The Physics of Control
A control valve's performance is a blend of physics, mechanics, and system dynamics. This interactive guide explores the three pillars of valve performance: the foundational "Why" of Bernoulli's Principle, the practical "How Much" of the Flow Coefficient (Cv), and the engineered "How" of Flow Characteristics.
Bernoulli's Principle: Energy in Motion
At its core, a valve works by forcing a fluid to trade pressure energy for kinetic energy (velocity). As the fluid speeds up through the valve's restriction, its pressure drops. This interactive diagram visualizes this fundamental energy exchange. Adjust the valve opening to see the relationship in action.
Upstream
1.0x
Vena Contracta
2.0x
Downstream
1.0x
Flow Coefficient (Cv) Calculator
The Flow Coefficient (Cv) is a practical metric that quantifies a valve's flow capacity. It's an empirical value that bridges the gap between ideal physics and real-world performance by accounting for friction and turbulence. Use this calculator to solve the standard liquid sizing formula.
Calculate Required Cv
Required Cv
31.62
Calculate Flow Rate (Q)
Resulting Flow Rate (GPM)
100.0
Inherent vs. Installed Characteristics
A valve's inherent characteristic is its lab-tested personality. When installed in a real system with pumps and pipes, friction losses change the pressure available to the valve, distorting its behavior into an "installed" characteristic. The goal is to select an inherent curve that becomes linear when installed.
Interact: Select an inherent characteristic, then increase the "System Piping Loss" to see how it transforms the curve. Notice how the Equal Percentage curve linearizes under high system loss.
1. Select Inherent Characteristic
Percentage of total pressure drop consumed by pipes at max flow.
Putting It All Together: Case Studies
The right valve choice ensures a stable, responsive, and easily-tunable control loop. This is achieved by matching the valve's inherent characteristic to the system's dynamics. Explore these common scenarios to see how the principles apply in practice.
Case 1: Liquid Level Control
Controlling level by throttling outflow. The pressure drop is mainly from the liquid's static head and is relatively constant.
System Type: Low Loss System
Analysis:
Since the pressure drop across the valve is nearly constant, the installed characteristic will be very similar to the inherent one. To achieve a linear installed response for stable control, the choice is simple.
Optimal Choice: Inherently Linear Valve
Case 2: Flow Control in a Long Pipeline
Controlling flow from a pump. As flow increases, friction in the long pipe consumes a large, and increasing, portion of the pump's energy.
System Type: High Loss System
Analysis:
The pressure drop available to the valve decreases significantly as it opens. An inherently linear valve would "flatten out" at high flows, losing control authority. We need a characteristic that counteracts this effect.
Optimal Choice: Inherently Equal Percentage Valve