Inputs and Options
Formula Used
This tool sizes liquid service valves using the standard Cv relationship:
- Q is flow rate in gpm.
- SG is specific gravity (water approx 1.0).
- dP is pressure drop across the valve in psi.
The calculator also shows Kv using Cv approx 1.156 x Kv and can estimate valve authority when other system loss is provided.
How to Use This Calculator
- Choose Size to compute required Cv from flow and pressures.
- Enter flow rate, inlet pressure, outlet pressure, and specific gravity.
- Add a safety margin to recommend a practical selection Cv.
- Optional: enter other system loss to estimate valve authority.
- Optional: enter pipe inside diameter to check velocity at design flow.
- Use Check mode when you already have a valve Cv.
- Download a PDF or CSV summary for submittals and reports.
Example Data Table
Sample sizing case for a hydronic control valve.
| Flow | P1 | P2 | SG | Safety | Required Cv | Recommended Cv |
|---|---|---|---|---|---|---|
| 300 gpm | 60 psi | 45 psi | 1.00 | 10% | 77.46 | 85.20 |
| 75 m3/h | 5.0 bar | 4.2 bar | 1.05 | 15% | 92.52 | 106.40 |
Values are rounded and intended for preliminary selection. Always confirm with manufacturer curves, minimum controllable flow, noise limits, and installation conditions.
Practical Guide to Valve Cv Sizing
Control valve sizing is a balance between stable control, acceptable pressure loss, and reliable operation across real operating conditions. Cv is a capacity index that links flow rate to pressure drop for a liquid at a given specific gravity. When Cv is too small, the valve must run near full open, leaving little control range and increasing the chance of noise and cavitation. When Cv is too large, the valve operates near closed at normal flow, which can cause hunting, poor resolution, and premature wear.
In construction projects, Cv checks commonly support hydronic balancing, booster pumping, chilled water distribution, and industrial water services. Start with the design flow at the coil, branch, or process load, then confirm the available differential pressure across the valve. Use measured or calculated inlet and outlet pressures at the selected operating point, not nameplate values. For water, a specific gravity of 1.0 is typically acceptable; for brines and glycol mixtures, use the expected operating temperature and mixture percentage. A modest safety margin is often applied to cover fouling, minor uncertainty, and future adjustment, but excessive margin can reduce controllability.
The calculator also reports valve authority when you provide other system losses at design flow. Authority indicates how much of the total branch pressure drop is “owned” by the valve. Higher authority generally improves stability, especially with modulating control, while very low authority can make the valve overly sensitive. Use the pipe diameter option to estimate velocity; unusually high velocity can indicate a line size constraint or an unrealistic flow assumption. Always validate the final selection against manufacturer data for rangeability, installed characteristics, and minimum controllable flow.
For higher pressure drops, confirm that the liquid will not cavitate or flash at the trim. Cavitation risk rises when the downstream pressure approaches the fluid vapor pressure, and it can damage seats and create vibration. Many manufacturers publish limiting pressure drop curves, pressure recovery factors, and recommended trim styles for quiet operation. Also consider installation details such as upstream strainers, straight pipe requirements, and whether the valve is located on the return or supply side. These factors can change the true differential pressure available and affect the installed flow characteristic.
Worked Example
Assume 300 gpm water flow, inlet pressure 60 psi, outlet pressure 45 psi, and SG = 1.00. The differential pressure is 15 psi, giving a required Cv of about 77.46. With a 10% margin, the recommended Cv becomes about 85.20, and the predicted valve drop at that selection is about 12.40 psi. This supports a practical selection step before confirming trim, valve type, and noise limits.
FAQs
1) What does Cv represent in valve sizing?
Cv represents valve flow capacity for liquids. It links flow rate to pressure drop at a given specific gravity, helping you compare valve sizes and trims consistently.
2) Should I add a safety margin to the required Cv?
A small margin can cover uncertainty, minor fouling, and commissioning adjustments. Avoid excessive margin, because oversized valves may control poorly at low openings and become unstable.
3) What specific gravity value should I use?
Use the fluid’s operating specific gravity. Water is about 1.0. Glycol, brine, and process liquids vary with temperature and concentration, so use project data or supplier information.
4) Why does valve authority matter?
Authority estimates how much total branch pressure drop is across the valve. Higher authority typically improves control stability. Very low authority can cause sensitivity and hunting during modulation.
5) Can I use this for gases or steam?
This worksheet is intended for liquid sizing. Gas and steam require additional compressibility and choking checks. Use the proper gas/steam sizing method or manufacturer sizing software.
6) What pressures should I enter for P1 and P2?
Enter pressures at the valve inlet and outlet at the design operating point. Avoid mixing static design values with measured running values, and ensure units are consistent.
7) Why does the calculator show pipe velocity?
Velocity is a quick screening check. High velocity can increase noise, erosion, and pressure loss. If velocity looks high, reassess flow assumptions, line sizing, and available differential pressure.