Make the most of WirelessHART

By Gareth Johnston and Alan Munns, ABB

ChemicalProcessing.com

Keywords: HART, wireless technology, Hart Communication Foundation, Gareth Johnston, Alan Munns, chemical processing, chemical engineering and process engineering

Plants can gain many benefits today from this open approach

Wireless technology is so much a part of our lives that we use words like Bluetooth and WiFi systems with confidence and familiarity. So why has it taken so long for the chemical industry to take advantage of the benefits a wireless network can offer?

Some things that we can tolerate as consumers — such as signal loss when driving through a tunnel or occasional interference from other wireless networks — are unacceptable at plants. The chemical industry requires reliable and secure transmission of information via a technique that’s easy to set up and flexible enough to adapt to changing conditions. WiFi, Bluetooth and others simply don’t meet these basic requirements. The result is that wireless networking has found a few specialized applications at plants but hasn’t been widely deployed.

The wireless instruments available to date are either proprietary or early pilot developments that only can exist within a single vendor network. However, an open approach is emerging.

Figure 1. At last year’s ISA show WirelessHART effectively linked instruments from multiple vendors.

Figure 2. A plug-in adapter can provide a simple, low cost way to get information from instruments.

In September 2007, the Hart Communication Foundation (HCF), Austin, Texas, released its latest specification, HART 7, which included wireless connectivity. HART 7 provides the chemical industry with a wireless network that can support instruments from multiple vendors in an industrial environment. This WirelessHART specification was demonstrated at last year’s ISA show in Houston, where a multi-vendor network of instruments worked together within the same wireless network connection to asset management applications running on computer workstations (Figure 1).

HART 7 gives plants a real incentive to consider wireless. So, we’ll examine the advantages and typical applications of a wireless instrument network, whether now is the right time to consider a wireless project, and how a WirelessHART instrument network works.

The advantages
Traditional 4–20-mA field instruments have been using the extremely successful HART protocol to assist during commissioning and scheduled maintenance work; in fact, more than 24 million such instruments have been installed to date. However, some 20 years since HART 5 became available we’re still not making the best use of the remote access HART offers — instrument information remains locked away for the vast majority of users. WirelessHART can provide the key to unlock this information and allow us to install instruments at a very much lower cost.

Retrieving stranded instrument and process information. The vast majority of 4–20-mA instruments installed have no mechanism to allow remote access to the information they hold. If you wish to retrofit a communications path back to an asset management system, then you typically would need to break into the field wiring to insert a HART multiplexer. This poses some risk and cost. The use of a WirelessHART plug-in instrument adapter (Figure 2) offers a more convenient, lower risk and lower cost alternative.

Reducing installation and planning costs. The need to run cabling around a plant makes installation and commissioning costly for traditional 4–20-mA analog and fieldbus (Foundation Fieldbus H1 and Profibus PA) networks. When you consider routing the cable, mounting cable trays, cutting holes and tunnelling, and generating the work permits, expenses can run to $5,000 per point. Wireless networks clearly can reduce these costs as there are no (or fewer) cables to run with instruments often being self-powered.

During the planning phase for traditional wired instrument loops, it’s common to add up to 20% spare capacity to account for future plant modifications because wired systems are expensive to modify and expand later. However, frequently much of this spare capacity isn’t actually used five or 10 years down the line. A wireless network offers a very scalable solution that can reduce the need for building in and paying for such spare capacity at the onset.

The intrinsic low-cost nature of wireless technology now allows you to tackle some of those process problems that only can be solved by measuring new process data or by monitoring device condition.

Target applications
The chemical industry historically has been cautious in adopting new technologies. Hopefully WirelessHART will jump-start the use of wireless instrument networks at plants. End-user surveys provide a good guide to likely wireless applications.

Upgrading existing instruments. Adding WirelessHART to an existing HART instrument in the field is very simple. Connect the adapter to the instrument at either a spare cable gland or even at a junction box. (The adapter could be self-powered or powered via the loop.) Add the network ID and password to the adapter and it will automatically join the existing WirelessHART mesh network. The original 4-20-mA signal remains intact and you now have remote access to the instrument information. This permits:

• Calibration check and over-range reading. For a pressure transmitter you can monitor the instrument process value and compare it to the 4–20-mA value. This can help confirm the instrument calibration status or provide a value when the 4–20-mA signal is out of range. (The WirelessHART value doesn’t depend upon the 4–20-mA value — it comes straight from the digital value.)
• Valve condition monitoring. For a positioner you now have access not only to valve position feedback but also to other data to help analyze the valve’s condition.
• Advanced diagnostics. You have remote access to instrument information such as asset signatures, level echo trace and other advanced diagnostics.
• Full use of complex multivariable instruments. For multivariable instruments (mass flow, for example), you now can read the process values previously hidden away.