Technical issues of gear pumps

The shaft power is proportional to the pressure difference between the inlet and outlet of the pump

Performance of screw pumps 3.1 Displacement The theoretical displacement of a screw pump can be calculated using the following formula: Qt=60Ftnm3/h Where: F is the effective cross-sectional area of the pump cylinder, in cm2； The lead of the t-screw thread, in meters ； n – The revolutions per minute of the driving screw. Internal leakage rate Qs of the screw pump: Qs=αp/σm, where p is the operating pressure of the pump ； σ – the viscosity of the liquid being pumped ； α—is a coefficient related to the screw diameter and effective length ； m=0.3-0.5. When pumping liquids of different viscosities, the displacement of the pump changes. The relationship between displacement and viscosity can be expressed by the following formula: Q2 = Qt – (Qt – Q1)(σ1/σ2)^m. Here, Q1 is the displacement at a viscosity of σ1 ； Q2—Displacement at a viscosity of σ2. 3.2 Power: The shaft power of a pump is generally the sum of the hydraulic power, frictional power, and leakage loss power. Water power Nc refers to the energy transferred by the pump to the liquid per unit of time; it is also known as output power. It can be calculated using the following formula: Nc = PQ × 10^-3 kW, where P is the difference between the discharge pressure and the suction pressure of the pump, in pascals ； Q – actual discharge volume of the pump, m3/s. Friction power refers to the frictional loss caused by the viscous resistance of the liquid, and can be expressed by the following formula: Nf = Kn1.5D2σm in kilowatts. Where: n is the rotational speed ； D―Outer diameter of the driving screw ； ―Viscosity ； K—a coefficient related to the screw length ； m=0.3-0.5. As can be seen from the above, when the viscosity of the liquid pumped varies, the shaft power of the pump also changes. Leakage loss refers to the power loss caused by liquid leaking from a high-pressure area back to a low-pressure area. Therefore, when calculating the shaft power of a pump, if the theoretical displacement is used, the shaft power of the pump is given by the following formula: N = NfPQ × 10^-3 kilowatts