Don’t Choose the Wrong Pump For Your Shear-Sensitive Product

When handling shear-sensitive products, the myth that slower RPM pumps are “more gentle” is not always true. There are several less-known factors that may be the culprits.

Shear-sensitive liquids will change their viscosity when put 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 do not change their viscosity, no matter how much force is applied. Water is an example of a Newtonian liquid.

Comparison of shear-sensitive liquids

When moving a shear-sensitive substance (ketchup, shampoos, polymers) through your process line, you want it to be handled gently throughout the system, to preserve the integrity of the product. You may think that you need a slower RPM pump to move the product so that it is not beaten up inside the pump casing. However, pump speed is only one consideration in the case of product handling.

A pump with a slower RPM doesn’t mean the pump is gentler on products. This is because the liquid needs to get a certain velocity and energy by the impeller to overcome the system losses. Velocity and energy are products of both impeller diameter and RPM.

Since we need the same amount of energy to get the job done, it really doesn’t matter how we achieve it—by using a big impeller or by using higher RPM. In fact, 1800-RPM pumps always have bigger impellers while 3500-RPM pumps have smaller ones, which is in agreement with the above explanation.

The factor that really matters in gentle product handling is pump efficiency at the given duty point! The efficiency actually tells us 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, if you have a long stretch of small-diameter tube and you try to push too high velocity of flow through it, you can encounter 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.

Because the pump is moving the product, many people incorrectly assume that the pump is to blame for product shearing. However, it is not that simple. Be sure you don’t discount these other important factors, such as velocity, energy, pump efficiency, and tube diameter. All of these decisions come together to determine how gently your product is handled.

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