Regenerative Turbine Pumps Introduction
Lytron's TB-5 turbine pump, offered with recirculating chillers, is a regenerative turbine pump.
Regenerative turbine pumps are often classified in the general category of pumps known as centrifugals. While this type of pump does borrow many of its operating principles from the "garden variety" centrifugal pump, the similarities end there because its performance characteristics are substantially different.
Figure 1: Centrifugal Pump
Figure 2: Turbine Pump Fluid
Flow (Casing Cross-Section)
In a common centrifugal pump, the fluid enters the center of the impeller (the eye) and is given a "push" by one of usually 4 to 8 rotating vanes, which impart a centrifugal force on the fluid (see figure 1). This fluid force is collected at the periphery of the impeller (volute) and is redirected towards the pump discharge to provide flow and pressure. In a regenerative turbine, the fluid enters the impeller much closer to its periphery where the first of a set of between 50 and 120 very small vanes gives the fluid a small push of centrifugal force in the radial direction towards the impeller periphery, much like a centrifugal pump. Instead of collecting the fluid force and redirecting it immediately out the pump discharge like a centrifugal, the water channel, which surrounds the turbine impeller, is shaped to deflect the fluid in a circular path back towards the inside diameter of the impeller vanes (see figures 2 & 3). Here it receives a second push of centrifugal force that increases the fluid velocity, and hence the potential pressure capability of the fluid. The term used to describe these multiple circular (actually helical) round trips is called regeneration, hence regenerative turbine. This regeneration principle is the key to the high pressure-producing characteristic of the regenerative turbine versus the centrifugal pump. In effect, the regenerative turbine achieves similar pressure performance to a multiple stage centrifugal pump, but with only one impeller and a much simpler casing design.
Figure 3: Regenerative Fluid Flow
The regenerative turbine pumps are preferred in applications where high pressure, low flow, and compact design are desired. The typical pressure versus flow (head-capacity) curve of a regenerative turbine is very steep, so, these pumps can easily overcome line restrictions, such as temporary blockages, or the friction of long lengths of piping or tubing. The steep pressure characteristic means that large changes in pressure (or restriction) have relatively little effect in flow rate. Another important characteristic of the regenerative turbine is pulsation free flow.
For high-pressure applications, positive displacement pumps (like piston, diaphragm, or gear) are also an option, however they typically suffer from two significant drawbacks. Many PD pumps have a pulsating flow output that can cause inconsistent performance in the end application, as well as, vibration, mechanical damage, and cavitation effects. PD pumps also tend to be mechanically intensive and often have friction and wear problems that increase maintenance and repair costs. Regenerative turbines do not suffer from either of these issues.
Figure 4: Regenerative Turbine
Impeller (Double-Sided Design)
Within the realm of available regenerative turbine pumps, Lytron's pump vendor has added several features to provide additional capability and functionality. The first is the use of a double-sided floating turbine impeller design (see figure 4). As pressure develops equally on both sides of the impeller, a thin hydrodynamic fluid film forms between the impeller and the casing. This film helps prevent impeller wear, and causes the impeller to self-adjust to its optimum axial position. An additional benefit of the balanced floating impeller is that very little axial thrust is applied to the motor shaft, which promotes long motor bearing life. These pumps have also undergone substantial performance optimizations to provide the highest pressure capabilities, while still maintaining optimal efficiency. Another feature of these pumps is their excellent low NPSH (net positive suction head) requirements. Where applications require the units to pump fluids at temperatures very close to their boiling points (or pumping low vapor pressure fluids) these pumps offer a specially designed pump inlet that gently accelerates the fluid to water channel velocity, dramatically reducing cavitation effects. Even if cavitation occurs, or the fluid already contains entrained vapor, these turbines can handle over 50% vapor by volume. The pumps are also offered with a wide variety of metallurgies and elastomers in easy to repair mechanical seal or sealless configurations.
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