Head(H), flow(Q), power(N) and efficiency(η) are the main performance parameters of centrifugal pump. The relationship between these parameters can be determined by experiments. The production department of centrifugal pump expresses the basic performance parameters of its products with curves, which are called characteristic curves of centrifugal pump.
Image1
The characteristic curve is measured at a fixed speed and only applies to that speed. Therefore, the value of speed n is indicated on the characteristic curve. Image1 shows the characteristic curve of domestic 4b20 centrifugal pump at n = 2900r / min. There are three kinds of curves on the graph.
① H-Q curve
The H-Q curve shows the relationship between flow Q and head H of pump. The head of centrifugal pump decreases with the increase of flow in a large range of flow. The shape of H-Q curve is different for different types of centrifugal pumps. If some curves are relatively flat, they are suitable for the situation where the head changes little and the flow changes greatly; if some curves are relatively steep, they are suitable for the situation where the head changes widely and the flow change is not allowed to be too large.
② N-Q curve
The N-Q curve shows the relationship between pump flow Q and shaft power N. N increases with Q. Obviously, when Q = 0, the pump shaft consumes the least power. Therefore, when starting the centrifugal pump, in order to reduce the starting power, the outlet valve should be closed.
③ η - Q curve
η - Q curve shows the relationship between flow rate Q and efficiency η. At first, η increases with the increase of Q, and then decreases with the increase of Q after reaching the maximum value. The maximum value of the curve is equivalent to the highest efficiency point. When the pump is operated at the corresponding pressure head and flow rate at this point, its efficiency is the highest. So this point is the design point of centrifugal pump. When selecting pump, we always hope that the pump will work at the highest efficiency, because the operation under this condition is the most economical and reasonable. However, in practice, it is impossible for the pump to operate exactly under this condition. Therefore, generally, only one working range can be specified, which is called the high efficiency area of the pump, as shown in Image1. The efficiency of high efficiency area should not be lower than 92% of the highest efficiency. The flow, head and power of the pump under the highest efficiency are indicated on the nameplate. The range of flow, head and power in the highest efficiency area is often indicated in the product catalog and instruction manual of centrifugal pump.
The characteristic curve of high pressure centrifugal water pump is measured at a certain speed. When the rotating speed changes from N1 to N2, the approximate relationship among flow, head and power is
Image2
Equation Image2 is called the law of proportion. When the speed change is less than 20%, it can be considered that the efficiency is unchanged, and the calculation error is not large.
When the impeller diameter does not change much and the rotating speed does not change, the approximate relationship among the impeller diameter(D), flow(Q), head(H) and power(N) is
Image3
Image3
Equation Image3 is called cutting law.
The characteristic curve provided by the pump production department is obtained by experiments with clear water. When the property of the liquid is different from that of water, the influence of viscosity and density on the characteristic curve should be considered.
① Effect of viscosity:
The higher viscosity of delivered liquid, the more energy loss in the pump body. Results the pressure head and flow rate of the pump should be reduced, the efficiency decreased, and the shaft power increased, so the characteristic curve changed.
Image4
② Effect of density:
The head of centrifugal pump has nothing to do with density, which can be explained conceptually. At a certain speed, the centrifugal force on the liquid is directly proportional to the density of the liquid. However, the pressure head obtained by the centrifugal force of the liquid is equal to the pressure formed by dividing the centrifugal force by the cross-sectional area of the impeller outlet, and then dividing by the product of the density of the liquid and the acceleration of gravity. In this way, the effect of density on the indenter is eliminated. However, the shaft power of the pump varies with the density of the liquid.
Image5
③Effect of solute:
If the delivered liquid is an aqueous solution, the change of concentration will inevitably affect the viscosity and density of the liquid. The higher concentration, the greater difference with clear water. The effect of concentration on the characteristic curve of centrifugal pump is also reflected in viscosity and density. If the conveying liquid contains suspended solids and other solid substances, the pump characteristic curve is not only affected by the concentration, but also by the type of solid substances and the particle size distribution.
image6
