Select the right pump early on to prevent later adjustments.
by Julien Bassett
April 30, 2018

Viscosity is the property of a fluid that resists the force causing it to flow. Because viscosity can significantly impact pumping performance and results, understanding it is essential. Disregarding viscosity could lead to the purchase of a pump that is too large, which is expensive to buy and run and can take up a lot of space. Conversely, if the pump is too small, it may not be able to complete the job at hand, or it can stall, which can burn out electrical components.

There are two types of fluid in terms of viscosity: Newtonian and non-Newtonian fluids. The viscosity of Newtonian fluids, such as water or alcohol, is not greatly affected by the pumping action and will not exhibit a significant change in flow or pressure over time. When pumping non-Newtonian fluids, such as nondrip paint or toothpaste, the viscosity changes during the process. This change can cause fluid resistance leading to underperformance or cavitation, which can ruin the material and pump equipment.

But viscous fluids are not going away anytime soon. So how does one address viscosity so that the integrity of the material remains intact? While pressure and flow correction methods can help keep things flowing when the viscosity changes, selecting the right pump at the outset can eliminate the need for adjustments later.

How Viscous Is It?

In a hose, water flows through and maintains its properties. Now imagine that a more viscous material, such as molasses, replaces the water. Getting the molasses to flow through the same size hose, at the same flow rate as the water, requires much more pressure. The combination of the flow rate and the viscosity of the material can result in turbulent or unpredictable flow, which can have a negative impact on the material and can cause system instabilities given the varying degrees of backpressure.

Table 1. An energy cost balance sheet for the operating systemTable 1. An energy cost balance sheet for the operating system

Because of these factors, understanding viscosity is critically important when determining how to size a pump and the diameter of the pipeline to use. To better understand, see Table 1 above, which illustrates the degree of viscosity (measured in centipoise) for several materials.

Pumping for Varying Viscosity Levels

Air operated double diaphragm (AODD) pumps can reliably pump fluids that have a relatively high viscosity (up to 50,000 cps) making them ideal for moderate viscosity applications. This is important in applications such as pumping molten chocolate where viscosity can change as a result of temperature variation. AODD pumps work for diverse applications such as transferring, batching, filtering and dispensing. AODD pumps can also handle shear-sensitive products, pass solids, run dry and provide variable flow rates.

While pump selection is the primary way to effectively manage viscosity, there are additional factors that can also have some bearing on the outcomes of working with high viscosity material. The following recommendations can help combat some of those issues: Use large suction lines when pumping viscous fluids to help minimize the risk of cavitation that can damage the pump or alter the fluid quality.

Position the pump close to or below the fluid level so that it causes inlet pressure to facilitate feeding the fluid into the pump to get the pumping action started.

Adjust the air control valve to slow the air flow and prevent the pump from stroking too fast to increase diaphragm life.

Image 1 Diaphragm pumpImage 1. Diaphragm pump viscosity correction factor for viscous fluids (Images courtesy of ARO)

Correctly Sizing Your Pump

Larger pump sizes typically handle higher viscosity fluids. To accommodate these fluids without compromising flow, consider oversizing the pump to compensate for any increased capacity or variation in the fluid type.

Luckily, most pump manufacturers offer recommendations to help end users size pumps for a given application as noted in Image 1.

Many industries manage fluids that have varying degrees of viscosity, including food and beverage, oil and gas, chemical, or other general industrial applications. The AODD pumps’ ability to pump a varying degree of viscosities make it a viable choice as it offers flexibility. Knowing this can help users assess how to correct for viscosity within the application and range of fluids that need to be pumped.