10 replies to this topic

[bazilbolia]

Hi guys,

 

I'm Bazil, a graduate chemical engineer and currently working at an oil and gas company. I was tasked in calculating the size of a  three phase separator and i need help in calculating a preliminary sizing of the separator. (I'm only given a day to calculate the size of the separator, so i'm trying to figure this out during my free time at night).

 

These were the data's given

 

Total liquid flow rate = 15,000 BLPD

Oil flow rate = 12,000 BOPD

Gas flow rate = 15 MMSCFPD

Water flow rate = 5000 BWPD

Crude API Gravity = 49 @ 35 ' C

Closed in tubing head pressure = 2050 psig

Flowing tube head pressure = 1440 psig

Flowing tube head temperature = 167 ' F (75 ' C)

Bottom hole pressure = 2050 psig

Bottom hole temperature = 210 ' F (98.89 'C)

Reid Vapour Pressure = 12 psia

 

I have little knowledge in separator calculations (I focused more in reactor designs when I was in uni), and I've read the API 12J on separator sizing, and the example at the back didn't do much help, as I don't know the flowing gas density. 

 

I could easily convert the crude API gravity into normal density and could I assume the flowing tube head pressure and temperature as the operating pressure and temperature of the separator as we do not know what operating conditions it would be? 

 

I've downloaded some notes from the forum, and i've read the notes and calculation from Marcel Dekker.

 

Using notes from Marcel Dekker, there were a few degree's of freedom, so I had to make some assumptions in order to use certain equations provided, such as the oil and water retention time (15-10min), gas density 650 kg/m3, Gas compressibility factor of 0.89, viscosity of 20 cP, water density of 1040 kg/m3 and a drag coefficient of 0.65 (i'm not sure how did the 0.65 Cd came about). I had to make these assumptions so that I could have a feel on how to use the equations.

 

I've roughly calculated the maximum allowable vessel diameter and got 180.776 inches which equates to 4.6m. I'm happy with the number that i've obtained but I was pretty sure that in order to get a real accuracy of the separator that I was tasked to calculate, I need the data's that I assumed earlier on, am I right? Or is there any other ways to do it?

 

I've attached the Marcel Dekker snippet which I took from the forums for easier reference and my preliminary calculations.  I apologize if it's in the wrong section and if there are any misunderstandings.

 

Am I in the right direction?

 

Thanks

Attached Files

•  DSC_2111.JPG   55.55KB   8 downloads
•  Separator Sizing Marcel Dekker.pdf   274.7KB   142 downloads

[Bobby Strain]

You should know that the gas density you indicate is not even close to the proper value. So, go back and start at the beginning before you waste more time.

 

Bobby

[MTumack]

You're happy with a Vessel diameter of 15 feet? That size is totally outside the realm of reality.

 

Who the hell would build that? Let alone ship that... Even ignoring the fact that it is friggen huge in size, the shell thickness required would be in the neighborhood of 10 inches thick easily. That's not only impossible to fabricate the materials for the vessel (shell /  heads) it would take probably 2 years to complete the Circular / Long seam welds required on it. And even if you could get it to that point, it would be impossible to heat treat (as required by ASME VIII on anything thicker than 1.5" in most applications). You'd have normalizes steel in some places, work hardened in others, a total nightmare really.

 

Your specifications are wonky too... 15000 barrels per day liquid with 5000 barrels water and 12000 barrels Oil? 1+1=3 I guess. Your gas assumptions are out to lunch at best.

 

I havn't done any calculations, but my experience says you could likely get 5 minutes retention time and acceptable separation in a 60" OD or 72" OD vessel. You should be looking for numbers closer to that range. It is also important to note, you will likely have to make some design choices here in terms of fabrication, the design pressure of 2050 psig @ 200 °F is wonky, would require CL 1500 Flanges, and still gets you a shell thickness in the range of 4 inches, but that is do able.

[bazilbolia]

You should know that the gas density you indicate is not even close to the proper value. So, go back and start at the beginning before you waste more time.

 

Bobby

 

Hi Bobby, 

 

I know that the gas density is not even close to the proper value since i don't have the composition, that's why im making the assumption. Could I just use the ideal gas formula to find out the density of the gas or wouldn't that be inappropriate? 

When you say go back and start at the beginning, what do you mean? Is that by knowing more of the design data's? 

 

Thanks

 

 

You're happy with a Vessel diameter of 15 feet? That size is totally outside the realm of reality.

 

Who the hell would build that? Let alone ship that... Even ignoring the fact that it is friggen huge in size, the shell thickness required would be in the neighborhood of 10 inches thick easily. That's not only impossible to fabricate the materials for the vessel (shell /  heads) it would take probably 2 years to complete the Circular / Long seam welds required on it. And even if you could get it to that point, it would be impossible to heat treat (as required by ASME VIII on anything thicker than 1.5" in most applications). You'd have normalizes steel in some places, work hardened in others, a total nightmare really.

