In today’s processing industry, plant operators and their suppliers must strike a balance between two different perspectives. On the one hand, there is an extremely broad range of requirements arising from the process applications requiring field devices with a high degree of flexibility. On the other hand, especially in large plants, the issues of standardisation and simple stock management are becoming increasingly important. And, devices must meet the normal everyday requirements of easy operation and universal use with minimal time and cost for maintenance. Every user has a common requirement for installation, commissioning and operation – the device must be cost-effective.
What about valves that must be automated? Control valves that influence the volume flow of process media (gases, vapours, water, etc.) in order to regulate process variables such as pressure, temperature or flow, are found in all process industries. They are a fundamental component of automated processing plants and are controlled by many different types of actuators. Hydraulic or electric actuators are two examples. However, the majority are pneumatic actuators. This is where the widest range of demands occurs and these are demands placed not only on the actuator or valve, but particularly on the field component that completes an automated application – the positioner.
The demands are for:
* The widest variety of actuator movements and ranges.
* Manufacturer-specific standards.
* Special applications.
* Controlled or ‘simple’ open/close operation.
* Fast and easy commissioning.
* Short travel times but with precise control.
* The ability to proactively receive information about wear and the state of the application.
Does one need to fall back on the widest range of devices or even manufacturers at the expense of the various requirements? Does this mean that the two different perspectives mentioned above, as well as the additional requirements, cannot be addressed through a single solution?
The answer is the Sipart PS2.
Easy mounting and initialisation
One great advantage of the Sipart PS2 positioner is its universal usability. This, however, requires a high degree of flexibility and mounting possibilities for various actuator and valve applications with a long stroke or part-turn range.
This requirement has been addressed by providing more than 400 mounting kits. These enable the device to be mounted on standardised actuators such as part-turn actuators according to VDI/VDE 3845, linear actuators according to IEC 60534-6-1 (NAMUR) and also on non-standard manufacturer-specific actuators.
The subsequent commissioning takes just a few minutes. This is made possible through the simple and direct pushbutton operation and a display, as well as through the adjustable friction clutch, which ensures smooth execution of the initialisation process without time-consuming dismounting and remounting of the device. The device learns the application properties automatically and adjusts itself to the relevant requirements. If changes develop over time as a result of environmental influences or wear, the positioner adjusts itself to the new requirements and informs the operator of the changes through alarm messages. This is done via HART, Profibus PA, Foundation Fieldbus, or digital output.
Flexibility for special applications /remote versions
The flexibility of the positioner is advantageous even for applications that are not typical for positioners, such as with pneumatic cylinders. The modular design and expandability of the device enable it to be connected to externally positioned feedback systems. It makes no difference to the positioner whether the external sensor works on a contact or non-contacting principle, or whether it is a potentiometer, current signal, or voltage value. All these different types of external sensors can be connected to the pneumatic filter/regulator module, regardless of whether it is installed inside or outside the actuator component.
This enables complex applications to be resolved in the simplest way, including those in which a positioner cannot be mounted because of the application environment (for example extremely high temperatures), the position or design of the actuator, or where the use of an external sensor is simply a more elegant and better solution.
Best practice
In the past, when level sensors at iron ore processor LKAB failed during the preparation of ore-water mixtures, the valve and positioner fitted below the tank became completely covered by sludge. This meant that many positioners had to be completely replaced. The solution was to install a remote version, a linear potentiometer for position feedback on the actuator. This is connected to the Sipart PS2 positioner via the filter module installed in a cabin well away from all undesirable influences.
The valve positioner is also especially impressive in applications subject to extreme vibrations, such as those caused by steam hammers. Using the giant magneto resistance (GMR) effect as a basis, a non-contacting sensor (NCS) detects the current position of a valve based on the magnetic field lines. Another advantage of the GMR principle is that it allows a greater distance between sensor and magnets. Other principles based on the field strength are more susceptible to this as well as to temperature influences, which diminish their reliability and availability. The sensing principle of the wear-free, non-contacting compact design of the GMR sensor conforms to IP68, which enables it to be used in highly demanding applications where conventional mechanical mounted solutions are stretched to their limits. The NCS sensor is also connected to the Sipart PS2 positioner via the above-mentioned, flexible modular solution.
The pulp and paper industry must deal with particularly high vibrations. Non-contacting sensors are used here to avoid the need to replace positioners due to wear on the mechanical position pick-off. Here, too, remote sensors can be installed, and the non-contacting position feedback of the NCS saves time and money.
The best thing is that the filter module for the connection of the NCS sensor or other external sensors can be easily retrofitted with the Sipart PS2, eliminating the need for the customer to order a device specially prepared that can thus not be used for standard applications.
In short, wherever a solution is needed for automatic control of a pneumatic actuator, irrespective of the form of the actuator movement, the Sipart PS2 positioner coupled with external sensors offers several advantages.
Low air consumption
An increasing number of plant operators are looking beyond the one-off costs when considering the cost-effectiveness of positioners and are instead examining the total cost of ownership. A fundamental component is the cost incurred from the compressed-air consumption. The biggest consumer of compressed air is not the control air that moves the actuator – all devices require the same quantity for this purpose. But why should a positioner consume large volumes of compressed air if it is only maintaining a constant position on the actuator?
This is why Siemens offers a positioner with extremely low holding air consumption. The following annual values have been calculated for a Sipart PS2:
* Energy consumption solely for generating compressed air, approximately 33 kW/h.
* Costs incurred solely for generating compressed air, approximately €2.
* Carbon emissions generated, approximately 21 kg.
This can represent significant potential savings – in both economic and ecological terms – when compared to other positioners.
Diagnostics and interoperability
Particularly in new plants, bus communication, or at least HART-based communication, is used. The Sipart PS2 excels on two fronts: First is the fact that it supports both common parameterisation concepts, based on EDDL (Electronic Device Description Language), and the FDT/DTM (Field Device Tool/Device Type Manager). Second is its proven interoperability in other control and asset management systems alongside the Siemens PCS 7 control system and Simatic PDM.
For plant operators, the issue of integration into control systems is closely linked with the additional diagnostic messages that the positioner sends to the control system and asset management system. This is where the customer can take preventive measures to generate added value by reducing costs.
The standard version of the Sipart PS2 has comprehensive diagnostics functions. It delivers reliable diagnostics data via the positioner and its adjacent units, i.e. the valve and actuator. This allows users to take action at an early stage before a serious valve or actuator fault occurs, thus avoiding a plant standstill. This is crucial, especially in the case of safety valves. An overview of the diagnostics functions includes:
* Pneumatic leaks.
* Stiff valves.
* Stiction of the stuffing box.
* Wear on the valve seat or plug.
* Deposits or caking on the valve seat or plug.
* Histograms, trends.
* Temperature measurement, stroke counter, etc.
Through a regular partial stroke test, the Sipart PS2 ensures that ESD (emergency shutdown) valves and open/close valves remain movable in emergencies.
Conclusion – everything is covered
Plant owners could not wish for simpler stock management than that provided by the Sipart PS2 because the product number of the standard device always stays the same. Regardless of whether for a linear or part-turn actuator, long or short strokes, control or open/close valves, manufacturer-specific actuators or applications with external sensors, solid, precise control, or additional diagnostics functions including the PST – everything is covered by the Sipart PS2 and the comprehensive functionality provided by the standard version, expandable modules and wide range of available mounting kits.
For more information contact Keshin Govender, Siemens Southern Africa, +27 (0)11 652 2412, [email protected], www.siemens.co.za
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