Automation systems in chemical and petrochemical plants must meet high and constantly increasing demands. The interface level of the automation system — between the field devices and the process control system — can contribute significantly to improving plant efficiency and availability. Newer interface products provide solutions for application-specific requirements. This can save valuable space, make installation faster and less prone to errors, and facilitate planning and documentation.
The interface level handles conventional functions such as electrical isolation, adapting and amplifying process signals, and filtering superimposed noise. The noise that arises from ground potential differences and inductive coupling can cause serious problems, especially in plants spread out over a wide geographic area or with varying electric potentials. Galvanic isolation of the individual field signal circuits ensures noise-free, precise signal transmission for effective plant operation.
Active or passive one- or two-channel isolation amplifiers and signal converters can offer integrated galvanic isolation for all necessary analog signal processing functions. Traditionally, these modules have measured ~0.5 to .9” (12.5 or 22.5 mm) wide, but newer circuit design with low power loss has resulted in modules just ~0.25” (6.2 mm) wide, reducing the required space on the mounting rail by 50 to 70 percent.
Industrial Control Devices for Explosive Environments
Most processing plants have certain areas where explosive atmosphere can occur. The measuring and control circuits in these plants must meet some type of Ex technology standard. With intrinsic safety isolators (Ex i), in addition to their conventional functions, the interface level also isolates the intrinsically safe (Ex i) field devices from the non-intrinsically safe input and output cards in the process control system. As “associated apparatus” for intrinsically safe circuits, Ex i isolation devices safely limit the energy that is fed into the Ex area so that no explosive sparks or any other thermal effects can occur. Because of the design requirements under ANSI/ISA 60079-11, or EN 60079-11 standard regarding intrinsically safe equipment, Ex i isolation amplifiers will be physically larger than comparable devices developed for non-Ex i applications.
Even so, newer Ex modules are available with a ~0.5” (12.5 mm) width per device. These devices can handle all functions from the supply isolator up to the temperature transducer, and either one or two channels. Compared with traditional Ex modules that range between ~0.63 and 0.9” (16 and 22.5 mm), these newer modules can save up to 45 percent space on the mounting rail. Modern transformer and circuit technologies, characterized by reduced power loss, make this narrow housing possible.
These devices, which also have precise response characteristics, provide consistently safe galvanic isolation for a 2.5 kV test voltage applied between inputs and outputs as well as to the power supply. For maximum protection, process engineers should verify that isolation amplifiers are certified to current national and international standards for installation in Div. 2, Ex Zone 2 and for Ex i circuits up to Div. 1, and Ex Zone 0 (gas) and 20 (dust).
Plug-and-Play Controls for Safety-Related Applications
If a measurement-and-control signal within the (Ex) plant structure is a component of the safety functionality, the plant operator must perform qualification of the signal transfer with regard to availability and quality in accordance with IEC/EN 61508 or IEC/EN 61511, and recommended by ANSI/ISA 84.00.01. Devices developed in accordance with IEC/EN 61508 can be used in safety-related applications up to SIL 2 and, in some instances, even up to SIL 3.
If it is necessary to switch higher power or current levels safely as implemented via explosion-proof technology, or in Ex applications in explosion protection type Ex d/e, the plant can instead use safety relays designed and certified especially for applications in accordance with IEC/EN 61508/61511, and ANSI/ISA 84.00.01, up to SIL 3. The safety relays are adapted to the relevant safety-related process control systems and controllers to allow the implementation of a variety of solutions from emergency shutdown (ESD) to fire and gas (F&G) applications. Features and functions such as positively driven contacts and line- and load-detection ensure comprehensive diagnostics and high availability.
Connecting a Large Number of Signals
Various device designs are used at the interface level. Ex i isolation amplifiers in rail-mounted housings are widely used. These devices, typically with one or two channels, are available for a wide range of functions. They allow the implementation of flexible, modular systems. Not only are newer rail-mounted devices slim, but some offer features such as pluggable connection technology. This makes for quicker troubleshooting and replacement if necessary. Modular designs typically are wired individually to the respective I/O cards.
Controlling and monitoring complex chemical and petrochemical plants requires a transparent, space-saving design to reliably process a large number of signals. Most technologies require separate wiring. This leads to increased installation and startup costs, especially if many signals are involved. Interface solutions employing motherboard or backplane technology can minimize these costs. Normally, eight or 16 interface modules are mounted, screwed on, or plugged onto the motherboard. On the system side, the signals are routed through plug connectors with high pin counts. This allows a plug-and-play connection to the control level via preassembled system cables.
Another approach for industrial applications uses quick cabling boards, or carrier termination, based on an aluminum profile specifically developed for this purpose. An integrated mounting rail contour can accept various standard interfaces (figure 1). Unlike motherboard solutions, the termination PC board is mechanically decoupled from the interface modules and integrated into the profile so that it is protected. Mechanical decoupling prevents PC board conductor breaks. This is why the termination carrier design is vibration-proof up to 2g in compliance with IEC 60068-2-6 and shockproof up to 15g in compliance with IEC 60068-2-27. Integrated screw clamps allow stable installation of the termination carrier on all commonly used DIN mounting rails.
High Plant Availability
A profile housing, adaptable to any length, permits adaptation to the required number of I/Os and to the width of the interface device type used. System connections on the control side optimize use of the existing space. The length on the control cabinet mounting rail is therefore defined only by the housing width of the DIN rail-mounted devices used. The component density in the control cabinet is defined not only by the length but also by the width. Just ~6.7” (170 mm) wide, the termination carriers also allow a transparent and maintenance-friendly design to be created in standard control cabinets, which are typically ~31.5” (800 mm) wide and have less than 7.88” (200 mm) between the cable ducts. When using the termination carrier and dual-channel intrinsic safety isolators (associated apparatus), a surface area between ~27.56 and 47.25” (700 and 1200 mm) can accommodate up to 384 Ex i signals (figure 2).
The maintenance-friendly design of the installed devices also contributes to high plant availability. The termination carrier design provides for eases access to all termination points, including the integrated test sockets. With power bridging connectors feeding the module from underneath, the individual devices are hot-swappable. Pluggable and coded cable sets make it possible to match the interface modules to the termination circuit board quickly, safely and unambiguously. The electronics for feeding in the redundant diode-decoupled supply voltage and providing fault signaling are located in the power supply module, which is connected to the isolators via mounting rail connectors. This means the termination PC board has no active components subject to failure, which would lead to replacing the entire module carrier (figure 3).
Rail-Mounted Controls Help Ensure Reduced Engineering and Startup Costs
Because standard rail-mounted devices are adapted to the termination carrier, only one device type for each function is required to address both commonly used system applications and single applications. This results in lower engineering and documentation costs, especially for SIL applications. It also reduces storage costs by cutting the number of required device models.
Used together with preassembled and tested system cables, the termination carrier design reduces the assembly time and troubleshooting during startup. This facilitates tasks such as the factory acceptance test (FAT) of entire control cabinets. System cabling products provide solutions for I/O boards used in various process control systems because connectors on the control side have been adapted for the specific I/O board design. In conjunction with the termination carrier concept, process engineers can implement an integrated and efficient plug-and-play solution (figure 4), extending from the signal processing level up to the process control system itself.