How to correctly select the model of a twin-screw pump

Thread Content

How to correctly select the model of a twin-screw pump: Pump selection involves choosing the performance parameters as well as the pump’s structural design. For information on the various structural designs available for twin-screw pumps, please refer to the relevant descriptions on that topic. I. Selection of performance parameters: 1. Flow rate Q: As a positive-displacement pump, the factors that affect the flow rate of a twin-screw pump are mainly the rotational speed n, pressure p, and the viscosity v of the medium. 1.1 Influence of rotational speed n: When a screw pump is in operation, a sealed chamber is formed between the two screws and the bushing. With each rotation of the screws, one sealed chamber moves from the inlet to the outlet; in other words, the liquid contained within one such sealed chamber is discharged. Under ideal conditions, with no leaks inside the pump, the pump’s flow rate is proportional to its rotational speed. That is: Qth=n*q, where n is the rotational speed ； q----theoretical displacement, that is, the volume of fluid discharged by the pump per revolution ； Qth----Theoretical displacement. 1.2 Influence of pressure △P: During the actual operation of the pump, there is leakage inside it, also known as slip. Due to the presence of a certain gap in the pump’s sealing chamber, as well as a pressure difference ΔP between the front and back of this chamber, a portion of the liquid flows back, resulting in leakage. The amount of leakage is denoted by ΔQ; thus, Q = Qth – ΔQ. It is evident that as the pressure difference ΔP between the front and back of the sealing chamber increases, the amount of leakage ΔQ also increases gradually. For different profiles and structures, the degree of influence varies as well. 1.3 Effect of viscosity v: Imagine: water and thick paste leaking out of a funnel-shaped container in equal volumes. Obviously, water leaks faster than glue. Similarly, for twin-screw pumps, fluids with higher viscosity exhibit less leakage compared to those with lower viscosity, and there is a certain proportional relationship between the leakage amount and the viscosity of the fluid. In summary, it is necessary to take all the above factors into account; only through a series of calculations can it be determined with accuracy whether the actual flow rate of the pump meets the requirements of the operating conditions. 2. Pressure △P: Unlike centrifugal pumps, the operating pressure △P of twin-screw pumps is determined by the outlet load, that is, by the outlet resistance. The outlet resistance is matched to the pressure at the pump’s outlet; the greater the outlet resistance, the higher the operating pressure. To determine the pressure, it is necessary to use knowledge of fluid dynamics to accurately calculate the outlet resistance. 3. Shaft power N: The shaft power of a pump is divided into two parts, namely: Nth——hydraulic power, which is the energy of the pressurized fluid ； Nr----Friction power. For a given pressure and flow rate, the hydraulic power remains constant; therefore, the factor that affects shaft power is the friction coefficient Nr. Friction power is the portion of power consumed due to the friction between moving parts. Clearly, these frictional powers increase as the working pressure difference increases, and an increase in the viscosity of the medium also leads to an increase in the liquid’s frictional power. Thus, in addition to the hydraulic power, the shaft power of the pump also includes frictional power, which increases with the viscosity of the medium and the operating pressure; therefore, when selecting a suitable motor, the viscosity of the medium is also a very important factor to consider. Especially when transporting high-viscosity media, relatively accurate calculations are required. After calculating the power, the appropriate motor should be selected in accordance with the relevant regulations specified in the sample table. N(KW) N≤10 10＜N≤50 N＞50 N＞100 K 1.5 1.25 1.15 1.1 Nm=N.K Nm----Motor power N----Shaft power K----Power reserve factor 4. Calculation and selection of suction performance The operation of the pump goes through the following stages: 4.1 Suction, during which the liquid moves continuously along the suction pipe ； 4.2 The rotating screw transfers energy to the working fluid ； 4.3, Extrusion: At this stage, the liquid is discharged from the pump at a pressure sufficient to overcome all the resistances in the extrusion pipeline system. Of the above three stages, the most important one is ensuring the suction conditions for the pump, as this is essential for its proper operation. It is a critical requirement for pump functioning; otherwise, cavitation will occur, leading to problems such as vibration and noise. 5. Calculation of net positive suction head: The pump’s net positive suction head NPSHr is related to factors such as the pump’s speed n, the pitch h, and the viscosity v of the fluid being pumped. For twin-screw pumps, it is calculated using the following formula: NPSHr = (1.5 + 0.253VF^1.84345 + 0.0572VF^1.55) * v^0.4146, where VF represents the axial flow velocity, and VF = n * h / 60 (m/s) ； n----rotational speed (r/min) ； h----lead (m) ； v----Working viscosity (°E). It can be seen that the pump’s NPSHr increases as VF and v increase. Therefore, in cases of poor suction conditions, it is advisable to choose a twin-screw pump with a small lead. This is very important when making a selection. 5.1 The calculation of the device’s net positive suction head available, NPSHa, will not be explained here. 5.2 To ensure the proper operation of the pump, that is, to avoid problems such as cavitation and vibration, the following condition must be met: NPSHa > NPSHr. This is the suction condition for the pump. 6. Selection of the rotational speed for twin-screw pumps: Choosing a different rotational speed often involves the following considerations: 6.1. By selecting an appropriate pump speed, it is possible to achieve suitable performance parameters such as flow rate. 6.2 The pump speed should also change depending on the viscosity. For twin-screw pumps, changes in viscosity are the main factor determining the operating speed; as viscosity increases, the allowable operating speed decreases as well. The selection of rotational speed is essentially a matter of suction performance; especially in cases of high viscosity, if the rotational speed is set too high, it will lead to insufficient suction, thereby causing problems such as noise and vibration. Therefore, it is essential to select the rotational speed in accordance with the relevant principles.