abolt
Chemical
- Oct 21, 2011
- 7
I haven't done any fluids in a while so bear with me, and I apologize for the length of my post.
We have an NG pipeline that has an upstream pressure of 50 psig and a downstream pressure at the sales line of 45 psig (has been as low as 40 psig). The flow rate had been around 80 MCFD but as of yesterday was 73 MCFD. I needed to calc what the pressure drop should be. We are concerned that the line is pinched and/or has liquid in it. An aerial view of the pipeline shows that its basically straight, and I've included a horrible paint rendition of the elevation view (not to scale). The total length is 2520' and drops 45' over a span of about 300' and then runs mostly flat. Near the middle, the line was bored 3' deeper to go under a stream and then rises 3' on the opposite side of the stream to the original depth. I'm not sure over what distance the 3' rise/fall takes place. 2000' of the line is SDR 11 3" plastic pipe. Where they bored under the stream they used 520' of SDR 11 4" plastic pipe. There is 1 90* elbow in the line.
Heres what I did: Found the flow to be fully turbulent and used the Blasius eqtn for f. The mach number was <<< 1, therefore I assumed it was incompressible and isothermal. For the kf(pipe) I just used 4fl/D for 2000' of the 3" section and 520' of the 4" section. For the kf(fittings) I used the 3k method for the 90* elbow and modeled the 3" to 4" transition as a sudden expansion at an angle of 30* and the 4" to 3" as a sudden contraction of 30*. I then solved EBE for P2 and found there should be a slight increase in pressure to 50.3 psig. Basically undetectable on the gauge. I found some basic pressure drop calculators on the web and they all came up with similar pressure.
My questions are:
1) I assumed the pipeline to be straight. Was this correct? There are some long bends in the line from the elevation changes but I didn't know if they would contribute any losses and I'm not sure how I would even measure them. Is there some rule of thumb where where you can neglect the bend and assume it to be straight (some r/D ratio?)
2) I made no provisions for the 3" rise/fall to go under the stream, mostly because I didn't know how to handle it since the net elevation change is zero. Was it safe to assume that the rise/fall was symmetric and that the gain from the drop was canceled out by the loss from the rise?
3)Did I handle the 2 different diameters of line correctly? Namely using the 4fl/D and the transistions as contractions/expansions? I don't know how the pipes were connected, so I figured this would be a "worst case" scenario.
4) Does my thought process/assumptions and answer (50.3 psig) make sense? We suspect that there may be liquid laying in the pipe where it goes under the stream. We think there isn't enough pressure to purge the line.
5)I also forgot to include in my calcs that there is a riser in the line, so probably a straight through T is in there as well, but I doubt that would contribute any appreciable losses?
Any input is greatly appreciated. Thank you.
We have an NG pipeline that has an upstream pressure of 50 psig and a downstream pressure at the sales line of 45 psig (has been as low as 40 psig). The flow rate had been around 80 MCFD but as of yesterday was 73 MCFD. I needed to calc what the pressure drop should be. We are concerned that the line is pinched and/or has liquid in it. An aerial view of the pipeline shows that its basically straight, and I've included a horrible paint rendition of the elevation view (not to scale). The total length is 2520' and drops 45' over a span of about 300' and then runs mostly flat. Near the middle, the line was bored 3' deeper to go under a stream and then rises 3' on the opposite side of the stream to the original depth. I'm not sure over what distance the 3' rise/fall takes place. 2000' of the line is SDR 11 3" plastic pipe. Where they bored under the stream they used 520' of SDR 11 4" plastic pipe. There is 1 90* elbow in the line.
Heres what I did: Found the flow to be fully turbulent and used the Blasius eqtn for f. The mach number was <<< 1, therefore I assumed it was incompressible and isothermal. For the kf(pipe) I just used 4fl/D for 2000' of the 3" section and 520' of the 4" section. For the kf(fittings) I used the 3k method for the 90* elbow and modeled the 3" to 4" transition as a sudden expansion at an angle of 30* and the 4" to 3" as a sudden contraction of 30*. I then solved EBE for P2 and found there should be a slight increase in pressure to 50.3 psig. Basically undetectable on the gauge. I found some basic pressure drop calculators on the web and they all came up with similar pressure.
My questions are:
1) I assumed the pipeline to be straight. Was this correct? There are some long bends in the line from the elevation changes but I didn't know if they would contribute any losses and I'm not sure how I would even measure them. Is there some rule of thumb where where you can neglect the bend and assume it to be straight (some r/D ratio?)
2) I made no provisions for the 3" rise/fall to go under the stream, mostly because I didn't know how to handle it since the net elevation change is zero. Was it safe to assume that the rise/fall was symmetric and that the gain from the drop was canceled out by the loss from the rise?
3)Did I handle the 2 different diameters of line correctly? Namely using the 4fl/D and the transistions as contractions/expansions? I don't know how the pipes were connected, so I figured this would be a "worst case" scenario.
4) Does my thought process/assumptions and answer (50.3 psig) make sense? We suspect that there may be liquid laying in the pipe where it goes under the stream. We think there isn't enough pressure to purge the line.
5)I also forgot to include in my calcs that there is a riser in the line, so probably a straight through T is in there as well, but I doubt that would contribute any appreciable losses?
Any input is greatly appreciated. Thank you.
