SMART WORK in control valves

According to a recent report from MarketLine International, the European valve and actuator industry is consolidating. Traditional valve companies are being transformed into process control companies in order to remain competitive across as wide a sector as possible.

The report The Future of the European Valve and Actuator Industry shows that the European mainstream valve industry has grown at an average 3 per cent per year for the last five years, but predicts that growth into the next century will be about half that figure. Highlighted by the report, though, is the higher growth rate for modulating control valves (3.9 per cent per annum) and the increasing market shares being taken by ball and butterfly valves.

Put these two factors together and you begin to understand the rationale behind some of the recent mergers and acquisitions across the worldwide valve business. MarketLine points to the alliance between the Flow Control Group of Tyco International and Keystone International (see panel on page 28) as an example of how the industry is responding to these market challenges. MarketLine's industrial analyst Martin Atherton says: 'significant market growth will now only be found outside Europe, by protecting domestic markets from cheaper imports and by encouraging the take-up of the value end of the industry including intelligent field devices and control systems.'

Within the Tyco set up, Keystone as well as its leading position in butterfly valves is now the designated quarter-turn actuator company for the whole group, including fellow group companies Hindle Valves and Winn Valves. And its latest product introductions in this area clearly follow the trends identified by MartketLine.

In collaboration with another Tyco partner, the Italian company Biffi, Keystone has launched the ICON (intelligent control open network) range of quarter- and multi-turn actuators. Competing in the same market as Rotork's IQ range, the ICON sits well with what Keystone Engineering Manager Steve Gibbs says is 'Keystone's philosophy of offering equipment that is compatible with open systems'. For remote control it uses the non-proprietary LonWorks two-wire fieldbus developed by Echelon, while local control is achieved via a simple integral push-button control station. A highly visible on-board display lets operators check valve status at a glance.

Position indication is the role played by Keystone's K-Block device. This magnetically-operated quarter-turn position indicator features a sealed sensor with two inductive proximity switches and an indicator with two metal screws located 90 apart. As the valve position changes, the sensor detects one or other of the screws, providing valve open and valve closed output signals. Unlike traditional devices based on electromechanical reed switches, the fully sealed IP67-rated K-Block is said to be very stable over a wide temperature range. Its non-contact design, with no mechanical components to wear or spark, minimises both maintenance requirements and the possibility of false signals. As well as Keystone actuators, the K-Block fits all competitive models. It uses the device-level ASi bus system to effectively take distributed control right out to simple on/off valves.

The latest product from Keystone, introducd only last month, is the KPP the Keystone Programmable Positioner. As its name implies, this device positions quarter-turn pneumatic actuators in response to programmable signals. The initial model responds to conventional 4-20mA signals but the next release will shortly have full digital capabilities, including Hart, DeviceNet, LonWorks and other non-proprietary buses.

Unlike competitive devices based on piezoelectric components, the KPP has dual solenoids for each direction of movement. Steve Gibbs describes it as a 'low powered, high flow switching device'. Supplied with standard Keystone and Namur mounting capability, it uses standard well-proven components and features zero air consumption at balance. The KPP's programmable options are many and varied, but are all configurable via a menu-driven, three-button control pad. An on-board LCD makes for easy and rapid commissioning.

One of the options allows for adjustable stroke limits. Stroke limits can be set to reduce the travel of the actuator, open and close, within the mechanical limits. The calibration routine sets the span to the mechanical limits of the valve/actuator package. Optimum control may demand that the valve is never either fully seated or open; adjustable stroke limits achieve this without need for adjustable mechanical stops.

Two setting modes are possible: static and dynamic. Static setting allows the fixed adjustment of upper and lower limits directly, while dynamic setting adjusts the valve position to achieve the desired process conditions. For example, the position of the valve can be adjusted using the raise and lower buttons on the KPP until the desired flow rate, temperature or pressure is achieved.

Travel time settings can also be adjusted so that, for example, a minimum time can be set to prevent rapid process changes being caused by valves modulating too quickly.

Other features of the KPP include an autotuned PID control algorithm, with manually adjustable override of parameters; double or single acting; direct or reverse acting; split range facility; IP65 enclosure; visual indication of position; and adjustable 'dead band' to prevent 'hunting'.

The dead band is basically the region above or below the setpoint in which a change of input signal produces no observable change in output signal. Obviously, for accurate positioning and process control it pays to minimise this region. This also applies to hysteresis within a control valve system. Taken together, both dead band and hysteresis play an important role in good valve positioning. This has now been addressed by Dresser Valves & Controls in the design of the Masoneilan SVI smart positioner.

According to Dresser's Thurman Payne, a conventional control valve system with no positioner would exhibit around 5-10 per cent hysteresis plus dead band increasing to as much as 40 per cent with the higher friction packings now being specificed to meet environmental emission limits. Putting a conventional positioner on the valve, with a non-adjustable control gain, can reduce hysteresis plus dead band to around 1-2 per cent. But Payne says this fixed gain is the 'weak link in the system and limits the gain of the entire loop'.

Because the same positioner may be used on different types of valve ball, plug, butterfly, globe etc its response will vary according to the features exhibited by the valve. To overcome this, the Masoneilan SVI has an autotuning feature that has been shown to deliver a hysteresis plus dead band of 0.04 per cent, even when tackling the 'stick-slip' behaviour normally associated with a high friction valve.