WirelessHART extends your reach: The easy way to add new instrumentation
A look at the benefits and considerations of deploying WirelessHART device networks. Minimize growing pains when installing new field sensors and actuators.
Any plant expansion will generally require an increase in I/O count. This seems straightforward enough, but in practice it soon runs into limitations. I/O cabinets have finite capacity, for one thing, and I/O is expensive. This leads the plant engineer with several choices: continue to add I/O points to the existing system until all the spares have been used up, add additional I/O cabinets, or increase capacity by adding WirelessHART capability. This article will show that this third alternative can be the fastest and most economical approach to increasing plant I/O counts. Wireless I/O systems have two major advantages over wired I/O: They save money and they save time. But choosing one will require answering some questions.
What infrastructure will be required?
A wireless system consists of a set of wireless transmitters that communicate to each other via a mesh network, and with a gateway, which connects to the control system, as shown in Figure 1. Many gateways offer a choice of protocols that can easily be integrated into a system interface, including HART-IP, Ethernet, or RTU-based communication protocols. The common protocols seen today are Modbus and OPC. One manufacturer now also offers the EtherNet/IP protocol, which eliminates the need for a Modbus-to-EtherNet/IP converter. The integration of the device information into the control system is, of course, the purpose of this activity so there are several available options to cover most control, monitoring, and asset management systems.
In addition to the primary variable (PV), the information from a WirelessHART device includes additional measurement variables, device condition status, device diagnostics, and in some cases, process diagnostics.
A WirelessHART transmitter may be battery, line or loop powered with battery life ranging from months to years, depending on how the device is configured and its update rate. Moderate update rates of several times per minute can support multi-year battery life. Adding a transmitter consists of the same activities as a traditional wired device, including mounting it and turning it on. In addition to the normal device configuration, two additional items need to be configured – the join key and the network I.D. There is no field wiring, so the job can be done quickly.
A wired HART device can be made wireless by adding a WirelessHART adapter on or near the device. These adapters can be battery powered or line / loop powered and in some cases, provide the power to the transmitter. Adapters can communicate device information from a single device to as many as eight devices connected using multi-drop HART configuration. Adapters may eliminate the need for batteries and allow the use of the existing plant inventory.
Adding a WirelessHART adapter to a smart valve positioner is one of the important applications for wireless technology. Since most plants have a limited or no spare wire infrastructure, accessing the intelligent information in a smart positioner has proven to save money and time during plant operation, maintenance and turnarounds. The ability to monitor and verify potential valve problems can avoid unscheduled shutdowns as well as eliminate unnecessary valve maintenance.
What about cost?
A battery operated WirelessHART transmitter typically costs more out of the box than an equivalent wired transmitter, but that cost difference quickly vanishes when the cost of wiring and installation is considered not to mention a significant reduction in project time. Putting in a conventional transmitter requires running the appropriate 4-20mA cabling, often in conduit, at a cost of about $20 per foot, which can exceed the cost of a transmitter.
Other costs avoided by a wireless system are the addition of I/O cards and additional drawings to show how the new wiring is routed and connected.
Experience has shown that wireless technology provides a cost-effective solution enabling some customer savings of 50% or more over the installed cost of a wired alternative. In addition, once a wireless network has been established at a site, additional wireless enabled instrumentation can be installed quickly to provide continuous monitoring or used temporarily to troubleshoot or identify particular issues. Your cost and savings may vary based upon the availability of spare wires, distances and other considerations.
A WirelessHART gateway can cost about as much as a few transmitters and a single gateway can handle from 100 to 250 transmitters.
What about reliability?
A common concern when considering a wireless installation is reliability. The signals travel via radio, but what if the plant has many tanks and other vessels and large amounts of metal everywhere, in both piping and supporting structures? How can a radio signal be expected to travel through such a labyrinth without being lost?
The solution can be found within the WirelessHART technology. This is because the system is configured as a mesh network: each transmitter is in constant communication with its neighbors. If a particular transmitter cannot send a message directly to a gateway, the mesh will pass the message from transmitter to transmitter until it reaches the gateway. This mesh is self-configuring within the gateway. When a transmitter is added, the system automatically computes how to get the messages from the device to the gateway in the most efficient manner possible. If something changes — a transmitter is taken down for maintenance or a physical obstruction is added — the gateway continuously and automatically reconfigures the mesh to maintain constant communication including primary and secondary routing paths. As a result, a WirelessHART installation typically will have end-to-end reliability greater than 99.9%.
