Gas Line Hydraulics Question

Hello All,

I have a system where i am compressing 10 mmscfd of gas from 45 psig to 190 psig. It then mix with 10 mmscfd of gas at 190 psig and flow into pipeline. The gas at the end of pipeline is 160 psig. However, I want to have gas at 190 psig at end of pipeline. Of course, I can increase compressor pressure discharge. However, when 10 mmscfd gas mix with this ...what will the pressure be? The compressor will build up pressure as per downstream resistance (reciprocating). however, i want pressure to be 190 psig at end of pipeline. WHat can be the options?? pleas see sketch also. Thanks.

Edited by james.corrobro@gmail.com, 22 July 2020 - 11:23 PM.

Currently both the streams are at 190 psig hence they are mixing, if you increase the pressure in one of the stream then you are creating additional back pressure in the other stream. Please note that the stream to mix both of them should be at the same pressure.

If you want less pressure drop in the downstream piping, you can increase the pipe diameter, but having said that if the inlet pressure is 190 psig you cannot get the outlet at 190 psig (unless you oversize the piping way more than it is required) coz there will be some pressure drop in the system.

Or:

1. Raise the pressure of both sources to 220 psig.

2. Add a compressor at the end.  190 --> 220 psig.

agree with Latexman. 

I am not sure why you say "However, I want to have gas at 190 psig at end of pipeline". I assume it is not a choice but 190psig may be an operating pressure in the equipment at downstream of pipeline. 

If so, you have to increase a compressor pressure as you already mentioned but, in that case the side stream you cant mix on compressor discharge. Looking at the compression ration in your case (i.e. 190/45 = 4.22), i think you anyway require two stage compressor in your current scheme. In that case, split the compressor stage optimum and mix your side stream in between the stages at appropriate location. For example, assuming optimum compresssion ratio is say, 3.5, design a compressor with first stage from 45psig to say, 160psig for 10mmscfd main suction stream. Drop the side stream pressure from 190psig to 160psig. Mix it with first compressor stage discharge stream and then put a second compressor stage to increase mixed stream (20mmscfd) pressure from 160psig to a value required to overcome hydraulic losses (along a pipeline) to deliver 190psig pressure at downstream of pipeline. 

Alternatively, you can drop the side stream pressure from 190 to 45psig and then increase the mixed stream (20mmscfd) pressure to a desired value as explained above. but i dont see any merit in that scheme as it may increase compressor cost and power requirement higher than above scheme.  

Note that, line size increase in your case wont help as you are already at a pressure (190psig) at the inlet of pipeline which you actually "want" at tht outlet of pipeline. 

Edited by Jiten_process, 26 July 2020 - 06:18 AM.