By Alan Austin
June 17, 2003 -- 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. In the event of a condition outside acceptable detection limits (an alarm), gas exposure data is communicated via hard wiring to a command or control station using either analog or digital signals.
While many of these gas detection and monitoring systems rely on traditional 4-20 mA current loops for their operation, their electronics are now often complemented by serial communications and networking. By combining analog technology with new digital protocols, important safety concepts have been both maintained and/or improved. These include (1) greater reliability and operational flexibility, (2) more rapid response and recovery, (3) easy availability of process data, and (4) reduced installation, calibration and maintenance concerns.
Typical applications suitable for gas detection and monitoring safety systems:
• Oil/gas production facilities
• Electric utilities
• Food & beverage processing
• Oil/gas refining plants
• Compressor stations
• Oil/gas drilling
• Multi-story parking garages
• Chemical processing
• Pulp and paper mills
• Wastewater treatment
• Steel mills
• Petrochemical
• Sulfur recovery plants
• Semiconductor manufacture
System scalability, flexibility, expandability
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 plant-wide distributed systems. If desired, the controller or main control panel may be remotely mounted.
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 which provide an intuitive operator interface for setup, calibration, and gas-reading functions. Some controllers even employ the popular ModBus protocol for complete status and control via 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 plant-wide distributed control system via a standard RS-485 output.
Installation, setup, calibration, ease-of-use
Ease-of-use has been a primary design concept. Setup of most systems has been reduced to a few basic steps, while 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 VDC, many panels carry an optional 115/230 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.
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 non-volatile 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 is the ModBus protocol which 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.
When used to control system operation remotely, one can send ModBus Read and Write commands to the controller registers to perform such functions as initiating gas check tests, zeroing and calibration of 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 by the ModBus master receive an exception code in the response message.
Summary
Because nothing is more powerful than an idea whose time has come, the time of arrival for full-featured, operator-friendly industrial safety detection and monitoring systems employing multiple gas detection sensors is now. Systems must be able to simultaneously process multiple signals from field-installed sensors, yet be simple to install, calibrate and operate with the press of a few front panel keys.
They must employ modularity for easy access, testing and service. And because they will probably embrace non-volatile digital logic (microprocessor-based) design and be able to respond to a digital communications device, safety systems will provide for reliable remote control. Finally, their design and the sensors used must be focused on fulfilling nearly all industrial gas detection and monitoring needs at the lowest possible cost of ownership, and over a long economic life.
About the Author:
Alan Austin is Manager, Product Line Management for General Monitors, Inc., in Lake Forest, Calif. The author may be reached at 949.581.4464, e-mail: [email protected].
