TAKING ETHERNET INTO HAZARDOUS AREAS

Phil Black - PII Editor 20/06/2011

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Communication between automation system components has migrated from proprietary to open fieldbuses, and more recently to Ethernet. The migration to Ethernet has delivered substantial cost savings to the end user as a result of lower hardware costs using standard IT technology and lower support costs with readily available expertise. As a widely adopted open standard supported by all industrial protocols, Ethernet also simplifies interoperability between separate systems.

The ability to construct redundant networks delivers the high availability required by critical control applications, and communication speeds from 10 Mbps to 10 Gbps meet the bandwidth requirements of even the most demanding applications. Ethernet supports copper cable with ‘Power over Ethernet’ option, fibre connections for long distances and noise immunity, and wireless to connect to mobile operators or where cabled networks are too costly.

But what additional considerations are required when the user wants to exploit these advantages in applications where there is a risk of explosive gases or dust being present? This article will investigate this by discussing typical hazardous area Ethernet applications, and the advantages and limitations of the different solutions available.

An offshore oil platform required an Ethernet connection from the control system to an operator terminal installed in a Zone 1 hazardous area.
The first option considered was to install a mechanically-protected Ethernet cable between the control room and the industrial PC housed in a purged enclosure. This could be achieved using an armoured Category 5E Ethernet cable but this option is expensive and would involve mixing intrinsically safe and mechanically protected cabling in this area of the platform. The operating company had a standard of using intrinsically safe equipment for instrumentation so it was decided to install an Intrinsically Safe isolator in the control room to provide an IS Ethernet connection to a second IS isolator in the purged enclosure housing the operator terminal. This solution had the benefit of providing a live-workable IS cable to traverse the process area.

A chemical manufacturer wanted to interconnect remotely mounted fire and gas controllers in various parts of a chemical plant.
The initial proposal was to use general-purpose Ethernet fibre switches, but this would have required the fibre optic cable to be routed around the outside of the plant to avoid the hazardous area. In hazardous areas the energy in a conventional fibre optic cable can be sufficient to cause ignition if an explosive gas or dust is present.

A new IEC and CENELEC standard was therefore published in 2006 to cover the protection of equipment and transmission systems using optical radiation. As this is a relatively new standard many end users may not yet be aware of these requirements for protection of fibre optic cables. Selecting inherently safe ‘Ex op is’ (inherently safe) certified Ethernet fibre optic devices allows standard fibre optic cable to be installed across the hazardous area. This is technically similar to the principle of intrinsic safety, where the energy entering the hazardous area is limited. In this application, it allows the user to install a site network with much shorter cable runs, significantly reducing the installed cost.

The alternative approach of using ‘Ex op pr’ certified equipment requires the fibre optic cable to be mechanically protected. The high cost and difficulty in handling inflexible, armoured fibre optic cable leads most end users to select the inherently safe fibre optic technique. This can be easily implemented with copper-to-fibre Ethernet media converters located at each controller node, to provide an inherently safe ‘Ex op is’ fibre optic connection that can typically support distances of up to 2 km between nodes using multimode fibre and distances of up to 8 km using single mode fibre.

A chemical distribution tank farm used an intrinsically safe bar code scanner to confirm the correct product was selected from the tanker loading matrix.
At the end of each shift the bar code scanner was returned to the recharging cradle and the history of tanker despatches downloaded to the Enterprise Resource Planning system. The customer wanted to move to a real-time update of the order processing system from the tanker loading matrix. As the IS bar code scanner supported 802.11b/g wireless it was decided that installation of a wireless LAN access point would provide wireless communications to the scanners and a network connection to the ERP system.

To provide wireless coverage of the tanker loading matrix the access points needed to be located in the hazardous area. The options considered were to house a general purpose access point in a flameproof (‘Ex d’) enclosure; however this approach required the antenna to be certified for operation in a hazardous area only a limited number of versions were available. The favoured approach was to install an Intrinsically Safe access point as this can be housed in a standard weatherproof enclosure with a wide choice of antennae. The IS access points can be powered over the Ethernet cable, simplifying installation.

The IS Ethernet connection is separated from the general purpose Ethernet network with an Intrinsically Safe Ethernet isolator. Security is a key requirement for today’s network design and this is supported by sophisticated access point features including encryption, network authentication, MAC address filtering and SSID broadcast control, together with the deployment of an Ethernet security appliance that can run software modules such as a firewall.

A major UK coal mine undertook a review of their monitoring and control.
A mine is an ‘enclosed space’ where explosive dust and methane gas can be present, with process areas often many kilometres from the nearest exit. The colliery decided to install an Intrinsically Safe Ethernet network – not only for the ‘at-face’ mining equipment but throughout the mine. This has now been operational for several years and has demonstrated significant economic advantages compared with conventional systems using other explosion protection techniques. In addition to the Intrinsically Safe Ethernet network, Intrinsically Safe PLCs and systems are used on all underground production and conveying equipment. The benefits of the open Ethernet network have delivered significant cost savings, with one down-time incident alone saving over £500k as a result of faster fault diagnosis. Following the breakdown of a conveyor, the conveyor manufacturer, motor manufacturer, mine control room and group engineering could all access diagnostic data from the mine. This allowed the cause of the breakdown to be identified and quickly resolved. The Ethernet network has used IS Ethernet switches for connections between IS devices, fibre to copper media converters for long distance communications, IP cameras for conveyor monitoring, and IS Ethernet to RS232 and RS485 gateways. This mining application has demonstrated the benefits of remote access that can be securely achieved with an Ethernet security appliance running a firewall and Virtual Private Network (VPN) tunnelling software.

A large refinery in Belgium had a requirement for temperature monitoring on over 2,500 points in the hazardous area.
Intrinsically safe I/O was selected to connect to the control system over an Ethernet network using the open Modbus TCP protocol. As availability of the I/O data was critical to the refinery operations, the highest levels of redundancy were built into the Ethernet network. Each I/O node had redundant Ethernet ports which communicated over two separate networks to the control system. Additionally, each of these 2 networks were designed as redundant rings so if any ring connection fails, the communication recovers within 200ms using the alternative ring path. As some of the nodes were more than 100m from the ring, switches with 3 fibre ports were selected to provide two ring connections and one I/O node fibre connection. Using fibre optic communications had the additional advantage of providing a high degree of immunity to electrical noise. As all the nodes and Ethernet network are located in Zone 2 hazardous areas, normally non-arcing (‘Ex nA’) certified Ethernet managed switches were selected to support the redundant ring topology.

Communications to Remote Terminal Units at oil and gas wells in a Saudi Arabian facility had traditionally used telecommunications technology.
Interest in providing video and voice communications along with I/O from the RTU led to the decision to deploy Ethernet technology. To meet the long-term bandwidth requirements, Gigabit Ethernet links were selected for the two redundant ring network links and for the oil/gas well link with up to 7 fast Ethernet ports for other equipment. In addition to the requirement for hazardous area approvals the switches have to be suitable for operation in the harsh desert environment with proven reliability at high temperatures.

This selection of Ethernet applications has demonstrated the flexibility of Ethernet with copper, fibre and wireless connections to provide cost-effective networking in hazardous areas. The key requirements for successful deployment are to select Ethernet hardware that is designed for the harsh process environments and is certified for use in hazardous areas. The complete network solution often requires a combination of the best network hardware, suitable enclosures, seals and glands and in-depth technical support that understands the demands of the hazardous area environment.

By Roger Highton – Product Manager at MTL Instruments