Multi-stage pump balance pipe pressure

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The pressure in the balance pipe of a multi-stage water pump is the pressure after throttling; it is a dynamic process (essentially slightly higher than the inlet pressure). Large multi-stage boiler feed water pumps are equipped with pressure gauges on their balance pipes, and this pressure is related to the throttling that occurs at the gap of the balance disc, which in turn is related to the load on the water pump. In short, (pressure inside the balance disc – inlet pressure) / area of the balance disc = axial thrust. The function of the balance pipe in multi-stage water pumps is to lead an external pipe from the outlet seal ring of the multi-stage boiler feed water pump to the inlet side of the pump; this pipe is called the balance pipe. Its main function is to balance the axial thrust of the water pump, reduce the axial movement of the rotor, and prevent friction between the impeller and the casing. When a multi-stage boiler feed water pump is in operation, high-pressure water is discharged from the outlet of the impeller; some of this water flows toward the back of the impeller, resulting in a pressure there that is roughly the same as that at the outlet. The side in front of the impeller’s blade assembly is the suction side, where the pressure is very low. In this way, a large pressure difference exists on both sides of the impeller, resulting in an axial thrust that points toward the pump inlet and is parallel to the shaft. This thrust presses the entire rotor toward the suction side; in severe cases, it can cause friction or collision between the impeller and the pump casing, affecting the safe operation of the feed water pump. Therefore, it is necessary to find a way to balance this force. There are many ways to balance axial thrust, such as using double-suction impellers or symmetrically arranged impellers (in multi-stage pumps), as well as using balance holes, balance discs, and balance drums. The balance pipe is merely a way to balance the axial thrust; it directs the pressurized water behind the impeller through the balance pipe to the inlet side, thereby balancing the pressures on both sides of the impeller. These methods have a simple structure, but they cannot fully balance the axial thrust; the remaining axial thrust must be absorbed by specially designed thrust bearings.