pressurisation lines are extended in length and made of stainless.
RE: blowdown of pipeline
RE: blowdown of pipeline
RE: blowdown of pipeline
While it is usually good to use a rule-of-thumb to estimate temp drop at lower pressures as many programs do, it should be noted the JT coefficient is not constant. It varies with pressure and this variation cannot be ignored at these pressures.
RE: blowdown of pipeline
Steve Jones
Materials & Corrosion Engineer
http://www.pdo.co.om/pdo/
RE: blowdown of pipeline
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
RE: blowdown of pipeline
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.