The gateway and the devices will communicate in the most reliable topology possible. A device can communicate to a gateway using either point-to-point (direct access with no mesh) or the mesh topology, or a combination of both on the same gateway. For fast changing process applications (for example 1 sec updates) point-to-point is typically used to reduce latency.
What about security?
A question that often comes to mind when considering a wireless installation is security. What is to prevent someone from monitoring the signals from the transmitters, or even introducing false data? The consequences of such a security breach could range from economic loss to a catastrophe. The designers of WirelessHART communications anticipated this, and included multiple layers of security, including end-to-end 128-bit encryption using the Advanced Encryption Standard (NIST standard FIPS-197). The gateway also provides authentication, verification, device key management, user-based log in, and activity logging.
When a field device is first turned on, the gateway sends it an invitation to join the network, including a request for the device’s specified encryption based on features installed in the gateway and the device. This ensures the gateway and the field device have matching passwords (join keys) and network identification information. Further back-and-forth communication is securely monitored by the gateway.
When a field device is ready to send a data packet to the gateway it will encrypt the packet, layer it, and publish it via WirelessHART link to the gateway. The gateway will identify the sending device, decrypt the packet, publish the device, then wrap the packet in a secured layer (an envelope, if you will), and send it on to the host system.
If a field device cannot contact the gateway directly, but instead uses another device as a relay station, the sending device will wrap its data in an envelope (analogy only) addressed to the gateway via the relay device. The relay device will package the packet into an envelope of its own and send it on to the gateway, which will verify, then unwrap everything to recover the original data packet.
Wireless is not only about replacing wired devices
Wireless is being used everywhere to make big improvements and is now widely implemented in refineries, oil fields, offshore platforms, chemical plants, tank farms, water and waste treatment centers and other industrial facilities around the world to obtain low-cost, reliable real-time data to optimize operations, improve worker and plant safety, and reduce emissions and other environmental impacts.
A wireless network can have a personnel safety advantage: Adding new field devices to a system means more data is delivered to the control room, minimizing clipboard rounds and keeping personnel off sometimes dangerous tanks.
Adding a wireless system can have environmental benefits, as well. Environmental compliance is streamlined by effective monitoring of overfill and leakage, avoiding high remediation costs, logging of an “event,” and by tank blanketing to control emissions.
Most of what has been said concerned wireless transmitters, but it is also possible in some cases to use wireless with control valves and actuators. This type of installation is suitable where the update rate of a wireless system (update rates can be set from once per second to once per hour) will not be a problem; tank level and tank temperature, which generally change slowly, are prime examples. When considering such an installation it is best to consult with the control equipment vendor before proceeding.
Today, installation of wireless field devices is seeing broad acceptance. What may surprise some is the fact that the cumulative hours of wireless device operation for WirelessHART has surpassed 1 billion hours. Over 10,000 installations exist globally and most of those are a mix with both wired and wireless field devices. The rapid adoption of this technology is driven by the low cost and quick installation, making this route the common choice for expansions during brief process outages where added downtime carries heavy financial penalties. And in the case of older plants, adding additional I/O or upgrading the system may not be practical or feasible.
Wireless has the additional advantage of being the right technology to monitor areas in a plant that just could not justify the cost of wiring a new device. For instance, bringing back vibration information from a non-critical motor may significantly reduce ongoing maintenance costs, but seem unjustified as compared to the cost of wired devices. Additionally wireless devices can be economically moved from one spot in a plant to another, permitting periodic checks on possible problem areas. This nomadic characteristic is unique to wireless devices and can greatly leverage the capital spend of process manufacturers.
WirelessHART has been designed to be simple and accessible for operations and maintenance technicians in a plant environment. Those that have gained experience working with traditional wired HART applications will have little difficulty making the change to wireless. It’s a way to expand your measuring and control capabilities easily, quickly, and economically.
Douglas Carlson is Smart Wireless gateway marketing engineer at Emerson Process Management.
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