Valve Positioners

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

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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.

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