Valve Position Detection in Modern Process Installations

Fig 1

Fig 2

In the process industry, valves and actuators are historically used for controlling and diverting fluid flows. Increasingly the plant engineers want a feedback signal of the current position of valves, taps and actuators in the control system. This feedback is generally obtained by use of inductive sensors which are built into a switchbox, or double sensors which are mounted directly on actuators or the manual valve. This article gives an insight in to the type of sensors that are applied in the field and the emerging trends in the market.

State of the Art with Regard to Standardisation
Industry standards can be viewed as both positive and negative toward technology development. On the one hand they limit creativity but on the other side they ensure that the apparatus of several manufacturers is compatible. Most Manufacturers now build position feedback equipment to VDI/VDE3845.

The Switchbox
The term “switchbox” commonly describes the integration of sensors (barrel, slot, micro-switch or dual styles), cam and connectors in an enclosure, this is then mounted on the pneumatic actuator. This solution was designed so as not to expose the sensors to the immediate environment and influences, and to be able to use standard sensors see Fig 1.

The Open Solution
The term “open solution” describes a double sensor mounted directly on automated or manual valves with no enclosure (see fig 2).

The design of this system takes into account the VDE/VDI 3845 standard. The sensors are activated by two stainless targets mounted at 90 degrees to each other on a “puck”. The puck itself is mounted by one screw directly onto the actuator stem.
This system shows many features including compact size and very simple assembly, no mounting kit, pre-wired, connector and terminal connection. Highly visible puck and LED’s for status indication.

Inductive Sensors versus Mechanical Switch
In all areas of industry where the mechanical switch was previously a much-used solution for a diversity of control problems, nowadays the approach has changed. The sensor is now the solution. This is a consequence of the ability of sensors to offer significant speed and reliability advantages:

- Entirely solid state, as a result, no wear and tolerance modifications
- Faster output switching
- Hysteresis and repeatability can be defined electronically much more successfully than in a mechanical device
- No welding or corrosion of output contacts
- Little or no influence of clogging or humidity
- No contact bounce (output echo)
- LED indication on most sensors to show when the sensor has switched. This makes maintenance and commissioning simpler and quicker

Typically the average life span of an inductive sensor is considerably longer than those of a mechanical switch. Historically, mechanical switches were used for valve feedback due to the difference in price, the fact that a switch polarity does not need to be considered and cut and paste designing. By use of modern techniques, Pepperl+Fuchs have succeeded in bringing sensors to the market which eliminate the few remaining advantages of the mechanical switch.

Possible Sensors for Valve Position Detection
Choosing the correct sensor can be achieved in 5 simple steps:

Step 1: Switchbox or open solution?
Weigh the aforesaid advantages and disadvantages against each other and make the choice. You either require the protection that the switchbox gives, or the simplicity and convenience of an open solution. Most valve suppliers offer as a matter of course the two solutions. You will find that switchboxes come in all sizes and weights, and in several qualities and classes.
direct top mount box solution

Standardization ~ 3
Terminal Compartment ~ 3
IP-classification 3 ~
Expense 3 x
Size 3 x
Number of components 3 x

Step 2: Define electrically the final termination a sensor will have to the control system which regulates the process (AC, DC, Namur etc). Of overriding importance is whether the system is used in an explosive area. We see the following electrical connection types:

- Three wire DC Sensors. These sensors have dedicated output wire beside the two for power supply
- Two Wire DC sensors. Here the load is connected in series with the sensor. Polarity independent. A direct replacement for mechanical switches. Pay attention to the off state current in the circuit and the voltage drop of the sensor
- AC voltage with Normally Open (NO) and/or Normally Closed contacts (NC)
- Two Wire AD/DC. The sensor is connected in series with the load
- Two Wire NAMUR; with an associated intrinsically safe barrier, these can be used in areas with an explosive atmosphere (area 0, 1 and 2). Lead breakage and short circuit monitoring are possible as standard
- Two Wire Failsafe sensors. As per two wire Namur but subject to use of a safety rated barrier, a SIL 3 approved circuit is obtainable

Step 3: Connection possibilities
This tends to be a personal choice of the end user. Some prefer glanded entry to a terminal strip and some prefer pre-wired or plug and socket connection. For example, the chemical and pharma sector tend to choose glanded entry to the box or sensor.

Step 4: Operation of the solenoid valve
Just like with the switchbox, one also has the option with double sensors to wire the solenoid by means of the sensor. The advantage lies in the fact that there is then only one cable that must be laid to the actuator.

Step 5: Finally, one must verify a number of general properties of the sensor to be certain the sensor will function perfectly in the application.
For example; off state current, voltage drop, environmental surroundings, vibration resistance etc.

Trends and Future Developments
Because installations are very often adapted to the newest technology in order to work more efficiently and reliably, all kinds of bus systems have entered the market. Of course producers of switchboxes and actuators want to anticipate this and we see that a number of these manufacturers have developed interfaces, in consultation with sensor manufacturers, to these bus systems.

A good example of this is switchboxes with AS interface incorporated because this bus system is particularly suitable for digital information. If the end user wishes to use standard sensors, appropriate field mountable modules can be found for AS interface, Profibus PA or foundation Fieldbus. It seems that more and more companies are realising the advantages of bus systems and particularly the combination of a field bus (for example Profibus) with AS interface is seen in installations more commonly.

We are increasingly seeing inductive sensors replacing the mechanical switch. In some markets, 60-70% of switchboxes are fitted with inductive proximity switches which have replaced mechanical forbearers. For valve position detection standardisation is still some way off. This is due to the starting point from which switchboxes have been devised and therefore a large mixture of mechanical assemblies and switch technologies exist. More commonly, end-users are frequently asking for solutions which lend themselves to greater reliability to allow for long term payback from the switchbox.

Sensor companies have continued development and innovation and an enormous mixture of sensors exist which can be assembled in all kinds of switchbox. In certain industries a clear tendency to the open solution is noticeable due to a price that comes close to that of the switchbox solution with mechanical contacts. Fitting valve position feedback on manual valves now also becomes a more attractive option with the size and cost of the open solution.

Pepperl & Fuchs GB Ltd.
Tel: 0161 633 6431

Published in Valve User Magazine Issue 12

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