The performance curve of a horizontal single-stage centrifugal pump is a plot of the basic performance parameters (flow, head, power, and efficiency) of a centrifugal pump at a fixed speed. The performance curve is measured at a fixed speed and is only applicable to this speed, so the value of the speed is indicated on the characteristic graph. There are three curves on the performance graph (two are now introduced): H-Q curve; N-Q curve.
A.H-Q curve
Trend: The pressure head of a centrifugal pump is reduced as the flow rate increases over a larger flow range. The shape of the H-Q curve varies with different types of centrifugal pumps. The flatter curve is suitable for occasions where the head change is not large and the flow rate changes greatly; the steeper curve is suitable for occasions where the head change range is large and the flow rate change is not allowed too much.
B.N-Q curve
Trend: The N-Q curve represents the relationship between the pump flow Q and the shaft power iV, and N increases as Q increases. Obviously, when Q is zero, the pump shaft consumes the least amount of power. When starting the centrifugal pump, in order to reduce the starting power, the outlet valve should be closed.
Energy loss of horizontal single-stage centrifugal pump
There are three energy losses in the flow of actual liquid from the pump population to the pump outlet. These energy losses reduce the efficiency of the centrifugal pump.
A. Hydraulic loss
Surface friction occurs when liquid flows through the contacted channels, and the resulting energy loss depends primarily on the length, size, shape, surface roughness, and flow rate characteristics of the liquid. It includes the impact loss caused by the liquid flow in the human leaf channel and the pressure chamber flow path, the frictional resistance loss along the liquid flowing through the liquid absorption chamber and the diffuser tube, and the liquid flowing through the above-mentioned places due to turning and contraction or expansion, etc. The resulting vortex loss.
B. Volume loss
The loss due to the leakage of the seal gap is called volume loss. The volume loss includes leakage loss of the seal ring, leakage loss of the balance mechanism, loss between stages, and loss of the loss of the packing. In order to reduce the volume loss, the sealing gap should be minimized and the wear resistance and assembly precision of the seal should be improved.
1. Seal ring leakage loss
When the pump is working, there is always a pressure difference between the two sides of the seal ring, so there will always be a part of the liquid from the impeller outlet to the leaf: the wheel population leaks. The energy of the leaking liquid is all used to overcome the resistance of the seal ring.
2. Balance mechanism leakage loss
In many vertical multistage pumps, there are mechanisms for balancing axial thrust, such as balance holes, balance tubes, balance plates, and the like. Since there is a pressure difference on both sides of the balance mechanism, a part of the liquid leaks from the high pressure region to the low pressure region.
3. Leakage loss between stages
In the volute multi-stage pump, the pressure on both sides of the stage spacer is not equal, and thus there is also leakage loss.
4. Mechanical loss
Mechanical losses include frictional losses between the pump shaft and the bearings and seals, and frictional losses (wheel resistance losses) between the outer surface of the impeller front and rear covers and the liquid. Mechanical losses are independent of flow.
