BY ALAN AUSTIN
Throughout industry—from chemical and petrochemical processing to wastewater utilities and pulp and paper mills—workplaces have the potential to be exposed to toxic gases, combustible gases and vapors, and oxygen deficiency. In many cases, the primary line of defense in protecting workers and equipment from these atmospheric hazards is fixed-point, gas detection safety systems. These systems typically consist of one or more catalytic-bead (CB), metal oxide semiconductor (MOS), infrared (IR), or electrochemical (EC) sensor(s) strategically placed at sensitive locations throughout the facility. If a condition outside of acceptable detection limits (an alarm) should develop, gas exposure data are communicated to a command or control station by hard wiring using analog or digital signals.
Many of these gas detection and monitoring systems rely on traditional 4-20 milliamp (mA) current loops for their operation, but their electronics are now often complemented by serial communications and networking. By combining analog technology with new digital protocols, important safety concepts have been maintained or improved. They include greater reliability and operational flexibility; more rapid response and recovery; easy availability of process data; and reduced installation, calibration, and maintenance concerns.
SIDEBAR SYSTEM SCALABILITY, FLEXIBILITY, AND EXPANDABILITY
Today’s modern safety systems provide multiple channels of continuous gas detection and monitoring. Modular, plug-in signal conditioning input cards provide for system scalability—from simple local setups to large plantwide distributed systems. If desired, the controller or main control panel may be remotely mounted.
 The MC600 controller is an example of a modern gas detection and monitoring system. (Photos courtesy of General Monitors, Inc.) |
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Plug-in cards, which handle catalytic bead, MOS, and 4-20 signals from field-mounted sensors, are designed for easy installation and removal from slots inside the cabinet for maximum sensor flexibility. Other features usually offered include easy-to-read, adjustable “daylight readable” LCD channel displays; LED Ready, Alarm, Warning, and Fault indicators; and keypad controls that provide an intuitive operator interface for setup, calibration, and gas-reading functions. Some controllers even employ the popular ModBus protocol, a widely used serial communications protocol in industrial applications, for complete status and control by dual redundant RS-485 serial communications.
Expandability possibilities and options are selectable through user-friendly hardware. Not only can the central controller operate on a “stand-alone” basis, it can be networked to a large plantwide distributed control system through a standard RS-485 output.
INSTALLATION, SETUP, CALIBRATION, AND EASE OF USE
Ease of use has been a primary design concept. Setup of most systems has been reduced to a few basic steps; menu-driven operation reduces operator training time and skill level. Gas table information is preloaded and stored in the controller at the factory to simplify setup. Operating on nominal power of +24 volts direct current (VDC), many panels carry an optional 115/230 volts alternating current (VAC) onboard power supply. Menu formats, LCD text display messages, and front panel navigation buttons comprise the usual operator interface for a flexible and reliable gas detection and monitoring system.
 Controller with cover open. |
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Calibration, or the process of applying a known level of gas to a sensor and having the sensor make adjustments so that its output signal matches the level of applied gas, is performed by means of a menu option that consists of a simple-to-follow, step-by-step procedure. “Remaining Sensor Life” is established during calibration with a message that indicates the approximate percent of expected life left for the sensor. Finally, all program and calibration data are stored in a nonvolatile memory that cannot be lost when power is turned off.
DIGITAL COMMUNICATIONS
As noted previously, ModBus is a widely used serial communications protocol in industrial applications. The simple master/ slave protocol is well suited for complex, small-to-medium systems that do not need to pass large amounts of data. Dual redundancy ensures the highest level of reliability.
The common language used in the ModBus protocol defines a message structure that the controller will recognize and use. It describes how the controller will respond to requests from the other devices and how errors will be detected and reported. A common format is established for the layout and contents of message fields.
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When used to control system operation remotely, ModBus Read and Write commands sent to the controller register to perform such functions as initiating gas check tests, zeroing, and calibrating connected detectors; configuring communication channels between the controller and connected units; and monitoring status information for connected devices.
In a normal communications query and response, the ModBus (master device) sends a query to the controller (slave), and the controller receives the query without a communications error.
The controller then handles the query normally within the master device’s allowable time-out and returns a normal response to the master. If an error occurs in receipt of the message or if the slave is unable to perform the requested action, the slave will construct an error message and send it to the master device as its response.
ModBus provides the capability to handle illegal operations in the form of exception codes. Invalid, illegal, or unsupported requests received by the ModBus master are answered with a message containing an exception code.
Today, there are many applications for full-featured, operator-friendly industrial safety detection and monitoring systems employing multiple gas detection sensors that provide reliable remote control.
ALAN AUSTIN, who has more than 25 years of experience with controls and instrumentation, is manager, product line management, General Monitors, Inc., Lake Forest, California. He has a B.S. degree in mechanical engineering from East Berkshire College in London, England.
