The emergence of digital and communications technologies means much more information can be communicated to a control system, with the added benefit of reduced cabling costs. So, I thought it might be useful to provide an overview of the various technologies available.
Gas safety is very important. Explosive hazards from hydrocarbon gases and vapours, toxic gases such as hydrogen sulphide and depleted oxygen environments can be significant risks for workers.
Gas detector heads are often connected to a centralized control system which shows the current gas levels and triggers any alarms. Detectors must sense gas in concentrations low enough to respond well before the situation becomes hazardous. While gas detectors are located in areas assessed to be at risk to gas build-up, the control system should be mounted where it can be monitored in relative safety.
Detectors have traditionally been connected to the control system via cables using ‘point-to-point’ cabling, where each detector connects to a separate input on the control system with an individual cable. These systems use analogue signals, which can only communicate a limited amount of information from the detector; typically gas value (4-20mA), fault (<3mA), and gas reading over-scale (>21.5mA).
The HART communications protocol (www.hartcomm.org) complements conventional 4-20mA systems by laying additional diagnostic information onto the 4-20MA signal. This data can be used to diagnose faults, manage system calibration and maintenance using a HART enabled device.
While the safety function of the detector is still performed by the analogue 4-20mA signal and conventional controller, the HART data enables access to extra information, including device temperature, serial number, calibration, last calibrated date, fault status, supply voltage, signal current, etc.
Modbus is an address-based protocol which communicates with each gas detector using a unique address. Information, such as gas level, alarm and fault status, is stored in registers within the detector, and the ‘Modbus Master’ control system routinely addresses individual detector ‘nodes’ to retrieve data – a full guide to Modbus can be downloaded from www.modbus.org/docs/PI_MBUS_300.pdf.
Like the HART communications protocol, Modbus can be used in conjunction with the analogue signal to provide additional detector information, or it can be used as the primary means of communication with a control system, such as a PLC (Programmable Logic Controller) or SCADA (Supervisory Control and Data Acquisition) system.
Another, well-established protocol is Foundation Fieldbus, which provides the opportunity to use alternative cabling interfaces. Data can be transferred via conventional copper or fibre-optic cables, or via Intrinsically Safe (I.S.) interfaces – more can be found at www.fieldbus.org.
All of these technologies allow detectors to be installed in an addressable cable configuration rather than the conventional point-to-point topology. This allows cable installation costs to be significantly reduced by connecting multiple detectors onto a single cable where the cable loops from one detector to the next.
Wireless communication is an attractive proposition where running additional cables to new or additional detectors is impractical. Detectors may be powered locally (via cables, batteries or solar panels) and transmit the gas levels and status information to a control system via a radio signal. Although various wireless products are available, a global ‘standard’ for the protocol and frequency deployed has yet to be established, so suitability will depend on local regulations.
For new installations, communications technologies such as Foundation Fieldbus may provide a good overall solution to improve on traditional analogue systems. However, each has its pros and cons, and is not a suitable option for every project. As always the individual safety case assessment is the critical determining factor.