 

Your specifications are wonky too... 15000 barrels per day liquid with 5000 barrels water and 12000 barrels Oil? 1+1=3 I guess. Your gas assumptions are out to lunch at best.

 

I havn't done any calculations, but my experience says you could likely get 5 minutes retention time and acceptable separation in a 60" OD or 72" OD vessel. You should be looking for numbers closer to that range. It is also important to note, you will likely have to make some design choices here in terms of fabrication, the design pressure of 2050 psig @ 200 °F is wonky, would require CL 1500 Flanges, and still gets you a shell thickness in the range of 4 inches, but that is do able.

 

 

Hi Tumack, these were the specifications that was given to me, and I assume that yes, the total fluid is 17,000 barrels instead of 15,000. I havent seen a proper separator yet, nor designing one so i'm taking a lot of assumptions with me. As I couldn't able to sort out the calculations using the equation from the notes by Marcel Dekker, i've calculated the size of the separator to be 2.5m x 0.5m, which is smaller that what you've mentioned. (I've attached the calculation that i've done- i've used some formula's from API 14E regarding pipe sizes. I've also attached the graph that i've used for the velocity assumptions).

 

I could probably say that if I would really want a proper sizing, I need some more data's.

 

Thanks for your input, Tumack. 

Attached Files

•  DSC_2115.JPG   63.87KB   5 downloads
•  Pipe.PNG   259.47KB   6 downloads

[Ajay S. Satpute]

Bazilbolia,

 

Below link shall help for quick calculations.

http://www.cheresour...parator-sizing/

 

Regards.

 

Ajay S. Satpute

[MTumack]

You need to factor in your liquid retention. Its pretty standard that you'll need 5 minutes of retention.

 

Think of it this way; 17,000 BBL/day of fluid needs to flow into your vessel, and the stuff that goes through there needs to be in it for a solid 5 minutes.

 

17000BBL/d * 5 min/(60min/hr*24hr/day) = 59.03 BBL of fluid eating up volume in your vessel. Thats 331.2 cu.ft, or 9.4 cu.M worth of space required in a Cylindrical vessel.

 

Your specified vessel has a volume of .5m^2*pi/4 * 2.5m = 0.491 cu.M.

Edited by MTumack, 11 February 2015 - 12:26 PM.

[paulhorth]

Bazil,

This is such a sad message. What has happened to training in process design these days? You don't seem to have any idea about the purpose of a separator, yet your boss expects you to do it in a day - most professionals would expect to take longer than that to do a proper design.

You have to design your system first. That is, you need to decide what happens to the product streams, the gas, oil, and water. Where do they go? Are you trying to make crude oil to meet a specification, such as vapour pressure, water content? Does the gas need to be compressed, or is it sent to flare? When you have thought this through, and drawn a flowsheet, you can consider the main design parameter for the separator, which is the OPERATING PRESSURE. You have to fix this first before you can size the vessel - and it certainly will be lower than the wellhead pressure, or else nothing could flow into it! Then, and not before, you can run a process simulation which will give you a material balance, that is, the flow and composition of the gas and liquid phases.

You can actually make a rough sizing without a simulation but this means estimating the gas density and gas flowrate at your chosen pressure, and making an estimate for the temperature. To make these estimates takes some experience.

 

Bobby's comment about gas density has nothing to do with the composition. 650 kg/m3 is a much higher density than any gas would have, in a process like this, by a factor of at least 20.

 

You also need to understand the basics of vapour-liquid equilibrium in multicompoment mixtures, like wellhead fluid. When the pressure goes down, more gas comes out, not just more volume but more moles. So, fixing the pressure is very important.

 

Paul

[Saxmomusic]

I am brand new in learning how to size oilfield equipment and trying to find resources (particularly 3 phase separators and heater treaters). Can anyone tell me what book the Marcel Dekker PDF came from that bazilbolia's original post?  Thank you.

[Zauberberg]

The quoted material is from the book "Petroleum and Gas Field Processing" by H.K.Abdel-Aal and M. Agour, published in 2003. The book is also available online at: http://www.icheh.com... Processing.pdf

Edited by Zauberberg, 09 March 2015 - 02:21 PM.

[Bobby Strain]

The best reference is Stewart and Arnold. They deal in oil.

 

Bobby

[exxterlin]

Dear  bazibolia,                      

 

 

you need to read the GPSA and DEPS shell. Besides the API 12J

 

If you want to tell me what you send me your email and will associate more information and some help if necessary.

 

The chromatography is recommended that araves analyze a program like HYSYS, etc.

 

I also disponfo payroll that I use on a daily basis to verify and size separators.

 

 

Greetings, Maximiliano