A pneumatic control valve is a process valve that uses compressed air to move a plug, diaphragm or piston inside a flow body, modulating the position of that internal element to regulate pressure, flow rate, temperature or tank level. The air supply acts on an actuator, which in turn shifts the valve stem to a precise opening between fully closed and fully open. In hygienic process lines, this is the workhorse component that holds a filler at constant pressure, meters a dosing stream, or balances a pasteuriser loop.
This guide explains the working principle, walks through the key parts, compares the main valve types you will encounter, and finishes with a practical selection checklist and CIP considerations.
What this article covers
- A pneumatic control valve uses compressed air on a diaphragm or piston actuator to modulate flow, pressure, temperature or tank level.
- Three Alfa Laval families cover most hygienic duties: the CPM for constant inlet or outlet pressure, the Unique RV-P for electro-pneumatic precision regulation, and the Unique RV-ST for third-generation single-seat regulation.
- Hygienic applications span dairy, beverage, food, personal care and pharmaceutical lines, including aseptic configurations for sterile processing.
- Selection hinges on the controlled variable, Kv requirements, connection type, temperature range, and the need for electronic or self-acting control.
- The Unique RV-P and Unique RV-ST are both built on the Alfa Laval Unique SSV platform, providing performance reliability across the hygienic valve range.
- All three families are designed for cleaning in place (CIP) using standard caustic soda and nitric acid cleaning regimes.
What is a pneumatic control valve?
A pneumatic control valve modulates a process variable by varying the opening of a valve plug inside a hygienic body. Unlike an on/off valve, which sits either fully open or fully closed, a control valve is designed to hold any intermediate position so it can regulate a setpoint continuously. The controlled variable is typically one of four: inlet or outlet pressure, volumetric flow rate, product temperature in a downstream heat exchanger, or the liquid level in a tank.
The energy that moves the plug is compressed air. For advanced positioner-driven valves like the Unique RV-ST, this usually involves a supply of 5 to 7 bar. Simpler constant-pressure designs like the CPM operate in an air pressure range of 0 to 6 bar. Compressed air is preferred in hygienic plants because it is safe, easy to route, and tolerates the wash-down environments common in dairy, beverage and food processing.
Some pneumatic control valves are entirely self-acting. They sense product pressure through a diaphragm and adjust position mechanically against a fixed pilot air pressure, without any need for electronic control. Others are electro-pneumatic: an electrical setpoint signal, typically 4-20 mA, is converted to a pneumatic signal that allows the positioner to manage the stem position precisely.
How a pneumatic control valve works
The working principle is easiest to follow if we split it into the air side and the product side.
On the air side, supply air enters the actuator. In a self-acting constant-pressure valve like the CPM, the supply air is set to a pre-set value that corresponds to the desired product pressure. A diaphragm unit inside the valve sees product pressure on one face and air pressure on the other. Any deviation between the two shifts the diaphragm, which moves the plug and re-establishes the balance of pressure. The valve reacts immediately to changes, quickly adjusting position to maintain the pressure at pre-set values.
On the air side of an electro-pneumatic valve, an integrated I/P converter receives a 4-20 mA setpoint and converts it to a pneumatic signal. This signal feeds a positioner that operates on the force-balance principle: the position of the actuator piston is held directly proportional to the input signal. A contact-free position sensor measures actual stem position and feeds it back to the loop, which trims air supply until the measured position matches the command. This ensures the valve achieve accurate modulation even under pressure shocks.
The integrated positioner operates by means of the force-balance principle, ensuring that the position of the actuator piston is directly proportional to the input signal.
On the product side, the plug moves inside a hygienic body. As the plug lifts off its seat, an annular flow area opens. The relationship between stem position and flow rate is characterized by the Kv value, which represents the flow of water at 20 °C in m³/h that the valve will pass at a 1 bar pressure drop. Different plug profiles and Kv values—ranging from as low as 0.5 up to 110 for the Unique RV-P—allow designers to pick a valve that covers the exact operating window of the duty.
Key components and parts
Most hygienic pneumatic control valves share the same building blocks, even if the implementation differs between self-acting and electro-pneumatic families.
Actuator
The actuator is the pneumatic motor. In the Unique RV-P, it is an electro-pneumatic actuator with an integrated I/P converter and positioner, requiring a max air pressure of 600 kPa (6 bar). The Unique RV-ST uses a third-generation single-seat platform with an advanced controller. In the CPM family, most components are simple and pneumatic, avoiding electronic control entirely.
