Variable-Speed Pumps Offer Finer Control

Low-flow, high-differential-pressure, variable-speed pumps improve response and simplify systems

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The traditional method for sizing a pump involves calculating the basic system requirements, 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.

A pump manufacturer sizes the pump to meet performance specifications and operate as closely as possible to the pump's best efficiency point. Because a 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 needs of a process.

It is possible to simplify this entire pump selection process and avoid oversizing and unnecessary components. The simpler approach eliminates the need for a control valve and a number of other parts and components. The pumping system can still operate at its most efficient point most of the time, and be able to adjust speed and flow to meet changing process conditions.

"The key is to use a low-flow, high-differential-pressure centrifugal pump installed on a variable-speed motor," says David Gill, PE, product manager at Sundyne (www.sundyne.com). "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."

A high-speed motor generates the necessary power and impeller speed for the application, thus eliminating the need for a speed-increasing gearbox. The design also reduces the noise associated with the gearbox and fan-cooled motor.

"The most significant advantage of a high-speed system is the drastic reduction in size of both the motor and the overall pumping system," says Gill. This size reduction greatly reduces the mass of the rotating assembly, allowing near-instantaneous response to process demand changes. In some applications, the response of the pump to a setpoint change can be four times faster than a traditional control valve-based system.

A digital controller allows the high-speed pump to operate in an envelope, rather than only along a single speed curve. The controller adjusts 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 BER include improved bearing life by reducing vibration and higher average pump efficiency by operating closer to the required pressure and flow. Gill notes that operating in the best efficiency range 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 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. 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.

Another set of algorithms can be programmed to maintain the pump operation in the BER while monitoring and performing diagnostic functions for the entire pumping system.

The pump control system can be packaged in a NEMA 4 enclosure and mounted on an integrated pump skid designed for installation in the pump's 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.

Gill explains that the variability of this pump and control, along with the simplicity of operation and installation, offers a good solution to difficult pumping problems, such as any process that is not steady-state or one that is frequently changing. In a standard application of a centrifugal pump, a bypass is needed so when the pump is being throttled by the control valve, it can route excess flow through the bypass.

Another good application is a washdown service. Here, a traditional pump running at a fixed speed produces a given amount of flow and pressure. As the number of wash stations on a line changes, the system resistance goes down, and the pump operates further out on the curve. The flow increases but the differential pressure decreases. With this new pump and control technology, the wash system pressure can be a setpoint, the header pressure a feedback, and the pump operation changed to provide a constant header pressure.

E-mail Rich at rmerritt@putman.net.

 

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