Valve Position Detection and Analysis in Modern Process Installations

In the process industry, valves and actuators have long been used for controlling and directing liquid flows. Increasingly, plant engineers are seeking feedback signals from valves, taps, and actuators regarding their current positions within the control system. Typically, these signals are captured either by proximity sensors embedded in the switch boxes or by dual sensors—installed directly on the actuator or manual valve itself. This article delves into the types of sensors being applied in response to emerging trends both in the field and in the market.

Current Status of Standardization

Industry standards present both advantages and disadvantages for technological advancement. On one hand, they can limit creativity; on the other hand, they ensure compatibility among devices from different manufacturers. After outlining numerous requirements, the VDE/VDI 3845 standard specifies critical mechanical connections between pneumatic actuators and their associated equipment. This standard defines installation holes as 30×80 mm and 30×130 mm, while the height of the mounting rods is set at 20, 30, and 50 millimeters. However, there’s no fixed standard for the diameter of the mounting rods, so users must pay close attention, especially when deploying dual-sensor applications.

The term "switch box" typically refers to the integration of sensors, cams, and connectors within a sealed enclosure. Sensors and pneumatic actuators are mechanically connected through it. This solution is designed to prevent sensors from being exposed to or affected by the surrounding environment, while still allowing the use of standard sensors. In these standard switch boxes, the following sensors are typically built-in:

● Cylindrical sensor with a typical length of 40 millimeters;

● Groove-type sensor;

● Rectangular V3-style sensor (similar to a microswitch);

● Dual sensors, compactly housed in a single box.

The switch box serves as a terminal device for connecting the valve position sensor and the solenoid valve. The board is already equipped with a coded connector and the necessary LED indicators. Its compact size makes it compatible with most standard boxes.

"Open-Loop Solution" specifies the installation of dual sensors on either an automatic or manual valve. Two inductive sensors are mounted in a single housing, accurately indicating the valve's open and closed positions.

Figure 1: Two inductive sensors are stacked and installed within a single housing, precisely indicating the valve's open and closed positions.

The system's design takes into account the VDE/VDI 3845 standard. These sensors are triggered by two stainless steel targets mounted perpendicularly at 90-degree angles on a "disc." The disc itself is securely attached to the actuator via a single screw.

In addition to being compact and easy to assemble, you can also discover the following advantages:

● The circular mounting holes (not slotted) on the sensor housing prevent damage caused by collisions or vibrations resulting from incorrect installation distances or movement.

● Two switching elements (discs) can accommodate a variety of pneumatic actuators. The only choice you need to make is the mounting pattern, such as: 30×80 or 30×130

● The stainless steel target has been designed onto the disc.

● Four LEDs indicate the sensor's output status, including whether power is present and the state of the solenoid valve output.

● The detection area of the dual sensors is narrow enough to prevent pinch points that could injure operators, as well as blockages caused by ice or dust.

● The wiring for the solenoid valve can be routed through the sensor, eliminating the need for a separate cable on-site.

Inductive Sensors vs. Mechanical Switches

In all areas of industry, mechanical switches were once the go-to solution for various control challenges—but today, the landscape has changed dramatically, with sensors undoubtedly becoming the preferred choice. This is because sensors offer clear advantages in terms of speed and reliability:

● Fully solid-state, ensuring no wear or error drift;

● Quickly toggle the switch state;

● Hysteresis and repeatability can be defined electronically to a greater extent than with mechanical devices;

● No welding or corrosion at the output end;

● Rarely or virtually unaffected by blockages or humidity;

● Contactless jitter (output echo);

● Most sensors feature LED indicators during switching, making maintenance and debugging simpler and faster.

Typically, the average lifespan of an inductive sensor far exceeds that of a mechanical switch. In fact, many mechanical switches were previously used for valve feedback—primarily due to their low cost, as well as the convenience of not having to worry about polarity issues when using them. Thanks to modern technology, Pepperl+Fuchs has successfully brought sensors to market, effectively eliminating even the minimal advantages that mechanical switches once had.

