The volume loss of the centrifugal pump includes leakage loss of the seal ring, leakage loss of the balance mechanism, and leakage loss between stages.
First, the leakage loss of the seal ring: At the inlet of the impeller, there is a seal ring. When the water pump is working, due to the pressure difference between the two sides of the seal ring, one side is approximately the impeller outlet pressure and one side is the impeller inlet pressure, so there will always be A part of the liquid leaks from the impeller outlet to the impeller inlet. This part of the liquid gains energy in the impeller, but the liquid is not delivered, which reduces the water supply to the pump. The energy of the leaking liquid is all used to overcome the resistance of the seal ring.
Obviously, the larger the seal ring diameter Dw, the greater the pressure difference between the two sides, the greater the leakage. For a fixed water pump, the seal ring clearance should be reduced with permission to reduce the amount of leakage and increase the efficiency of the pump. The general total clearance is approximately 0.002 of the seal ring diameter. If Dw=200 mm, the total clearance is 0.40 mm. During assembly, the seal ring must not be too large eccentrically; otherwise, the leakage will increase. In addition, the leakage resistance can be reduced by increasing the resistance of the seal ring. The main measure to increase the resistance is to make the seal ring into a labyrinth, a zigzag shape, etc. This also increases the seal length of the seal ring and increases the resistance along the way.
Leakage of the seal ring, in some cases, can cause disturbance of the impeller inlet, so it is necessary to rationally design the seal ring form.
Second, the leakage loss of the balance mechanism In many centrifugal pumps, there are institutions to balance the axial thrust: such as balance holes, balance tubes, balance plates and so on. Since there is a pressure difference on both sides of the balance mechanism, a part of the liquid leaks from the high pressure area to the low pressure area. Leakage of the balance hole can reduce the efficiency of the pump by about 5%. In the balance plate mechanism, the leakage amount accounts for 3% of the working flow, but the high pressure pump is somewhat larger than this value; in order to reduce the leakage loss, the diameter D′ of the balance plate can be reduced without affecting the balance force.
Third, the leakage loss between stages In the scroll-type multi-stage pump, the pressure on both sides of the interval plate is not equal, and therefore there is a leakage loss, according to the arrangement of the institutions, the pressure difference between the two sides of the stage plate may be one, The second or third stage, the more the difference in the number of stages, the more severe leakage between the inter-stage separators, so step-type interstage seals are widely used here.
In addition, in the sectional multi-stage pump, there is also leakage between stages. However, this is different from the above-mentioned inter-stage leakage, since this part of the leaked liquid does not pass through the impeller and therefore does not belong to the volume loss. Here, the pressure difference before and after the stage spacer is caused by the supercharging effect of the diffuser portion of the vane and the suction effect of the impeller side clearance (equivalent to a centrifugal impeller). Under the influence of the pressure difference, the leakage liquid enters the gap of the front stage impeller along the gap of the stage partition plate, passes through the guide vanes, and the counter guide vanes (intake guide vanes) flow back to the gaps between the stages, repeating the above process. Although the stage-to-stage multi-stage pump leakage does not belong to the volume loss, it does so by reciprocating flow, which is to consume the power of the pump. In addition, when the portion of the liquid passes through the guide vane, the effective section of the guide vane throat is reduced (ie, the leaked liquid takes up a portion of the cross section), so that the flow rate there increases, causing additional hydraulic loss. According to "The basic design of centrifugal pumps," the book describes: a multi-stage pump, at a flow rate of 20 liters / second, the interstage gap from 0.75 mm to 0.25 mm, the inter-stage leakage q reduced 0.7 liters / second Due to the decrease of q, the flow through the guide vane is reduced, the flow rate at the guide vane throat is reduced, the hydraulic loss in the guide vane is reduced, and at the same time, the relative velocity of the liquid in the vane and guide vane side clearance is reduced due to q reduction. Reduced, thus reducing the impeller disc friction loss, so the efficiency of the pump increased by about 5%.
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