Equal Percentage vs Linear Control Valve: Inherent Characteristic, Installed Gain, Rangeability and Selection

Equal-percentage and linear control valves differ in how Cv changes with travel, but neither characteristic is universally better. A linear inherent characteristic gives approximately proportional Cv change with travel, while an equal-percentage characteristic gives approximately equal percentage changes in existing Cv. The practical selection depends on what happens after installation: valve differential pressure, system resistance, required flow range, predicted travel and installed gain can reshape the response.

Equal Percentage vs Linear Control Valve: Quick Comparison

Five Questions to Answer Before Choosing the Characteristic

What Is an Inherent Control Valve Characteristic?

A control valve’s inherent flow characteristic describes the relationship between valve travel and flow capacity when differential pressure across the valve is maintained under defined conditions. ISA distinguishes this inherent characteristic from the installed characteristic, which reflects the actual pressure drop available to the valve in the complete system. ISA guidance on control-valve flow characteristics provides additional background on this distinction.

Linear Inherent Characteristic

With a linear inherent characteristic, equal increments of valve travel produce approximately equal increments in Cv. Under constant valve differential pressure, the relationship between travel and capacity is approximately proportional. This does not mean actual installed plant flow will remain proportional to travel after piping and process losses are included.

Equal-Percentage Inherent Characteristic

With an equal-percentage characteristic, equal increments of valve travel produce approximately equal percentage changes in the existing Cv. The absolute capacity change is smaller at lower capacity and becomes progressively larger as the valve opens.

Why Installed Characteristic Matters More Than the Datasheet Label

Once installed, a регулирующего клапана operates as one resistance element within a complete piping and process system. For a broader engineering framework covering characteristic selection, boundary conditions and project evidence, see Raymon Valve’s Flow Characteristic Selection Guide.

Why Valve Pressure-Drop Share Matters

A control valve has greater hydraulic influence when a larger share of available system pressure drop occurs across the valve. As piping and process equipment consume more available pressure at higher flow, less differential pressure may remain across the valve. Valve ΔP should therefore be reviewed at minimum, normal and maximum operating conditions rather than only at the maximum-flow case.

Why a Linear Valve May Not Produce Linear Installed Flow

Changing system resistance can alter the relationship between Cv and actual flow. A linear inherent characteristic can therefore produce a nonlinear installed characteristic if valve differential pressure changes through the operating envelope.

Why Equal Percentage Can Compensate for Falling Valve ΔP

Equal-percentage trim provides progressively larger absolute capacity changes as the valve opens. In suitable systems this can partially compensate for declining valve differential pressure, but it does not guarantee a linear installed flow response.

Installed Gain: The Link Between Travel and Control Response

Installed gain describes how strongly actual process flow changes in response to a change in valve position at a particular operating condition. ISA’s installed-gain discussion illustrates why gain should be reviewed across the required operating range rather than at only one design point.

Why High Installed Gain Can Be Difficult

Relatively high gain means that a small valve-position change can produce a comparatively large process-flow change, increasing sensitivity to friction, stiction, position error and loop tuning.

Why Very Low Installed Gain Also Matters

Very low installed gain can require substantial valve movement to create a useful flow change, consuming available travel and reducing control authority.

Rangeability Is Not the Same as Process Turndown

Rangeability is a valve characteristic under specified conditions, while usable process turndown is a system-level result. ISA75.11 addresses inherent control-valve flow characteristics and inherent rangeability; see the current ISA75.11 scope and publication information.

Usable process turndown can also be limited by valve sizing, pressure drop, actuator and positioner resolution, friction, deadband, hydraulic limits, instrumentation and process dynamics.

A Different Characteristic Cannot Correct These Problems

• incorrect Cv/Kv sizing;
• excessive valve size;
• unsuitable trim geometry;
• inadequate actuator or positioner resolution;
• excessive friction or deadband;
• cavitation or flashing;
• insufficient available valve ΔP;
• incorrect controller tuning;
• rapidly changing process gain.

When Does a Linear Characteristic Make Sense?

