The application of fieldbuses to valve automation

Fieldbus systems and their application to valve automation, and associated instrumentation: this review showshow these systems fall into one of three categories

Fieldbus systems and their application to valve automation and associated instrumentation fall into one of three categories.

CATEGORY 1 (eg AS-interface).

The first is at a relatively simple level where the requirement is to distribute digital I/O (inputs and outputs) to monitor inputs from switches or sensors (level, travel limit, fault alarms etc) and/or control outputs to on/off devices, such as solenoid valves or relays.

In such applications the requirement for high-speed data transmission is not critical and the need to process significant amounts of data at the node does not exist.

Generally the control function is on/off and the speed at which the related equipment operates is far slower than the electronic data transfer rate.

Electronic hardware cost is an important factor with local nodes needing to be relatively simple generally with a maximum I/O count of 4.

AS-interface is a typical system in this category.

CATEGORY 2 (e.g HART).

The second category is data acquisition or transmission to and from 4-20mA process instruments such as temperature and flow transmitters or valve positioners and position transmitters where data additional to the process variable is captured in digital form (useful for remote diagnostics or calibration).

Digital data is transmitted by superimposing it on top of the 4-20mA analogue signal.

HART from Emerson Process Management is the leader in this field.

(HART stands for "Highway addressable remote transducer").

CATEGORY 3 (eg Profibus or Foundation fieldbus).

The third category is by far the largest with systems transmitting process control data at high speed to potentially large numbers of nodes.

The functionality of the nodes can range from distributing discrete digital and analogue I/O, providing gateways to lower level systems (AS-interface), through to remote processing and local loop control.

Systems in this third category are full function fieldbuses capable of handling cyclic data, events and messages.

Examples include Profibus or Foundation fieldbus (FF).

The requirements of control valves and on/off valves differ.

CONTROL VALVES.

HART, Profibus or Foundation fieldbus can be directly applied to positioners on modulating valves because the benefits of remote calibration and diagnostics on critical control valves could in some cases justify the cost and complexity of having the technology as an integrated part of a control valve assembly.

ON/OFF VALVES.

Power constraints and the relatively high cost of the field electronics is limiting the integration of full function fieldbuses (eg Profibus and Foundation fieldbus) in switch solenoid units for on/off valves.

There are a number of alternative approaches that are being specified: AS-INTERFACE.

Profibus or Foundation fieldbus gateways can be used to convert fieldbus protocol to AS-interface which is a more effective solution for on/off valves.

Integrated AS-interface switch/solenoid control monitors can then be connected directly to an AS-interface bus using either DIN connectors or a cable piercing connector.

REMOTE I/O SYSTEMS.

These systems allow hard wired, integrated, valve actuator control monitors to be connected to remote I/O modules situated close to clusters of valves.

The remote I/O modules are then connected back to the Distributed Control System (DCS) using a field bus link (eg Profibus, Foundation fieldbus, Modbus or Ethernet).

I/O systems can lead to significant reductions in wiring and installation costs but the system is viewed simply as an extension of the Distributed Control System.

There are also HART transparent remote I/O systems which allow users access to most of the intelligence built into certain field devices such as temperature transmitters or valve positioners.

Redundancy is often built into the power supply, central processing unit (CPU) and field bus links of these systems.

Remote I/O systems are sometimes seen as less complex than full function fieldbuses; they usually require minimal staff re-training and can be easy to validate.

On/off field devices (eg the K Controls 007C) are suitable for single acting, double acting or "stayput" applications.

They can be fitted to rotary or linear valve actuators and are usually wired to the remote I/O module using a single multi-core cable.

Special variants for pharmaceutical clean rooms allow the multi-core cable and pneumatic supply line to pass into the field device via a single conduit.

VALVE COUPLERS.

This approach is particularly useful in hazardous areas as intrinsically safe versions are available which can be used in combination with standard certified field devices.

FIELDBUS IN HAZARDOUS AREAS.

One of the inherent problems with fieldbus is that more devices need to be powered via a single cable than would otherwise be the case.

In addition an addressable circuit in each field device increases the power requirement still further.

Intrinsic safety (EExi) relies on limiting the power than can pass from the safe to hazardous area via a safety barrier.

The power in a particular circuit must not exceed the ignition energy limits of a particular hazardous gas.

As intrinsic safety is a "power limiting" technique and fieldbus is a "power hungry" technique a specific EExia system has been developed for fieldbus applications.

With conventional intrinsically safe applications the safety of a circuit comprising a field device, safety barrier and interconnecting cable is determined using a number of defined parameters (eg voltage, current, power, inductance and capacitance).

This is known as the Entity Concept.

Once the calculations have been performed for each circuit the results have to be clearly documented (see EN 60079-25: 2004).

The increased power requirements of a field bus system, combined with the need to frequently modify the often complex arrangement of trunk and spur cables has resulted in the development of FISCO.

FISCO (Fieldbus Intrinsically Safe COncept).

FISCO Technical Specification IEC TS 60079-27: 2002 sets out the design parameters for field instruments, terminators, power supplies and interconnected systems.

It was developed by PTB in Germany.

Unlike conventional parallel wired installations safety has been determined as a result of practical testing of all the parts used.

Using FISCO the inductance and capacitance of a particular cable does not need to be considered in the safety analysis provided a particular cable type is used.

As fieldbus cabling architecture is usually quite complex and can be subject to frequent modification, this simplifies matters considerably.

Different FISCO power supplies have also been developed that permit higher levels of voltage and current to pass into a group IIB hazardous area.

