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Pumps & Systems, April 2007

Rotary screw pumps have existed for many years and are manufactured around the world. More demanding service requirements impose challenges on screw pump manufacturers to provide higher pressure or flow capability, better wear resistance, improved corrosion resistance, improved efficiencies and lower leakage emissions. Better materials and more precise machining techniques, as well as engineering innovation, have led to improvements in all these areas. 

Twin screw pumps achieve flow rates to 18,000-gpm (4000-m3/h), pressures to 1450-psi (100 Bar) and can handle corrosive materials, again at good efficiencies. 

Multiple screw pumps provide remarkably good operating efficiencies versus centrifugal pumps when handling viscous liquids such as heavy crude oil, bunker or residual fuel oils and low sulfur fuels. The high efficiency performance is a clear advantage over centrifugal pumps where liquid viscosity exceeds 100 SSU (20 centistokes). See Figure 1.

Figure 1. Multiple Screw Pump Efficiency

Not only are annual power savings substantial, but the initial driver, starter and cabling costs are also lower for multiple screw pumps. Operating liquid temperatures as high as 600-deg F (315-deg C) have been achieved in twin screw pumps for the ROSE® deasphalting process (see Figure 2).

Figure 2. A ROSE® process 600-deg F twin screw pump.

Timing gears and bearings are force cooled while the pump body is jacketed for a hot oil circulating system to bring the pump to process temperature in a gradual, controlled manner. Three screw pumps have been applied to the same elevated temperature, more commonly on asphalt or vacuum tower bottoms services in refineries.

To quote Dushyant Mehra, research analyst with Frost & Sullivan, "The primary challenge for oil & gas pump manufacturers is to improve the energy costs and efficiency of their pumps." 1 The pump user can also contribute to their bottom line by using the most efficient technology to move liquid products. Many times this is a positive displacement pump, not a centrifugal pump. Figure 2A shows excellent efficiency over a broad range of discharge pressures.

Figure 2A. A typical multiple screw pump performance curve.

 In multiple screw pumps, each wrap of screw thread effectively forms a stage of pressure capability. High pressure pumps have 5 to 12 stages, or wraps, whereas low pressure pumps may have only 2 or 3 wraps. This staged pressure capability is illustrated in Figure 3. More wraps are incorporated into pumps designed for higher pressure service.

Three Screw Pumps: Principle Applications

Three screw pumps are the largest class of multiple screw pumps in service today. Typical applications include machinery lubrication, hydraulic elevators, fuel oil transport, fuel oil burner service, power hydraulics and refinery processes, such as high temperature viscous products including asphalt, vacuum tower bottoms and residual fuel oils.

Three screw pumps also find extensive use in crude oil pipeline service, as well as gathering, boosting and loading of barges and ships. These pumps are also used for high pressure distillate fuel injection for gas turbines.

Designs are now available in sealless configurations, such as magnetic drives and canned arrangements, to allow customers to reduce emissions and meet government guidelines. Three screw pumps are renowned for their low noise levels, high reliability, ease of repair and long life.

Design and Operation

Three screw pumps are manufactured in two basic styles, single suction and double suction (see Figure 4). The single suction design is used for low to medium flow rates and low to very high pressure. The double suction design is really two pumps in parallel in one casing, used for medium to high flow rates at low to medium pressure.

Figure 4. Single and double suction three screw pumps.

Three screw pumps generally have only one mechanical shaft seal and one, or perhaps two, bearings that locate the shaft axially. Internal hydraulic balance is such that axial and radial hydraulic forces are opposed and cancel each other. Thus, bearing loads are very low.

Another common characteristic of three screw pumps is that all but the smallest, low pressure designs incorporate replaceable liners in which the pumping screws rotate. Minor repair kits (seals, gaskets, bearing) and major repair kits (all wearing parts, including those in the minor repair kit) allow easy field repair of most three screw pumps. So field repair is a simple matter and the piping does not have to be disturbed. 

The center screw, called the power rotor, performs all the pumping. The meshed outside screws, called idler rotors, cause each liquid-holding chamber to be separated from the adjacent one, except for running clearances.  This effectively allows staging of the pump pressure rise. Because the center screw is performing all the pumping work, the drive torque transferred to the idler rotors is only necessary to overcome viscous drag of the cylindrical rotor spinning within its liner clearance.

The theoretical flow rate of these pumps is a function of speed, screw set diameter and the lead angle of the threads. Basically, flow rate is a function of the cube of the center screw diameter. Slip flow is the volumetric inefficiency due to clearances, differential pressure and viscosity. It is a function of the square of the power rotor diameter, resulting in larger pumps being inherently more efficient than smaller pumps. In crude oil pipeline service, the power savings is greatest compared to centrifugal pumps.