Process Flow Diagram
Experiment Controls
Live Readings
| S.No. | Valve Opening (%) | Flow Rate (%) | Observation |
|---|---|---|---|
|
Start the experiment and adjust the valve to record observations |
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Construction
An Equal Percentage control valve consists of a valve body with inlet and outlet ports, a valve seat providing the sealing surface, a specially contoured equal percentage valve plug (such as an logarithmic/exponential contoured plug or slotted V-port plug), a valve stem connecting the plug to the actuator, a diaphragm or piston actuator that converts signal pressure to stem movement, and a return spring providing fail-safe action.
The plug profile is engineered with a logarithmic/parabolic geometry that increases the open annular area exponentially as the stem lifts — producing an exponential-type flow area progression with stroke.
Working Principle
When the control signal increases, the actuator lifts the valve stem and moves the plug off the seat. Due to the plug's exponential contour, each equal increment of valve travel produces the same percentage increase in flow relative to the previous flow.
Initially, at small openings (e.g. 10% to 20%), the flow increases very slowly (2% to 5%). As the valve approaches full opening (e.g. 80% to 100%), the flow increases much faster (76% to 100%), resulting in a distinct exponential-type curve on a flow vs. opening graph.
Equal Percentage Characteristic
The Equal Percentage flow characteristic is mathematically expressed by:
Q / Qmax = R(x/L - 1) or (dQ/dx) / Q = c, where R = valve rangeability, x = fractional travel
Key features of this characteristic:
- Exponential flow response — flow change per step is proportional to the current flow rate
- Constant percentage gain — equal relative flow increase per unit stem movement
- Exponential-type curve — slow initial rise, accelerating sharply near full opening
- High dynamic stability — prevents control loop hunting under varying pressure drop conditions
Characteristic Data Comparison
| Valve Travel | Equal % | Linear | Quick Opening |
|---|---|---|---|
| 10% | 2% | 10% | 35% |
| 20% | 5% | 20% | 55% |
| 40% | 15% | 40% | 80% |
| 60% | 40% | 60% | 93% |
| 80% | 76% | 80% | 98% |
| 100% | 100% | 100% | 100% |
Advantages
- Provides excellent control over a wide operating range (high rangeability)
- Maintains a nearly constant total control loop gain when line pressure drop varies with flow rate
- Prevents instability and hunting at low flow rates / small valve openings
- Ideal for process systems where major pressure drop shifts from valve to piping as flow increases
- Equal percentage increments compensate for non-linear process dynamic characteristics
Disadvantages
- Non-linear flow relationship complicates tuning in systems with strictly constant differential pressure
- Low flow resolution at small openings requires precise valve positioning mechanism
- Slower initial flow response compared to linear or quick-opening valves
Industrial Applications
Equal Percentage valves are the most widely used control valves in industrial process control because they provide excellent control over a wide operating range.
Pressure Control Loops
Widely used in steam lines, gas headers, and liquid pressure control loops where system pressure drop varies with process load.
Heat Exchanger Control
Applied in heat exchanger cooling and heating water loops to offset non-linear heat transfer characteristics.
Variable ΔP Piping Systems
Ideal for long piping networks where friction losses increase significantly with higher flow rates.
General Process Control
The standard choice across chemical, oil & gas, power generation, and water treatment process control loops.