8 replies to this topic

[x127127@hotmail.com]

Hello Everyone,

 

I am trying to evaluate the  existing blowdown system of a natural gas compression station. And I really need some help and your expertise. I have two major questions here.

 

It is a controlled type blowdown. The gas inverntory in high pressure segment(3000 psig), intermidiate pressure segment (1000 psig) and low pressure segment (50 psig) is throttled by one pressure control valve (PCV) and the set pressure is 20 psig for flare to reach full capacity of 50 MMSCFD. The vent gas is routed to the ground flare for combustion, or vented by venting stack if flare is not operable. There is a SDV in each blowodown line and no orifice or other pressure reducing device.

It is not a good design. The PCV is oversized and can not control the downstream pressure with accuracy, and we have to do blowdown mannually adjusting the PCV. I am trying to solve the problem, one option is replace the existing PCV with two pressure regulator in series. However in this design, the upstream pressure regulator is still subject to wide range of upstream pressure from (100-3000 psig). I still doubt if the option of two pressure regulator can handle the situation. I am thinking of a second option, replace the existing one PCV with three parallel control loop comprising of a shutdown valve and one pressure control valve. The 1st control loop is designed to handle high pressure (1500-3000 psig), 2nd control loop for medium pressure (500-1500 psig) and 3rd for low pressure (0-500 psig). A pressure transmitter and a new pressure controller will be used to switch between the three lines. I am not sure if the option can work or not. Do you have better idea on how to modify the blowdown control system?

 

Another question I have is the vent stack. The vent stack is 16"x10", 10" for the tip. I have back calculated the velocity and found that at 10" vent stack tip gas flowrate will be Mach No 1 @ 66 MMCFD. With 400 ft long 16" pipe all the way downstream to PCV, the pressure will be 23 psig. However, the pressure setting of PCV is 29 psig when routed to vent stack. I am very confused here. The back pressure is calculated to be 23 psig , and PCV set @ 30 psig. What action PCV will take when the downstream pressure is 23psig, PCV will open more to increase the downstream pressure to 28 psig?. If PCV really fully open, what is going to happen - the blowdown header will be subject to high pressure of the bulk gas, for example 3000psig?  I am assuming that if PCV fully open and the pressure discontinuity will occurs at each blowdown line with 3" or 4" pipe.

 

I hope I have explained my questions clearly. Any advice will be appreciated. Thank you guys.

[Bobby Strain]

A sketch would help.

 

Bobby

[x127127@hotmail.com]

Please find the sketch attached. Thank you very much.

Attached Files

•  20171216175217.pdf   43.4KB   44 downloads

[latexman]

I am not very familiar with blowdown systems.  You'll probably get advice from others who are more familiar later.  It seems to me that one would need to know the pipe size and design flow rates of each of the high, medium, and low pressure segments to get started and to determine what control scheme modification is needed to improve controllability.

[Bobby Strain]

This looks like an accident waiting to happen. It's hard to tell what a mess it is without a P&ID showing all valves and piping specifications along with spec breaks. Without that information it is not possible to make a recommendation.

I estimate that you can vent about 100 MMSCFD through the vent with 23 psig at the pcv outlet.

Bobby

Edited by Bobby Strain, 17 December 2017 - 05:59 PM.

[x127127@hotmail.com]

Thank you Bobby.

 

The upstream pip of PCV is 900# carbon steel, downstream PCV is #150 carbon steel. 

I calculated 66 MMSCFD (molecular weight 17.2) at 10" vent exit and velocity is Mach 1 and 23psig at the PCV outlet. May I ask 100 MMSCFD is based on 10" or 16" exit? PCV pressure setting is at 30 psig when routed to vent stack. However the backpressure calculation is 23 psig. I don't know what action PCV will have, since 23 psig is smaller than setting 30 psig. Will PCV open more to increase outlet pressure until full open?

 

The original design is not feasible. That's why I am trying to modify. I am thinking of using three control branches for high, medium and low pressure. All blowdown from high medium and low pressure are vented to common header and throttle by a common PCV, which is not a good design. 

 

Thank you for your help.

[Bobby Strain]

You must be careful how you calculate pressure profile at high mach number. I have my own software that evaluates the pressure loss at the exit of the pipe. I used 10-inch. I calculate the mach number at less than one using a k value of 1.2. Obviously the k value is important. The mach number at the pipe exit will never be more than 1. And yes, you should provide independent depressuring valve to each source.

 

Bobby

[x127127@hotmail.com]

Thank Bobby for your advise. I am going to design dedicated pressure or flow control for high pressure source, medium pressure source and low pressure source. 

[Glauco_Tirollo]

I believe this is outdated.

 

But I would like to comment,.

 

"The back pressure is calculated to be 23 psig , and PCV set @ 30 psig. What action PCV will take when the downstream pressure is 23psig, PCV will open more to increase the downstream pressure to 28 psig?"

 

I'm pretty sure that the backpressure is only a reference for sizing the valve. You should be controlling the pressure upstream the PCV and not downstream. If the backpressure is lower than expected  your PCV will open a little bit less.