Sign up for the ControlAir newsletter.
Get news, updates, and offers direct to your inbox.
A pressure regulator’s Cv, or flow coefficient, describes its flow capacity, but it does not tell you how well the regulator will maintain outlet pressure as demand changes. For proper regulator sizing, review the manufacturer’s flow curves alongside Cv to confirm that the required flow and outlet pressure can be achieved together.
Choosing a regulator based only on a high Cv or maximum flow rating can result in an outlet pressure that falls below your application’s requirements.
What Does Cv Mean for a Pressure Regulator?
Cv is a flow coefficient used to compare the flow capacity of valves and regulators. It is defined as the flow of water in U.S. gallons per minute at 60°F that produces a 1 psi pressure drop across the device.
Under comparable conditions, a higher Cv indicates greater flow capacity. However, Cv is not an SCFM rating. Calculating air or gas flow also requires operating conditions such as inlet pressure, outlet pressure, temperature, and gas properties.
Why Is Cv Alone Not Enough to Size a Pressure Regulator?
A regulator’s published maximum Cv represents its fully open flow capacity. During normal pressure regulation, the valve adjusts its opening to respond to downstream demand.
Once the regulator reaches its fully open limit, it cannot open farther to compensate for increasing demand. Flow may continue, but the regulator can no longer maintain the desired outlet pressure.
This is why maximum flow capacity and usable flow at a required pressure are different selection criteria.
What Is the Difference Between Cv and a Flow Curve?
Cv helps compare flow capacity. A regulator flow curve shows how outlet pressure changes as flow increases under specified test conditions.
| Selection Tool | What It Shows | How to Use It |
|---|---|---|
| Cv | Flow capacity at a specified valve opening; maximum Cv describes the fully open condition. | Compare capacity alongside pressure-regulation performance. |
| Flow Curve | Outlet pressure at different flow rates for stated operating conditions. | Check whether the regulator maintains acceptable pressure throughout the required flow range. |
What Is Pressure Regulator Droop?
Droop is the decrease in a regulator’s outlet pressure as downstream flow increases while the regulator setting remains unchanged.
Droop is different from the pressure difference between the inlet and outlet. A pressure-reducing regulator intentionally lowers supply pressure; droop describes the additional change in outlet pressure as demand rises.
A flatter flow curve indicates less variation in outlet pressure across that flow range. The acceptable amount of droop depends on the equipment and process being supplied.
How Do You Read a Pressure Regulator Flow Curve?
- Check the test conditions. Identify the regulator configuration, port size, supply pressure, and test medium.
- Find the relevant pressure curve. Use the curve corresponding to the stated initial outlet-pressure setting.
- Locate your required flow. Find the flow rate on the horizontal axis and trace upward to the curve.
- Read the outlet pressure. Trace across to the vertical axis and compare the result with your minimum acceptable pressure.
- Check the full operating range. Evaluate normal and peak demand, keeping operation away from the steep capacity-limit region.
Example: Reading a Type 7000 Regulator Flow Curve
The illustrated ControlAir Type 7000 flow curve uses a 3/4-inch port and a supply pressure of 100 psig.
Following the curve that starts at approximately 80 psig under no-flow conditions, the outlet pressure falls to approximately 50 psig at 200 SCFM.
If an application requires 80 psig while consuming 200 SCFM, that operating point would not meet the requirement under these conditions—even though the regulator passes the requested airflow.
These values are approximate readings from the illustrated curve, not performance guarantees for every Type 7000 configuration or supply condition.
What Information Do You Need to Select a Pressure Regulator?
Before selecting a regulator, establish:
- Available supply pressure, including expected variations.
- Required outlet pressure and acceptable pressure variation.
- Normal and peak flow requirements.
- Operating medium and material compatibility.
- Port connections, exhaust needs, and environmental requirements.
Use those requirements to evaluate the appropriate flow curves, then compare Cv and other specifications. A larger port or higher Cv alone does not confirm that a regulator will provide the pressure control your application needs.
For additional guidance, read How to Determine the Appropriate Regulator or contact ControlAir at [email protected] for help reviewing your operating requirements.