gas pipe inner wall temperature calculation

[atayto]

Hi all
I need help. Quick response is highly appreciated. I am currently trying to solve one problem in on production platform. during the well start up gas cap from the well is being bleed off through the flowline manifold and main line to Separator. Gas temperature in well is 6deg C and pressure is 300barg. when gas passes to the separator through the choke valve its pressure drops to 26.5barg. due to joule Thomson effect the temperature drops down to -68 deg C.

In the winter the ambient temperature is -7 deg C.

The pipe embrittlement temperature is -51 deg C.

In reality I know that gas flow will not reduce pipe wall temperature to -68 deg C because of heat transfer from air. Also I know that pipe itself has some heat.

does any body knows how I can calculate pipe inner wall temperature? I have some more questions but it will be easier for me to ask when I have answer to this question.

Thanks a lot.

 

[casflo]

You must calculate the heat q transmitted from the air to the gas, taking into account free convection for the air, outside the pipe, the pipe metal resistance and the forced convection inside the pipe for turbulent gas flow, but it is necessary you have some knowledge on heat transfer.
With the correlations for the film coeficients inside and outside the pipe, it is possible to determine U, the global transmission coefficient and with U it is possible to calculate q.
When q and the film coefficient inside the pipe have been calculated you can calculate the inner pipe temperature.

 

[EmmanuelTop]

It is a trial and error procedure - first assume the surface temperature of the pipe, than calculate heat transfer with ambient air, then use that Q figure to calculate inner wall temperature (convection) and outer wall temperature (conduction). Repeat till the assumed outer wall temperature equals the calculated value.

Keeping the flow below certain limit will limit temperature excursion - at least on paper. Please bear in mind that what happens in reality, and especially if there is liquid phase in the system, can be different from what the calculations say. It seems that the designer probably did not include cold startup in system design temperature calculations, and this can be potentially very dangerous thing. The system being on an offshore platform raises the risk further. Calculating the maximum flow that can prevent from overcooling is a good thing, but I wouldn't place my bets on that - there should have been a startup heater or a different piping class used.


Dejan IVANOVIC
Process Engineer, MSChE

 

[LittleInch]

I would add a TI ds your choke and control flow based on temperature.

Bigger issue is probably hydrate formation. If you need to add meg upstream, it will throw your temp calcs all over the place.

Any calculation always seems to fall short of reality hence just control on temperature. Temp falls too far, reduce flow.

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

 

[georgeverghese]

As pointed out in some previous discussions, these low temp startup transients due to gas depressurisation are handled by start stop mode operation under work permit.

 

[atayto]

Thanks all.

 

[MortenA]

To answer your question: Yes this can be calculated and is routine done using standard software. You can also make a fairly good approximation yourself using textbook heat transfer and excell.

If you dont have the knowledge or the software i would recommend that you hire a consult for this type of work.

Best regards, Morten