Isentropic Efficiency in Depressuring 6

[JoeWong88]

Dear all,

Low temperature embrittlement due to system blowdown shall always be considered during material selection study. During Basic Engineering phase, normally depressuring unit in HYSYS and / or PRO-II will be used to carry out blowdown and it low temperature effect. One of the parameter seriously affect the results especially temperature is ISENTROPIC EFFICIENCY.

The higher the isentropic efficiency, the lower the final temperature. In some case , when we use Isen. Eff = 80%, its final temperature is lower than the pre-selected material (LTCS) low temperature limit (-46 degC) and a much expensive material (SS) is required. If we apply lower isen. eff=50%, its final temperature is still higher than the CS LT limit.

I have gone through several project. Some project use isen. eff= 50%, some use 80%, etc. Some use 100% for gas system and 50% for vapor-liquid system, etc...

I really would like to take this opportunity to gather some informations, thoughts and advices from all of you.


Looks forward your advice.

JoeWong

 

[EmmanuelTop]

Hi Joe,

In my opinion, isentropic efficiency is closely related to the ratio of compression work VS friction losses during depressuring process. When estimated efficiency is high (close to 100%), fluid temperature falls down to the lowest expectable values. This happens because no amount of energy is transfered to heat. But maybe my assumption is not correct.

I think it is very hard to estimate isentropic efficiency during depressuring process, which is dynamic in its nature. During first stage calculations, I always use the most conservative approach (98%), because it will result in the most safe (but not cost effective, sometimes) design approach.

http://antwrp.gsfc.nasa.gov/apod/astropix.html

 

[sailoday28]

Heat transfer to the control volume in question will keep the temperature above that of an isentropic process. If the process is rapid, then the isentropic process should be considered. Otherwise find a program that incorporates the effect of heat transfer.

Regards

 

[JoeWong88]

EmmanuelTop,
Thanks for your inputs.

I do agree with you that it is hard to estimate isentropic efficiency during depressuring process as it is dynamic in nature.
Using isen. eff. of 98% (or even 100%) no doubt is most safe and conservative but costly approach...

sailoday28,
Thanks for your inputs.

Yes. Heat transferred from metal mass potentially prevent the fluid temperature drop too low. My opinion would be heat transfer (heat transfer coefficient, heat transfer efficiency, heat potential from metal mass, etc)is another factor by itself from cold depressuring perspective. Weak relationship between heat transfer and isen. eff. (just opinion).

JoeWong

 

[Latexman]

There has been significant experimental work over many years by vendors of compressors, pumps, and turbines to measure the isentropic efficiency of their products. Of course, they had to do this to sell their products. Plus, the isentropic efficiency of a compressor is a lumped parameter of just the compressor. This makes it easier to measure.

But, to blowdown a pressure vessel or pipeline, all that is needed is a valve and some controls. Since the valve and controls do not play much of a role in determining the isentropic efficiency of the blowdown process, similar vendor data is not readily available. And, the isentropic efficiency of a blowdown process depends on the fluid, the vessel, the pipe, if the vessel is insulated, if the piping is insulated, its surroundings, etc. It's a much more complex distributed parameter problem. Wall friction, fitting losses, heat transfer, relative amounts of fluid and steel, and shock effects, if present, will cause deviations from isentropic behavior.

Over the last 10-20 years there does seem to be some limited literature articles on this, and they do refer to some experimental data. The references I found quickly are:

Rapid depressurization of pressure vessels, Journal of Loss Prevention in the Process Industries, Volume 3, Issue 1, January 1990, Pages 4-7, Afzal Haque, Stephen Richardson, Graham Saville and Geoffrey Chamberlain.

Modelling of two-phase blowdown from pipelines—I. A hyperbolic model based on variational principles, Chemical Engineering Science, Volume 50, Issue 4, February 1995, Pages 695-713, J. R. Chen, S. M. Richardson and G. Saville.

Modelling of two-phase blowdown from pipelines—II, A simplified numerical method for multi-component mixtures
Chemical Engineering Science, Volume 50, Issue 13, July 1995, Pages 2173-2187, J. R. Chen, S. M. Richardson and G. Saville.

A numerical blowdown simulation incorporating cubic equations of state, Computers & Chemical Engineering, Volume 23, Issue 9, 1 November 1999, Pages 1309-1317, Haroun Mahgerefteh and Shan M. A. Wong.