Sensor for valve position detection

Choosing the right sensor can be done in just 5 simple steps:

Step 1: Switch box or open-style solution?

Weighing the advantages and disadvantages outlined above, make your choice. You may opt for the protection offered by a switch box, or embrace the simplicity and convenience of an open-style solution. Most valve suppliers provide both types of solutions. You’ll find that switch boxes vary significantly in size and weight, as well as in the number and variety of options available.

Step 2: Define the sensor terminals electrically; the control system to which the sensor is connected regulates the operational process (AC, DC, Namur, etc.). A critical point to consider is whether the system will be used in an explosive area. Refer to the electrical connection types listed below.

● DC three-wire sensor. These sensors have one additional wire besides the two power lines?

A separate line. The third line connects to the PLC input card or the output terminal on the control system.

● DC two-wire sensor. Here, the load is connected in series with the sensor. Many sensors feature polarity tolerance, meaning you no longer need to worry about the wiring polarity. As a result, connecting them is just as straightforward as linking a standard mechanical switch. When using these sensors in practice, however, be sure to pay close attention to the circuit’s leakage current and the sensor’s voltage drop.

● AC sensors with normally open and/or normally closed outputs. Here, the load is connected in series with the sensor.

● AC/DC dual-line universal sensor. This sensor is connected in series with the load and can be used with either AC or DC voltage inputs.

● Two-wire Namur sensors, paired with compatible intrinsically safe products, are designed for use in hazardous (explosion-proof) environments (Zones 0, 1, and 2). They enable monitoring for open-circuit and short-circuit conditions.

● Two-wire fail-safe sensors. Compliant with the two-wire Namur sensor standard, but utilizing an isolation barrier that meets safety requirements, these sensors can achieve SIL3 safety certification.

Step 3: The Possibility of Connection

● The connection method depends on the end user. For example, sealed tubes or plugs with cables. In the chemical and pharmaceutical industries, cable connections are often preferred, with cable-sealed tubes routed into boxes or sensors.

Step 4: Solenoid Valve Operation

Just like with switch boxes, using dual sensors allows wiring the solenoid valve directly through the sensors—its key advantage being that the actuator requires only a single cable connection.

Step 5: Finally, you must verify the sensor’s various characteristics to ensure it operates flawlessly in your application.

Similar examples include leakage current, voltage drop, ambient conditions, maximum output current, and operating voltage. Whenever possible, you can verify this by selecting sensors with specifications that ensure compliance with the allowable limits for static current (interference), as well as the maximum values of shock and vibration.

Future developments

As the installation process often needs to keep pace with the latest technological advancements to enhance efficiency and stability, various bus systems have begun entering the market.

Of course, manufacturers of switch boxes and actuators are eager to join. Many of these manufacturers have already collaborated with sensor makers to develop interfaces for these bus systems. The AS-i-enabled switch box is a great example, as this particular bus system is ideally suited for handling digital information. If end users prefer to use standard sensors, they’ll need the appropriate field installation modules, such as AS-Interface, ProfibusPA, or Foundation Fieldbus.

On the other hand, you can also see electrical actuators and positioning systems equipped with Profibus or Foundation Fieldbus interfaces. Clearly, more and more companies are recognizing the advantages of bus systems, particularly the widespread adoption of fieldbus combinations—such as Profibus paired with AS-i.

The conclusion of this article

Increasingly, people are noticing the trend of inductive sensors replacing mechanical switches. In certain markets, 60% to 70% of switch boxes equipped with inductive switches have already taken over from their mechanical counterparts. However, standardizing valve-position detection still has a ways to go—largely because the original designs of switch boxes were rooted in a complex blend of mechanical assembly and switching technologies. More often than not, end users also consistently demand more reliable solutions, focusing on the long-term value and performance of the switch boxes.

The continuous development and innovation of the sensor company have led to the widespread integration of diverse sensors into various switch boxes.

In certain industry sectors, open-loop solutions are clearly becoming a dominant trend, as their price is very competitive compared to mechanically actuated switch-box designs. Now, thanks to the size and cost advantages of open-loop solutions, integrating position feedback directly into manual valves is also growing more appealing.