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J-T Effect When Starting up Gas Well?

J-T Effect When Starting up Gas Well?

J-T Effect When Starting up Gas Well?

(OP)
Hi everyone; keep up with your great advise, they have been very helpful to me. I need some advise concerning JT effect when starting up gas well. Any feedback will be greatly appreciated:

I've a incoming subsea (gas only) flowline 10" SDV that opens in 20 sec. with 220 mmscfd, .7 sg gas @ 340 barg and 50 dC (start-up conditions). There's no by-pass around the SDV. No doubt that there will be a JT effect. My questions are:

1. What would be the lowest P&T I've before hydrates start forming as a result of JT effect?
2. What happens if opening time is changed to 30 sec? How does time affects JT?
3. Do I need a by-pass to first equalize the line before opening SDV?
4. Dilemma is whether to use methanol injection downstream or change flowline material, say duplex sstl or LT cs.

Any feedback is welcome and take care.

lf

RE: J-T Effect When Starting up Gas Well?

Pressure up slowly without significantly blowing down your supply line in the process.  Offhand I would think that you don't have enough volume in your facility to substantially reduce the pressure in the pipeline from 340 Barg, as long as the well was flowing first.  What's downstream?

You probably don't need a bypass for a 10", but that pressure diff is pretty high.  In any case I would assume that being an SD it has a properly sized O/C actuator.

Maybe you have a flow assurance study somebody did for that pipeline that you can refer to for hydrate formation characteristics.
 

**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/

RE: J-T Effect When Starting up Gas Well?

you should have done a proper flow assurance study that covers special situations such as this before getting this far. Find it and see what it says.

Best regards

RE: J-T Effect When Starting up Gas Well?

(OP)
Hi and thanks again for your efforts.

These are all opportunities arised from a Hazid and I just wanted to get ahead of the game for my own info. Under normal operating conditions, the gas stays above the calculated 21 dC required for hydrates to start forming and pipe material is suitable. However, there is special case where we might have back-flow from a packaed riser due to to possible shutdown of a gas injection well or LNG loading terminal shutdown. Eventually, a FA report will be available, only that we are getting ready to start ordering long lead items; i.e. SDV, duplex pipe, etc.

How do you calculate dT across a control valve? I'm having a hard time finding a real equation do do this. Any feedback will be greatly appreciated.

Keep up the good advise and be safe.

...lf

RE: J-T Effect When Starting up Gas Well?

Maybe you should get a consultant or find an in-house expert?

Calculating JT is fairly easy - since most of just would "fire-up" our trusty old simulation tool. If you dont have that, then its either half a degree celsius pr bar (convert to your favorite unit here) or quite difficult.

I would say that its no more special than it should have been included. Today i would assume that a sub-sea project would include some transient multiphase simulation - including the scenario you mentions.

Best regards

Morten

RE: J-T Effect When Starting up Gas Well?

btw: Its either very hot waters you live in - out your assumption re. temperature is incorrect. How would you prenet the top part from cooling?

Best reards

Morten

RE: J-T Effect When Starting up Gas Well?

Quote:

Dilemma is whether to use methanol injection downstream or change flowline material, say duplex sstl or LT cs.

One is thermodynamic hydrate inhibition and the other is materials selection to avoid brittle fracture - how is that a dilemma?

When deciding upon a minimum design metal temperature, really examine the metal temperature and not a flowing gas temperature under J-T cooling as they may not be the same.

Steve Jones
Materials & Corrosion Engineer
http://www.linkedin.com/pub/8/83b/b04
 

RE: J-T Effect When Starting up Gas Well?

leofig,

Per my understanding, you have to look at your fluid properties to determine the dT due to JT effect (throttling or isenthalpic expansion). Given a specific dP you will get a particular dT. Determine the lowest temperatures the equipment will be subjected to on the basis of maximum pressure differential.

Note that valve opening is a dynamic process, where you are changing the dP (and hence dT) across the valve. If you open slower, the valve will take longer to equalize pressures, and the fluid and equipment will be subjected to high differential temperatures for a longer time.

Also, consider that when you open the valve slowly, the flow rates are quite small, so the initial coldest "slug" of fluid will be very small in volume. This will be followed by a larger "slug" which will be slightly warmer ... and so on.
 The equipment and fluid downstream will be at a much higher temperature. This will represent a greater thermal mass than the initial cold "slug". So your temperature transients may not be excessively severe.

You may simulate this in OLGA or something similar to get an idea of the length scales and suitably select piping material for a specific length downstream of the valve.

You will have to look at the hydrate envelope to determine if fluid is likely to form hydrates after it goes through max. dT.

You idea of continuous methanol injection may work as it will introduce additional volume at a higher temperature (run numbers to determine the volume rae) thus helping you avoid hydrates and preventing exposure of downstream equipment to excessively cold temperatures.

Wow .. I wrote quite an essay there. Hope it helps.  

RE: J-T Effect When Starting up Gas Well?

Also, SJones is correct in suggesting that the dT realized in the fluid will not be the same as dT realized in the equipment. At low flow rates, convection heat transfer will be quite low.

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