Flow of gas in pipes

I was going through my old diary and found some information I had noted down from a book on Gas Technology I had come across in a library about nine years ago. I don’t recall the title and author of the book but it was one of those out of print old books you get attracted to for some secret/sacred knowledge. :)

The information was about flow of gas in pipes and the simple diagram and explanation there was extremely useful and it helped me throughout my career.

I am recreating the diagram here, hope you will find it useful.

Assumptions:
1) Horizontal pipe, uniform diameter
2) Four pressure gauges attached to points A, B, C and D at equal distances of 8m.
3) When valve E is opened, gas flows in the direction of the arrow.

The graph below shows what happens under four different sets of conditions: (Please click on the image to zoom in.)

1) Standing Pressure: When gas is standing still in a pipe i.e. when valve E is turned off, the pressure throughout the whole length of the pipe is the same. 

So, pressures at A, B, C and D = 20 mbar 

2) Working Pressure: Valve E is turned on to allow a ‘normal quantity’ of gas to flow through the pipe. In this case the pressure drop between point A and B is 2 mbar. 

So, pressure at point B = 18 mbar 

But, distance between A and B is same as distance between B and C as well as C and D. So pressure loss will be same i.e. 2 mbar. 

So, pressure at point C = 16 mbar 

Hence, Pressure at point D = 14 mbar 

3) Working Pressure with increased quantity: If the quantity of gas flowing is now increased then the pressure loss will also increase. The line 3 shows, with a 40% increase in quantity, pressure loss has doubled i.e. 4 mbar. 

So, pressure at point D = 8 mbar 

4) Normal quantity with partial blockage: Line 4 shows readings obtained for a ‘normal quantity’ of gas flowing but with an obstruction in pipe between point A and B. This reduces the effective diameter of pipe between point A & B and this has greatest effect on the pressure. 

Point B onwards pressure drop is back to normal i.e. 2 bar. 

However, the actual pressure drop between point A and B has increased. 

Pressure at point D = 4 mbar

Let there be light gas! :)