None of these articles contain the single source answer to this question, so a literature search and study will be needed by those who have a need to understand this issue. I also saw a reference to more experimental data by Overa, Stange, and Salater in 1994 in one of the above references, but could not find it in the databases available to me. Maybe others can find it.



Good luck,
Latexman

 

[sailoday28]

It would also be interesting to have the definition of the efficiency for the blowdown process.

 

[JoeWong88]

Latexman,
Thanks for your great inputs to this subject.

sailoday28,
My understanding of isentropic efficiency in depressuring process...

Isentropic efficiency for an expansion process is defined as

[tab]Isen. Eff. = Actul work / Isentropic work

Depressuring is an dynamic expansion process and i view it as step wise expansion as the vessel pressure is gradually decreased. Isen. eff. will be used as constant throughout the depressuring...

JoeWong

 

[EmmanuelTop]

Thanks Latexman. Great inputs from your side, as usually.

For the purpose of estimating minimum temperature excursions during depressurization of process vessel or pipeline, I think there is no single source or criterion that can be applied without risk of getting insufficiently accurate results. To many factors are involved, and for each system very rigorous modeling (if possible) should be performed - in order to predict fluid and metal behavior during blowdown. At this point, one can realize that having too many factors/criteria to be considered is as confusing as having no criterion at all. Hand-on-experience and "sixth sense" can play a master role in this kind of calculations.

Every time I encounter extremely low temperatures in software simulation, I try to recheck them manually - assuming 100% conversion of energy without degradation to heat. And I play it safe, specifying MOC which is suitable for such temperature levels. This way I know the system will work 100% safe, and I sleep quietly at night.

http://antwrp.gsfc.nasa.gov/apod/astropix.html

 

[Latexman]

EmmanuelTop,

Sage advice!

Good luck,
Latexman

 

[sailoday28]

If material selection is the main concern as a result of low temperatures during a blowdown, then conservative analyses whether using moc or quasi steady can be done. However, one should also remember that if the process is considered adiabatic with regard to the fluid, then the material is not undergoing a temperature transient.
If one considers the heat transfer effects, then a more realsitic model should include temperature gradients in the structural material.
For example, blowdown of a long hollow cylinder with heat transfer, considering radial gradients as governing should include solving DEL^4 phi, the biharmonic equation.

 

[JoeWong88]

EmmanuelTop,
Yeap...there's no single source can be applied without risk of getting insufficiently accurate results. That's the main purpose of this post to gather information, practices, thought & experiences from all.

ALL,
From low temperature embrittlement and material selection perspective, taking Isen. eff. of 100% is rather conservative.

Well...from depressuring rate & flare capacity design, Isen. eff. of 100% may not be conservative. What's do you think from fire depressuring perspective ?

Some information you may already awared. There are at least 2 softwares in the market available for depressuring study. They base on rigorous model.

1) LNG-DYN from TECHNIP
2) BLOWDOWN from IMPERIAL COLLEGE.

Personally i have used the LNG-DYN. Good (and also bad thing) is NO Isen. Eff. required.


JoeWong

 

[Latexman]

The practical side of me suggests if the blowdown lasts on the order of an hour duration or longer, metal temperatures should approach isentropic, and if the blowdown lasts on the order of seconds, metal temperatures will not approach isentropic. This leaves us to struggle with blowdowns that last minutes, which is probably the majority of them.

Good luck,
Latexman

 

[sailoday28]

Latexman (Chemical)Are there typos in you last post?
If the blowdown is rapid, lasting seconds, there should be little Q and process should be approaching isentropic.
Regards

 

[dcasto]

remember, during blowdown, there is reduced hoop stresses. The most critical thing to watch is stress due to contraction.

If you look at blowdown of a saturated liquid, you can only have 100% efficency as the boiling liquid needs lots of energy to vaporize. In long pipelines, the probability of failure from cryogenic temperatures during a blowdown is not much of an issue.

 

[sailoday28]

I still would like the formal definition of efficiency during a blowdown process.

 

[dcasto]

There isn't really an efficency, the delta H is zero across a valve or port. Efficency is the change in ethalphy. The efficency would be based on the heat transfer rate from the surroundings into the system. If you insulated the system, then no heat transfer, no delta H 100%.

 

[Latexman]

sailoday28,

No typos. I was specifically talking about metal temperatures, not process temperatures.