Diaphragm Unit
The diaphragm is the moving element that translates air pressure into linear motion. In CPMI-2 and CPMO-2 valves, the diaphragm unit consists of two flexible PTFE and EPDM diaphragms supported by 12 stainless steel sectors. The upper diaphragm is typically NBR for standard temperatures but can be PTFE-covered EPDM for ranges up to 140 °C.
Plug and Seat
The plug moves inside the valve body. On CPM valves, the valve body and the seat are welded together. For the Unique RV-ST in sizes 38 mm to 101.6 mm, the plug carries an EPDM plug seal, allowing the unit to also function effectively as a shut-off valve. On 25 mm sizes, no plug seal is included.
Positioner
The positioner is the intelligent control unit. The Unique RV-ST is available with the Type 8692 (with display) or Type 8694 (without display). Both utilize a contact-free, wear-free position detection module. These units are rated for protection classes IP65 and IP67.
Optional Booster
When product pressure exceeds the available air pressure, a booster can be fitted to the CPM. This allows for accurate regulation where the product pressure is equal to 1.8 times the available air pressure. The booster consists of a housing, piston, diaphragm, and cover, secured with a clamp.
Hygienic Body and Seals
Product-wetted steel parts are standard 1.4404 (316L) stainless steel, while other steel parts use 1.4301 (304). The standard inside surface finish is bright polished with an Ra ≤ 0.8 µm for the Unique series. Product-wetted elastomers are EPDM as standard, with options for HNBR or FPM to suit specific chemical requirements.
Types and variations
Hygienic process plants typically utilize three principal valve families. Each addresses a specific control duty.
Self-acting constant-pressure valves
The Alfa Laval CPM is a pneumatic regulating valve that maintains a constant pressure in hygienic lines at the inlet or outlet. It lacks electronic controls, reacting immediately to alterations in product pressure via its diaphragm unit. Variants include the CPMI-2 for inlet control, CPMO-2 for outlet control, and the CPM-I-D60 for high-capacity applications. Common uses include filling and bottling equipment where uniform pressure is required.
Electro-pneumatic precision regulating valves
The Alfa Laval Unique RV-P is an automatic regulating valve with an electro-pneumatic actuator for duties requiring precision control of flow, pressure, temperature, and tank levels. Built on the Unique SSV platform, it offers flexible Kv values because the lower elements can be exchanged. It is available in manual, standard, and aseptic versions; the aseptic model Max 8 bar configuration is often requested for sterile processes.
The Alfa Laval CPM maintains a constant pressure quickly adjusting position to maintain the pressure at pre-set values without any need for electronic control.
Single-seat regulating and shut-off hybrids
The Alfa Laval Unique RV-ST is the third generation of the single-seat regulating valve line. It is designed to meet the highest process demands for hygiene. Sizes from 38 mm to 101.6 mm (1.5 inch to 4 inch) come equipped with a plug seal to function as a shut-off valve. It offers a vast range of Kv-values and can be ordered with either a Type 8692 or Type 8694 positioner.
| Family | Control type | Primary duty | Max Product Pressure |
|---|---|---|---|
| CPM (CPMI-2/CPMO-2) | Self-acting | Inlet/Outlet constant pressure | 1000 kPa (10 bar) |
| Unique RV-P | Electro-pneumatic | Precision flow, level, temp | 1000 kPa (10 bar) |
| Unique RV-ST | Electro-pneumatic | Flow/Pressure control & Shut-off | 10 bar (1000 kPa) |
Quick selection guide
- To maintain constant pressure without electronic signals, select a CPM (CPMI-2 for inlet, CPMO-2 for outlet).
- For computer-controlled (4-20 mA) regulation of flow or level in hygienic processes, specify the Unique RV-P.
- If you need both accurate regulation and a shut-off function in a single valve, choose the Unique RV-ST in sizes above 25 mm.
- Specify Aseptic versions when your application requires hermetic sealing for sterile processing.
Industries and applications
Pneumatic control valves are necessary wherever a hygienic process requires a stable, regulated environment. In the dairy, food, and beverage industries, these valves maintain uniform pressure for filling equipment and bottling lines. Their hygienic design also makes them suitable for personal care and pharmaceutical manufacturing.
In pharmaceutical and biotechnology applications, aseptic versions are critical. For the Unique RV-P, an aseptic version is available to prevent atmospheric contamination. The CPM family is also safe and reliable for these highly regulated fields, providing excellent valve cleanability across all models.
Hygienic process lines for the dairy, food, beverage, personal care and many other industries rely on these valves for uniform inlet or outlet pressure.
