Pumps & Systems, July 2013
Pump system controls have a long and evolved history. An examination of the industry reveals water transportation as the ultimate goal. Whether transporting freshwater or wastewater, all the hardware, technology and engineering has one outcome: moving water from one location to another.
Water is moved for many reasons, including:
- Treatment to clean, filter, cool, heat, aerate or purify for use or consumption
- Use it as a transfer medium tool of kinetic energy to generate hydroelectric power and/or to pressure
- Use it as a transport medium (for example, logging, pressure sewage, machined filings)
History of Pump System Controls
The discovery that man and mules could be replaced with steam, gas and diesel engines and, ultimately, electric motors began the electric age, which led to present-day pump control, automation and instrumentation.
Control of pump systems was accomplished with a simple, electromechanical pressure switch. These pressure control switches were—as are most electrical controls—a mechanical device designed to perform an electrical function. The switch was a hardware spring mechanism that sensed and reacted to pressure bellows to open or close electrical contacts in relation to water pressure in a closed pump system. These contacts were then used to activate or deactivate an electric-motor-driven pump and are still the most commonly used pump control device.
This simple control of the functionality of an electric pump then led to the development of multiple-system pump control devices—such as alternators, water sensors, seal leak detectors, intrinsically safe relays and a myriad of level control and detection devices. These devices may be mechanical, electromechanical, electronic/solid-state and integrated circuits, which execute a limitless number of functions.
In standard pump control systems, these devices are used as control functions to carry out the operating sequence and functionality of a pump system. A standard control panel would use electromechanical relays and contactors to define the control sequence per the electrical schematic design and via the physical wiring of the control components.
A modern custom control panel with PLC, VFDs and SCADA
Programmable Logic Controllers
The present state of technology for system sensors and monitoring devices allows for information gathering of any aspect of a water system. Using mechanical, electromechanical, solid-state, load-cell, transducer, capacitive, sonic, radar or radio frequency (RF) technology, many metrics can be monitored—including liquid level, pressure, temperature, flow, pH, turbidity, dissolved oxygen, torque, kilowatts, power factor, density and weight.
Automation controls, specifically programmable logic controllers (PLCs), have expanded and streamlined electrical control. Originally designed to alleviate automotive industry model-year manufacturing changeover-time delays and expenses, PLCs have filled a niche in controls technology. PLCs provide a way to use and monitor the system sensors that are available as input devices. They then use real-world information gathered from the input devices to perform a function based on a specifically designed, internal system “user” program. The user program then makes decisions based on the currently available real-world data from the inputs and activates or deactivates output devices—such as contactors, starters, lights and alarms.
Because PLCs are solid-state automation systems, they can receive or send digital (on/off) and/or analog (0 to 10 volts direct current or 4 to 20 milliamperes) signals, making them perfect for monitoring any analog sensing input device. Since a PLC is basically a computer processor designed to be a control system, it has the-in ability to communicate on multiple hardware and software platforms using:
• Supervisory control and data acquisition (SCADA)
• Remote I/O
• Industrial Ethernet
The PLC’s inherent design has the ability to gather and report system operating data and performance information and display it via human machine interface (HMI) or operator terminal interface (OTI). These HMI terminals use a keypad or touchscreen and are available in multiple configurations and sizes. This system data can then be recorded in a database and transmitted in any configuration to provide system performance data, trending and alarms in multiple formats to remote locations.
PLCs continue to develop with another version that evolved from better processor hardware technology—the process automation controller (PAC). While the PLC requires a program scan time measured in milliseconds to perform a user program scan, the PAC can perform this function in microseconds. This makes PACs well-suited for high-speed control, safety and data processing applications as required by large municipal treatment plant and petrochemical process control applications.