As an example using the Entity Concept and assuming 20mA per device, 3 or 4 devices could be fitted on a fieldbus trunk in a Zone 1 hazardous area IIB/IIC gas groups.

Using FISCO this can be extended to 6 devices in a IIC application and 13 devices in a IIB application.

Using FISCO live maintenance is still permitted and the administration of safety documentation is simplified.

INTRINSICALLY SAFE REMOTE I/O SYSTEMS.

In hazardous areas these systems are suitable for use in Zone 1.

If we take as an example an installation of 16 valves a typical system would consist of an isolating repeater, a gateway, a 16 channel input module, four output modules a terminal base for each module and two EExd power supply units.

The ATEX Certified 007C Control Monitors used with remote I/O systems are fitted with conventional EExia 24vdc solenoids 0.7W (0.1 to 7 bar).

Exi VERSION OF THE AS-I INTERFACE.

A recent development enables AS-interface to be taken into hazardous areas.

AS-interface is a simple two-wire bus with a maximum network length of 100m and is ideal for valves and actuators.

A typical system would use Profibus DP in conjunction with an AS-interface gateway and power supply in the safe area.

The gateway would be located near the junction with the hazardous area.

The AS-interface cable would then be taken into a Zone 1 or 2 hazardous area using increased safety (EExe) techniques.

EEx m AS-i I/O connector modules are then sited close to small clusters of valves.

The modules use increased safety (EExe) and encapsulation (EExm) as the method of protection, no additional barrier to the safe area is required.

The signals are converted to intrinsically safe signals (EExi).

The modules are available in two I/O combinations, 4 inputs/2 outputs or 4 outputs only.

Up to 31 modules can be fitted to a single AS-i bus (62 valves).

Individual network segments can be up to 100m, repeaters enable network extensions of up to 100m but safe area mounting is required for the repeater.

The ATEX Certified 007C Control Monitors used with AS-interface Ex valve connector modules are fitted with conventional EExia 12vdc solenoids 0.7W (0.1 to 7 bar).

EExd AS-INTERFACE CONTROL MONITOR.

As an alternative to intrinsic safety K Controls can offer the EExd Control Monitor complete with an integrated AS-interface I/O module.

Like all EExd equipment EEx cable glands and armoured interconnecting cable has to be used.

No live maintenance or calibration is permitted.

PROFIBUS PA AND FOUNDATION FIELDBUS (FF) EEx VALVE COUPLERS.

PROFIBUS PA.

Often used where on/off devices are to be mixed with instruments on a single bus.

This option takes Profibus DP (93.75 k/bits/RS485) and passes it through a segment coupler which converts it to Profibus PA (31.25 kbits/sec IEC 1158-2).

(The maximum current per segment is 100mA).

The Profibus PA valve coupler conforms to the FISCO model and has 4 outputs and 8 inputs using the 2:1 wiring mode (four valves per coupler).

A maximum of three valve couplers can be attached to one segment coupler.

This allows 12 valves per segment.

The coupler features short circuit and lead break monitoring, limit position and actuation time monitoring and a full stroke counter.

It can be installed in a Zone 1 hazardous area.

FOUNDATION FIELDBUS(FF).

Often used where on/off devices are to be mixed with instruments on a single bus.

This option takes FF directly from an H1 card via a FISCO power repeater.

(The maximum current per segment is 100mA).

The FF valve coupler conforms to the FISCO model and has 4 outputs and 8 inputs using the 2:1 wiring mode (four valves per coupler).

A maximum of three valve couplers can be attached to one segment coupler.

This allows 12 valves per segment.

The coupler features short circuit and lead break monitoring, limit position and actuation time monitoring and a full stroke counter.

It can be installed in a Zone 1 hazardous area.

The ATEX Certified 007C Control Monitors used with Profibus PA or Foundation fieldbus valve couplers is fitted with low powered Piezo technology solenoids 0.007W (2 to 6 bar).

2:1 WIRING MODE.

K Controls has recently obtained ATEX Certification for EExia intrinsically safe switchboxes, switch terminal boxes and switch solenoid control centres that are fitted wiring with solid state proximity sensors that permit two signals to be monitored through one dual lead via a 2:1 mode of operation.

The benefits of this approach are particularly significant when using fieldbus valve couplers as the number of inputs per coupler can typically be increased to 8 thereby saving the number of couplers needed per installation.

Up to four 007 switchboxes or four 007C switch solenoid control centres (fitted with special low powered pilot solenoid valves) can therefore be connected to each fieldbus valve coupler.

MONITORING THE POSITION OF MODULATING VALVES.

Where the position of a valve needs monitoring throughout its travel a position transmitter is required.

The addition of a fieldbus capability to a 4-20mA position transmitter can allow a number of devices to be added to a fieldbus spur and also for remote calibration to take place.

EExi or EExd versions are available, HART, Profibus or Foundation fieldbus enabled.

CONCLUSION.

The implementation of fieldbus technology in the process markets has been slower than in the factory automation markets.

Investment cycles are often longer and the problems of software validation and operating in hazardous or hostile environments are taking longer to overcome.

However full function fieldbuses are already being integrated into valve positioners and valve position transmitters.

They are also being utilised by the latest remote I/O systems for on/off and modulating valves.

This trend is set to continue.

The user can benefit from lower installation costs and the enhanced valve performance information which can then be made available directly to the hierarchy of other factory management networks.

These new valve automation techniques will make a positive contribution to the implementation of fully computerised manufacturing systems which aim to improve customer service, reduce costs and increase market share.

The authors of this review are Brian Prince and David Yates, Joint Managing Directors of K Controls.

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