Perfect combination

Nick Denbow

The specification for the design of the Fisher GX control valve range was based on input from control valve users in Europe, and views were sought from across the whole industry, rather than just from existing Fisher valve customers. The major feature of the Fisher GX valve is that it is designed to meet European customer requirements. It is an integrated valve and actuator with a simplified selection procedure, and Fisher have also introduced a new version of their Digital Valve Controller, the Fieldvue DVC2000, to complement the GX design and make the inherent advantages of digital valve control available to general service control valves.

The major requirement from customers in the survey was for simplified sizing and selection procedures for valves in this general service category, a reduced numbers of parts and increased commonality of spares across the range. All these features were seen to reflect on the total cost of ownership of the valves, particularly by reducing the time and expertise required to manage them on site. This simplification is immediately apparent on opening the multi-language CD containing the sizing selection and configuration programme for ordering the Fisher GX. The CD language options are English, German, French, Spanish, Italian and Chinese; the same languages are also available on the DVC2000 operator display. A cus-tomer using this selection CD experiences a simple sizing and selection procedure that enables decisions to be made based on body sizes and materials options.

The GX valve is a compact and rugged globe-style valve, suitable for either throttling or on-off control of liquids, gases and vapours: it is available in nine metric sizes between DN 15 and DN 200 to cover the normal flow rate and pipeline sizing requirements. Within the valve body the flow passages have been engineered to provide maximum capacity with a stable flow pattern, ensuring smooth operation. Carbon and stainless steel materials are the GX standards, with a choice of several alloys for more corrosive applications. The design conforms to European PED requirements. Metal-to-metal seating is also standard supply with the GX valve, with the available options including PTFE soft seating for Class VI shutoff, and hardened trim with a Stellite overlay for mildly erosive applications.

The Fisher GX was designed with a minimum number of parts for an integral valve and actuator system, in order to reduce maintenance and complexity: many parts are common across different valve sizes, which reduces the spares inventory. Actuator sizing calculations are not necessary: specifying the port size determines the actuator required, which is universal for pressure drops up to 51 bar. The multi-spring design is field-reversible from normally closed to normally open without needing extra parts. The stem packing uses live loaded PTFE V-rings as standard, giving excellent stem sealing and extended service life by compensating for normal wear. Ultra-low friction graphite based packings and an extension bonnet are available for high temperature duty above 232 °C. To avoid external corrosion, the actuator bolting is stainless steel as standard, and the external surfaces are covered with an anti-corrosion finish applied using powder coating techniques.

Instrument mounting on the Fisher GX is in accordance with Namur standards (IEC 60534-6-1), so that any Namur-compliant positioner can be specified, as well as auxiliary solenoids, limit switches or other accessories. The recommended option is to use the Fisher DVC2000 digital valve controller, which mounts directly onto the actuator yoke, without needing any mounting brackets, and connects to an air passage inside this yoke. In the air-to-open mode the DVC2000 introduces actuator loading pressure through this passage, avoiding the need for any tubing external to the casing. Based on the highly successful Fieldvue product family, this specially designed digital valve controller provides the simplicity of a local user interface, together with the ability to work within a digital plant architecture such as PlantWeb. The DVC2000 also offers the further options of an integrated position transmitter and limit switches, which reduce the complexity of product selection and specification.

Instead of the conventional mechanical linkage, a non-contact technology is used utilising a magnet array which is connected to the valve stem. The sensor elements within the DVC2000 housing are twin Hall-effect magnetic field sensors, used for the travel feedback and position transmitter as well as for the limit switches. The output from the sensors provides a stable, dimensionless value related to the angle of the field detected by the stem magnet, which changes as the stem rises. Simple zero and span travel calibration can be performed on the DVC2000 itself in the valve and controller set-up routine. This approach eliminates all the hysteresis effects that can occur with mechanical position sensor linkages, and makes removal and replacement of the valve controller very simple. It also further reduces the number of parts used and the overall mounting complexity.

The functions available with the DVC2000 interface provide a direct replacement for older analogue instruments: the valve stem position transmitter can output a 4…20 mA signal, and the two integral limit switches are fully configurable throughout the travel range. All models are suitable for type ‘n’ or intrinsically safe operation conforming to Atex and European EMC directives. Approvals to other standards such as JIS, CSA, FM and SAA are available. The electrical housing meets IEC 60529 IP 66 and also Nema 4X. The real advantages arise where the DVC2000 is installed in a digital environment using Hart digital communications: a further release of Foundation Fieldbus-compatible units will follow. Using this technology and the Emerson AMS Suite Intelligent Device Manager software system, multiple instruments can be monitored for alerts and maintenance alarms. This information can be integrated with the control system to provide a view into the valve and other intelligent plant sensors for plant operational management, directly from the safety of the control room. Diagnostic tests can be performed online, with no interruption to the process. Alternatively, these tests can also be performed when the valve is offline and bypassed if necessary. The snap-on software known as AMS ValveLink is typically used to diagnose faults such as increased valve friction, incorrect seating or sealing forces, non-linearities, travel deviation errors or pneumatic supply problems. The valve travel history can be monitored, trends derived and travel totalised, to predict when packing will need replacement.

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