Good luck,
Latexman

 

[JoeWong88]

Thanks to all active involvement in this discussion...

Latexman,
Please see my 2-cents opinion...

From metal perspective, quick depressuring will cause latent heat in metal have insufficient time to transmit to fluid. If depressuring is slow enough (infinite time), sufficient heat transfer and approaching "isentropic" (not sure if this term apply here).

From process perspecitve, rapid expansion put system in metastable state where expansion should approach isentropic expansion. If depressuring is slow enough (infinite time), then the process will not be isentropic.

Please correct or confirm my understanding.


dcasto,
Please correct me if i wrongly interpreted your post.

The efficiency that you mentioned is heat transfer efficiency from surrounding to fluid. Whilst the Isentropic efficiency is related to work efficiency due to expansion.

[tab]Isen. Eff. = Actul work / Isentropic work

Assuming a vessel is perfectly insulated (system is perfectly isolated from surrounding), during depressuring, fluid temperature drops and latent heat in the metal will transfer from metal to fluid. As liquid having very high transfer coefficient, metal temperature (liquid contact part) is merely same as fluid temperature. Thus, the heat transfer efficiency is high (theoretically close to 100%) in liquid contacted part. However, as vapor is having very low heat transfer coefficient, latent heat in metal (vapor contacted part) have difficulties in transferring heat to vapor. Thus, the heat transfer efficiency is low (theoretically close to 0%). One of the valid real world example is we normally notice icing formed at the bottom of depressured vessel.

In my opinion, the pipeline example would have less low temperature problem if it is subsea pipeline. Nevertheless, most (if not all) subsea pipelines are wrapped with multiple coating which will act as insulator. It will decrease heat transfer efficiency from surrounding to pipeline / fluid.

If the pipeline is expose to ambient air, my opinion is the air will have a very low heat transfer efficiency. Thus heat contribution from ambient to the pipeline would be low especially the depressuring is quick in nature.

In my opinion, the great contributor of heat (metal temperature do not drop too low) is the latent heat from the metal itself.

One of the example is the long distance subsea pipeline transfer production gas, condensate or full-well stream fluid, pipeline depressuring may take upto few hours and not compliance to API Std 521 of 15 minutes.

You have highlighted a valid point. "During blowdown, there is reduced hoop stresses. The most critical thing to watch is stress due to contraction.".

sailoday28,
I have uploaded some definition of "PV work contribution term" used in depressuring unit (HYSYS)...

http://files.engineering.com/getfil...d5e-034370e22ce9&file=Optionpage_Isen_Eff.PNG

I believe the efficiency is referring to Isentropic efficiency as stated in previous post.



JoeWong

 

[dcasto]

there is no work, so Eff = 0 / 0 = infinite efficency.


Now lets get real life what the pipeline wall temperature will be. I've blowndown lots of pipelines. If its gas, the pipe will get to the isentropic temperature (or the JT coefficient temp). If its a liquid line, you'll get the bubble point temperature.

 

[sshep]

I read through this thread and only at the end does dcasto finally mention the real issue that perplexes me: What sort of a normal depressuring process recovers the pressure drop as shaft work such that isentropic efficiency would become an issue in the calculation?

I may have it wrong, but the whole issue of isentropic efficiency does even seem neccessary to be applied to normal depressuring via a valve. When throttled across a valve, the energy from the pressure drop must be contained in the outlet fluid per standard engineering throttling model, although in practice some pressure energy may be dissappated as velocity, noise, vibration, etc. If you really want to exploit the possibility of low temperature at the outlet you must extract shaft work, but this is not normally a desired objective of depressuring.

Isentropic efficiencies of 80% to 100% (as cited above for calculations) require the extraction of shaft work via such equipment as an expander. This equipment achieves low temperatures in a pressure letdown process by removing the extra energy of the pressure drop as shaft work.

In otherwords, I think you are making the issue of downstream temperature in the depressuring line more complicated than it need be for vapor depressuring. With respect to cold temperature (ice cited above) forming in an upstream vessel being depressurized, this is usually due to vaporizing liquid in the vessel. Vaporization requires heat which is taken from the environment. The temperature (and corresponding boiling pressure) obtained inside the vessel is a function of heat transfer and has nothing (that I can see) to do with the notion of isentropic efficiency.

best wishes,
sshep