Since for gas flow, we have chosen to call the horizontal axis the вЂњxвЂќ axis instead of the delta p axis, we define the pressure drop ratio at which flow becomes fully choked as the Terminal Pressure Drop Ratio, and give it the symbol xT, T standing for вЂњTerminal.вЂќ At this point we say that flow has become choked. At low pressure drop ratios the flow follows the straight line, but then it deviates more and more until at last, further increases in pressure drop ratio do not yield any additional flow. If we were to conduct a flow test, the actual relationship between flow and pressure drop ratio would be as shown by the curved blue line, not the straight one. Graphing the equation results in the upward sloping green line.

The equation at the top of Figure 1 tells us that flow is proportional to the square root of x. Making this change makes the expression вЂњО”pвЂќ which would appear in the equivalent liquid equation, equal to вЂњx p1.вЂќ (О”p/p1 X p1 = О”p) This change from the liquid equation is not absolutely necessary, but we will see later that it makes the prediction of gas choked flow much more convenient. We then substitute the single character, x, for О”p/p1 to make the expression simpler. (Note that the subscript, 1, for pressure and density indicate that they are the conditions upstream of the valve.) The only difference is that instead of using the square root of pressure drop (О”p) in the equation and for the scale of the horizontal axis of the graph, we use the square root of the вЂњPressure drop ratio,вЂќ О”p/p1. pressure drop ration through a control valve.The equation for flow at the top of Figure 1 is almost identical to the equation we would use for liquid in cases where flow was given in pounds per hour.

Pipesim equations for gas lift