Overcome low inlet pressure issues and minimize maintenance.
by Jeff Sporer
January 25, 2018

Viscous products such as processed cheese, peanut butter, meat, chocolate and whipped toppings can leave operators with cavitation issues when using traditional pumps. Low inlet pressures, high pumping pressures and process lines that may have relatively low levels of product in them all create issues that increase maintenance overhead and reduce pump reliability.

Twin screw pump technology is designed to handle low inlet pressure applications, operate reliably at high differential pressures and increase run times. Although these units usually mean a higher initial investment, they can run at higher flows and speeds to reduce the size of pump needed.

When a process has a low inlet pressure, rotors or lobes within a pump cut across its inlet. This can increase the inlet pressure required for the pump to run. Rather than blocking the inlet, however, the screws in a twin screw pump create an auger effect that actually helps the fluid into the pumping chamber. By using non-galling internal materials and precision engineering, the pumps can be constructed with exceptionally tight internal tolerances to create even greater suction and increased pumping efficiency. The smooth operation of this style of pump further reduces slip pulse in low inlet pressure, low viscosity and high pressure conditions.

twin screw pumpA twin screw pump works better with low inlet pressures (Image courtesy SPX Flow)

Twin screw pumps are designed to operate reliably for long periods of time with minimal maintenance. They can run over a wide range of operating speeds and pressures, making them a flexible choice to meet different process requirements. A single pump can operate at the necessary duty point for a viscous process flow as well as running at higher speeds (typically up to 4,000 revolutions per minute [rpm]) to push clean-in-place (CIP) fluids at required speed and volume for effective cleaning. This can simplify the design of a process line with fewer pumps, valves and associated process controls.

Some models offer bi-directional flow capability without modification such as reversing the bearings or turning the pump around. This helps increase the operational efficiency in hygienic applications where, during cleaning cycles, the pump needs to run in the opposite direction and may further reduce the overall number of components required for installation. The operating principle of the twin screw also eliminates pressure fluctuations and therefore reduces operating noise levels.

Twin screw pumps are ideal for challenging, hygienic applications. They have high strength bearings that maintain axial loads and heavy duty shafts for increased torque. No bearings are used in the product zone and front loaded access to product side seals simplifies routine maintenance tasks.

Real World Application Examples

Meat processing: One meat processor wanted to convert their process from conveyor belts to pumping through an extrusion device. The challenge was compounded by the highly viscous meat being in a nearly frozen state. Using External Circumferential Piston (ECP) technology, meat could not be pulled into the pump nor could the pressure necessary to move the product through the pipeline be achieved. To overcome this issue, a twin screw pump was attached directly to the outlet of an auger unit. This resulted in sufficient flow at the low inlet pressure and necessary differential pressure to achieve a smooth, consistent flow rate of the meat. The pump also did not cavitate or require re-priming when the process ran out of meat to fill the auger.

Non-dairy whipped toppings: A global processor was looking to streamline its production. It changed its separate product and CIP pumps for a single twin screw pump. The operating range of the pump provided sufficient flow at low inlet pressure and the necessary high differential pressure to handle the whipped toppings while also providing higher speed CIP pumping service.

Processed cheese: At one particular processed cheese plant in the U.S., large volumes of processed cheese are supplied to a pump from an evaporation flash cooker vessel which operates under vacuum to cook and dry the cheese. The vessel has a low inlet pressure and at times the level of the product in the vessel is also low. Processed cheese is pumped at around 350 pounds per square inch (psi). The challenging conditions of this application meant the processor was experiencing problems with cavitation and damage of existing pumps.

By changing technology, the processor experienced greater reliability. The design means pressures on the shaft are axial rather than radial, making it less prone to break. The tight internal tolerances of the pump also reduced cavitation issues, significantly lowered maintenance overheads and increased pump runtimes.

Summary

Twin screw pumps have a wide operating range, enabling the use of smaller units. Their ability to run at different duty points also means they offer a flexible solution that can handle changes in flows or process fluids. Models that offer bi-directional capability without modification further increase the operational efficiency of cleaning cycles.