Intrinsically-safe fieldbus provides chemical plant savings

Similar conventional and fieldbus installations are compared

May 1999 Supplement
Feature Article

Intrinsically-safe fieldbus provides chemical plant savings

Similar conventional and fieldbus installations are compared

H. Krummen, Wacker-Chemie, Cologne, Germany

he new, intrinsically-safe PROFIBUS-PA fieldbus system offers savings of 30–50% over conventional wiring techniques. These savings result from reduced hardware and planning costs. They encompass both new installations and maintenance.

In 1997, Wacker-Chemie equipped a plant section in their Cologne works with PROFIBUS-PA technology in an EEx Zone 1 environment. Following is our experience with the world’s first real EX Zone 1 fieldbus application.

User requirements and advantages of the PROFIBUS-PA fieldbus system. A technological leap forward was made when the 4-20 mA live zero signal was introduced which made intrinsically-safe fieldbus technology possible. The main requirement, besides that of being functional, was that "the communications system must be economical to run." In the pilot plant at Wacker-Chemie in Cologne, tests were carried out to measure economic effect and functionality. The results will be compared with those of an earlier, similar but conventional project.

Installation. Wacker-Chemie’s particular application places heavy demands on field equipment with fieldbus interfaces, since gases and vapors are produced when chemicals and process materials are manufactured, transported and stored. These create an explosive atmosphere if mixed with oxygen in the air. For this reason, only electrical equipment with "intrinsically-safe" EEx i protection is used here. In contrast to other protection types, "intrinsically-safe" refers not simply to individual pieces of electrical equipment, but to the entire electrical circuit. PROFIBUS-PA field devices have been certified for intrinsic safety by the PTB in Braunschweig, Germany, in accordance with CENELEC guidelines. PROFIBUS-PA for intrinsically-safe applications is based on the IEC 1158-2 "physical layer." Intrinsically-safe operation of fieldbus and field devices is assured by the fieldbus intrinsically-safe concept (FISCO) model formation. With PROFIBUS-PA it is possible to power 10 field devices over the bus when used for intrinsic safety applications, and up to 32 for nonintrinsic safety applications. The segment coupler converts the physical layer from PROFIBUS-DP using RS 485 to PROFIBUS-PA with IEC 1158-2, and at the same time adapts the baud rate, as transmissions are made on the IEC layer at a fixed rate of 31.25 kbps.

Flow of information, availability. In process control applications such as Wacker-Chemie, the requirements of a fieldbus are quite different from those in other areas of production processing. It is not the extremely low cycle times — in the region of a few milliseconds — that are of principal importance here; it is the increased data transfer security, safe operation in explosion-protected plants and power transmission via the bus cable. In process technology about 90% of the cycle times demanded are between 100 ms and 1 s.

An important aspect, besides communications technology requirements, is device parameter standardization. A maintenance engineer at Wacker-Chemie Cologne usually looks after more than 1,000 devices. These involve up to 200 different type groupings. When maintenance is being carried out, it must be possible to check or set parameters for a device while it is working without affecting the intrinsic safety. It would not be acceptable to close down the plant to carry out maintenance.

Economy, potential for capital savings. Early evaluations have resulted in a potential saving on investment costs of about 30–50%. This is because of the way conventional wiring is structured (Fig. 1). And, if you look at the cost of transmitting a process variable from the sensor to the control system, the figure of 50% is exceeded in some cases. Here, we are taking into account sensor connections, wiring to the local distribution box, the connection to the control room, Ex barriers, power supply, controller analog input modules and, of course, space requirements. In addition, it must be taken into account that all these components have to be assembled.

In contrast, fieldbus technology is a total concept, and many of the bottlenecks and high-cost elements of conventional field device single wiring are automatically eliminated. As shown in Fig. 1, wiring is drastically reduced. Only two wires are required for PROFIBUS-PA to connect a large number of devices.

Expenditure on hardware is, therefore, also reduced. For example, with conventional installation technology, the complete schedule for a measuring point complete with a CAD system runs several pages. With PROFIBUS-PA it is possible to summarize several measuring points on an A4 sheet, or to transfer parts lists into an Excel file. This is the savings potential in chemical plant construction. In technical support, further unevaluated savings result from minimizing risk of errors in existing instrumentation and control equipment because installation hardware is reduced. Thus, with PROFIBUS-PA, switch boxes with meter-long jumper distributors belong to the past. In certain cases, especially where equipment is retrofitted, users believe that the savings potential is even greater because structural alterations are not necessary.

