10 replies to this topic

[dgoyal]

I have system (gas compressor station) having volume almost 500 cubic meter this need to be depressurized by a common vent in emergency like gas leak , fire etc. The station condition would be as below ;

Blow down initial pressure =65 Barg
Blow down initial temperature = 30 Deg C
Final pressure required = 6.89 Barg ( 100 PSIG )
Maximum allowable blowdown time = 5 Min
As this is automatic blow down and can be activated in case of LEL alarm , so its suggested to consider adiabatic depressurization .
As this is a compressor station major volume is of piping.
MOC of system component is Carbon steel.

By using hysis Iam getting very low temperature in system so i want to make sure whether this blowdown time is achievable or not.
Normally in actual practice the temperature drops only after orifice not in upstream of orifice.
Please suggest is hyssy is right tool for determing this or not.

DGOYAL

[Art Montemayor]

DGOYAL:

Your query is not complete in describing or explaining what you are proposing to do:

1) What is the gas (or its composition) that you are venting?
2) For clarification, are you venting pure gas? Or Liquid + gas? Or liquid?
3) Is your compressor a reciprocating or a centrifugal type?
4) What is your "system" composed of?

If you are "blowing down" pure, discharge gas from the compressor's discharge piping system (as is usually done), then what you are doing is creating a free, adiabatic expansion of the high pressure gas to atmosphere. As such, then, you will experience the free, adiabatic expansion of a real gas - which is explained thermodynamically by the Joule-Thomson expansion. The Joule-Thomson expansion of a real gas creates a low-temperature, low-pressure gas product downstream. It does not affect the temperature of the source gas high pressure reservoir. The low temperature produced only affects the downstream piping and equipment. HySys should simulate the Joule-Thomson effect quite well. Why are you concerned with the downstream conditions going to flare or atmosphere? You can design low temperature piping (sometimes stainless steel) for this application. Your blowdown time is certainly achievable. All you require is the right Cv in your blowdown valve.

Please furnish an engineering schematic diagram and a complete explanation of what you propose.

[dgoyal]

The gas I am venting is natural gas and its pure gas compressor is centrifugal
System is composed of piping and compressor and two filter separator.
In hysys I modeled it as a horizontal vessel of 1 meter dia and 500 cubic meter vessel and I used dynamic depressing utility.

In steady state what you are proposing is correct but not during depressisation because during depressing the system temperature drop due to conduction and if system temperature drop the downstream temperature further drop.

I have gone through some more blog and I understand this is something related to isentropic efficiency but nobody sure what value should be chosen if you can proviso some thought on this this will be great

[Art Montemayor]

dgoyal:

Concise and detailed explanations are vitally important in order to communicate an engineering problem. What you now describe is probably NOT A PURE GAS expansion. In other words, the vessel being blown-down is not filled 100% with gas alone.

You are now alluding to another type of blow-down problem: the blowdown of a pressure vessel filled with liquid (condensed) hydrocarbons and a vapor phase above it composed primarily of natural gas. If this is the case (as I suspect), then YES, YOU WILL BE EVAPORATING A CONDENSED HYDROCARBON and consequently creating a refrigeration evaporator out of the pressure vessel. This cooling of the vessel is due not to the Joule-Thomson effect, but to natural refrigeration evaporation. Also, you can forget about any practical heat conduction through the piping if you are dealing with a 5-min blow-down. Unless you have an academic problem (homework) instead of a real, industrial application, then I wouldn't worry or bother with the heat conduction through the pipe for the brief, 5-min period.

This subject of "isentropic efficiency" has been kicked around and chewed up in this Forum in a thread originated by JoeWong. It has also been discussed at length in another Forum, Eng-Tips. You should refer to what was discussed or "resolved" in those threads and what is the definition of "isentropic efficiency". As I understand it, it is an experienced factor applied to the expansion in order to predict a more accurate, lower temperature than that calculated by simply applying isenthalpic expansion theory.

Hopefully, Joe is reading this and can join in.

[dgoyal]

THANKS SIR

This is pure gas because gas dew point is -20 Deg C. Now I have done further calculation and I have used the detailed heat loss model in HYSYS and i have calculated approximate metal mass in contact of vapor and the result is up my expectation.
What I have learned I am consolidating;
• We have to use the isentropic efficiency between 50-80. In my case I have considered 80
• Use detailed heat loss model in HYSYS.
• Make proper calculation of metal mass contact with vapor /liquid.
• Model the length and height of vessel properly based on considering piping involved in system. ( Remember HYSYS uses only single vessel in blow down calculation)

[JoeWong]

Mr. Montemayor,
Thanks for your excellence explanation and guidance...

dgoyal,
I don't have much to add to the excellence explanation and guidance by Mr. Montemayor . I would like to invite you to read this thread "Isentropic Efficiency And Its Implications (Click HERE)". The points brought-up in this thread may assist you in tackling your depressuring issue. Hope it don't confuse you.

