Expanding plant networks with WirelessHART

With process instruments getting ‘smarter’ every year, their capabilities are often underutilised. Important data that could help save costs remains unused. This potential can be tapped by using WirelessHART.

Intelligent devices, whether they are valve positioners, temperature devices, flow meters or level meters, supply additional data such as secondary process variables or device diagnostics that can be used to obtain a better insight into the process. This data can be made accessible using a HART data transfer protocol. Although new control systems are HART enabled, the many legacy control systems in the field frequently lack the ability to collect HART data. The WirelessHART standard can be used in both retrofit and greenfield installations, providing distinct advantages for each.

Wide-ranging advantages

The time to engineer and develop the expansion or construction of a process unit can be drastically reduced by installing wireless systems to replace both infrastructure and signal cabling. Moreover, investment costs are reduced, as are the costs associated with obtaining the necessary approvals. A signal that previously took days to bring online using traditional wiring can now be commissioned within just a few hours using WirelessHART. Less time and higher flexibility allows maintenance crews to deploy wireless nodes for temporary troubleshooting or adding ‘stranded’ measurement points to increase safety or improve efficiency.

By utilising previously unused HART diagnostic data, a maintenance department can detect issues; for example, plugged lines or worn valve seals. Leaks and contamination may also be detected before they ever cause a problem to other instruments or the complete process. What’s more, plant personnel are protected against safety risks. A logistics department can better plan when to turn assets on and off. What also should not be forgotten is that the efficiency of critical parts of plants and systems can be monitored from remote locations using WirelessHART.

Interaction between state-of-the-art industrial networks

It is important for users considering utilising a communication solution based on WirelessHART to understand its optimal position in the process and industrial network structures prior to installation. By doing so, it is possible to simplify the network architecture and system management, resulting in better connectivity along with both capital and operational cost savings. The Purdue Model of Process Control explains how components in modern industrial networks collaborate. It distinguishes between five levels in the hierarchical structure of a network of control systems. With this model, users obtain a common model as well as standard terminology so that networks in all industrial application areas can be identically structured.

Figure 1: Purdue Model of Process Control mapped into a functional network diagram.

Each level has specified functions and actions for which it is responsible. These responsibilities range from creating preventive maintenance schedules and equipment renovation cycles in level 4, to different types of local instrument control found in levels 0, 1 and 2. Levels 0, 1 and 2 are directly connected to level 3. This involves manufacturing operations and control, which is directly responsible for establishing immediate production schedules and maintenance routines. Having a better insight into the diagnostics data of the instruments installed in the plant or system makes it possible at level 3 to schedule maintenance routines more efficiently than in the past. Level 3 can be considered a plant network, as it essentially acts as a master of operations to the lower instrument control levels (Figure 1).

Simple coupling

A process plant network is fairly complex, as different applications such as visualisation, asset management and data historians require access to measurement and control data from the field. WirelessHART facilitates flexible integration of diagnostics data using a WirelessHART adapter - or from process measurements via WirelessHART instruments (Figure 2a). A WirelessHART gateway is used to simply establish a connection to the network (Figure 2b).

Figure 2: a) The WirelessHART adapter from Phoenix Contact can support up to four Hart devices. b)The WirelessHART gateway has an integrated WLAN interface to connect with a wireless network and to enable TCP mapping from HART to Modbus.

In the simplest case, the WirelessHART gateway is directly connected to the control system (Figure 3). In this case, the wireless standard represents an alternative to the 4-20 mA wiring to transfer the process variables from the field instruments. The simplicity of the system is derived from the interface between the control system and the gateway; for instance, Modbus or Profibus. The HART commands are mapped to the I/O register format of the interface protocol. The control system can then use the measured data for simple closed loop functions or various calculations.

Alternatively, the user can connect the WirelessHART gateway directly to the control network level (Figure 3), allowing the distributed control system (DCS) to directly access the gateway data. To achieve this, a commonly used communication protocol is used, such as Modbus. By integrating the WirelessHART gateway directly into the control network, additional controls are not required at the lower levels and new measurement points can be set up faster.

Figure 3: The WirelessHART gateway can be installed in the network in various ways.

Direct access

Directly connecting the WirelessHART gateway with the plant network has proved advantageous. This is because the DCS as well as other higher functioning plant tools, such as the asset management system (AMS), HMI screens or a data historian, can access process and diagnostics data. Generally, the gateway interface is based on a protocol such as HART IP or FDT/DTM, which is suitable for calibrating, configuring and transferring measurement data. This means that communication with the field devices is realised without converting or mapping to another protocol. As a consequence, it is possible to provide more efficient maintenance schedules so that it is no longer necessary to shut the plant down at regular intervals, but rather only when devices need maintenance. Further, safety of operating personnel is increased as devices can be wirelessly calibrated. As a consequence, personnel no longer have to venture into hazardous areas. Further, it is guaranteed that device calibrations are up to date, which in turn ensures improved accuracy in the process.

Summary

Expanding plant networks using the WirelessHART standard offers numerous advantages. As a consequence, field devices and instruments that are difficult to access can be directly coupled to the higher-level control system of the plant. This means that personnel obtain far wider access to a significantly more comprehensive database.