What is a balance hole? Why are the balance holes on turbine blades odd in number?

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I. What is a balance hole? What is its function? To balance the axial thrust of the turbine, small holes (balance holes) are usually made in the rear cover plate of the impeller, near the hub, so as to maintain a constant low pressure in the annular chamber on the back side of the impeller. To reduce leakage, a sealing ring is also installed on the rear cover of the impeller, with its radial position matching that of the sealing ring at the inlet. Generally, the total area of the balance holes must be 4 to 5 times greater than the area of the clearance around the sealing ring on the back cover of the impeller. However, as the fluid in the annular chamber on the back side of the impeller flows into the impeller through these balance holes, it disrupts the suction condition of the fluid at the impeller’s inlet, increasing the flow losses within the impeller and thereby reducing its flow efficiency and cavitation resistance; for this reason, this approach is only used in small pumps. This method is simple and reliable, but it cannot fully balance the axial force; it can only balance 70% to 90% of the axial force, with the remaining portion having to be borne by the thrust bearings. Although, in theory, there is no pressure drop on the rotor blades of an impulse turbine, steam leakage through the diaphragm seal still creates a certain pressure difference before and behind the rotor, resulting in an increased axial thrust. Installing balance holes on the impeller helps to reduce the axial thrust. II. Why are the balance holes on turbine blades odd in number? An odd number of balance holes are provided on each impeller, which prevents the placement of two balance holes on the same radial section and thus avoids excessive weakening of the impeller’s cross-sectional strength. Usually, there are 5 or 7 openings. III. How many methods are there to balance the axial thrust of a steam turbine? Methods to balance the axial thrust of a steam turbine, aside from the axial thrust being supported by thrust bearings: 1. Balance pistons are installed at both ends of the high-pressure shaft seal with opposing pressure differences. 2. The high-pressure and medium-pressure cylinders are arranged in reverse order. 3. Shunted layout of the low-pressure cylinder. 4. Provide balance holes on the impeller.