First plant section. This obvious cost reduction is what persuaded Wacker-Chemie to equip its first plant section with storage tanks using PROFIBUS-PA technology in early 1997 (Fig. 2) at a chemical plant in EEx zone 1. The part of the project involving the storage tanks consists of two identical plants equipped with seven level transmitters, two temperature transmitters, one mass flow transmitter and a pilot valve for ball valve control. All components are interlinked by PROFIBUS-PA and connected via three segment couplers to the overlaid bus system — PROFIBUS-DP.

Currently, a second plant section with storage tanks (Fig. 2) is being built with PROFIBUS-PA technology — with more than 100 analog values, such as pressure, temperature, flow, level, etc., and 200–300 binary values. Another project is automating a production line costing several million DM for instrumentation and control. Cost savings with PROFIBUS-PA are an important goal for this large investment.

PROFIBUS-PA from the instrumentation and control point of view. PROFIBUS-PA is based on IEC 1158-2 transmission technology. IEC 1158-2 defines strict limitations on power consumption for field devices. This causes great demands on device design, especially for actuator manufacturers. Until recently, power consumption less than 2 watts was sufficiently low for field devices, but now regulations insist on power consumptions of less than 10 to 20 mW for fieldbus-operated positioning elements in explosive atmospheres. The available power has to be split between bus hardware and the actuator system. A quantum leap in power consumption technology was, therefore, required for intrinsically-safe, multi-channel pneumatic valves for pneumatic actuator technology, such as drives and valves; low power devices are required.

Actuators best suited to low-power operation work on the piezoelectric principle. Piezoelectric actuators show efficiency grades of up to 50%, and when they are maintaining a certain static position, they require no power at all. Low switching current and operating voltage mean that a control system can operate with low power and with straightforward switching technology. In most cases it is possible to avoid installing power electronics.

Thus, for example the NAMUR pilot valve has a short-duration pick-up current of 5 mA at 24 V for a few milliseconds, as well as a typical rest / stop current of less than 0.2 mA at 24 V. The pilot valve is connected to PROFIBUS-PA by two-wire technology. NAMUR position sensors have two inputs. The PROFIBUS-PA hardware uses a microprocessor-controlled system. As this processor is not fully occupied with communications tasks, it can take on other work. For example, it can set the monitoring period between the "valve open" signal and the "valve closed" position response synchro. If the period is exceeded, an alarm is triggered and sent to the control system via the fieldbus. It is also possible to detect hysteresis. Diagnostic messages for maintenance intervals are then passed to the process control system. Thus, an "intelligent displacement" is made to the pilot valve at field level, relieving the control system.

Process instrument requirements have changed in that a PROFIBUS-PA interface must be integrated into the instrument itself. Since the two-wire technique is already standard instrumentation technology, the problem of low energy balance does not occur. The necessary basic components (ASICS) for integrating the PROFIBUS-PA interface are available.

The first process instrumentation implementation stage in the storage tanks includes continuous level and flow instruments. The instruments provide hydrostatic level measurement and Coriolis flow metering.

Using standard profiles fulfills the user’s well-known requirement for manufacturer-independent field device exchangeability; this also minimizes the service tools required, e.g., handhelds. PROFIBUS-PA field devices at Wacker-Chemie can either be operated locally at the device or centrally from an engineering station with a universal operating tool for PROFIBUS-PA devices. All data from the devices are archived, documented and modified centrally. This increases plant transparency and safety.

Outlook. Like the development of programmable controllers from pneumatic control, fieldbus systems signify a quantum leap in process technology. Signal converters and positioning devices are integrated via PROFIBUS-PA in process control systems. Miles of cables and diameter-dominant cable harnesses belong to the past. Jumpering becomes a foreign word. The future will be marked by savings potential and ease of servicing together with greater availability. FB

The author

Krummen Dipl. Ing. Helmut Krummen is the responsible plant manager at the Wacker-Chemie GmbH Cologne site. His tasks are planning and maintaining the EMSR-installations, analyzing techniques and communication networks of the Cologne site.