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.
The first screw pump built was probably of an Archimedes design used to lift large volumes of water small vertical distances. They are still manufactured and used for this service. Modern three screw high performance pump deliver liquids to pressures above 4500-psi (310 bar) and flows to 3300-gpm (750-m3/h) with long term reliability and excellent efficiency.
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
