APPENDIX F – Air Valves and Common Misconceptions
Surge Analysis and the Wave Plan Method
Supplementary Material: Example Problems and Solutions
Appendix F – Problem 1
F.1 A surge analysis study of a pipeline system with one kinetic air valve of 200 mm inflow and outflow diameters indicated air-slam condition occurs, with a sharp increase in pressure of about 4 bar at the time of air valve closure. The surge analysis was repeated with a three-stage non-slam air valve to reduce the extreme air-slam pressure. The three-stage air valve had the same inflow and outflow orifice diameters (and same discharge coefficients) as the kinetic air valve for its primary orifice and had a 40 mm diameter secondary outflow orifice. The switch from the larger outflow orifice to the smaller secondary outflow orifice occurs when the pressure in the air cavity exceeds 0.05 bar (gage). The surge analysis for the pipeline system with the three-stage air valve indicated a full air-slam pressure with a sharp increase in pressure of about 4 bar at the time of air valve closure (equal to the air slam observed with the kinetic air valve). The analysis was repeated with a switch pressure of 0.025 bar (gage) and the slam pressure dropped to 0.25 bar. Which one (or more) of the following explains this phenomenon?
i. Bug in the software
ii. The switch pressure has no bearing on the performance of the non-slam air valve
iii. The primary outflow orifice was too large for the air cavity and the lack of adequate resistance to the outflow of air prevented the pressure in the air cavity from rising to the switching pressure level, thereby letting all air out through the primary orifice which in turn lead to the full slam pressure.
iv. Non-slam air valves are not trustworthy and exhibit random behavior in surge analysis tools.
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