Overall, the wireless devices in your self-organizing network will have good connections if it meets the following criteria:
- At least 99% of messages sent by each device reach the assigned gateway.
- At least 70% of transmissions between two nodes (one “hop”) are successful.
- Device batteries last as long as expected.
- Radio signal strength in the gateway diagnostics is good. This check can be misleading on its own (weak signals can still get through if the path is stable), but it can help identify a problem when it arises.
When it comes to adding devices to your first WirelessHART self-organizing wireless network, remember that, in general, bigger really is better. In fact, the more wireless nodes in the network, the easier it is to expand. It really is that easy.
Rules of Thumb for Manual Network Design
Although a new breed of automated tools is available to streamline wireless network design (see sidebar, p7) manual rules of thumb can also be used. Starting with a scale layout of the process unit or area, draw connecting lines between each planned wireless device and neighboring wireless devices that meet any of the following criteria:
As a best practice during the design phase, each wireless device should be connected to three other wireless devices, even though the wireless connection distances may vary by direction. Having three connections during the design phase ensures each device has two alternate connections after installation.
If a wireless device does not have three connections during the design phase, then add additional measurement points or use a range extender to fortify connectivity.
There should not be any connectivity lines between wireless devices in the following situations:
Australian Terminal Keeps Bitumen Flowing
At Terminals Pty.’s facility in Geelong, Victoria, Australia, bitumen is unloaded from ships through a pipeline 3,000 ft (900 m) long and 8 in. (200 mm) in diameter. Because bitumen solidifies at ambient temperature, electric heaters operate all along the pipeline to keep the bitumen hot (160 ºC) and fluid. If a heater fails, a cold spot could form, causing the bitumen to solidify and plug the line, an expensive problem.
“We needed to monitor the bitumen line,” according to Bitumen Terminal project manager Joe Siklic, “to make the operators aware of cooling anywhere in the line from the ship to the storage facility, which could result in an emergency shutdown. Any delay in unloading could keep a ship at the pier longer than planned with demurrage costing up to $30,000 per day.”
The terminal chose wireless technology, Siklic says, for its lower initial cost and minimal maintenance as compared with hard wiring. Eight Rosemount wireless temperature transmitters are evenly spaced along the pipeline, sending temperature readings on one-minute intervals to a Smart Wireless Gateway on shore that channels data to the AMS Suite predictive maintenance software used for instrument configuration and performance monitoring. The collected data also are forwarded to a SCADA system in the terminal control center via fiber-optic cable.
Due to the self-organizing nature of this technology, each wireless device acts as a router for other nearby devices, passing the signals along until they reach their destination. If there is an obstruction, transmissions simply are rerouted along the mesh network until a clear path to the Smart Wireless Gateway is found. All of this happens automatically, without any involvement by the user, providing redundant communication paths and better reliability than direct, line-of-sight communications between individual devices and their gateways.
“This is an ideal application for wireless,” Siklic says. ”Since numerous paths exist to carry the transmissions, the network would easily compensate for a transmitter failure, and the operators would be warned. This wireless network has proved to be reliable, compatible with existing control equipment and cost-effective.”
New Tool Eases Wireless Network Design
While there are well-developed rules of thumb for manually validating that a WirelessHART network configuration will provide adequate connectivity (see sidebar, p5), a new engineering tool from Emerson Process Management now makes the job even easier.
Called the AMS Wireless SNAP -ON, the tool allows users to drag and drop devices and gateways onto a plant layout, then easily validate and optimize the network design against known best practices. Further, once the network is up and running, the tool allows the user to maintain the network easily by graphically displaying network traffic and diagnostic data.