In many process automation applications, the pressure and flow of gases are controlled. Explosion protection requirements mean that an inert atmosphere is often created and maintained in reactors and process tanks in a plant. This is usually done with nitrogen and in some cases also with argon. Other applications include the transfer of materials between process tanks and filling, using inert gases, air, sterile air and carbon dioxide. Pressurising mechanical seals with gas is another application.
To cover these functions, dedicated lines with regulating valves and flow meters are required in many existing applications for each pressure and gas level. Depending on the process and industry requirements, the investment costs for this type of control can be between € 3,000 – 10,000 per control circuit.
Proportional valve technology represents an interesting alternative for closed-loop control of inert gases in process automation. It achieves the same accuracy as the existing technology, but the level of installation and investment required is lower. Installing a control station in a compact control cabinet also saves space and simplifies access to all process components. The ability to integrate proportional valves on valve terminals reduces space requirements, simplifies automation and provides more diagnostics options. In some cases, processes which have previously been carried out manually can be automated simply and cost-effectively.
What are Proportional Valves?
Proportional valves or proportional pressure regulators are dynamic control valves which not only enable discrete switching positions like open/closed, but, when combined with a proportional solenoid or two switching valves as pilot valves, also enable the dynamic closed-loop control of the outlet pressure.
Proportional valves can be classified into two types: direct-acting and pilot-operated. Direct-acting proportional valves use a proportional solenoid to directly control the position of the valve seat, which determines the outlet pressure. Pilot-operated proportional valves use two switching valves as pilot valves to control the pressure in a control chamber, which in turn controls the position of the main valve seat. Pilot-operated proportional valves can handle higher flow rates and pressures than direct-acting proportional valves.
The following table compares the main features and advantages of direct-acting and pilot-operated proportional valves:
| Feature | Direct-acting | Pilot-operated |
|---|---|---|
| Flow rate | Low to medium | Medium to high |
| Pressure range | Low to medium | Medium to high |
| Response time | Fast | Moderate |
| Hysteresis | Low | Low |
| Power consumption | High | Low |
| Installation | Simple | Complex |
| Advantages | High accuracy, fast response, simple installation | High flow rate, high pressure, low power consumption |
How are Proportional Valves Used for Closed-Loop Control of Inert Gases?
Proportional valves can be used for closed-loop control of inert gases in process automation by replacing the conventional regulating valves and flow meters. A closed-loop control system consists of four main components: a sensor, a controller, a positioner, and a proportional valve.
The sensor measures the actual value of the controlled variable, such as pressure or flow, and sends it to the controller. The controller compares the actual value with the desired setpoint value and calculates the deviation. The controller then sends a signal to the positioner, which adjusts the proportional valve accordingly. The proportional valve regulates the outlet pressure or flow of the inert gas, and the process repeats until the deviation is minimized.
The advantages of using proportional valves for closed-loop control of inert gases in process automation are:
- Reduced installation and investment costs: Proportional valves can replace multiple components, such as regulating valves, flow meters, and pressure transmitters, and reduce the number of lines and fittings required. This can save up to 70% of the installation and investment costs compared to the conventional technology.
- Increased flexibility and control quality: Proportional valves can handle a wide range of pressures and flows, and can be easily adjusted to different setpoints and process conditions. Proportional valves can also provide high accuracy, fast response, and low hysteresis, which improve the control quality and stability of the process.
- Space-saving and easy access: Proportional valves can be installed in a compact control cabinet, which saves space and simplifies access to all process components. Proportional valves can also be integrated on valve terminals, which further reduce space requirements and simplify automation and diagnostics.
Examples of Proportional Valves for Closed-Loop Control of Inert Gases in Process Automation
Proportional valves can be used for various applications that involve closed-loop control of inert gases in process automation. Some examples are:
- Inertisation and pressure blanketing: Inertisation is the process of replacing the contained gas volume in a reactor or a process tank with an inert gas, such as nitrogen or argon, to prevent explosions or chemical reactions. Pressure blanketing is the process of maintaining a constant pressure in a reactor or a process tank with an inert gas, to protect the contents from oxidation or contamination. Proportional valves can be used to control the pressure and flow of the inert gas during inertisation and pressure blanketing, and to adjust to different tank sizes and process phases.
- Overflowing and material transfer: Overflowing is the process of filling a container or a process tank with an inert gas, such as air, sterile air, or carbon dioxide, to prevent the formation of foam or to create a protective layer. Material transfer is the process of transferring materials between containers or process tanks using an inert gas, such as nitrogen or argon, to avoid contact with air or to prevent sedimentation. Proportional valves can be used to control the flow and pressure of the inert gas during overflowing and material transfer, and to adapt to different filling levels and material properties.
- Pressurising and monitoring mechanical seals with gas: Mechanical seals are devices that prevent leakage of fluids from rotating shafts in pumps, compressors, or mixers. Mechanical seals are often pressurised and monitored with an inert gas, such as nitrogen or air, to enhance their performance and reliability. Proportional valves can be used to control the pressure and flow of the inert gas for pressurising and monitoring mechanical seals, and to compensate for pressure fluctuations and seal wear.