Marshalling issues in process control

Distributed Control Systems
A Distributed Control System (DCS) is the control platform for all hardware and instrumentation that is needed to transmit signals back and forth and process control actions for continuous processes with high numbers of control loops. The DCS is the infrastructure not only for all advanced control strategies but also for the lowest control system. Typically a DCS consists of functionally and/or geographically distributed controllers capable of executing from 1 to 256 or more regulatory control loops each. The input/output devices can be integral with the controller or located remotely via a field network.

Figure 1: Brief architecture of a DCS. Reference: www.isa.org

The controllers have extensive computational capabilities and, in addition to proportional, integral, and derivative control, can generally perform logic and sequential control. Most control loops are redundant due to safety and reliability requirements, which is also one important feature of a DCS. Modern DCS are equipped with optimization, high-performance model-building and control software as options. A large number of local data acquisitions, Human Machine Interfaces (HMI) and Engineering Workstations can be found distributed all over the plant. They all communicate through a data highway/field or Ethernet network.

The worldwide market for Distributed Control Systems is mainly driven by Oil & Gas and Power Generation Industries. More than 50% of all DCS sales are for applications in power plants, refineries, Oil & Gas fields and terminals, etc... There are already existing DCS installations worldwide worth $ 65 Billion, which will reach the end of their lifetime within the next years. To guarantee reliable and economical availability of Oil & Gas and electricity, these installations have to be replaced and even extended to fulfill growing demands worldwide, particularly with regard to the BRIC (Brazil, Russia, India and China) countries. In 2008, more than 10.000 DCS installations were sold worldwide. Most of the worldwide growth in Process industries will take place in Asia.

Both innovation and differentiation can be achieved by product features that support customer’s efforts to save installation space and time and other trends for lean production like reduced product complexity and easy handling.
 
Where are DCS used?
In general DCS are used for large continuous production processes with a high number of control loops. Ranges of applications can be found in all major types of process engineering where materials, often liquids or gases, are to be produced, transported, stored, processed or finished. So Electric Power, Chemical, Oil&Gas, Petrochemical, Food&Beverage, Pharmaceutical, Pulp&Paper, Mining&Metals, Water&Wastewater and Cement&Glass are typical industries for such control systems. DCS are applied for general process control and automation, safety and surveillance control, fire-extinguishing systems and many other kinds of control processes.

Managing complicated wiring
The Classic concept of DCS systems follows in many cases the next schematic:

Typical DCS wiring architecture. Wire mapping is done between the marshalling
rail and the Field Terminal Assemblies (FTA).

Marshalling is needed to connect all wires coming in with cables (single pair, multi-pair or multi-conductor) from the field to their counterpart on the control side. Usually analogue and digital signals are grouped to structure the wiring. Therefore it is convenient to have at least two connection levels in between the needed wire combinations can be made. This work process is called mapping.

Sometimes, mapping is only a one to one combination from the field side to the control side, but in most cases mapping needs to be done crosswise.

The mapping can also be done without a separate marshalling rail. Then wires from the field have to be directly connected to the FTA (Field Terminal Assemblies) or I/O modules. This is possible but has some disadvantages like direct access of likely low skilled personal to higher value components like FTA or I/O modules or less functionality in case of maintenance or later extensions. The advantages are much less space needed and much less connections to be made, both implying lower costs.

So basically for marshalling only rail mounted feed-through blocks are needed. Nevertheless end customers often want to have more functionality than only feed-through on the marshalling rails. Disconnect function is required for measurement and maintenance later in operation. Fuse blocks are often required to protect field devices and controls, although this kind of protection is often completely included in the I/O-modules or the FTA of the different manufacturers. So fuse blocks on the marshalling panels are in most cases a redundant feature, nevertheless often required by end customers. As the Process market is fairly conservative, also legacy system design plays an important role in decision making so that marshalling design or even the general need for marshalling is often just derived from existing or older system that have to be updated or extended.

Power, Signal and Data connectivity
Being the electrical connectivity specialists in Industrial environments including Process, we offer diverse solutions for Control applications, be it DCS or Emergency Shut Down and Fire & Gas systems. Some of our most recent examples can be seen here below.

Our Customer Specific Solutions (CSS) business unit designs and produces assembled Junction Boxes for both standard and Ex areas, Terminal Block rails completely finished to fit in the Marshalling and Control cabinets, customer tailored FTAs suiting the DCS vendors’ controllers or cable harnesses for the interconnection of such FTAs.

