Air or gas tapped in the pipeline carryiny a liquid can cause problems,eg reduce the effective flow,aggravate the effects of surge,and cause pump cavition.Possible cause of air (or gas) entrainment are:
- The pipeline was fully charged with air/gas when empty.
- Air is entrained at pump suction.
- Air is drawn in through faulty joints or gland.
- Air/gas is trapped in pockets during pipeling filling.
- Air/gas in solution is released due to change in pressure and temperature.
The problem of dealing with air/gas entrainment is not usually demanding one.Entrained air/gas will trend to collect at high pointin the system.It can then be removed by introducing air release valves at these points.These may by simple,manually- operated valves(bleed valves),or fully automatic.In the latter case the valve should perform the following functions:
a. Release of air/gas accumulating in pipeline during normal pressurized operation,to prevent restriction of fluid flow.
b. Retention of the fluid in the pipeline without loss under all operating conditions.
c. Release of air/gas during pipeline filling at a volume rate sufficient to prevent pressure restricting the filling rate.
e. Admission of air to the pipeline during emptying at arate sufficient to prevent excessive vacuum pressure in the pipe.
Single orifice air valves are capable of of performing funcions(a) and (b).They are normally used where only relatively small volumes of air/gas are to be releaseed,or where it is desirable to provide additional ventilation at operating pressures.
Dual(Double) orifice air valves are capable of performing all four functions.They can normally provide complete protection againt air/gas entrainment under all system operating conditions.