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.