In the process of depressuring from a vessel, the process in the vessel could be isenthalpic, isentropic or so called "polytropic" where in between isenthalpic and isentropic. The "Isentropic efficiency" in PRO-II is relating the theoretical isentropic process and actual process. However, the PV term used in HYSYS is weak in thermodynamic meaning. This parameter is subject to many factors such as vessel dimension, fluid composition, liquid level, configuration of vessel-piping, depressuring time, pressure and temperature level, liquid swelling, foamy / gassy, etc.

However, the vapor passing through the RO on depressuring line is normally isenthalpic.

Above is mainly the process in vessel and RO. Another great subject here is the heat transfer efficiency which is still yet to be studied. Metal contact with vapor, metal contact with liquid mist/annulus and metal contact with bulk liquid will have difference heat transfer efficiency. This further complicate the depressuring process.

Honestly, i am still struggling in understanding the impact of each parameters and phenomena and to find a way out to have a "Rule-of-thumb" for this magic "isentropic efficiency". What i really need is time and sufficient cases to derive the "rule-of-thumb". God bless. This day will come...or it may not come forever.

[Qalander (Chem)]

Dear Joe/Respected Art. Hello/Good Afternoon,

I must admit a very pertinent and analytical explaination here indeed!

Best regards
Qalander

[dgoyal]

Thanks
very mcuh really it was great expiantion.

[aju_1807]

Dgoyal,

Normally, when we perform depressurization calcs in HYSYS we consider 15 min as a depressurization time. However, in your case suprisingly ypu have considered 5 min (which is quite fast which may lead to quite lower temperature). Secondly, the temperaure of the fluid also drops because of gradual reduction in the pressure. This will be helpful in selecting vessel metallurgy. Normally, we consider wall temperature instead of fluid temperature for selecting metallurgy of vessel.

Hope my reply is helpful.

[JoeWong]

Dgoyal,

Normally, when we perform depressurization calcs in HYSYS we consider 15 min as a depressurization time. However, in your case suprisingly ypu have considered 5 min (which is quite fast which may lead to quite lower temperature). Secondly, the temperaure of the fluid also drops because of gradual reduction in the pressure. This will be helpful in selecting vessel metallurgy. Normally, we consider wall temperature instead of fluid temperature for selecting metallurgy of vessel.

Hope my reply is helpful.

Aju,
Depressurization considering 15 min as depressurization time either from initial pressure to 1/2 design pressure or 6.9 barg is just a recommendation and guideline. This probably as very good start for depressurization studies. All depressuring system designed according to these may NOT 100% safe and issue free.

Detailed stress analysis still required during detailed design to confirm depressuring system achieving depressurization objective and plant SAFE at reasonable and acceptable risk.

I have no surprise that some system may need to depressurized within 5 minutes. This typically applicable to thin wall vessel. Following post described more-or-less same idea...

Depressuring within 15 minutes no longer applicable ?

[logic]

Hi

I have also a similar situation where in i need to calculate the BDV CV using Hysys

can you please forward me with the procedure of performing the blowdown calculations for a centrifugal compressor loop

which consistes of a suction scrubber, compressor, air cooler

Your help will be much appreciated

Thanks a lot

I have system (gas compressor station) having volume almost 500 cubic meter this need to be depressurized by a common vent in emergency like gas leak , fire etc. The station condition would be as below ;

Blow down initial pressure =65 Barg
Blow down initial temperature = 30 Deg C
Final pressure required = 6.89 Barg ( 100 PSIG )
Maximum allowable blowdown time = 5 Min
As this is automatic blow down and can be activated in case of LEL alarm , so its suggested to consider adiabatic depressurization .
As this is a compressor station major volume is of piping.
MOC of system component is Carbon steel.

By using hysis Iam getting very low temperature in system so i want to make sure whether this blowdown time is achievable or not.
Normally in actual practice the temperature drops only after orifice not in upstream of orifice.
Please suggest is hyssy is right tool for determing this or not.

DGOYAL