Switching to wireless level detection

08 November 2011

New developments in wireless communications offer a solution for level monitoring without the need for cabling or the associated risk of damaging the existing installation, and enabling configuration and troubleshooting from a central location.

Emerson process Management reports that intelligence inside its switches are now also able to distinguish between material build-up on the fork and a high product level, reducing the need for inspection in the field. Electronic Device Description Language (EDDL) is used to enable the level switches to be configured and monitored from the same device management software as a plant’s other intelligent devices such as pressure and temperature transmitters.

Adding hardwired level switches into an existing plant can be costly due to the cost of laying and connecting new cabling, as well as possibly additional cable trays, system input cards, and system tag license costs.

Many tanks around the plant will not originally have been fitted with instrumentation connected to the control system. Similarly, coolant and lubricant level in various assets have not been monitored continuously. Using wireless technology this type of information can now be better utilised.

Wireless level switches overcome the limitations of hardwiring. They can communicate using the IEC 62591 (WirelessHART) protocol and can be deployed without running cable or using up spare wires and system input cards. Because there are no wiring connections to be made, commissioning is also easier.

A wireless level switch will share the same network infrastructure as other wireless transmitters, with information transmitted via the same gateway. One gateway can support up to 100 IEC 62591 transmitters. Once a gateway is in place, plant personnel can expand the network at will, enabling level switches to be installed on points previously not monitored by the control system, to enhance operation and worker safety.

Because IEC 62591 devices all use the same common application protocol, devices and gateways from different manufacturers should work together seamlessly, self-organising to form a mesh network where each device maintains communication with multiple neighbours – establishing multiple communication paths and relaying data even from the most remote devices all the way to the gateway. If devices are added or removed, the network is able to automatically adjust its communication path, without interrupting data flow.

Modern DCS will have native support for wireless. However, older control systems can also make use of wireless level switches, or other IEC 62591 transmitters, using a wireless gateway that converts the signal to Modbus/RTU, Modbus/TCP, or OPC. Wireless support on the control system engineering console is not required as the network setup is done through a web server embedded in the gateway and devices are configured through intelligent device management software. No additional software needs to be loaded onto the control system or other PC for operations or security. All that is required are the existing HART configuration tools, including asset managers and hand-held configurators, to bring the network online.

A vibrating fork level switch operates on the principle of a tuning fork. An internal piezo-electric crystal oscillates the external fork at its natural frequency. The frequency changes depending on the medium in which it is immersed and these changes can be monitored. Unlike many other level switch technologies, the vibrating fork technology does not have parts that can get stuck and therefore is less prone to failure.

With a simple on/off signal from a hardwired float level switch it was not possible to tell the difference between a stuck switch and an actual high-level condition. Similarly, it was not possible to tell if the level switch was damaged or had failed and the signal was therefore invalid. For this reason, technicians are periodically required to go to the field to perform checks just to be sure, often to find nothing wrong.

With intelligent devices, however, changes in frequency are used to detect high or low level, as well as media build-up on the fork, external damage to the fork, internal damage to the piezo, and excessive corrosion.

Having such intelligence in the field can reduce the necessary trips to the field for inspection purposes, as many suspected problems can be remotely verified from the control room, and cleaning or service scheduled accordingly.

Remote set up
Remote setup is a relatively new development in level switches. Manufacturers can now use EDDL to define how a device is to be displayed in the system. This technology is used for continuous devices such as transmitters and positioners as well as with discrete devices such as level switches, on/off valves, and electric actuators.

The use of EDDL enables IEC 62591 level switches to be set up and checked using the same intelligent device management software as the other devices in the plant. The information from level switches can be displayed side-by-side with information from wireless transmitters for pressure, temperature, and other process parameters. They are displayed the same way as FOUNDATION fieldbus and PROFIBUS devices.

Systems based on EDDL are said to make managing the mix of devices easier, eliminating the errors and learning curve associated with using different software or different driver for each one. Manufacturer know-how, in the form of text and illustrations, is brought into the system through the EDDL file.

A device overview page will clearly indicate the process state as ‘wet’ or ‘dry’ and this is accompanied by device health status, which will indicate the validity of the information. Because the operator is able to tell the difference between media build-up on the fork and an actual high level they can act accordingly.

Material build-upon the fork can be detected in its early stages and flagged as an advisory alarm, so that cleaning can be scheduled before build-up accumulates to the point where it causes a false process state indication. As a supporting troubleshooting tool, the fork’s frequency is also displayed as a dynamic needle gauge with a colour band on the scale to distinguish normal from abnormal operation. The health of the internal power module is also indicated.

EDDL is key to interoperability, providing complete access to all device functionality through a hierarchical menu structure. The EDDL file from the device manufacturer is copied onto the system to tell it how to interface with the device. Unlike other device integration technologies, no software installation skills or license key management are required.

Each version of each device from every manufacturer has a unique EDDL file. There are no shared files, ensuring that the addition of a new device will not overwrite another. Because EDDL is a compressed text file, independent of the Windows operating system, existing device files are not made obsolete by new Windows versions. Conversely, new device files do not force a Windows upgrade for the system.

Time delay can be configured to minimise false switching due to turbulence or splashing, such as in the presence of agitators. To set the delay time on traditional level switches they must be opened up and a potentiometer adjusted by screwdriver. This is inconvenient in the field and exposes electronics to potentially harsh environments. With a WirelessHART vibrating fork level switch, the delay time and other settings can be checked and adjusted remotely from the control room, with the EDDL technology enabling the device management software to maintain a single audit trail for all devices, including level switches where configuration changes are logged.

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