While centrifugal pumps have been the first choice, screw pumps have their own advantages.
by Josh Pepper & Michael Moore
September 5, 2018
Screw pumps can easily transfer fluids of varying temperatures and viscosities.Image 2. While the chemical-processing industry has long relied on centrifugal pumps for fluid-handling activities, screw pumps can easily transfer fluids of varying temperatures and viscosities. This means that there is no need for the operator to have to manipulate the fluid-transfer process in order to fit the needs of the pump, which can be time-consuming and costly.

Additionally, meeting production rates and quotas has always been job No. 1 in chemical processing operations, but as operating costs continue to rise, increased operational efficiency is required so that the amount of energy that is consumed during pumping operations can be optimized.

In this area, centrifugal pumps can also fall short:

  • Oversized pumps are often chosen as a way to lessen the complexity of pump selection, which can result in increased operation and maintenance costs, inefficient operation and higher-than-necessary energy consumption.
  • In centrifugal pump operation, flow rates decrease as differential pressures increase. Therefore, time-sensitive operations that are dependent on maintaining a constant flow rate will take longer, which may be costly. Centrifugal pump performance will be adversely affected when pumping a fluid that has a viscosity greater than 100 cSt.

In the end, these inefficiencies will lead to higher utility bills and potentially compromised production rates, which will undoubtedly have a negative effect on the chemical processing operation’s bottom line.

The Solution

One alternative can be positive displacement (PD) screw pumps. The difficulties with increasing the use of screw pumps in chemical processing are twofold: the built-in advantages of the centrifugal pump’s vast installed base, and convincing design engineers to acknowledge that there are alternatives like screw pumps.

Many engineers are also of the opinion that screw pumps are capable of producing only low flow rates.

In reality, today’s screw pumps have experienced remarkable advancements in terms of the flow rates that they can create, with flow ranges from 220 gallons per minute (gpm) (833 liters per minute [L/min]) to 11,000 gpm (41,635 L/min) increasingly common.

The design of PD screw pumps makes them capable of handling various liquids, even those with higher viscosities. The screw pump’s method of operation allows opposed screws to engage in order to form a sealed cavity with the surrounding pump casing.

As the drive screws turn, the fluid is shifted and steadily and constantly conveyed to the discharge port of the pump, which creates a volumetrically consistent flow rate regardless of the pumping pressure.

The list of benefits provided by screw pump technology in chemical processing fluid transfer applications is a long and significant one:

  • ability to handle a wide range of flows, pressures, liquid types and viscosities
  • constant flow, even in the presence of varying backpressures that are created by viscosity changes
  • high volumetric and overall operating efficiencies, resulting in reduced operational costs
  • controllable output or capacities possible through high pump turndown speed ratios
  • low internal velocities
  • self-priming operation and good suction characteristics
  • low mechanical vibration that lengthens service life
  • intrinsically smooth and quiet operation
  • extremely low pulsation that reduces stress and prolongs the life of associated fluid transfer components (piping, hoses, seals, bearings, etc.)

Specifically, two types of screw pump models can excel in chemical processing fluid-transfer activities: twin screw with timing gear (WTG) and triple screw.

WTG pumps are designed with external bearings and a timing-gear transmission, which produces double-suction, self-priming operation with no metal-to-metal contact between the pump’s internal components. This design helps the pump achieve the highest flow rates of any rotary PD pump, even at varying backpressures and viscosity levels.

In fact, the deliverable flow rate of a PD screw pump, unlike a centrifugal pump, actually increases as the fluid’s viscosity increases. These design characteristics also make screw pumps suitable for all types of transfer applications, including low- or high-viscosity, lubricating or nonlubricating, neutral or aggressive, and clean or contaminated fluids.

Triple screw pumps are built to handle clean lubricating fluids without solid content across a wide range of viscosities, temperatures and pressures. They are designed with a male drive spindle, two female secondary spindles and a case that contains the screws, which allows the fluid to move smoothly and continuously in an axial direction from suction to discharge. This method of operation delivers smooth, constant product
flow with low noise and high levels of energy efficiency.

Conclusion

No unit operation is more important than another in chemical processing, but if manufacturers want to optimize the performance of their fluid-transfer applications, they would be wise to consider the benefits offered by positive displacement screw pumps.

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