13 replies to this topic

[ikramreduzan]

Im involve in a gasfield development project. We are currently in the conceptual design stage and one of the concept that need to be study is JT Process. I do need explanation regarding the relation between pressure drop at choke valve and its effect to the JT Process. The case that I try to figure out is as following:

 

FTHP = 2500 psig

Required Export Pressure = 1560 psig

 

by considering pressure drop in equipments to be very small and the main pressure drop is cause by choke valve and JT valve. the question is:

 

1) which pressure drop we need to determine first, Choke Valve or JT Valve pressure drop???

 

we can see in the results section (Attachment 1) that the higher pressure drop on the choke valve & lesser pressure drop on the JT Valve; will results in: 

 

-the water contents in the product gas will be higher.

 

-temperature will be higher

 

-the condensate flow rate will be lesser.

 

Can anyone describe what is the criteria that need to be taken into account when we deal with this problem. Thank you.

 

 Attachment 1.xlsx   10.42KB   86 downloads

 

 

[Bobby Strain]

It looks as though all these conditions will result in hydrate formation. So you need to resolve that issue.

 

Bobby

[ikramreduzan]

I do Injected MEG into the Gas stream upstream of the heat exchanger and the JT Valve... so there is no issue for hydrate formation... I have set that the MEG injection rate to ensure the LTS temperature is well above the Hydrate formation temperature. the issue is i dont know which pressure drop should we determine first whether it is choke pressure drop or JT valve pressure drop...

[Nasiruddin]

It depends on number of factors,

 

1. What is the requried water dew point specification of sales gas?

2. What is the maximum sales gas delivery temperature?

3. What are the potential products (Sales gas , LPG or no LPG, NGL, condensate, etc.)

4. What is overall system design pressure of gas processing facility (lb rating, 900# or 1500#)

5. You have to optimize the available pressure drop between choke and JT valve to consider overall design.

 

You also mentioned lower condensate flow in case of higher pressure drop across choke valve. This you can optimize through operating pressure of the 1st stage separator (I assume there should be at least one separator which receives the well fluids at the plant inlet thorugh manifold).

 

Nasiruddin

[ikramreduzan]

the gas is NGL. It will be transport to another platform and the minimum landing pressure required at the other platform is around 1400 psig.  the gas will be mixed back with the condensate before being transport to the other platform. the transport gas temperature is around 90 to 105 Deg F.. the transport water contents is expected to be 7 lb/mmscf gas even we can get it much lower. for that amount of water in the gas;the water dew point spec is at 40-45 Deg F....

 

Mr Nasirudin, so you r suggesting that I need to optimize the choke and jt valve pressure drop to fulfill all the requirement that you have listed on your post? 

 

Can you correct me if im wrong with this statement:

 

if the pressure drop at choke valve is low that's mean that all the gas processing facility before the JT valve will have higher design pressure which will surely increase the cost. but it has the advantage for lots more pressure drop at JT valve which will results in lower temperature and more condensate. 

 

but in case of we choose to increase the pressure drop in the choke valve, we will have lower design pressure for the facility upstream of the JT valve which will reduce the cost. the disadvantage is less pressure drop at JT Valve  which will results in less condensate..

 

for your suggestion to optimize the first stage separator pressure to get more condensate. i do think the pressure at the first stage separator is related to the pressure drop at choke valve... the higher pressure drop, the lower separator pressure and less condensate...the lower pressure drop.. the higher separator pressure and higher condensate

[Nasiruddin]

Why you are mixing condensate back in to gas, its better to have overall system sketch / PFD for correct understanding of the system first.

[ikramreduzan]

Attached is the process scheme...we mix the condensate and the gas into the same line because of onshore have a device to measure amount of condensate and gas even they are in the same line.

Attached Files

•  HYSYS-Print.pdf   24.66KB   100 downloads

[Nasiruddin]

What is the purpose of this whole facility, first you are separating gas and condensate after cooling and then mixing again for transport (without any pressure boosting device. i.e. pump or compressor) only taking out Fuel Gas. Is this possible to simply mix gas and condensate in the pipeline like this.

 

If these are well fluids to be transported to other platform for processing, simply choke can be adjusted depends on downstream requirement.

 

Will you please explain the purpose of this whole system

 

Nasiruddin

[Sathya R]

1.  Considering 100 kpa pressure drop across the choke valve is acceptable. However this will give maximum Cv.

 

2.  Also consider the FTHP profile in the design of the JT Valve.

 

3. The FTHP of the gas wells tend to decline in few years.

[Bobby Strain]

If you knew how to use Excel VBA to automate HYSYS, the solution could be reached quickly. If you are going to stay in this business, if you don't know Visual Basic, you should learn.

 

Bobby

[ikramreduzan]

The purpose of this platform is to make sure the export gas fulfill the pipeline dew point requirement. the amount of water is expected to be below 6lb/mmscf gas. the platform will remove acces amount of H2O and make sure it comply to export requirement.

 

Sathya: the high pressure is expected to continue for more than 10 years... what is the relation between the pressure drop at the choke and Cv???  and what is it consequence to overall process with maximum Cv???

 

Bobby: im quite new in conceptual, FEED & HYSYS simulation... involve in detailed equipment design in most of my previous time. Dont even know about excel VBA. 

[Nasiruddin]

Still I feel the choke pressure drop should be governed by delivery / tie-in pressure. You have to calculate the pressure drop in the export line by fixing the delivery point pressure. Then back calculate all the pressure drops inside the facility (Exchangers, vessels, etc.) will give you the downstream pressure of choke valve. Pressure drop across JT you can select based on Gas water dewpoint requirement.

 

Alternatively, to optimize condensate production you need to run different cases for production separator at various pressures, draw a graph between Pressure and condensate production, it will give you an idea at which pressure maximum condensate recovery is possible. Based on that you can chooose the pressure drop across choke valve.

 

I hope this will help.

 

Nasiruddin

[ikramreduzan]

Thank you Nasiruddin. Ive done as what you said regarding back calculate to get the choke pressure drop. will do the graph of pressure and condensate soon. your comments help me a lots. thanks. can you also help me on my new post regarding the compression curve in HYSYS. thanks

[Erwin APRIANDI]

I just read this thread and I'm questioning, Why you should optimize the condesate production by play around with pressure and temperature at JT valve outlet if at the end you will mix the gas and condensate in the export line?
 
For me it is better to optimize the pressure and temperature at JT valve to gain the required water content of 7lb/MMscf or water dew point spec of 40-45 deg F (at what pressure???) so to make sure that the water content meet the most stringent spec. And to be sure that water from the condesate will not increase the dew point and water content.
 
In the other hand while you are doing a conceptual study, you also have to consider the turn down, since a JT does not have high turndown ratio. To achieve high turndown ratio you have to install several valves. And for JT valve application, you also should consider the pressure available from the well, whether it is enough to supply enough pressure for the whole operating case during whole plant lifetime?
 
Conceptual design, FEED is about predicting something that may happen, doing this kind of project required a lot of experience since you have to ensure that all possible scenario of operating are cover so you would not land on adding new equipment at the end (during detail design) which add more cost to the overall project.
 
Hope it help.

Edited by erwin.apriandi, 26 March 2013 - 09:35 PM.