This is the design to consider for difficult applications, particularly when two or more of the above factors are important.
Untimed multiple-screw. This technology most often comes with three screws — one power rotor and two idlers (Figure 6). It also is available in two-, four- and five-rotor versions. All designs operate with the same principle — that is, with the rotor and idler(s) run in a close-fitting housing. The epicycloidal geometry of the rotor set assures a rolling contact; the rotor set runs inside the bore of the casing much like a journal bearing. A film of the liquid being pumped prevents contact of the rotating elements and the bore. Most designs feature axial and radial balancing. Thrust bearings aren't required. When applied and operated properly, such pumps provide a very long service life between maintenance cycles. Units typically are made of iron and steel, and so shouldn't be used in corrosive applications. Also, except for some special designs, this technology only suits contaminant-free fluids.
Unlike the other screw designs, the untimed multiple-screw can accommodate a magnetic coupling for guaranteed leak-free performance. Such pumps handle applications such as machinery lubrication, fluid power hydraulics, fuel injection and compressor seal systems; some special designs are used for high-pressure water-based applications.
Advantages of untimed multiple-screw pumps include:
• very low noise and pulsation;
• high efficiency;
• operation at full motor speed even at high flow rates (allowing application of a smaller pump without gear reducer);
• self priming;
• high reliability (balanced design); and
• ability to be applied on rotating equipment driven by auxiliary shaft at speeds above two-pole motor speeds.
When evaluating the pump type for your fluid handling system, focus on four key aspects, namely, fluid, discharge, supply and operating objectives. The following general guidelines indicate when a positive displacement pump may be a good choice:
Fluid. Will your pump see liquid viscosities of greater than 20 centistokes, entrained gas or deal with a shear-sensitive liquid? If so, consider a positive displacement pump for its ability to handle high viscosity liquids more efficiently, i.e., with lower annual energy costs, than centrifugal pumps.
Discharge. Will pressure vary in your system? If so, consider a positive displacement pump for its ability to deliver a nearly constant volume of liquid over the pressure range.
Supply. Will supply conditions vary in your system? If so, consider a positive displacement pump for its versatility in handling a wide range of NPSHA (net positive suction head available), fluid characteristics and ability to adjust speed efficiently.
Operating. Will your flow or pressure demands change occasionally or even frequently? If so, consider a positive displacement pump because of its ability to respond immediately and efficiently to pressure changes and varied speed.
Selecting the right pump type is important — but so too is properly sizing the pump. Many engineers don't appreciate the importance of sizing a pump only to meet the application's requirements. It's common to oversize a pump, for example, in anticipation of future planned expansions. This leads to higher initial and energy costs.
In making a decision on the best pump for the job, take the time to consider suitability, efficiency, reliability, and, perhaps most importantly, the total cost of ownership. A pump's original price may not amount to much compared to the cost of energy and maintenance over its life.
SEAN McCANDLESS is oil and gas market manager for Colfax Fluid Handling, Monroe, N.C. RICHARD MEIGHAN is director, product sales, power generation and industrial markets, for Colfax in Monroe. E-mail them at email@example.com firstname.lastname@example.org.