Is Twin Screw Pump Right for You?

In recent years, twin screw pumps have boomed in popularity. We have not only seen this in our own portfolio, but also in overall market trends. This rise in market share is not without warrant since twin screw pumps are often used to simplify processing systems. The traditional method of using one positive displacement pump for product and one centrifugal pump for CIP has been replaced with one twin screw pump to handle both cycles. Furthermore, our own Waukesha Twin Screw (WTS) has lower suction head requirements than our Universal Series, has self-priming capabilities, and runs at speeds up to 4,000 RPM to deliver high flow rates in demanding applications.

However, just because a twin screw pump can be used, that doesn’t necessarily mean you should. SPX FLOW offers ECP rotary lobe pumps, traditional rotary lobe pumps, twin screw pumps, and centrifugal pumps. With that in mind, it is essential to use the pump best suited for the application. And SPX FLOW’s team of application engineers and sales specialists ensure that you not only get a properly sized and configured pump, but also the correct pump technology for your unique application.

The most common issues we see in twin screw applications are solids handling and pumping high viscosity products at low speeds with high-speed CIP requirements.

In solids handling applications, twin screw pumps can struggle to process both abrasive solids and solids where product integrity must be maintained. When pumping abrasives and solids, slip is the most critical factor. As abrasive fluids slip through tight pump clearances, the fluid causes those clearances to open and result in an exponential loss of efficiency. Additionally, twin screw pumps are significantly less efficient than our Universal Series. Universal series pumps are 100% efficient at products at-or-above 500 centipoise, whereas a twin screw pump will be in the range of 80-90% efficiency. Note that twin screw pumps are maximally efficient at higher speeds, which can further exacerbate the rate of wear. This lack of efficiency in twin screw pumps can lead to costly down time, slowed production rates, and premature maintenance.

Those same clearances and slip pathways can also damage particulates in applications where the integrity of solids is critical. Twin screw pumps also have more pinch points by design - which can be detrimental when pumping solids with particle integrity as the priority. As the screw rotates in the pump body, each rotor pitch means that there’s an opportunity to damage the particulates. The screws also have small carry-through volumes which means that larger particulates may not fit in the screws. When we contrast that with the Universal Series, the potential for damaging the product is minimized. With a shovel-like rotor tip design and large carry-through volumes, particulates can be pumped with ease.

The principal factor when sizing twin screw pumps is sizing motors correctly. Consider this application as an example: After sizing, it is determined that the product requires 8 horsepower at 300 rpm and the CIP cycle requires 10 horsepower at 1200 RPM. This application would require a 40 horsepower 1200 RPM motor!

This is due to motor turndown ratios. Positive displacement pumps and twin screw pumps operate under a constant torque principle. However, they still require adequate horsepower to generate the necessary torque to operate efficiently. As we turn a motor down in speed, the horsepower varies directly. If we need 8 horsepower at 300 rpm, we require 32 horsepower at 1200 rpm to have enough horsepower available at the lower speed. This is where twin screw pumps can become expensive. Generally speaking, higher viscosity products are pumped at lower speeds, with higher flow CIP duties at higher speeds. These higher speed CIP duties push the selection to a larger pump model to accommodate the flow rates, while the lower flow product cycle gets pushed to even slower speeds due to the larger pump displacements. This routinely results in a vicious cycle requiring both an oversized pump and motor. In a scenario such as this, the cost of a motor on a twin screw pump can be as much as 50% or more of the total pump cost – which means it can be a more expensive option than a smaller PD pump and centrifugal pump combo.

As you can see, choosing the right pump for the right application is critical to finding that sweet spot between performance and efficiency. One way to ensure that you have the right pump for the job is with a partner who knows best. Reach out to me today and I’ll personally help you decide what’s best for your scenario. Contact me using the email link below.