The six basic performance parameters of the pump

The basic performance of the pump is usually composed of 6 individual parameters.

1, flow rate (capacity, discharge)

The flow rate of the pump refers to the volume or mass of the liquid that flows out of the outlet section of the pump per unit time, which is called the volume capacity and the mass capacity, respectively. Volume flow is represented by the symbol Q and mass flow is represented by Qm. Volume flow is commonly measured in liters per second (L/s), cubic meters per second (m3/s), or cubic meters per hour (m3/h); mass flow is commonly measured in kilograms per second (kg/s) or Tons per hour (t/h). According to the definition, the volumetric flow has the following relationship with the mass flow: Qm = ρQ, where ρ is the density of the liquid to be transported (kg/m3).

Due to the different flow requirements for various applications, the vane pump design has a wide range of flow rates, ranging from less than 1 litre per second, to as large as tens or even hundreds of cubic meters per second.

In addition to the above-mentioned pump flow, the concept of pump theoretical flow QT and leakage flow q is also encountered in the study of impeller theory.

The so-called theoretical capacity (theoretical capacity) refers to the flow through the pump impeller. Leakage capacity refers to the part of the theoretical flow out of the impeller, which is the gap between the rotating Parts of the pump and the stationary part, such as the gap between the inlet ring of the impeller and the pump casing, and the pump shaft and packing in the stuffing box. The clearance between the balance hole in the balancing device and the axial force balance device or the gap between the balance plate and the housing flows back to the impeller inlet and out of the pump. It can be seen that the relationship between the pump flow rate, the theoretical flow rate and the leakage flow rate is as follows: QT=Q+q.

2, head (head)

The lift, denoted by the symbol H, refers to the energy gain obtained after the unit weight of the delivered liquid flows through the pump, ie, the total energy actually delivered to the unit weight by the pump. The unit is m (N·m/N=m). . Therefore, by the definition of the pump lift, the lift can also be expressed as the unit energy difference of the inlet and outlet sections of the pump.

3, power (power)

Power refers to the amount of work done by the pump on the flow in unit time. The unit is Watts (W) or kilowatts (kW). The power of the pump includes four kinds: shaft power, effective power, matching power of the power machine, water power, and power loss in the pump.

(1) Shaft power (input power) P

Axis power refers to the power transmitted to the main shaft of the water pump after the power machine passes through the transmission equipment, that is, the input power of the water pump. The power listed on the pump's nameplate usually refers to the pump shaft power.

(2) actual power, effective power, Pe

The effective power refers to the energy obtained from the flow out of the pump per unit time, that is, the actual effective work done by the pump on the delivered flow.

(3) Matched power of motive power machine (Pg)

The matching power of the power machine is the output power of the prime mover matched with the pump. Considering that the pump may be overloaded during operation, the matching power of the power machine is usually selected to be greater than the shaft power of the water pump.

(4) Water power Pw

Water power refers to the power remaining after the shaft power of the water pump overcomes the mechanical resistance, that is, the power delivered by the impeller to the liquid passing through it.

(5) Lost power within pump

The input power of the water pump (ie, shaft power) is only partially transmitted to the liquid being delivered. This part of the power is the effective power, and the other part is used to overcome the various losses in the pump during pump operation, ie loss of power. The power loss in the pump can be divided into three categories, namely mechanical losses, volumetric losses and hydraulic losses.

4, efficiency (efficiency)

The effective degree of energy delivered by the pump is called efficiency. The input power of the water pump (shaft power P) cannot be fully transferred to the liquid due to mechanical loss, hydraulic loss, and volume loss. The liquid can only obtain the effective power Pe through the water pump. The efficiency is a parameter used to reflect the magnitude of the loss power in the pump and measure the effective utilization of the shaft power P, that is, the ratio of the effective power Pe to the shaft power P.

5, the speed (pump speed) n

That is, the speed refers to the number of times the water pump shaft or impeller rotates per minute. Usually expressed as N, the unit is r/min. The rotational speed of the water pump is closely related to other performance parameters. A certain rotational speed generates a certain flow and head, and corresponds to a certain shaft power. When the rotational speed changes, other performance parameters will cause corresponding changes.

The pump is designed according to a certain speed, so the power of the matched power machine should meet the working conditions of the pump, and it should be consistent with the pump speed in the speed.

6、Allow suction vacuum height [Hs] or required NPSH Δhr

Allowed suction vacuum lift and net positive suction head required are parameters characterizing the cavitation performance (inhalation performance) of the water pump under standard conditions. When the pump works, often due to improper device design or operation, there will be too low pressure at the inlet of the pump, resulting in cavitation, resulting in reduced performance of the pump or even intermittent flow and vibration. After the phenomenon of cavitation in the pump, the pump will not work properly, and even the cavitation may not work. In order to avoid the occurrence of water pump cavitation, it is necessary to correctly determine the geometric installation height of the pump and design the pump system through the cavitation performance parameters of the pump.

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