Valve Position Detection in Modern Process Installations

In the process industry, valves and actuators have always been used to control and transfer the flow of fluids. More and more plant engineers want feedback signals on the current position of valves, taps and actuators in the control system. Usually this signal is obtained by proximity sensors built into the switch box or dual sensors directly mounted on the actuator or manual valve. This article takes a closer look at the types of sensors used in the field and new market trends. The current situation of standardization Industry standards have both advantages and disadvantages for the development of technology. On the one hand, it limits creativity, but on the other hand it ensures compatibility of equipment from different manufacturers. Among other things, the VDE / VDI3845 standard specifies the important mechanical connection between pneumatic actuators and connected equipment. The standard includes the definition of mounting holes: 30×80mm and 30×130mm. The height of the mounting rod is 20, 30 and 50 mm. There is no fixed standard for the mounting rod diameter, so users must pay special attention to the application of dual sensors.

Switchbox

The term "switchbox" generally describes the integration of sensors, cams and connectors in a closed box. Sensors and pneumatic actuators are mechanically connected via it. The solution is designed to avoid exposure of the sensors to the surrounding environment or to influences, while enabling the use of standard sensors. In these standard switchboxes, the following sensors are typically built-in:

● Cylindrical sensors with a typical length of 40 mm;
● Slot-shaped sensors;
● Rectangular V3-style sensors (similar to micro switches);
● Dual sensors, compactly placed in one box.

The switchbox is used as a terminal device to connect valve position sensors and solenoid valves. The coded connectors as well as the necessary LED indicators are already on the circuit board. The dimensions of the circuit board fit into most standard boxes.

Open solution

The "open solution" stipulates the installation of a dual sensor on an automatic or manual valve (see Figure 1). Two inductive sensors are mounted one on top of the other in a housing and accurately display the switch position of the valve.

Figure 1. Two inductive sensors are mounted in one housing, accurately displaying the switch position of the valve. >

The system has been designed with VDE / VDI 3845 standards in mind. The sensors are triggered by two stainless steel targets mounted at 90 degrees to a "disc". The disc itself is mounted on the actuator with a single screw.

In addition to the compact size and simple assembly, the following advantages can be found:

● The round mounting holes (not slotted) on the sensor housing avoid servicing due to collisions or vibrations caused by incorrect mounting distances or movements.
● The two switching elements (discs) can be adapted to a wide range of different pneumatic actuators. The only choice that needs to be made is the mounting pattern, e.g. 30×80 or 30×130 (see Figure 2)
● The stainless steel targets are already designed on the discs. (see Figure 1).
● 4 LEDs indicate the output of the sensor, the presence of power and the state of the solenoid valve output.
● The detection area of ​​the dual sensor is narrow enough so that there are no pinch points that could cause injuries to the operator or blockages caused by ice or dust.
● The wiring of the solenoid valve can be routed through the sensor, without the need for separate cables on site.

Figure 2. Two switching elements (discs) can adapt to a variety of different pneumatic actuators.

Inductive Sensors vs. Mechanical Switches

In all areas of industry, mechanical switches were once the preferred solution for a variety of control problems, but today the approach has changed and sensors are undoubtedly the preferred choice. This is because the advantages of sensors in terms of speed and reliability are obvious:

● All solid state, so no wear or error modification;
● Fast output of switch status;
● Hysteresis and repeatability can be more electronically defined than mechanical devices;
● No welding or corrosion on the output terminals;
● Little or no influence from clogging or humidity;
● No contact bounce (output echo);
● Most sensors have an LED display when switching, making maintenance and commissioning simpler and faster.

In general, the average life of an inductive sensor is much longer than that of a mechanical switch. In fact, many mechanical switches were used for valve feedback in the past because of their low price and because polarity issues did not need to be considered when using switches. Through modern technology, Pepperl+Fuchs has successfully brought sensors to the market that eliminate the only advantage of mechanical switches.

Sensors for Valve Position Detection

Choosing the right sensor can be achieved in 5 simple steps:

Step 1: Switch box or open solution?

Weigh the advantages and disadvantages above and make your choice. You may need the protection of a switch box or the simplicity of an open solution. Most valve suppliers offer both solutions. You will find switch boxes of varying size and weight, as well as a wide variety of quantities and types.

Step 2: Electrically define the terminals of the sensor. The sensor is connected to the control system that regulates the process (AC, DC, Namur, etc.). This is particularly important if the system is used in an explosive area. See below for the types of electrical connections.

● DC three-wire sensors. These sensors have a separate line in addition to the two power lines. The third line is connected to the output on the PLC input card or control system.
● DC two-wire sensors. Here, the load is connected in series with the sensor. Many sensors are polarity tolerant, meaning that the polarity of the wires does not matter. Therefore, it is as simple as connecting a normal mechanical switch. When using these sensors, you must be aware of the leakage current of the circuit and the voltage drop across the sensor.
● AC sensors with normally open and/or normally closed outputs. Here, the load is connected in series with the sensor.
● AC/DC two-wire universal sensors. This sensor is connected in series with the load. Both AC and DC voltages can be connected.
● Two-wire Namur sensors, together with the related intrinsically safe products, are used in explosion proof environments (zones 0, 1 and 2). Wire break and short circuit monitoring is possible.
● Two-wire fail-safe sensors. As standard with two-wire Namur sensors, but with a safety barrier that meets the safety level, safety level SIL3 can be achieved.

Step 3: Connection possibilities

● The connection method depends on the end user. For example, a gland with cable or a plug. The chemical and pharmaceutical sectors tend to choose a cable connection, which enters the box or sensor through a cable gland.

Step 4: Solenoid valve operation

As with the switch box, the use of dual sensors can also be used to wire the solenoid valve through the sensor, with the advantage that only one cable connection is required for the actuator.

Step 5: Finally, you must confirm many characteristics of the sensor to ensure that the sensor works perfectly in the application.

Similar examples are leakage current, voltage drop, ambient environment, maximum output current and operating voltage. If possible, you can confirm by selecting the sensor specifications to ensure that the maximum values ​​of static current (interference), shock and vibration are within the permissible range.

Future developments

As the installation process often has to adapt to the latest technological developments to improve work efficiency and stability, various bus systems have begun to enter the market.

The manufacturers of switch boxes and actuators certainly want to join in. Many of these manufacturers have already developed interfaces to these bus systems in consultation with sensor manufacturers. Switch boxes with AS-i are a good example, as this bus system is particularly suitable for digital information. If the end user wants to use standard sensors, suitable field-mounted modules such as AS-interface, Profibus PA or Foundation Fieldbus are required.

On the other hand, it is also possible to see the use of electrical actuators and positioning systems with Profibus or Foundation Fieldbus interfaces. It is clear that more and more companies are aware of the advantages of bus systems, especially fieldbus combinations such as Profibus with AS-i are very common.

Conclusion of this article

There is a growing trend towards inductive sensors replacing mechanical switches. In some markets, 60%-70% of switch boxes equipped with inductive switches have replaced mechanical switches. Standardization of valve position detection still has some way to go. This is because the starting point of the original switch box design is a large mixture of mechanical assembly and switch technology. More often, end users also often require more reliable solutions with an eye on the long-term return of the switch box.

The continued development and innovation of sensor companies has resulted in a large mix of sensors that can be assembled in various switch boxes.

In some industry sectors, open solutions are a clear trend because their price is very close to the mechanical contact switch box solution. Now, due to the size and cost advantages of open solutions, it has become more attractive to assemble position feedback on manual valves.