Testing Positive with EX-I Interfaces

Process Automation optimization with EX-I Interfaces

Field Instruments required to provide safe explosion protection.
Field Instruments required to provide safe explosion protection.

Phoenix Contact explains how modern Ex-I Interfaces can help to optimise process automation and manage explosion safety

In process plants with explosive areas, intrinsically safe (Ex-i) interfaces form a bridge between the control centre and the installed field sensors and actuators.

The requirements placed on modern Ex-i signal isolators are outlined here in reference to the MACX MCR-Ex product line, with examples cited from a typical application in a refinery.

The primary task of Ex-i signal isolators is to safely limit the energy conducted into the Ex area to an amount that is below the minimum ignition threshold of the surrounding explosive atmosphere.

According to the design specifications of the EN 60079-11 standard for intrinsically safe explosion protection (Ex i), these signal isolators need to ensure that all the parameters affecting intrinsic safety – such as the maximum no-load voltage (Uo), maximum short circuit current (Io), and maximum power (Po) – are adhered to even during malfunctions.

In the new MACX Analog Ex product line, these parameters have been further optimised and minimised, meaning that the devices can be used as compatible electrical equipment for the widest possible range of intrinsically safe field devices. MACX Analog Ex also offers very large Co values, as the maximum permitted outer capacitance values directly affect the cable connection lengths.

MACX Ex isolators are certified according to the current ATEX standards marked II(1)G [Ex ia] IIC and II (1) D [Ex iaD] for Ex-i circuits up to Ex zones 0 (gas) and 20 (dust), which makes them suitable for all Ex zones and gas groups. Additional certification for Ex n explosion protection, marked II 3G Ex nAC II T4, means the devices can also be installed in Ex zone 2 areas.

Saving Space
No matter if it’s single-channel or dual-channel HART-compliant repeater power supplies and output isolating amplifiers, NAMUR switching amplifiers, signal duplicators, solenoid drivers, or temperature transducers: All MACX Analog Ex modules are just 12.5 mm wide.

This means that up to 45 percent less space is used on the mounting rail compared to the usual module widths of 16–22.5 mm (Figure 2).

The narrow housing design is made possible thanks to the modules’ innovative transformer and circuit technologies, ensuring there is minimal power loss. In addition, the Ex-i isolators provide highly accurate signal transmission with a typical error margin of just 0.05 percent, and they can drive loads up to 800 Ohm.

Irrespective of the mounting position, these devices cover a wide temperature range from -20 °C to +60 °C. Other positive aspects for system designers are the patented transformer technology and the low coupling capacitances, which result in very good EMC properties in accordance with EN 61326.

Safe electrical isolation in accordance with EN 61010 with a 2.5 kV test voltage between input, output, and supply voltage both protects the control system and reliably prevents interference such as equalising currents from ground loops.

The MACX Analog Ex modules thus help to ensure faultless signal transmission, which in turn facilitates efficient process control and maximum system availability.

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Compatibility
The Macx Analog Ex interfaces are developed for use in safety-oriented circuits that comply with EN 61508 and have been certified accordingly by external inspectors such as TÜV Rheinland. The interfaces all qualify for SIL 2, some even for SIL 3.

For any device compliant with IEC 61508 and subject to Safety Live Cycle Management, the entire hardware as well as any firmware is thoroughly examined for error rates using FMEDA (Failure Modes, Effects and Diagnostics Analysis).

These error rates are used to calculate the key safety parameters required for SIL classification. The key parameters documented in the device’s Safety Manual, such as Probability of Failure on Demand (PFD) and Safe Failure Fraction (SFF), enable faultless design of the safety-oriented measuring circuit. The rugged DIN rail housings of the MACX Ex devices provide pluggable, coded connection terminals, available in screw or spring-cage technology.

This simplifies installation as well as prewiring. Socket connectors are already integrated, for example, to connect HART handheld devices. Auxiliary power and general error messages can be implemented quickly and flexibly thanks to the T-connector concept.

Best in class
As part of ongoing plant upgrades, Oil Refinery Oberrhein (MiRO) has recently introduced a new automation solution in its blending plant. One of the changes was to replace the Blend Ratio Control concept (BRC) with an Experion Blend Controller system (EBC) from Honeywell.

This changeover entailed replacing the control system’s I/O level, which is located in the central switchgear room. For the functional specification of the Ex-i interface level, which was also being upgraded, the MCR planners defined the following main goals:

  • Lowest possible component width so that there is minimal use of control room space, with ample spare room for future system extensions
  • Less product diversity in order to minimise warehousing and documentation costs Suitability for safety-oriented measuring and control circuits
  • Following a period of extensive testing of Ex-i solutions,
  • Processing Involves Approx. 750 Ex Signal Isolators
  • The 30 decentralised blender stations located at MiRO’s Karlsruhe plant are classed as Ex zone 1 and in parts as Ex zone 0.

The field instruments, which consist of various flow meters and HART-capable 4–20 mA control and separator valves, are required to provide intrinsically safe explosion protection (Ex i).

The blending process also entails pump control and the analysis of tank level gauges, signaling contacts, and 4–20 mA signals coming from the analysis station’s two-wire transmitters. The field instruments themselves were not exchanged as part of the plant upgrade.

The signals’ point-to-point connections are provided via four-core cables up to 600 metres long. These start at the honeycomb routing terminals located in the blending plant’s central distribution station.

From this station, the signals are directly routed to the Ex i isolator level, where they are passed through approx. 750 Analog Ex signal isolators and then processed via C200 and C300 controllers.

The Ex i isolators are arranged into device groups of 16 and mounted in open routing frames on DIN rails. The frames, which are built in-house at MiRO, have a very compact design and are easy to maintain. In addition, the compact 12.5 mm MACX isolators use 38 percent less space on the mounting rail than the modules used previously.

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Meeting requirements
Highly precise signal processing is crucial for properly instating the formula specifications. Rolf Bechtold, Project Engineer in Automation Technology at MiRO, confirms the MACX devices’ excellent signal quality: “The isolating amplifiers with transistor outputs, used for analysing the counter pulses, convinced me due to their clean edges.

So far, we have not encountered any processing issues with the C200 counter cards from Honeywell.” Bechtold adds, “The MACX isolators can be used flexibly and cover all our requirements with a manageable number of product types. This is an important advantage in terms of device standardisation.”

The MACX MCR-EX-SL-RPSSI-I Ex-i repeater power supply offers a supply Ex-i input for two-wire transmitters to convert signals from the analysis station, and also a non-supply Ex-i input for the 4–20 mA signals from the flow meters.

The device passes on the HART-transparent 0/4–20 mA signals to the passive and active analog C300 input cards.

Says Bechtold, “This flexibility was fantastic for migration within a running system, and it also provides a number of advantages for wiring. Again, it is very useful to us that the isolating amplifiers feature two relay or transistor outputs.

Thanks to the MACX devices’ Ex properties, we were able to retain all of our field instruments. This would have been much less straightforward using any other range of Ex isolators available on the market today.”

The MACX Analog Ex product line is a perfect match for MiRO’s requirements. This is proven by how well the blender upgrade has been performing in production over the past 18 months.

The Ex isolators are also designed and certified for use in safety-oriented applications in accordance with EN 61508, up to SIL 2 and in parts up to SIL 3. Because of this, engineers can flexibly include them in planning for a wide range of plant upgrades and refits.  

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Oil & Gas Middle East - September 2020

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