Wireless instrumentation reliability

WirelessHART instruments and networks can equal or exceed their wired counterparts in terms of reliability.

Image 1. Wireless monitoring is often the best solution for pumps in remote locations because they are difficult to monitor with standard wired instruments.
Image 1. Wireless monitoring is often the best solution for pumps in remote locations because they are difficult to monitor with standard wired instruments.

Flow monitoring and control systems must exhibit a high degree of reliability, which in the past has made wired instruments and networks the first choice. Advancements in wireless instruments and networks, particularly those conforming to the WirelessHART standard, make wireless a viable option in many applications.

WirelessHART is an open standard supported by the independent FieldComm Group, an organization with hundreds of members. A WirelessHART installation typically includes:

  • A self-organizing network of WirelessHART instruments
  • A gateway to receive wireless signals from up to 100 instruments
  • A wired connection — such as Ethernet or serial Modbus — for the gateway to send data to one or more control and monitoring host systems

No wiring, cables, conduit or power supplies are needed for WirelessHART devices because each contains a power module and a radio. A WirelessHART network can integrate into the plant’s control system as a source of wireless input/output (I/O) or operate independently through direct connections to data historians, asset management systems and other host systems. In most cases, gateways are integrated in parallel to multiple host systems to extend benefits beyond process control by providing incremental information to operations.

Multiple WirelessHART devices form a self-organizing mesh network (see Figure 1), where each device communicates with the gateway, while relaying the signals of devices that cannot directly reach the gateway. This allows WirelessHART to mitigate obstacles, cover long distances and provide network redundancy. Organizations that use traditional 4-20mA HART instruments can use the same tools for managing wired HART and WirelessHART devices.

Traditional wired 4-20 mA loops have native subsecond update rates to the control systems, but the update rate needs to be configured into the WirelessHART devices in addition to a network ID and join key for security purposes. Update rates are typically set at eight seconds or slower to maximize power module life and network capacity, but can operate as fast as once per second. Update rates of eight seconds are sufficient for most control and monitoring loops in flow control and other process applications.

Self-organizing mesh technology

Every device in the WirelessHART network has redundant connections back to the gateway through the self-organizing mesh network, which is self-forming and self-optimizing. The mesh network leverages 15 channels in the 2.4-gigahertz spectrum with a protocol that includes several advanced technologies to provide maximum coexistence and security.

Whether an obstacle is in the physical or the radio frequency space, the network automatically reroutes to ensure high availability. When using redundant gateways with redundant power supplies, as should be done with any critical I/O component, total system availability can be calculated and proven to be greater than 98 percent.

WirelessHART is not the only low-power wireless network based on the IEEE 802.15.4 standard. But, other networks using this standard are point-to-point, where devices either:

  • Only communicate to a gateway.
  • Have manually configured mesh capability so devices can communicate with each other, but the user must manually configure each redundant connection during implementation and any time a substantial change to the environment occurs, such as adding an obstacle.

These systems are inherently less reliable and require more maintenance because they lack the self-organizing mesh capability of WirelessHART.

For example, suppose a measurement point is out of reach of the gateway or behind an obstacle. The only options in this case with a point-to-point wireless network would be to add gateways or move the wireless instrument, which are costly alternatives.

Similarly, changes in the environment, such as scaffolding or inclement weather, can change the viability of communication paths and disrupt point-to-point signals. The self-organizing mesh of WirelessHART eliminates these issues and provides a superior alternative by autodiscovering the optimal paths back to the gateway. These paths are therefore redundant and continually and automatically modified as necessary without loss of data. If a weak point ever occurs in a WirelessHART network, any device can act as a repeater to fortify the network.

With manually configured mesh networks, the user must select and configure multiple paths for each wireless instrument after installation and manually manage any disruption. Network design cannot be done on paper without explicit point-by-point field validation because not all obstacles are perfectly documented in relation to where the measurement point is located. In the case of capital projects with new infrastructure, this requires the user to wait until all infrastructure and temporary obstacles, such as scaffolding, are removed before final installation and configuration. WirelessHART addresses these issues, and the network gets stronger as more devices are added because more potential paths for optimization in the self-organizing mesh are available.

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Wireless can be more practical

Wireless is often the only practical solution in brownfield applications where instruments need to be added to comply with regulations and to improve safety and performance. Pump monitoring is a good example.

Pumps in chemical, petrochemical, refining and other industrial plants need to be monitored, even if they are in remote locations, such as a lift station or pipeline. When a pump fails, it can cause catastrophic failures that lead to expensive repairs, fires and plant downtime. A pump or seal failure can start a fire or shut down a complete process, requiring expensive, unplanned emergency maintenance.

In many facilities, automated monitoring is used for only 10 percent of the most critical pumps or not at all. That leaves nearly 90 percent of pumps in a typical facility prone to relying on manual rounds, unnecessary maintenance or running to failure.

Many pumps are in hazardous areas, remote locations or areas where no spare I/O or power is available (see Image 1). In these cases, using wired instruments for monitoring cannot be cost-justified, and wireless is the only practical alternative to enable early detection through remote monitoring. Typically, wireless installations will take one-fourth of the time and cost of a wired 4-20 mA or FOUNDATION Fieldbus installation for providing flow, pressure, level, temperature and/or vibration measurements.

Wired instruments require a supporting wired infrastructure, which can include a power supply, cabling, conduit and cable trays to bring the signal to a field junction box or marshalling cabinet, along with input devices at the control and monitoring system to accept the instrument’s 4-20 mA or FOUNDATION Fieldbus signal. This makes wired instrument and network installation expensive and time-consuming, particularly when design and installation labor are also considered. Many mature facilities have no spare input points in the existing infrastructure, making wireless the only practical alternative.

Wireless networks eliminate the complexities and costs of wired instrumentation and are just as reliable. Unlike 4-20 mA wired systems, each WirelessHART network component has built-in diagnostics for remote monitoring and early detection of potential issues.

Wireless technology has changed the landscape of remote pump monitoring. At a fraction of the installed cost of wired instruments, plants can enable preventive maintenance by augmenting manual rounds with online condition monitoring. This not only gives early indication of failure, but also avoids unnecessary maintenance. The same benefits for pumps can be extended to other critical assets such as heat exchangers and cooling towers.

WirelessHART performance is comparable to wired systems for most flow monitoring and control applications, and it has been in widespread use for more than 10 years. More than 30,000 networks are deployed globally, with more than 40 different types of WirelessHART instruments for measuring flow, pressure, level, temperature and other key process parameters.

Dan Carlson is a solution architect at Emerson Automation Solutions. He holds a Bachelor of Arts in physics from Carleton College and an MBA from the University of Minnesota. Carlson has been with Emerson for 15 years, working on wireless technology and applications.

Learn more about the FieldComm Group at fieldcommgroup.org.

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