Formation of air pockets
The term "formation of air pockets" describes the accumulation of air bubbles in certain areas inside pipes or centrifugal pumps which can seriously impair pump operation. They occur due to gas (mostly air) contained in the fluid handled in dissolved or undissolved (bubble) state (see Gas content of fluid handled).
To avoid the formation of air pockets, suction lift lines must be laid with a rising slope towards the pump. If possible, each pump should have its own individual suction line so that no air can penetrate the suction line via a non-operating pump when only one pump is running. Shut-off gate valves (see Valve) in suction lift lines should be fitted with their stem in a horizontal position.
Eccentric reducers and branch fittings should be used in horizontal suction lift lines.
See Figs. 5, 6 Fitting
Valve stems' gland packings should preferably be connected to a water supply under pressure. For long discharge lines incorporating rising and falling stretches of piping, each high point must be provided with an automatic vent valve.