Typical applications include metering flow into heat exchangers for temperature control or regulating the liquid level in storage tanks. Because they are easy to clean and maintain, they support high hygienic levels across various hygienic processes.
How to choose the right pneumatic control valve
Choosing the correct valve requires evaluating several technical factors to ensure the component matches your process duty.
1. Define the Control Variable
Determine if you need to regulate pressure, flow, temperature, or level. For simple pressure setpoints with no external controllers, a CPM is often sufficient. If your system relies on a control signal from a PLC, you will require the electro-pneumatic Unique RV-P or Unique RV-ST regulating valve.
2. Calculate Kv Requirements
The Kv value defines the valve's capacity. Choosing the correct size ensures the valve operates in its recommended working area. For example, a CPMI-2 with a Kv 23 requirement and a 200 kPa pressure drop at 8 m3/h flow will land perfectly at 48% open.
3. Select Size and Connection
Valves are available in various connection types including ISO, DIN, SMS, BS, and Clamp. The Unique RV-ST supports sizes from 25 mm up to 101.6 mm. Ensure the valve dimensions match your existing piping to avoid installation stress from overloading pipelines.
4. Verify Temperature Range
Standard EPDM seals typically cover -10 °C to +95 °C. For higher temperatures, such as +140 °C, the CPM requires both diaphragms to be solid PTFE or the upper diaphragm and O-ring to be FPM/PTFE covered EPDM. The Unique RV-ST EPDM range extends to +140 °C.
5. Consider Air Supply
Electro-pneumatic models like the Unique RV-ST require air pressure between 5 to 7 bar. If your product pressure is significantly higher than your available air, remember to add a booster to your CPM order to maintain control.
6. Review Documentation Requirements
If you require further technical specifications, our control and check valves collection offers detailed documentation across all hygienic families.
Cleaning, CIP and maintenance
Hygienic valves are designed for Cleaning in Place (CIP) to ensure sterility between batches. The CPM features a self-draining design and excellent cleanability, while the Unique series utilizes self-draining bodies built for high-volume hygienic processing. For CIP specifically, you must ensure the valve is fully open to allow maximum cleaning flow. For a CPMI-2, this means no air pressure is required to be fully open, whereas a CPMO-2 requires sufficient air pressure.
A typical CIP regime uses clean water free from chlorides and two primary agents: Caustic soda (NaOH) at 1% by weight and Nitric acid (HNO3) at 0.5% by weight, both at 70 °C (158 °F). Cleaning agents should be dosed gradually to avoid excessive concentrations. Always rinse well with clean water after the cleaning cycle.
The recommended cleaning (CIP) for these valves utilizes Caustic soda and Nitric acid, typically at concentrations of 1% and 0.5% by weight at 70 °C.
Preventive maintenance is key to minimizing downtime. Diaphragms in CPM valves should be replaced at least every 12 months. When servicing, always keep spare O-rings in stock. For the Unique RV-P, lubricate the O-ring and lip seal with Alfa Laval Lubricant during assembly. Torque specifications are critical: CPM clamps should be tightened to 10-15 Nm, while the Unique RV-P spindle and plug require 30 Nm using two 17 mm spanners.
Finally, always observe safety warnings. If an actuator is marked SPRING UNDER LOAD, do not attempt to disassemble or cut it open. This presents a severe danger of injury. Always release compressed air and ensure pipelines are depressurized and cooled before maintenance work begins.
Alfa Laval pneumatic control valves from Euroflow
Euroflow is an Alfa Laval Master Distributor for postal codes 66-99 and 07-08. We maintain a stock of CPM, Unique RV-P, and Unique RV-ST families along with the necessary service kits, positioners, and boosters to keep your process lines running. Whether you are specifying a new dairy line or upgrading a pharmaceutical filler, we can help map your duty points to the right Kv value and material options. Explore our full regulating and check valves range to find the correct model for your application, or look for specific positioners and position feedback units for your existing equipment.
Summary and next steps
A pneumatic control valve is the primary tool for automated regulation in hygienic environments. By using compressed air to modulate a plug's position, these valves provide accurate control of pressure, flow, and temperature. Whether you choose the self-acting CPM for pressure duty, the high-precision Unique RV-P, or the hybrid regulating/shut-off Unique RV-ST, you are selecting a platform designed for safety and ease of maintenance.
To move forward, calculate your required Kv value and identify the connection standard that matches your facility. If you are working with English-language documentation, our control and check valves overview provides further details on sizing and model variations to help you make an informed decision.
