Box 9.2

Reduction in Positive Displacement Pump Output Due to Gas Cutting

Note: The following estimates are minimum values. As gas cutting increases, the flow properties of the fluid increase and the head above the suction decreases, all of which will further degrade the performance of a positive displacement pump.
The following would apply to Moyno feed pumps as well as duplex and triplex mud pumps. What would be the reduction in output of a positive displacement pump if the gas cut at the suction of the pump reduced the fluid density from 12 ppg (1440 kg/m3) to 10 ppg (1198 kg/m3)
1. Given: The displacement of the pump is 13 volume. The reduction in liquid volume due to gas at the suction is :

12 ppg *y^3 vol = 100 ppg * 1^3 vol

where Y3¼0.83, so 83% of 12-ppg mud and 17% gas by volume will give a 10-ppg drilling fluid.

2. Given: The suction pressure is 1 atm absolute (14.7 psi gage or 101 kPa), and the discharge pressure is 5 atm absolute (73.5 psi gage, 518 kPa). The reduction in volume as the gas is compressed to the discharge pressure would follow Boyle’s law:

P1V1 = P2V2

1 atm abs * 17V1 = 5 atm abs * V2

V2 = 0:03V1

Then the final discharged volume of fluid would be only 86% of the uncut volume. If the pump under consideration were supercharged (as a triplex mud pump would be) part 2 of this example would have to be calculated first for compression of the gas as a result of the charging pump, then for the new volume used in part 1, and then recalculated for part 2 and summed to get a solution.