Control Valve Characteristic Curves Explained — Inherent vs Installed Characteristics

Valve characteristics curves [5] | Download Scientific Diagram

🔹 Introduction

Control valves are the final control elements in every process control loop.
No matter how advanced your DCS, PLC, or PID controller is — the loop’s performance ultimately depends on how well the control valve responds to control signals.

However, one area that many engineers overlook is the valve characteristic curve — how the valve’s flow rate changes in response to a change in its opening (stem position).

Misunderstanding this concept leads to poor loop control, instability, and energy losses.
This article will explain inherent vs installed characteristics, the types of control valve curves, and how to choose the right one for your process — in simple, real-world terms.

🔹 What Is a Control Valve Characteristic?

A control valve characteristic describes the relationship between valve opening (or travel) and the flow rate passing through it — assuming the valve is operating under certain conditions.

In short:

💡 “It shows how much the flow changes for every change in valve position.”

🔹 Why Valve Characteristics Matter

If you choose the wrong characteristic:

The process may overreact or underreact to small controller outputs.
The control loop can become non-linear and hard to tune.
Flow may be unstable at low openings.
Energy may be wasted due to throttling in the wrong range.

Correct valve selection ensures:
✅ Smooth control response
✅ Stable process behavior
✅ Proper use of valve’s full control range

🔹 Types of Valve Characteristics

There are two main types of control valve characteristics:

Inherent Characteristic
Installed Characteristic

Let’s understand each one clearly.

🧩 1. Inherent Valve Characteristic

The inherent characteristic is the flow behavior of a valve tested under constant pressure drop (i.e., lab conditions).

It shows how the flow rate (Cv or % of max flow) changes with valve stem position when the pressure across the valve remains the same.

This is purely a valve design property, independent of the process it’s installed in.

The most common inherent characteristics are:

Type	Description	Typical Use
Linear	Flow rate increases linearly with valve travel. 50% open = 50% flow.	Level control, split range loops
Equal Percentage (EQ%)	Each increment in travel produces a constant % change in flow — small flow at low openings, large at high openings.	Pressure, temperature, flow control
Quick Opening	Large flow change at small openings.	On/off or safety service

📊 Inherent Characteristic Curves (Constant ΔP)


Flow (%)
│
│             EQ%
│            /
│           /
│          /
│         /
│        /
│       /
│      /
│     /
│    /
│   /     Linear
│  /
│ /       Quick Opening
│/________________________
         Valve Travel (%)

Linear: Straight line
EQ%: Exponential curve
Quick Opening: Steep at start, then flattens

🧩 2. Installed Valve Characteristic

Once the valve is installed in the process, the pressure drop across it (ΔP) no longer stays constant.
It changes with flow and system pressure losses.

So, the installed characteristic shows how the valve actually behaves in the real system — including effects of pipelines, friction losses, pumps, and fittings.

In most real cases:

System pressure drop increases with flow.
The installed curve becomes flatter (less sensitive) than the inherent one.

Example:

An equal percentage valve, when installed, often behaves more like a linear valve due to system resistance.

🔹 Relationship Between Inherent and Installed Characteristics

Aspect	Inherent	Installed
Test condition	Constant pressure drop	Varying system pressure drop
Controlled by	Valve design	System + valve combination
Used for	Valve specification	Actual system performance
Shape	Ideal	Realistic
Purpose	For manufacturer rating	For loop tuning and performance

🔹 How System Pressure Drop Affects Valve Behavior

When flow increases:

More pressure is lost in the pipeline.
Less pressure remains across the valve.
Hence, for the same valve opening, flow increases less than expected.

This causes the installed characteristic to flatten out.

🔹 Choosing the Right Valve Characteristic

Selecting the correct characteristic is key to stable control.
Here’s a practical guide:

Process Type	Typical Dynamic Behavior	Recommended Valve Characteristic	Reason
Flow control loop	Fast, low lag	Linear	Gives stable flow control with proportional signal
Pressure control	Self-regulating	Equal Percentage	Compensates for pressure drop changes
Temperature control	Slow, lagging	Equal Percentage	Smooth response, prevents overshoot
Level control (in tanks)	Integrating process	Linear	Predictable change in flow
On/Off service	No throttling	Quick Opening	Provides full flow quickly

🔹 Real-World Example

At a petrochemical plant, an engineer noticed a temperature loop oscillating heavily.
The installed valve was linear trim, but the process required a slow, smooth control because of thermal lag.
Replacing it with an equal percentage trim stabilized the temperature and improved product quality.

Moral of the story:

✅ The right valve characteristic can make a bad loop good — and a good loop perfect.

🔹 Equal Percentage Valves — Why They’re So Popular

Most industrial loops (especially pressure and temperature) prefer equal percentage valves because:

Small changes at low opening → small flow change
Large changes at high opening → big flow change

This compensates automatically for system non-linearities and gives more uniform control across the valve’s travel.

🔹 Typical Valve Characteristics in Formula Form

Type	Formula (Normalized Flow)
Linear	$q = C \times x$
Equal Percentage	$q = C \times (R^{x} - 1) / (R - 1)$
Quick Opening	$q = C \times (1 - e^{-kx})$

Where:

$q$ = Flow fraction (0 to 1)
$x$ = Valve travel fraction (0 to 1)
$R$ = Rangeability (typically 50–100)
$k$ = Constant depending on valve design

(Don’t worry — these formulas are mainly for sizing software, not field use.)

🔹 Common Mistakes in Valve Characteristic Selection

Mistake	Effect	Recommendation
Choosing linear valve for temperature loop	Oscillation and overshoot	Use equal percentage
Using equal percentage for level control	Slow response	Use linear
Ignoring system pressure losses	Poor installed characteristic	Simulate or estimate system curve
Using quick opening for throttling	Unstable flow	Use only for on/off service

🔹 Key Technical Terms Explained

Term	Meaning
Inherent Characteristic	Valve’s flow behavior under constant ΔP (manufacturer’s curve).
Installed Characteristic	Actual flow behavior in process due to varying pressure losses.
Rangeability	Ratio of maximum to minimum controllable flow (typically 50:1 or 100:1).
Equal Percentage	Each increment in stem travel causes equal percentage change in flow.
Quick Opening	Provides large flow with small movement — best for on/off applications.

🔹 SEO Keywords (integrated naturally)

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🔹 Conclusion

Understanding control valve characteristics is crucial for every instrument and control engineer.
Never select a valve based only on size or pressure rating — the flow characteristic defines how your loop behaves in operation.

To summarize:

Inherent characteristic → manufacturer’s curve under constant ΔP.
Installed characteristic → actual performance in your process.
Linear for level/flow, Equal Percentage for pressure/temperature, Quick Opening for on/off service.

By matching valve characteristics to process behavior, you ensure smooth control, longer valve life, and stable operation — the hallmarks of a well-designed control system.