What is Pump Shear and What Can We Do to Minimize it in Sanitary Pumps?

March 12, 2014

In sanitary and high purity process applications, maintaining product integrity is critical. In food applications, product handling can affect texture, taste, and product appearance. In pharmaceutical applications, maintaining delicate cell structure and protein folding is critical. For this post, we will focus on products that create challenges when pumping due to shear sensitivity and give a high level overview of the products we offer to pump them.

Generally speaking, fluids fall into one of two categories: Newtonian and Non-Newtonian. Among other things, a Newtonian fluid holds its viscosity regardless of shear rate. When you stir a glass of water, no matter how fast you stir, the water doesn’t really change much.

Conversely, the viscosity of non-Newtonian liquids changes depending on shear rate. Thixotropic liquids are shear thinning, meaning that as shear increases, fluid viscosity decreases. Examples of thixotropic fluids include ketchup, whipped cream, blood, mayonnaise, and paint (both ketchup and mayonnaise are also examples of Bingham plastics- materials that behave as a rigid body at low stresses but flow as viscous fluids as high stress).

The opposite of a thixotropic fluid is an anti-thixotropic fluid or shear thickening fluid. Also known as dilatant, these fluids viscosities increase with the rate of shear strain. There are a few different theories on what causes this, but the prevailing opinion is that a colloid’s ability to order and thicken in the presence of shear forces is due to interparticle forces. As long as interparticle forces, such as Van der Waals forces dominate, the suspended particles remain in ordered layers. Once shear is applied, however, particles begin to flocculate, or come out of colloidal suspension and begin to behave like a solid. Examples of these fluids include corn starch and water and also silly putty.

Ok, that’s nice, but let’s be concise: what is shear? Shear is defined as relative motion between adjacent layers of a moving fluid. When one layer of a fluid moves adjacently to another layer of fluid, it can begin to exhibit the deleterious effects of shear. Shear rate is defined as the measure of the extent or rate of relative motion between adjacent layers of a moving fluid. Therefore:

Shear Rate= Velocity/Distance

(Equation 1.1)

So what does this mean when selecting a pump? To answer that question, it is important to understand the impact of shear on your product. Some fluids require shear to get them to a viscosity where they can be transferred easily. Others can be irrevocably damaged. Understanding your fluid and what you are trying to accomplish through pumping is essential. This does not mean just moving a fluid from point A to point B, but can also include altering or maintaining rheological properties.

Most of the concern in today’s process industry is focused on shear sensitive products. This begs the question, what is the lowest shear pump? That’s a great question and one without a clear answer (and one that usually varies from sales mongrel to sales mongrel). When trying to address this question, it’s important to remember that very little of the product is actually being sheared. Most product is moved through large cavities and only a small amount moves along the OD of the pump, the area where maximum shear occurs.

Waukesha Univeral Pumps can be Run at Slow Speeds, Minimizing Shear

Waukesha Univeral Pumps can be Run at Slow Speeds, Minimizing Shear

The most important factor in minimizing shear is pump speed. That basically eliminates centrifugal pumps from consideration when handling shear sensitive fluids. 3500 RPM really imparts a lot of energy into these delicate fluids. Positive displacement pumps are often the preferred route when handling shear sensitive products. Waukesha’s Universal 1 and Universal 2 external circumferential piston pumps can be run at speeds below 100 RPM. Referring to equation 1.1 above, we can see that by reducing speed, we will reduce our rate of shear.
Lower shear pumps aren’t limited to lobe and twin wing pumps. Other lower shear pump options include Graco’s AODD pumps, which essentially just “push” fluid through the pump cavities, imparting very little radial energy.

Twin Screw Pump

Sanitary Twin Screw Pump

Other pumps with a reputation for low shear include progressive cavity, eccentric disc, and sine pumps. Another pump we’ve found best suited from this in these applications is the sanitary twin screw pump. Using axial instead of radial motion to move fluid, the twin screw pump allows us to handle high viscosity fluids at high pressure differentials. Sanitary twin screw pumps are also available with varying screw pitches to pass large solids and minimize product damage. These parts are ideal for handling shear sensitive products like mayonnaise and can even pass whole strawberries, making it an excellent pump choice for fruit preparations and yogurt applications.

Inner Workings of Quattroflow Pump

Inner Workings of Quattroflow Pump

Another product gentle pump we’re excited about for pharmaceutical applications is the Quattroflow pump. This pump is a quaternary diaphragm pump that uses an eccentric disc to move fluid through the pump chamber. Available with USP Class VI pump heads, it have been predominately used in pharmaceutical applications that require the movement of sensitive cells and lysates containing valuable proteins. While Quattroflow use has been largely limited to biologics applications, Holland is excited about exploring alternatively dosing and filling applications with the technology.

In sum, product integrity is an integral part of fluid handling. A pump is, by definition, a mechanical device that takes energy from a motor, be it electric or air, and imparts it into a fluid to move it from point “A” to point “B”. Some products are especially sensitive to this energy. In order to preserve delicate product integrity, we need to minimize shear. For help doing this in your next application, contact a Holland Sales Engineer today to help you weigh the advantages and disadvantages of a variety of pump types. Lean our over 60 years of experience and let us help you select the right pump for your shear sensitive application.