![]() ![]() Liquid viscosity decreases with an increase in temperature. The recommended viscosity limit with cast iron rotors depends on pump size and model. Sometimes a pump with a cast iron rotor may be prone to gear tooth failure in high viscous application where the pump manufacturer may recommend a pump with a high-strength steel rotor. Horsepower & Torque -Besides the power required by the pump to deliver certain flow and speed, there will be additional power required to compensate frictional loss within the pump due to fluid viscosity. For low speed gear pumps, additional power means increase in torque to the rotor and shaft.Image 2 shows the percentage of rated speed compared to viscosity for internal gear pumps. Pump manufacturers have recommended maximum pump speed for various viscosities. Higher viscosity liquids require a lower pump speed to allow the liquid to fill the space between the rotating gear teeth. Pump Speed-The second way to decrease velocity gradient in Equation 1 is by decreasing the pump speed.However, there is no direct relation between viscosities and clearance, and it ultimately depends on pump model and size. The end user may consult with the manufacturer for the best clearance for the particular application. One way to do it is by increasing the clearance. But as viscosity increases, shear stress (τ) given by Newton’s law of viscosity increases (Equation 1). For high viscosity value (µ), shear stress can be decreased by reducing shear rate or velocity gradient (∂u/∂y). ![]() Generally, it is desired to have a small clearance to prevent fluid from seeping back.
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