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How to Size a PD Pump for a Range of Viscosities

August 7, 2014
Waukesha Universal 2 Pump Side Mounted on a Stainless Steel Frame, with Motor, Gear Reducer and a Variable Frequency Drive

Waukesha Universal 2 Pump Side Mounted on a Stainless Steel Frame, with Motor, Gear Reducer and a Variable Frequency Drive

At Holland, we size pumps for sanitary applications every day. We also work with many customers who are having issues with pumps someone else sized for them. This experience has made us well aware of many common oversights made when selecting a rotary positive displacement pump. One of the most common errors made when sizing an ECP pump, such as the Waukesha Universal 1 or Universal 2 series pump, is failing to take into consideration the range of product viscosities the pump will handle. This post will focus on that issue and what steps we can take to ensure adequate drive speed range is selected to handle a variety of products.

To begin, a sanitary process system may require that the same pump handle both viscous and thin products. A simple example could be a shampoo process- imagine a facility wants to transfer shampoo from one tank to another and then follow the transfer with a system water flush. An excellent pump for this application is the Waukesha External Circumferential Piston pump. The Universal 1 and Universal 2 pump, utilizing Waukesha Alloy 88 rotors, have long slip paths compared to other PD pump types, allowing them to handle both thick and thin fluids. Initial pump selection, however, must consider the RPM required for both thick and thin products.

Why do we need to consider both products? As products become thinner, or less viscous, the fluid tends to “slip” back through the pump. While the U1 and U2 series pumps are nearly perfectly efficient with fluids over 300 cps, some “slip” does occur with thin fluids. This “slip” must be compensated for with additional motor RPM. Optional pump features, such as front face or hot clearance rotors and CIP features only exacerbate this problem. Other challenges, such as high pressure conditions that also increase slip should also be considered.

Additionally, pump condition and wear should be considered as well. The tolerances on brand new pumps tend to be much tighter than used pumps. This wear opens clearances and increases slip, decreasing displacement capacity. We will need to increase pump speed to compensate for this.

To avoid these issues, it is critical to ensure we are able provide enough speed for the low viscosity product and enough torque for the high viscosity products. PD pumps are driven by motors coupled to gear reduction units. A gear motor uses a series of gears to increase torque, resulting in decreased shaft speed. We want to avoid using a drive that is “maxed” out when running the thick product and can’t spin fast enough to move our thin product. If we don’t have a large enough gear reduction, however, we will be unable to generate the torque to move the high viscosity product.

So how do we solve this problem? It’s actually not that difficult. First, we size a pump and motor that will spin faster enough to move the high slip product. Then we check that the torque the higher speed motor can generate against the torque required to move the high viscosity product. Finally, we’ll utilize a variable frequency drive and a motor with at least 10:1 constant torque turndown (we need to avoid motor slip as well), which will allow us to effectively adjust speed of the pump to handle the complete range of product viscosities and displacement requirements. Finally, an over speed safety factor will should be considered to compensate for pump wear over time.

To recap, when sizing a sanitary pump for multiple products, it is critical to take into account the viscosities of all products the pump will hand. At Holland, we use our over 60 years of experience sizing sanitary pumps to dictate how much speed compensation is required for a given set of conditions. For help with your next sanitary pump application, contact a Holland Sales Engineer today.

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