25 replies to this topic

[sshah]

Dear Sir, I want to install new natural gas piping. I have got data from suppliers that i am able to receive the gas at 60 barg, amb temp and 11000 scm flow rate. Now as the incoming press is to high I have to reduce the pressure up to 4 - 5 barg. when i follow ideal or real gas low it shows me reduction of pressure up to 4-5 barg the temp of gas will reduced to -243 °C but this is not viable. Please you are requested to educate me on this problem. How do i consider to reduce pressure press of incoming natural gas? and how do i maintain temp at amb?

[ankur2061]

sshah,

 

If you reduce the pressure of the natural gas across a control valve or any throttling device the temperature will definitely drop due to the free adiabatic expansion of gas. The question is to what extent? At very low temperatures there may be even liquid dropout which you may have to remove using a Liquid Knockout Drum / Vessel.

 

Have you modeled the pressure reduction of your natural gas from 60 barg to 5 barg using some kind of a simulator such as HYSYS or any other to arrive at -243°C? How did you arrive at -243°C? Could you furnish your gas composition and pressure and temperature conditions before the pressure reduction for any of the other Cheresources members to check the outlet temperature after pressure reduction?

 

Regards,

Ankur.

[sshah]

Thank you very much Ankur.

 

No I have not done any modeling for this. As per ideal gas law P1/P2 = T2/T1, i have considered 1 for inlet condition n 2 for outlet condition if solve this i got -243 °C.

 

Compositions of Natural gas is

 

C1 = 92.64%

C2 = 4.52%

C3 = 1.24%

iC4 = 0.21%

iC5 = 0.03%

C6+=0.17%

CO2 = 0.52%

N2 = 0.31%

NC4 = 0.30%

 

Sp.Gr = 0.6

NCV = 8660

GCV = 9594

 

Will u plz help me to get the correct temp at outlet after reducing press up to 5 barg?

 

Thank you in advance

[sshah]

after metering station:

Inlet press is 60 barg

Inlet Temp is amb

flow is 11000 scm

 

Required outlet paramaters for burners inlet is

 

Press: 1 - 4 barg

Temp: amb

 

Total length of pipe is 300 m approx.

 

I have to install piping network for RLNG fired burners in place of Fo fired burners.

 

I haope u will get all data from above.

[neel_avi]

Dear Sshah,

 

As some data were assumed 

Flow rate - 11000 sm3 per day.

 

Diameter of the pipe considered as 6".  

 

The temperature downstream of the control valve is coming to be -7.4 Deg C. I guess if its through an expander, the temperature will drop further. 

 

Please find the schematic of the model which was simulated on TGNET. 

 

Regards

 

Avijit

Attached Files

•  NG pressure.png   419.71KB   148 downloads

Edited by neel_avi, 26 March 2013 - 01:14 AM.

[sshah]

Dear Avijit,

 

Thank you very much.

 

I am reducing press through PRV from 60 barg to 5 barg. I have considered line of 4'' considering velocity of 9.5 m/s sec for 11000 SCM flow rate, the pressure drop derived is 0.7 kg/cm2

 

I am planning to put Pre heater before PRV to raise temp of inlet gas from amb to approx. 50°C then reducing the press from 60 barg to 5 barg.

 

Plz give ur suggestion

[sshah]

Dear Avijit,

 

I m sorry there are two nos of header each having 230 SCM/hr flow and 4'' of line size other are as mentioned above

[ankur2061]

sshah,

 

HYSYS provides the temperature drop at 25°C inlet temperature for your composition to be -6°C for a pressure drop of 55 bar (60 to 5 barg). Below is the snapshot. 

 

Regards,

Ankur

Attached Files

•  Pr_Drop_60barg_5barg.JPG   288.58KB   151 downloads

[sshah]

Dear Ankur,

 

Actually the flow rate is 1100 SCM/day i.e 230 SCM /hr. sorry for inconvience caused. I am very greatful if you are educate me for the reducing system squence also.

[ankur2061]

sshah,

 

Flow rate plays no role in determining the temperature due to pressure drop. It is the gas composition, inlet temperature and the pressure drop that determine the outlet temperature. Even considering 11,000 Sm3/day the temperature due to pressure drop from 60 barg to 5 barg the temperature remains -6°C considering an inlet temperature of 25°C.  Obviously, if your minimum gas temperature is even lower than 25°C, the outlet temperature will also be lower.

 

Regards,

Ankur.

[Dacs]

I think the issue that you have to look at is the possibility of having a temperature lower than your system MDMT. In your case however, I think you're still good to go.

 

And to add, you can't use ideal gas laws for high pressure system. You can't even use that without considering JT-effect anyway.

 

All that said, I think you have to focus on how to achieve that pressure reduction. IMHO you need to have 2 control valves in series (1 pressure and 1 flow CV) to properly control the letdown.

 

[sshah]

Dear Ankur,

 

Thnak you very much for your reply.

 

As per ur calculation gas comesout at -6°C and 5 barg. but as i have to distrubute gas at 5 barg and at amb temp furthet i have to heat it up with HE.

or

I have to pre heat it up to 60°C before reducing pressure from 60 to 5 barg to achive gas after pressure reduction at amb and 5 barg. am i right?

 

Please  share ur points.

 

Is ther any hand made or excel calculation or reference books available for manual calculation of temperature drop from pressure drops and gas compositions?

