This article is intended to give insight into the safe and appropriate application of wireless technology in the process industry, writes Endress+Hauser marketing manager, Greg Ferrar.
Wherever you look, the world is becoming more and more wireless. Our PC’s communicate with each other and to our printers while connecting us to the web via WiFi. Our mobile phones chat to our PCs and hands-free devices in our cars via Bluetooth while allowing us to talk to each other through thin air. So what about sensors and instrumentation in our industry? How are we progressing?
Firstly, it’s important to distinguish what we mean when we refer to ‘wireless’. As we’ve already said, wireless seems to be everywhere with an ever increasing barrage of buzz words and jargon. So let’s take a look at what’s out there:
• Wireless Local Area Networks (WLAN): e.g. wireless Ethernet in an office environment
• Global Wireless Networks (WAN): e.g. gateways with GSM/UMTS communication
• Personal Area Networks (PAN): e.g. handhelds based on Bluetooth technology
• Wireless Industrial Sensor Networks (WSN): e.g. WirelessHART
In our business, when we talk about wireless, we mean WSN and more specifically WirelessHART.
Despite the extensive marketing hype from some suppliers promoting the implementation of WirelessHart devices everywhere, the global standard for WSN is only just being finalised. Because of this, the technology is yet to be accepted for true plant control — this is still some time away.
Most modern field devices, instruments, valves, drives and analysers are ‘intelligent’ in that they have self-diagnostic functions which allow them to regularly check their own health and report any problems back to the plant’s supervisory system. Many devices are now also multi-variable, so a pressure transmitter may also measure temperature, some flow meters may also measure density, etc. However this extensive assortment of useful information is confined to the device and is only available to plant systems if HART or one of the other fieldbus technologies has been implemented.
In existing plants that use the conventional HART based 4-20mA process variable (PV) layout, attaching WirelessHART adaptors to strategic devices has some merit — the 4-20mA cable already exists so there are no new wiring costs and the useful HART data can be made available plant-wide for predictive maintenance, trouble shooting, manipulation, etc. The down side to this approach is the need to buy and install a considerable amount of new hardware including a wireless adaptor for each device and WirelessHART gateways, positioned strategically around the plant to receive the signals. However in some operations the value of convenient access to device information and configuration can justify the cost, but these are the exceptions.
An alternative way to access the useful HART data in these installations is by tapping into the 4-20mA signals at the control room. Devices can be connected to HART multiplexers in parallel with the 4-20mA signals and the HART data can then be re-transmitted from the multiplexer to the plant — quite possibly a cheaper solution. In fact, most existing wireless installations are actually ‘wired’ wireless which then raises the question — ‘Why do we need wireless at all?’
The ultimate benefit of ‘wireless’ is the total elimination of cables, so the only way to enjoy this benefit is with ‘true’ wireless. However, it is important to address issues of safety and reliability when using wireless technology along with the responsible application of wireless devices in process industries. If the WiFi or Bluetooth link between your home computer and printer fails and you can’t print a document it’s annoying, but what if a level sensor fails to communicate the ‘high high’ alarm in a tank of sulphuric acid to your plant’s control system? The results could be catastrophic.
In view of potential catastrophic failures, a common concern amongst users considering this technology is the vulnerability of wireless communications to malicious ‘wireless jamming’. One only has to do an internet search for ‘wireless jammer’ to be amazed by the abundant availability of low cost wireless jammers. This concern is likely to limit the acceptance of wireless to only monitoring rather than control and safety applications.
It’s estimated that 70 to 80 per cent of all measuring devices in a typical process plant or tank farm are loop powered. Not all measurements are critical to safe plant operation; many relate to monitoring and inventory control. The devices making these non-critical measurements are ideally suited to accept a ‘true’ WirelessHART adaptor (why use cables at all?) which is attached to the device and transmits the HART data as well as powering the device from its integrated battery.
[Greg Ferrar is the marketing manager at Endress+Hauser Australia.]