Choose the Right Pump for Shear Sensitive Product

Slower rpm pumps necessarily handle shear sensitive products gently. There are several lesser-known factors that contribute to product handling.

Shear sensitive liquids change viscosity when under stress or pressure, such as when they are hit by the impeller inside a pump. Some liquids become less viscous with increased force (called shear thinning or pseudoplastic), while others become more viscous with increased force (called shear thickening or dilitant). By comparison, Newtonian liquids, such as water, do not change their viscosity, no matter how much force is applied.

When moving a shear sensitive substance (ketchup, shampoos, polymers) through a process line, it needs to be handled gently throughout the system to preserve the integrity of the product. It is commonly thought that a slower rpm pump prevents product from being beaten up inside the pump casing, however, pump speed is only one consideration in the case of product handling.

Liquid needs to receive a certain velocity and energy by the impeller to overcome system losses. Velocity and energy are products of both impeller diameter and rpm. Since the same amount of energy is required to get a job done, it doesn’t matter how this is achieved—by using a large impeller or by using higher rpm. 1800-rpm pumps always have bigger impellers, while 3500-rpm pumps have smaller ones.

Pump efficiency at the given duty point is the most important factor for gentle product handling. The efficiency is directly related to the ratio between the amount of liquid that comes into the pump and the portion of it that leaves the pump. When efficiency is low, the product stays in the casing and recirculates—meaning the impeller beats it for a longer time.

In addition to rpm, there are other system factors such as tube diameter that can cause damage to gentle products. For example, pushing too high of a velocity through a long stretch of a small-diameter tube can cause turbulent flow as a result of the friction between the product and the inner walls of the tubing. Turbulent flow can shear the product, whereas laminar (smooth) flow preserves the integrity of the product.

Many people incorrectly assume because the pump is moving the product, that the pump is to blame for product shearing. However, it is not that simple. Other factors, such as velocity, energy, pump efficiency, and tube diameter all determine how gently product is handled.