 

As I have to install new facility of RLNG piping distribution pipe lines for utility help to understand property and calculation of Natural gas behavior to carry out line sizing and pressure reduction of the same.

 

Thank you in advance.

 

Thank you all.

[PaoloPemi]

sshah,

if you model with ideal gas law dt/dp = Joule Thomson = 0

 

as Ankur correctly suggested you can solve a constant enthalpy flash (Hin=Hout) to calculate outlet temperature,

almost any simulator gives this possibility with different models as Soave-Redlich-Kwong, Peng-Robinson etc. which are reasonably accurate for hydrocarbons,

 

I write in relation to your question for excel calculation or reference book,

yes there are both Excel Applications,

for example I utilize Prode Properties

and good books,

I recommend the Properties of Gases and Liquids, it's old but still a reference.

[ankur2061]

sshah,

 

Unfortunately I do not have any excel sheet for such a calculation. If you let me know what temperature you desire at a reduced pressure of 5 barg then I can tell you what temperature to maintain at the inlet of the PCV.

 

Regards,

Ankur.

[sshah]

Dear Ankur,

 

I required amb temp, as i have to distribute RLNG to different location by approx 400 m long pipe lines.

[shan]

Not P1/P2=T1/T2,

It is (P1*V1)/(P2*V2) = T1/T2 if Z1=Z2, You know V1 (based on inlet T1 & P1)but you don't know V2.  You may determine the letdown gas T2 based on isenthalpic expansion.

 

You may heat the gas either upstream or downstream of the pressure letdown valve depending on the available heating medium temperature.

[ankur2061]

sshah,

 

You keep on repeating "amb". Even ambient has to have a numerical value depending on the location of the plant or unit and again ambient would be different during winter and summer. So a numerical value has to be defined for the gas temperature and only you can do that. Please provide the numerical value for anybody to make some sense.

 

Regards,

Ankur. 

[sshah]

Dear Ankur,

 

Temp for Natural Gas are

Summer = 35-38°C

Winter = 10-12°C

Moonsoon = 28-30°C

[sshah]

plz take average temp is min 18 °C and Max is 40 °C

[ankur2061]

sshah,

 

Any process design engineer would require one design value to be used as a design case and not the ambient temperature range. I believe you are an engineer and hence you need to provide me the temperature for your design case for the pressure reducing skid. 

 

Normally for natural gas consumers the temperature should be well above the hydrocarbon dewpoint of the natural gas. Some companies require it to be a minimum of 15-20°C above the HC dewpoint of the natural gas. If natural gas is being fed to a Gas turbine or gas engine often a tempertature of 60°C at the B/L of the turbine or engine is specified. In such a case the gas would require to be heated before it goes to the turbine or engine.   

 

Now can you decide what temperature do you require for the gas after the pressure reduction or you still want somebody else to decide for you?

 

If you can't decide then I shall assume the required temperature to be 20°C above the HC dewpoint. Let me know.

 

Regards,

Ankur.

[sshah]

Dear Ankur,

 

Please consider 15 C above dew point. 

[ankur2061]

sshah,

 

The dewpoint of your gas corresponding to a pressure of 5 barg is -30°C. If we consider that the minimum temperature that needs to be maintained for the gas downstream of the pressure reducing skid should be 15°C above the HC dewpoint then you cannot allow the gas temperature to fall below -15°C.

 

In order to maintain a minimum temperature of -15°C at the downstream of the pressure reducing skid, the minimum upstream temperature required is 18°C. All what I have mentioned is theoretical based on calculations of HC dewpoint and the requirement of temperature at upstream of pressure reduction at 60 barg in order to have a minimum temperature of -15°C downstream of pressure reduction at 5 barg.

 

Practically you should not allow your gas temperature to fall below 5°C in order to prevent icing on the line and subsequent practical problems related to icing such as corrosion and low temperature stresses on the piping. Hence, I would suggest that you do not allow the gas temperature to fall below 5°C. In order to do that, the minimum gas temperature that should be maintained at 60 barg upstream of the pressure reduction is 34°C.

 

I am hoping that this will be the end of this thread and I have addressed your major concerns.

 

Regards,

Ankur.

[ankur2061]

sshah,

 

There is one another point that I missed out and that needs to be highlighted. At a pressure of 5 barg and -15°C you will have hydrate formation downstream of the pressure reduction which are ice-like crystalline structures and will plug the gas piping over a period of time.

 

Hence the suggestion stands that you should maintain a minimum gas temperature of 34°C upstream of the pressure reduction.

 

Regards,

Ankur.

[sshah]

Dear Ankur than you so much for your explanation.

 

Just last information i want to know is:

 

Inlet pressure of gas is 60 barg with temperature of 28°C is available, now i will heat inlet stream before reducing station by 32°C so the inlet steam temp of reducing station is 60 °C and pressure is 60 barg so outlet temp near to 30°C will be achived art 5 barg and due to friction pressure drop pressure reduced up to 1 barg in pipe line even i m able to get nearer to 26°C gas at outlet of transfer pipe. am i correct?

 

Thank you very much for knowlegde sharing and this is the end of this thread.

 

Once again thank you all.

[hoyar]

I am going to design a gas reducing system. inlet gas pressure is 55 barg in 40 ^oC. I have considered a K.O.Drum after that . gas temperature leaving drum must be above 10 ^oC.

 

1. criteria of handbook for KO drum sizing

2. how i can find out heater is required or not? what kind of heater is applicable?

 

thanks