by Joe Evans, Ph.D
September 30, 2009

The screw pump differentiates itself from other rotary pumps in the way fluid moves through its pumping chamber. Fluid flows axially in the screw pump, but circumferentially in others. They are available in single and multiple rotor designs and offer a wide range of flows and pressures. Figure 5 (image courtesy of seepex Inc.) is a cross section of a single rotor screw pump.

Cross section of a single rotor screw pumpPositive Displacement Pumps Figure 5 (image courtesy of seepex, Inc.)

It consists of auger-like rotor with lobe shaped surfaces that mesh with a mating stator made of rubber or some other synthetic elastomer. Its pumping action creates a number of moving seals as CTIO volumes move axially through the stator. Since each CTIO volume appears to move intact through the entire length of the pumping chamber, this particular design is often referred to as a progressing cavity pump. These pumps will accommodate a wide range of liquids and viscosities and are often used to pump sewage sludge and other process solutions with concentrated solids.

 

Operating Characteristics

 

Pressure

As with reciprocating pumps the maximum pressure (P) generated by a rotary pump is determined by the application and the pump and driver components. The manufacturer specifics maximum working pressure, and maximum differential pressure depends on the pump's fluid sealing capability. For this reason, some of these designs are also often referred to as "semi" positive displacement pumps.

Capacity

The capacity (Q) of a rotary pump is proportional to its displacement (D) times it driven speed (rpm) less slip (S).

Q = (D x rpm) - S

The displacement of a rotary pump is defined as the net volume of fluid transferred from OTI to OTO during one rotation. Believe me when I say that this is all you will ever want to know! Because of the complex geometry that exists between the rotor and pump case, calculus is often required to compute actual displacement. It can be so complex that displacement is sometimes approximated.

Slip is similar to that in a reciprocating pump and is defined as the quantity of fluid that leaks from OTO to OTI per unit time. It depends on the clearances between the rotors and case and the operating pressure. Generally, slip increases in direct proportion to pressure and is most marked in designs like the flexible member pump. Flexible vane pumps (aka rubber impeller pumps) are especially subject to slip at higher pressures and tend to be inherently protected against over pressure.

Power

The power required for rotary pumps is calculated in the same manner as it is for reciprocals.

bhp = (Q x P) / (1714 x ME)

where 1714 is the bhp conversion factor and ME is mechanical efficiency.

Mechanical efficiency is the ratio of pump power output to pump power input.

 

October 2009, Pumps & Systems

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