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Safe process automation

Applications of the programmable safety control system in the process industry
Safe process automation

A practical method of implementing plant control tasks at low cost is to use PLCs. The programmable safety control system – PSS takes care of safety-orientated functions and standard functions within a control system.

Bart Monnée

The specifications of the control concept and the peripheral devices of the control system vary according to the type and size of a plant. It must therefore be decided at the outset of the plant planning stage whether a central or a standalone concept is to be implemented. In the case of central control system concepts there are two structural options with regard to a PSS (programmable safety control system) (Fig. 1):
• the PSS 3032 and PSS 3056 compact control systems for use in small to medium-scale applications,
• the PSS 3000 and PSS 3100 modular control systems for complex applications.
A series of input and output PC cards are available for the modular control systems. Communication cards can also be plugged into the modular systems, which allow data to be exchanged – generally for diagnostic purposes – with operational field buses. The construction and functioning methods of the programmable safety control systems are, however, totally compatible. Safety-orientated networking can be realised with the SafetyBUS p safe bus system (Fig. 2).
Three-channel diversity construction
The highest degree of safety is attained with a three-channel diversity construction. Information arriving at an input port is passed to the CPU via three channels. The information is processed by three different processors and sent to the output port in turn via three channels. The output port is activated if the same result is received from all three processors. Safety-orientated modules (FS = fail-safe) are selected via the three-channel backplane buses of the modular systems, while standard modules (ST) are selected via the single-channel bus. The user programmes for the fail-safe and standard parts are executed separately. They communicate without interaction – an error in the ST section, for example, has no effect on the FS section.
PSS in process automation
The programmable safety control system (PSS) takes care of safety-orientated functions (up to Category 4 in accordance with EN 954-1) and standard functions within a control system. In addition to an extensive history of applications in the machinery industry, the PSS is also frequently used in the process industry. The typical applications include burner control and tank storage of dangerous liquid gases. These types of process demand immediate shutdown in case of an error. This requirement complies exactly with the fail-safe functionality of the PSS, which changes to the safe condition as soon as it detects a wiring fault (short circuit, open circuit or cross-connection) or when the output results of the three processors do not match. In practice, the PSS can be used for any process where fail-safe control is required and the safe condition is defined by “power off“. Thanks to the wide range of PSS products and their programming flexibility, a safe solution can be found for nearly every process.
Software packages
There are a number of standard function blocks for burner control and tank storage. For tank storage, for example, standard function blocks are available for level monitoring, gas detection and valve control. For burner control, there are burner-specific function blocks such as sequence control, flame monitoring, pre-purge, ignition and blower control. The PSS is especially interesting for multiple-burner installations, as the complete installation can be controlled by a single device and there is no longer any need to install a separate burner management system for each burner (Fig. 3). The ready-to-use function blocks offer the customer a quick and flexible way of programming. If the installation is modified, the user often only needs to add or delete a function block or change the parameters.
Hardware features
The PSS is provided with various hardware features specifically for the process industry, such as a fail-safe analogue input card and the selective shutdown functionality. The analogue input card can be used whenever fail-safe monitoring of analogue values is required. The approved analogue input card is available with a range from 0 to 10 V or 0/4 to 20 mA and contains six channels. The analogue input signal is converted into three digital signals for each channel by three A/D converters from different manufacturers. A comparison algorithm and tolerance can be configured for these signals. If, according to the algorithm, the difference between the signals exceeds the configured tolerance value, the PSS changes to “stop“. A continuously changing test signal is transferred to all the inputs as a means of verification. The operating system compares this test signal with the resulting three values after the A/D conversion. If the difference between the test signal and the result signals exceeds a fixed limit, the PSS will also change to “stop“. In addition, the card detects open circuits and overcurrent. One obvious application of the input card is limit switches, for example safe monitoring of an analogue value to prevent a tank containing dangerous liquids from overflowing or to avoid explosions that might be caused by extremely high pressure.
Normally, the PSS will change to “stop“ whenever a wiring fault is detected. With selective shutdown, the user also has the option of partially shutting down the PSS, thereby increasing its availability without compromising safety. The philosophy on which selective shutdown is based is that the process is subdivided into a series of autonomous units. If an error occurs in one unit, only this particular unit is then shut down. This is achieved by splitting the I/O into a maximum of 32 signal groups. The user can configure the reaction of the PSS in case an error occurs in a signal group, separately for each group. This reaction can be a PSS stop, a shutdown of all sub-slots containing the group’s I/Os, a shutdown of all the group’s I/Os or a shutdown of the defective outputs only. One typical application of selective shutdown is a multiple-burner installation where only the burner concerned is switched off if a fault occurs.
Communication
The PSS has several different options for communication, such as Interbus, Profibus, Devicenet, Modbus and SafetyBUS p – the recently developed safe field bus system. SafetyBUS p enables safe communication both between PSSs and between PSSs and distributed I/O modules. SafetyBUS p is an approved fail-safe bus system that complies with both DIN AK 6 (DIN 19 250) and Category 4 (EN 954-1). In addition, input devices or actuators from other manufacturers can be connected directly to SafetyBUS p if the SafetyBUS p chip sets are used. The advantages of SafetyBUS p are less wiring, longer distances and more I/O capacity.
Modbus is still frequently used in the process industry. The PSS can operate as a slave in a (multidrop) Modbus network by using the Modbus software module and the RS232 interface on the CPU. In a DCS-controlled plant, the PSS can perform the safety-related functions and exchange data with the supervising DCS over the Modbus connection, for example in a power plant where the DCS controls the plant as a whole and the PSS performs the safety-related burner management functions.
Pilz
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Further information cpp-203
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