Pump selection includes the choice of performance parameters and the choice of pump structure type, the choice of pump structure type see double screw pump structure introduced. The choice of performance parameters: 1. Flow Q: As a positive displacement pump, the factors that affect the flow of twin-screw pump are speed n, pressure p, and medium viscosity v. 1.1 speed n impact: screw pump at work, the formation of a sealed chamber between the two screws and bushings, the screw every one revolution from the import to export a sealed chamber, that is, a sealed chamber volume of liquid is discharged. Ideally, there is no leak inside the pump, so the pump flow is proportional to the speed. That is: Qth = n * qn ---- speed; q ---- theoretical displacement, that is, the volume of liquid discharged per pump per revolution; Qth ---- theoretical displacement. 1.2 pressure △ P of the impact: pump the actual work process, its internal leakage, also known as slip. As the pump seal cavity has a certain gap, and the seal chamber before and after the existence of pressure △ P, therefore, there is a part of the liquid backflow, that there is leakage, the amount of leakage with Q, Q = Qth- △ Q Obviously, with Before and after the sealed chamber, the pressure difference △ P increased, the amount of leakage △ Q gradually increased. For different types of lines and structures, the impact size varies. 1.3 Effect of viscosity v: Consider: Leach clear water and viscous paste from funnel-type containers in the same volume. Clearly, water leaks faster than paste. Similarly, for the twin-screw pump, viscous fluid smaller than the viscosity of liquid leakage is small, the amount of leakage and medium viscosity has a certain proportion. To sum up, to comprehensively consider the above factors, through a series of calculations to accurately know the actual pump flow meets the requirements of the working conditions. 2. Pressure △ P: Unlike centrifugal pumps, the working pressure of twin screw pump △ P is determined by the outlet load, which is the outlet resistance. The outlet resistance is matched with the pressure at the outlet of the pump. The greater the outlet resistance, the greater the working pressure. If you want to know the pressure, you need to use the fluid mechanics knowledge of export resistance accurate calculation. 3. Shaft power N: Pump shaft power is divided into two parts, namely: Nth ---- hydraulic power, that is, the pressure of liquid energy; Nr ---- frictional power. For a given pressure and flow, the hydraulic power is constant, so the factor that affects the shaft power is the frictional rate Nr. Frictional power is the fraction of power consumed by the friction of moving parts. These frictional powers apparently increase with the increase of working pressure difference, and the increase of medium viscosity also leads to the increase of liquid frictional power. As a result, the pump shaft power in addition to hydraulic power, friction power which increases with the media viscosity and working pressure, so the choice of matching motor, medium viscosity is also a very important reference data. Especially in the transport of high viscosity medium, the need for more accurate calculations. After calculating the power, select the matching motor should follow the sample table specified in the relevant provisions. N (KW) N≤10 10 <N≤50 N> 50 N> 100 K 1.5 1.25 1.15 1.1 Nm = NK Nm ---- motor power N ---- shaft power K ---- power reserve factor Suction on the performance of the calculation and selection Pump work is divided into the following stages: 4.1 inhalation, this time continuously moving along the suction pipe liquid; 4.2 rotating screw to the work of the liquid pressure; 4.3 pressure out, when the liquid with Overpressure required to relieve all resistance to the piping system is drained from the pump. In the above three stages, the most important stage is to ensure that the suction pump conditions, the pump can work properly, which is an important pump conditions, or else cavitation occurs, which causes vibration, noise and other issues. 5. Calculation of NPSH: The pump NPSHr and the pump speed n, lead h and the viscosity of the pump medium v ​​and other factors are related to the introduction of our factory Bornemann twin-screw pump with the following Formula: NPSHr = (1.5 + 0.253VF 1.84345 + 0.0572VF 1.55) * v 0.4146 VF ---- axial flow rate, VF = n * h / 60 (m / s); n ---- speed (r / min); h ---- lead (m); v ---- working viscosity (° E). Thus, pump NPSHr is with VF, v increases. Therefore, the suction conditions are not good, it is appropriate to choose a small lead twin screw pump. This is very important in the selection. 5.1 NPSHa device NPSHa calculation, not elaborated here. 5.2 If you want to keep the pump working properly, that is, cavitation, vibration and other issues do not occur, you must ensure that the following conditions: NPSHa> NPSHr This is the pump suction conditions. 6. Twin screw pump speed selection: Choose different speeds often involve the following problems: 6.1 by selecting the appropriate pump speed, in order to achieve the appropriate performance parameters such as flow and so on. 6.2 With the different viscosity, pump speed should also be changed. For Boremann twin-screw pumps, the change in viscosity is the primary condition that determines the speed, and the lower the allowable speed, the higher the viscosity. The choice of speed is essentially the issue of suction performance, especially in the case of high viscosity, if the speed is too high, it will cause insufficient inhalation, resulting in noise and vibration and other issues. Therefore, be sure to select the speed according to the relevant principles.
