English Explanation | What is Control Valve CV | Easy & Simple Explanation

Flow Coefficient (Cv)

The flow coefficient (Cv) represents a valve’s capacity to pass fluid. It is defined as the number of U.S. gallons per minute (GPM) of water at 60°F that can flow through a fully open valve with a pressure drop of 1 psi across the valve.

In simple terms, Cv indicates how much fluid a valve can handle for a given pressure drop. The relationship between flow rate and pressure drop through a valve is conveniently expressed using the flow coefficient, Cv.

Valve Selection Based on Cv

When selecting a control valve, the nearest Cv value to the calculated requirement is typically chosen—usually rounding down, unless the project allows a higher maximum pressure drop (ΔP).

Oversized valves can cause control problems such as hunting, unstable operation, and poor heat transfer (ΔT) across coils due to excessive flow.

Undersized valves may restrict flow, fail to meet process demand, and exceed the available pressure drop.

Proper Cv selection is critical for stable control and efficient system performance.

Cavitation in Control Valves

Cavitation occurs in a control valve when the local liquid pressure drops below the fluid’s vapor pressure, causing vapor bubbles to form. As the pressure recovers downstream and rises above the vapor pressure, these bubbles collapse violently, potentially leading to noise, vibration, and valve damage.

Flashing in Control Valves

Flashing occurs when the liquid pressure falls below its vapor pressure and remains below the vapor pressure downstream of the valve. In this case, the vapor bubbles do not collapse and the fluid permanently changes phase, often resulting in erosion and material loss.

Choked Flow in Control Valves

Choked flow is the condition where further reduction in downstream pressure does not increase flow through the valve. At this point, fluid velocity reaches its maximum, and cavitation can become extremely severe, posing serious risks to valve integrity.

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