Pumps for low-flow, high differential pressure applications have changed dramatically over history. The first of their kind were positive displacement pumps. As electric motor and pump technology improved, synchronous speed, multistage centrifugal pumps came into vogue and remained the pump standard until the 1960s, when the single-stage, high-speed centrifugal pump made its debut.
It was a pump originally developed for aircraft to provide turbine boost during takeoff, but its short duty cycle left it running dry for the remainder of the flight. It differs from the standard multistage design in that it combines an induction motor with a speed-increasing gearbox that develops the velocity head needed for the duty point. This single-stage technology offers many advantages over its predecessors, including smaller size and ease of maintenance. Its disadvantages include increased noise and slightly lower efficiency.
But pump installations can be further simplified by combining high-speed motors, liquid-cooled power electronics and advanced digital controls.
The new concept
This pump design combines proven high-speed pump hydraulics, inducer technology, an integral high-speed motor and associated control logic to push the low-flow, high-differential-pressure centrifugal pump into a new operating range. As shown in Figure 1, this pump differs significantly from the standard single-stage, high-speed centrifugal pump. In the configuration shown, the pump is installed directly on the motor housing, and the impeller mounts directly to the rotor shaft, eliminating couplings, guards and alignment issues. The rotor shaft and impeller assembly are the only rotating pieces. The high-speed motor generates the necessary power and impeller speed for the application, thus eliminating the speed-increasing gearbox. The design also reduces the noise associated with the gearbox and fan-cooled motor. It includes an integral oil sump and lubrication system to ensure proper lubrication and extended bearing life. The most significant advantage high speed offers is the drastic reduction in the size of both the motor and the overall pumping system.
Figure 1. The high-speed centrifugal pump.
Selection and operation
The traditional method for sizing a pump involves calculating the basic system requirements and then adding a fudge factor or two to cover the unknowns. This frequently results in a pump that's oversized in terms of differential pressure and flow. Figure 2 illustrates the effect of fudge factors on a pump. The red line represents the original pump curve as calculated from the process requirements. The green line represents the pump curve, including fudge factors. The oversizing is obvious.
Figure 2. Fudge factor effects
A pump manufacturer sizes the pump to operate as closely as possible to the pump's best efficiency point. Because the traditional pump is oversized and operates at a fixed speed, it needs a control valve to adjust the system resistance and shift the pump's operating point along the pressure-flow curve to meet the process needs.
Figure 3 shows a fixed-speed pump curve in green. The blue arrow shows how the operating point will move on the curve as the control valve affects the system resistance. The red arrow shows how much pressure the control valve consumes as it throttles the pump to meet the real flow conditions.
Figure 3. Control valve effects
Jettisoning extraneous hardware
The pump is no longer constrained by an operating curve because the motor in the new design not only operates at high speed, but also operates at varying speeds as it serves the process in an optimal manner.
Pump selection can be reduced to a few simple questions that define the operating requirements. The important questions are the minimum and maximum flows and pressures. Knowing how the system responds to changes in flow and pressure determines the pump's operating envelope. This knowledge leads to pump hydraulics and motor selection that meet a range of operating points. A typical operating envelope is shown in Figure 4.
Figure 4. A typical operating envelope
Although the pump operates within its envelope, some pumping systems still require a control valve. For example, maintaining a constant pressure at varying flow may require a valve. The pump speed can be adjusted in concert with the control valve position to achieve a desired flow and pressure point, thus minimizing the power dissipated across the control valve.
An additional advantage that running within an operating envelope offers is that a pump can be turned down from its contract rating for energy and cost savings and turned up for greater flows or pressure. This capability effectively avoids any need to oversize the pump to account for unknowns.
Advanced pump control
It's the advanced digital controller that allows the high-speed pump to operate in an envelope rather than only along a single speed curve. The controller adjusts the pump operation in response to system changes. Not only is the pump responding to the controller to meet process changes, the controller keeps the pump operating in its best efficiency range (BER) for the system requirements.
Advantages of operating in the best efficiency range include improved bearing life because of reduced vibration and higher average pump efficiency because of operating closer to the required pressure and flow. Operating in the best efficiency range also keeps the pump away from damaging conditions, such as cavitation and low-flow instability.
The controller operates in either a passive or an active control mode. In the passive mode, it responds to a demand signal and adjusts the pump operation to meet the system requirements. The signal to the controller may be a flow or pressure signal from a process control system. In the active control mode, the process signals and setpoint are inputs to the controller. Proprietary algorithms adjust the pump operation to meet system demands.
Each control approach has its own best use. When a sophisticated process control scheme needs additional pumping or flexible pumping capacity, the passive mode is the choice. The sophisticated controller determines the demand on the pump and makes output adjustments. On the other hand, the active mode is best suited to plain vanilla control schemes in which the pump system controller runs the pump and auxiliaries.
Taking further advantage of the advanced pump control system, another set of algorithms can be programmed to maintain the pump operation in the best efficiency range while monitoring and performing diagnostic functions for the entire pumping system.
Pump system integration
The goal of an integrated design is a simple plug-and-play installation. Component modularity makes it easy to produce an integrated pumping system. The integrated design simplifies many installation problems seen on larger belt-driven or coupled pumps, such as:
Mounting and grouting,
Motor coupling alignment,
The pump control system can be packaged in a NEMA 4 enclosure, mounted on an integrated pump skid and designed for installation in the pump-operating environment. This eliminates the need for additional space in the plant's motor control center. As part of the system integration, the controller is prewired to the motor, and the only connections are process piping, primary power to the electrical disconnect and control signals.
The variability of this modern pump and controls, along with the simplicity of operation and installation, offers a complete solution to difficult pumping problems when the system demands change or are unknown at the outset.
Dave Gill is manager of business development and strategy, AnySpeed products, Sundyne Corp. For more information, call 888-504-8301.
Figures: Sundyne Corp.