| High Efficiency Multiple Screw Pumps |
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| Written by James R. Brennan, Colfax Corporation | |
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Design and OperationThe vast majority of twin screw pumps are of the double suction design.The opposed thread arrangement provides inherent axial hydraulic balance due to symmetry. The pumping screws do not touch each other, thereby lending themselves to manufacture from corrosion resistant materials. The timing gears serve to synchronize the screw mesh as well as transmit half the total power input from the drive shaft to the driven shaft. Each shaft effectively handles half the flow and thus half the power. Each end of each shaft has a support bearing to overcome the radial hydraulic loads, which are not otherwise balanced. The timing gears and bearings are external to the liquid pumped. They need not rely upon the lubricating qualities of the pumped liquid nor its cleanliness. Four mechanical shaft seals keep these bearings and timing gears isolated and operating in a controlled environment. Figure 9 illustrates hydraulic radial forces on a twin screw pump rotor due to differential pressure. These forces are uniform along the length of the pumping threads and cause deflection for which running clearance must be provided in the surrounding pump body.
Figure 9. Radial forces in twin screw pumps.Deflection must be kept to a minimum because greater deflection requires larger clearances, resulting in more slip flow or volumetric inefficiency. Excessive deflection damages the surrounding body and/or contributes to rotating bend fatigue, which ultimately results in shaft breakage. Large diameter shafts and screw root sections are used to maintain minimum deflection. Depending on the machining direction of the threads (left or right hand) and the direction of shaft rotation, the pump manufacturer can cause deflection in either of the two radial directions, up or down for a horizontal pump. These radial deflection loads are absorbed through externally lubricated antifriction bearings. Radial loads are proportional to differential pressure across the pump. Higher differential pressure produces higher radial loads or forces. Smaller lead angles of the screw set reduce these radial loads and reduce the flow rate. Larger lead angles increase the flow rate as well as the radial loading. Bearings are usually sized to provide 25,000 or more hours L10 bearing life at maximum allowable radial loading and maximum design operating speed. Because of this pumpage-independent bearing system, twin screw pumps with external timing gears and bearings can handle high gas content as well as light oil flushes, water, etc. Twin screw pumps are manufactured from a broad range of materials, including 316 stainless steel. When extreme galling tendencies exist between adjacent running components, a slight increase in clearance is provided to minimize potential for contact during upset conditions. In addition, the stationary bores in which the screws rotate can be provided with a thick industrial hard chrome coating that further reduces the likelihood of galling and also provides a very hard, durable surface for wear resistance. Such coatings do, however, require the capability of inside diameter grinding to achieve finished geometry within tolerances. For highly abrasive services, the outside diameter of the screws can be coated with various hard facings to better resist wear. Among these coatings are tungsten carbide, stellite, chrome oxide, alumina titanium dioxide and others. Medium and high viscosity operations are not the only regions where multiple screw pumps bring advantages to the end user. Low viscosity combined with high pressure and flows less than approximately 450-gpm (100-m3/h) are excellent screw pump applications. The combination of modest flow, low viscosity and high pressure is a difficult service. This is a typical application for reciprocating pumps in crude oil pipelines. Reciprocating pumps require pulsation suppression devices in the suction/discharge to avoid excessive vibration on the piping system, which is a potential environmental concern. Ongoing research and development efforts will further extend the capabilities/capacities of these machines, allowing better performance over a broader range of applications. Multiple screw pumps are uniquely suited to many of the applications in the oil and gas market and offer long term benefits to their users, including higher efficiency, pulseless flow, smaller footprints and equipment that is easily maintained. References1 "Oil & Gas Industry Report," Oil & Gas Pump Showcase, Pumps & Systems Magazine, February 2007, p. S4. James R. Brennan is projects manager for Colfax Americas, 1710 Airport Road, Monroe, NC 28110-5020, 704-289-6511, Fax: 704-289-9273, http://www.colfaxcorp.com/. Comments (0)
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