Intelligent and intrinsically safe

Heap & Partners has developed a smart positioner built around Hoerbiger-Origa's piezo-pneumatic technology with microprocessor control - an intrinsically safe option

The market for intelligent process control valves is one which has grown rapidly in recent years, with these products offering huge advantages over manual valves. This is not to say there aren't drawbacks, however. Many of these intelligent valves offer levels of sophistication that many users simply don't need, and offer options such as fieldbus for monitoring and diagnostics that are unused in a large number of process applications. Some products even necessitate the use of fieldbus for their operation, yet only a small percentage of process valve users have actually adopted fieldbus communications. These added extras and unnecessary sophistication can add significant cost to the installation, and increase set up times.

Fluid handling equipment specialist Heap & Partners set out to design a product which would offer all the advantages of an intelligent valve, whilst addressing the need for easy installation, simple set up and maximum operating efficiency. And recognising that 4-20mA was still a much more widely used control option than fieldbus, the company determined that use of fieldbus should not be an implicit requirement for use of the valve - rather, it was decided that users should be able to specify options for the likes of fieldbus communications, diagnostics and flow monitoring.

The result of the development work is the Storm valve - a smart valve positioner, typically costing 30-40% less than alternative products. The design comprises an analogue pneumatic positioner, based around piezo pneumatic technology from Hoerbiger-Origa. Intelligence is provided by an in-built 16-bit processor, which is configured via an integrated three-button keypad and LCD display. Use of the RISC processor ensures accurate control, and enables a greater degree of functionality to be built into the unit as standard without adding cost.

Automatic calibration

Upon installation, the unit is automatically calibrated and requires no fine adjustment of either the air inlets or the electronic controls. With a typical setup time of just 2.5min, the Storm positioner can provide a time saving of around an hour per valve installation. The set-up routine includes auto-tuning of the PID control loop, and there is also an option to calibrate the 4-20mA loop.

Using a PID control algorithm is the key to providing enhanced operation. Three-term control eliminates overshoot whilst increasing the speed and accuracy of response. An attractive feature of the Storm positioner is that the standard unit can be readily programmed to suit a wide range of options, such as direct reverse, limited travel and limited range, all of which are often optional extras on alternative designs. The unit can also be supplied pre-programmed to match specific performance curves for individual applications.

At the heart of the positioner is a Hoerbiger-Origa piezo valve - a simple three port, two position valve in which the air flow is switched by a piezo element. Operating rather like a bimetallic strip, the piezo element deflects in proportion to an applied voltage, so that the valve may be used in a simple on/off switching function or in a proportional mode.

Ideal alternative

For the Storm positioner, the piezo technology offers an ideal alternative to solenoid operation. Because it operates on a minute voltage, the piezo valve can be driven directly from the 4-20mA data signal, removing the need for a separate power supply. And there is no requirement for the circuit protection normally associated with solenoid valves. The technology offers very fast operation, with reaction times typically less than 2ms. In addition, a power rating as low as 0.014mW means that piezo valves are intrinsically safe. Fully certified to EEx 1A IIC T5, the Smart positioner can be specified for use in hazardous areas without added explosion protection costs being incurred. Further protection is provided by a specially designed enclosure sealed to IP66.

Another advantage of the use of piezo technology is that it eliminates the problem of static air bleed. The pneumatic positioning system is extremely efficient in its air consumption, with the control system flowing measured volumes of air during the positioning operation only. Once in position, the air flow is totally closed off, which eliminates the air wastage associated with designs that require a constant air bleed.

 

Piezo technology explained

Pneumatics control valves are most commonly operated by solenoids, to facilitate the easy interfacing with electronic controllers. While convenient and relatively low cost, solenoids are not particularly efficient - they heat up, and even with low power solenoids, typically have an electrical consumption of several Watts. In explosion-proof environments, remote control and battery operated systems, energy consumption is particularly important and demands careful consideration when specifying products. By comparison, piezo-pneumatic operators offer a low energy electro-pneumatic interface. This technology has minute power demands, is extremely fast in operation and, with practically no moving parts, is extremely reliable. At the heart of all Hoerbiger-Origa piezo operators is a tiny piezo valve - a simple three-port, two-position valve in which the air flow is switched by a piezo element. Operating rather like a bimetallic strip, the piezo element deflects in direct proportion to an applied voltage, so that the valve may either be used in a simple on/off switching function or in a proportional mode to produce an analogue pressure output. By integrating the piezo valve into an ISO standard pilot operator, Hoerbiger-Origa offers a low energy piezo technology, as an alternative to solenoid operation for most standard pneumatic valves, providing significant application advantages: Fast operation - reaction times are typically less than 2ms No requirement for the circuit protection normally associated with solenoid valves A single pilot operator may often be used where two conventional solenoids were previously required, as piezo valves hold their switched position With a power rating as low as 0.014mW, piezo valves are inherently intrinsically safe Piezo valves can be actuated directly by data signals using simple two-wire technology and are compatible with most types of fieldbus systems Switching, proportional and pulse width modulation control modes are all possible. Standard units are offered with protection to EEx ib II CT6/T5 and EEx ia II CT6/T5 and also with NAMUR connections. Piezo technology is finding an increasing number of uses in a great variety of applications. Its low power consumption and non-sparking characteristic makes it ideal for usage in intrinsically safe environments where no spark can ever be tolerated due to the high risk of explosions. Its tiny size and virtually indestructible ruggedness means it is also being adopted for use in mobile equipment. The fact that it can emulate both analogue and digital systems is also proving a great user attraction.

Hoerbiger-Origa
f106@industrialnetworking.co.uk