• valve differential pressure remains relatively stable over the required operating range;
• system resistance does not strongly distort installed response;
• approximately proportional Cv changes support the required control objective;
• minimum, normal and maximum operating points remain in a useful travel range;
• installed gain remains suitable across the operating envelope.

When Does an Equal-Percentage Characteristic Make Sense?

• valve differential pressure decreases substantially as flow increases;
• the required operating envelope is comparatively broad;
• smaller relative capacity changes are useful toward lower capacity;
• the inherent characteristic helps compensate for changing system pressure distribution;
• installed-flow and gain analysis supports the proposed characteristic.

Equal Percentage vs Linear Selection Matrix

Valve Sizing Should Come Before Final Characteristic Selection

A characteristic comparison requires confirmed minimum, normal and maximum operating cases, including flow, P1, P2, temperature, relevant fluid properties and calculated Cv/Kv. For a broader discussion of required sizing inputs and calculation review, see the Руководство по подбору регулирующего клапана.

Where liquid-service conditions create cavitation or flashing risk, also review Control Valve Cavitation vs Flashing. Characteristic preference is secondary if the proposed hydraulic condition already creates unacceptable cavitation, flashing, noise or choked-flow constraints.

Control Valve Characteristic Selection Workflow

1. Define the control objective. Identify the required process response.
2. Define minimum, normal and maximum operating cases.
3. Confirm P1, P2, temperature and fluid data.
4. Calculate required Cv or Kv.
5. Review valve differential pressure.
6. Compare candidate characteristics.
7. Check predicted valve travel.
8. Evaluate installed characteristic and installed gain.
9. Review cavitation, flashing, choked flow, noise or velocity where applicable.
10. Confirm the exact supplier datasheet and trim data.

What Should the Supplier Provide?

• sizing basis and calculation method;
• selected valve size and trim size;
• exact proposed trim designation;
• required Cv/Kv at minimum, normal and maximum flow;
• selected rated Cv/Kv;
• predicted travel at key operating cases;
• proposed inherent characteristic;
• Cv-versus-travel curve or data for the exact proposed trim where available;
• inlet and outlet pressure assumptions;
• calculated valve differential pressure;
• cavitation, flashing or noise review where required;
• actuator and positioner selection basis;
• technical deviations;
• calculation revision, date and assumptions used where relevant.

RFQ Data Needed Before Selecting Linear or Equal Percentage

• среда и фаза;
• минимальный, нормальный и максимальный расход;
• P1 and P2 for each operating case;
• operating temperature;
• relevant fluid properties;
• line size and connection;
• control objective;
• required operating range;
• shutoff requirement;
• fail action;
• noise or severe-service limits where applicable;
• available valve datasheet or existing supplier sizing sheet.

Normalize the Sizing Basis Before Comparing Price

Типичные ошибки при выборе

Assuming Equal Percentage Is Always Better

No characteristic is universally superior. Installed system behavior determines whether the proposed characteristic provides useful control response.

Assuming Linear Trim Means Linear Plant Flow

Linear describes the inherent valve characteristic under defined conditions. Changing system pressure losses can reshape installed flow response.

Choosing Characteristic Before Sizing

Characteristic selection cannot correct a valve that is fundamentally oversized or incorrectly sized.

Treating Catalog Rangeability as Plant Turndown

Usable process range is a system-level result affected by sizing, valve ΔP, actuation, instrumentation and process dynamics.

Checking Only Maximum Flow

Maximum flow confirms capacity, but minimum and normal operating cases often determine whether the valve provides useful controllability.

Ignoring Installed Gain

Two valves that both satisfy maximum Cv requirements can still behave very differently across the required operating envelope.

Final Selection Guidance

A linear characteristic may perform well where valve differential pressure remains comparatively stable and the installed system supports a suitable response. An equal-percentage characteristic may be worth evaluating where valve ΔP falls as flow increases or where a broader operating range must be controlled.

Neither characteristic can compensate for incorrect sizing, unsuitable trim, hydraulic limitations or incomplete process data. Final selection should be confirmed against the actual datasheet, sizing calculation, exact trim curve and project requirements.

Часто задаваемые вопросы

For broader product information, see Raymon Valve Control Valves. For company background, see О Raymon Valve.