What is actually meant by sizing of a control valve?
In engineering terms, sizing a control valve is the mathematical process of determining the correct Flow Coefficient (Cv) required for a specific process.
what is control valve leakage class?
In an ideal world, a control valve would be bubble-tight when closed. In reality, because control valves are designed primarily for throttling (regulating flow) rather than isolation (shutting off flow), some amount of leakage is expected.
The ANSI/FCI 70-2 standard establishes specific “Leakage Classes” to define exactly how much fluid is allowed to pass through a valve in the fully closed position.
The 6 Leakage Classes
The standard breaks leakage down into six classes, ranging from Class I (most lenient) to Class VI (most stringent).
How do we select the proper flow characteristics for control valves? Specifically, how does the selection process differ between Globe valves and Rotary valves (like Ball and Butterfly valves)
when the trim cannot be easily changed?
- Selection by Mechanical Trim (Globe Valves)
In Globe valves, the flow characteristic is determined by the physical shape of the plug and seat (the trim). If the process requirements change, you physically swap the trim to one of the following:
Linear Trim: Best when the valve handles more than 40% of the total system pressure drop. The flow is directly proportional to the valve travel.
Equal Percentage Trim: The most common choice. Used when the valve handles a small portion of the pressure drop (less than 25%) or in systems with long pipe runs. It compensates for the loss of system pressure as flow increases.
Quick Opening Trim: Used primarily for On/Off or emergency applications where full flow is needed immediately.
- Selection by Positioner (Ball and Butterfly Valves)
Unlike Globe valves, Ball and Butterfly valves have fixed internal geometries. You cannot “swap a plug” to change how they behave. Instead, we use Digital Positioners to modify the characteristic.
The Inherent Shape: A Segmented Ball valve or a Butterfly valve naturally has an Equal Percentage characteristic.
The Electronic “Fix”: If your process requires a Linear response from a Butterfly valve, you don’t change the hardware. Instead, you program a Characterization Curve into the digital positioner.
what is cavitation?
Cavitation is the phenomenon that occurs in liquid flow when the static pressure falls below the vapor pressure of the liquid, causing vapor bubbles to form, and then subsequently rises above the vapor pressure, causing the bubbles to collapse. Cavitation is a two-stage process: the first stage is the formation of vapor bubbles, and the second stage is the implosion of these bubbles during pressure recovery.
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