Valve Positioners Flexible in Any Position — Valve Positioners as Versatile Multi-Taskers

Author / Editor: Thorsten Jung / Dr. Jörg Kempf

An extremely broad range of process applications requires field devices with a high degree of flexibility. Likewise, the issues of standardization and simple stock management are becoming increasingly important. And the devices must meet the normal everyday requirements: easy to operate and capable of being used universally, while requiring minimal time and cost for maintenance. So how does this apply to automated valves?

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Sipart PS2 is currently the most widely used positioner for linear and part-turn actuators in a wide range of process industries. The proven all-round design has a particularly flexible stroke range and intelligent diagnostics and it communicates via Hart, Profibus PA, or Foundation Fieldbus.
Sipart PS2 is currently the most widely used positioner for linear and part-turn actuators in a wide range of process industries. The proven all-round design has a particularly flexible stroke range and intelligent diagnostics and it communicates via Hart, Profibus PA, or Foundation Fieldbus.
(Picture: Siemens)

Control valves that influence the flow of process fluids are a fundamental component of every automated process plant. They are controlled by many different types of actuators, including hydraulic and electric designs, but the majority have pneumatic actuators. This is where the widest range of demands occurs — not only on the actuator itself and the valve, but particularly on the positioner. The latter is the field component that transforms a collection of hardware into an automated application and it has to provide 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.
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Does one need the widest range of devices or even manufacturers to meet these various requirements for a positioner? Does this mean that the two different perspectives mentioned above, as well as all these additional requirements, cannot be addressed through a single solution? The answer is emphatically no, as the Sipart PS2 positioner demonstrates.

Easy Mounting and Initialization

One great advantage of the Sipart PS2 positioner is its universal usability. This in turn requires a high degree of flexibility and mounting options for various actuator and valve applications. This requirement has been addressed by providing more than 400 mounting kits. These enable the device to be mounted on a very wide range of actuators, both standardized — such as part-turn actuators according to VDI/VDE 3845 and 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 push-button operation and display, plus an adjustable friction clutch which ensures smooth setup without time-consuming dismounting and remounting of the device. The positioner 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 about the changes through alarm messages. This is done via Hart, Profibus PA, Foundation Fieldbus, or digital output.

Flexibility for Special Applications

The Sipart PS2 positioner’s capabilities do not stop there. 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-based 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 EMC filter module, regardless of whether it is installed inside or outside the actuator component, for example in a pneumatic cylinder.

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This enables complex applications to be resolved in the simplest way. This includes those in which a positioner cannot be mounted directly 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.

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. For this reason many positioners had to be completely replaced.

Remote Sensors for Dirty Plants

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 EMC filter module and thus transmits information about the current position. The remote positioners are installed in a cabin well away from all undesirable, critical influences.

The performance of the Sipart PS2 is especially impressive in applications subject to extreme vibration, such as those caused by steam hammers. Using the giant magnetoresistance (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 direct-mounted solutions are stretched to their limits. The NCS sensor is also connected to the Sipart PS2 positioner via the flexible modular solution referred to above.

The pulp and paper industry must deal with particularly high vibration levels. This is certainly the case in one of Norske Skog’s plants, where non–contacting sensors are used to avoid the need to repeatedly 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.

Another advantage is that the EMC filter module for connecting the NCS or other external sensors can be very easily retrofitted to the Sipart PS2. This eliminates the need for the customer to order a special device that cannot 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 different approaches.

Low Air Consumption

An increasing number of plant operators are looking beyond the one–off costs when considering the claimed cost–effectiveness of positioners, and are instead examining their total cost of ownership. A fundamental component of this is the cost incurred by the compressed–air consumption of the positioner. It turns out that in most cases the biggest consumer of compressed air is not the control air that moves the actuator — all devices require the same quantity of compressed air for this purpose — but the air used by the positioner as it holds the valve at a constant position.

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This is why Siemens offers an exemplary positioner with extremely low air consumption. With an intrinsic consumption of just 0.036 Nm3 per hour and allowing for typical supplementary conditions, the following annual values can be calculated for a Sipart PS2:

  • energy consumption solely for generating compressed air: approximately 33 kWh;
  • costs incurred solely for generating compressed air, approximately €2;
  • resulting CO2 emissions: approximately 21 kg.

Compared to older positioners, this represents a potential saving of more than 90 percent.

Diagnostics and Interoperability

Particularly in new plants, communications based on fieldbuses — or at least the Hart protocol — are common. Here the Sipart PS2 excels on two fronts. First is the fact that it supports both common parameterization concepts: those based on EDDL (Electronic Device Description Language) and the FDT/DTM (Field Device Tool/Device Type Manager), respectively. 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 diagnostic 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 shutdown. This is crucial, especially in the case of safety valves.

Typical diagnostic functions are:

  • 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 or an additional solenoid valve test, the Sipart PS2 ensures that ESD (emergency shutdown) valves and open/close valves remain movable in emergencies.