While centrifugal pumps have been the first choice, screw pumps have their own advantages.
by Josh Pepper & Michael Moore
September 5, 2018

In the universe of industrial manufacturing, the general definition of the term unit operation is rather benign: a basic step in a process. In the world of chemical processing, however, unit operations are much more than that. Namely, they are the blueprint for the series of production stages that must be precisely completed for the manufacturer to turn a collection of raw materials into a viable end product.

There are five classes of unit operations in chemical processing. This article will focus on fluid-flow processes, which most commonly consist of transfer, filtration, solids fluidization and transportation activities. For reference, the other four unit operations are heat transfer (evaporation and heat exchange), mass transfer (distillation, extraction, adsorption and drying), thermodynamic (refrigeration and gas liquefaction) and mechanical (crushing, pulverization, screening and sieving).

Over the years, centrifugal pumps have built a reputation as the default pumping technology for fluid transfer applications in chemical processing. In all fairness, centrifugal pumps do possess the basic capability to meet the operator’s needs in these situations since their method of operation makes them well designed for the high-volume transfer applications that are common in the industry. Centrifugal pumps also work well with thin, water-like fluids that must be transported through networks of piping with variable flow rates.

In chemical processing, unit operations are the blueprint for a series of production stages.Image 1. In chemical processing, unit operations are the blueprint for a series of production stages. One of these stages is a series of fluid-flow processes that consists of transfer, filtration, solids fluidization and transportation activities. Screw pumps, which are available in twin- and triple-screw configurations, have exhibited the capability to perform reliably, efficiently and safely in these critical applications. (Images courtesy of Blackmer)

This article, however, will illustrate how different types of pumping technology—positive displacement twin- and triple-screw pumps—can be a more reliable, efficient and economical choice than centrifugal pumps in critical fluid handling operations in the chemical processing industry, many of which involve the handling of highly abrasive, corrosive and hazardous materials.

The Challenge

The biggest challenge in making the screw pump more prevalent in chemical processing applications is moving the industry’s operators away from the ingrained perception that centrifugal pumps are the one and only choice for their fluid transfer requirements.

In many cases, chemical processing systems have been designed around the pumping technology, rather than the other way around.

This means that engineers are first familiar with centrifugal pumps and attempt to work within their operational limits. They know how centrifugal pumps operate, know their benefits and are confident that they are the best technology for what they are trying to accomplish.

Based on this mindset, design engineers will design systems that can blend or heat raw chemical materials or compounds as a way to manipulate the process and bring the viscosity of the fluid down to less than 300 centistokes (cSt), which makes the fluid more manageable for the centrifugal pump to handle. In this case, what they are doing is reconditioning the fluid to fit the pumping technology—irrespective of the cost impact.

Despite the ability to recondition the fluid to meet the operational needs of the centrifugal pump, operators must still ensure that the centrifugal pump is operating on or near its best efficiency point (BEP). Centrifugal pumps rarely operate at their exact BEP because pristine pumping conditions are seldom experienced. With this in mind, a centrifugal pump is assumed to be operating at its highest level of efficiency in a window between 80 percent and 110 percent of its BEP.

However, when the operation of the pump moves too far to either side of the BEP, uneven pressure will be applied to the impeller, which can lead to increased radial thrust that will cause the pump’s shaft to deflect. When deflection occurs, higher loads will be placed on the bearings and mechanical seal, which will damage the pump’s casing, back plate and impeller.