Furthermore, we have the complete range of products for the Control cabinet: from Power Supplies and Overvoltage Protectors, through Analogue and Digital Signal Conditioners, to terminal blocks for Power, Signal and Data, and all the needed accessories and tools for marking and installing. And on a higher communication level, a complete Industrial Ethernet range is available, including Managed and Unmanaged Switches, Routers, COMservers and all the related connectivity with the Steadytec range of connectors, as well as cabling.

The new series of Intrinsic Safety Isolators, the ACT20X, feature in a very few variants most of the required signal conditioning configurations. The beauty of this range is not only in it’s electronics but also in it’s housing and configuration software.

The housing is of new design and has up to 40% more PCB surface, allowing more circuits without increasing the used space in the cabinet. One innovative new feature we find on the PCB connectors. They are not only finger safe on both sides (male and female), but also include a leading pin contact and integrated, captive encoding system with unique “autoset” function. Furthermore, they offer simple and fast operation through integrated and colour-coded release frame with finger grip and including screwdriver clip. These and other interesting technical advantages are all designed in for the user’s added value, to prevent from errors, to ensure correct functioning of the installations. These features stand for safety and reliability.

The other very interesting innovation is related to software. Due to the many suppliers in the market, preferences of technology and communication protocols, and no real global harmonised standards, users many times must learn how to configure each type of equipment via the own devices’ software. This is normally unpractical and requires longer times to put the devices into service, plus it generates a less organised information system due to different types of configuration files and software applications.

These modules are configured with the widely standardised Field Device Tool (FDT) technology and DTM files. This way, operators can use the same software platform and structure as with other of their field devices, reducing the learning time and improving the file system. Efficiency and availability come to mind once more.

Another of the outstanding market launches from Weidmüller is the Varitector family of Surge Protectors for signals. There are two different variants, the Varitector SSC which includes all features in a Terminal housing, or the Varitector SPC which is a Pluggable surge protector in a compact housing. Some of the advantages of the SSC apart from the 6.2 mm pitch is the direct PE-contact fit on the rail, the Torx-slot screw and connection of cross sections between 0.5 and 6 mm2, with versions for Analogue and Digital signal protection, and usable in Ex applications.

The SPC is in a 17.5 mm pitch design, with pluggable arrester, includes a remote fault indicator, PE rail contact, allowing 2 analogue or 4 digital signals, and it’s also appropriate for intrinsic safety applications.

The international standard IEC 61508 “Functional safety of electrical / electronic / programmable electronic safety-related systems (E/E/PES)” is intended to be a basic functional safety standard applicable to all kinds of industry. It provides the framework to reach an acceptable level risk in any such system. In Process it can be applied to ESD and F&G systems in case of emergencies or interruption of a process. The standard defines Safety Integrity Levels using requirements grouped into two broad categories: hardware safety integrity and systematic safety integrity. A device or system must meet the requirements for both categories to achieve a given SIL.

Many times the use of relays is required to drive Valves for the correct shutdown. The relays will normally have a SIL3 rating.

Weidmüller launches its new SIL3 rated Relays, with a Triple relay structure and remote plus local function monitoring. The output continuous current is 5A.

We have recently launched the new Catalogue of interface boards or FTAs for use with Yokogawa’s Centum and ProSafe controllers. Weidmüller’s interface products allow managing the wiring of the I/O Card from the field to the DCS in a much easier way and to minimize the wiring mistakes. The boards have been designed in order to be compatible with the standard connectors used in the Yokogawa I/O Cards. These Interfaces provide additional features as redundancy for the signals (double connectivity) and for the power supply, which can be monitored remotely via a relay contact. There is also the possibility to use dry or wet contact in many I/O modules. Furthermore, we provide customized pre-wired harnesses with whatever length is desired, making the installation fast, easy and error-free.

Weidmüller’s expertise solving Marshalling issues
Weidmüller is developing several solutions in the area of Marshalling always with the idea to reduce space, reduce complexity and save time, therefore operative costs.
Multi-level terminals with compact connection technology, Multi-Functional terminal blocks to reduce the number of used terminals in the marshalling cabinet, Terminals with on-board cross-wiring to reduce cabling and even Interface modules with Electronic cross-wiring to reduce commissioning times are some of the ideas and upcoming products we work on.

For Terminal Block assemblies we can also provide the RailDesigner® software, which makes the assembly of terminal rails child’s play. The new version of RailDesigner® is a time-saving program which makes it much easier for you to select, create 2-D and 3-D plans and order assembled terminal rails with a high level of precision. It also speeds up the process of acquiring quotations and placing orders, for example by eliminating the need for time consuming fax orders: all processes can be initiated directly using the software.

Ask us how we can help you improve your installations, we will find ways to bring in our expertise and technology into play!