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Valve Actuator Sizing Check - Force Balance, Air Pressure & Spring Range Verification

Verify that the actuator can open, close, and hold the valve against process forces at available air supply pressure

Verify control valve actuator sizing by checking the force balance between actuator output, spring force, process thrust, packing friction, and seating force. Enter valve body size, trim type, shutoff class, process pressures, actuator model, and available air supply to determine whether the actuator can reliably open, close, and throttle the valve under all operating conditions. Flags undersized actuators that may fail to seat, fail to open, or lose controllability at high differential pressure.

Pro Tip: The most dangerous actuator sizing mistake is ignoring the worst-case process condition. An actuator sized for normal operating pressure may not have enough force to close the valve during a process upset when the differential pressure doubles. Always size the actuator for the maximum shutoff differential pressure, not the normal operating differential. Add a 25% force margin above the calculated requirement to account for friction increases from packing degradation and spring relaxation over the service life.

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Valve Actuator Sizing Check
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How It Works

  1. Enter Valve Data

    Input the valve body size, trim type (globe, ball, butterfly), plug/disc area, seat diameter, shutoff class (ANSI Class II through VI), and stem/shaft diameter. These determine the process forces and seating force requirements.

  2. Enter Process Conditions

    Input the upstream pressure (P1), downstream pressure (P2), and maximum shutoff differential pressure. The calculator determines the unbalanced force on the plug or disc that the actuator must overcome to move and seat the valve.

  3. Select Actuator Type

    Choose the actuator: spring-diaphragm (fail-close or fail-open), pneumatic piston (spring-return or double-acting), or manual override. Enter the diaphragm or piston effective area, spring range, and bench set pressures from the actuator nameplate.

  4. Set Available Air Supply

    Enter the air supply pressure available at the actuator. The calculator determines actuator force at minimum and maximum air pressure and checks against the required forces for opening, closing, and throttling at all operating conditions.

  5. Review Force Balance

    The force balance summary shows actuator force available vs required at each critical condition: breakout from seat, mid-stroke throttling, and seating at shutoff. Margin percentages below 25% are flagged as marginal, below 10% as undersized.

Built For

  • Instrument engineers verifying actuator selections on valve specification sheets
  • Maintenance engineers diagnosing valves that fail to fully close or seat under process pressure
  • Reliability engineers evaluating actuator capability after process condition changes or uprates
  • Safety engineers verifying ESD valve actuator sizing for fail-safe operation at maximum shutoff pressure
  • Controls engineers troubleshooting valves with poor controllability at high differential pressures
  • Project engineers reviewing vendor valve-actuator packages for compliance with design specifications

Features & Capabilities

Force Balance Analysis

Calculates all forces acting on the valve stem: actuator thrust, spring force (varies with travel), process unbalanced force, packing friction, and seat load. Shows the net force margin at every critical position.

Multiple Actuator Types

Handles spring-diaphragm (direct and reverse acting), pneumatic piston (single and double-acting), and scotch yoke actuators. Each type has different force-vs-travel characteristics that affect the analysis.

Shutoff Class Verification

Verifies that the actuator provides sufficient seat load for the specified shutoff class per ANSI/FCI 70-2. Class IV requires metal-to-metal seat load, Class V requires higher load, and Class VI (soft seat) has specific seat force requirements.

Fail-Safe Verification

For spring-return actuators, verifies that the spring alone can drive the valve to the fail-safe position against maximum process forces when air supply is lost. This is the critical safety check for ESD and fail-safe applications.

Air Pressure Sensitivity

Shows how actuator performance changes across a range of supply pressures. Identifies the minimum air pressure required for reliable operation and flags installations where supply pressure variations could cause intermittent problems.

Frequently Asked Questions

The required actuator force equals the sum of all opposing forces: process unbalanced force (differential pressure times plug area for unbalanced trims), packing friction (depends on packing type, stem diameter, and bolt torque), seat load (force needed to achieve the specified shutoff class), and dynamic forces from flow. For a fail-close valve, the spring must overcome all these forces at maximum process pressure without air supply. For the air-opening direction, the air pressure must overcome spring force plus packing friction. Always use the worst-case process condition, not the normal operating point.
The bench set (or bench range) is the air pressure range required to stroke the actuator from fully closed to fully open when the valve is on the bench with no process forces. For a fail-close (air-to-open) spring-diaphragm actuator, a bench set of 5-13 PSI means the valve begins to open at 5 PSI and is fully open at 13 PSI applied to the diaphragm. The bench set is determined by the spring rate and preload. Under process conditions, the effective range shifts because process forces assist or oppose the actuator. The bench set must be compatible with the positioner output range to maintain controllability.
An undersized actuator exhibits several failure modes. The valve may fail to fully seat, allowing leakage that exceeds the shutoff class rating. It may stick at high differential pressures because the actuator cannot overcome the sum of process force and packing friction. Controllability degrades because the actuator has insufficient force margin to make precise position changes against varying process forces. In the worst case, a fail-safe actuator with inadequate spring force may not drive the valve to the safe position during an emergency, creating a safety system failure.
Packing friction is the single largest variable in actuator force calculations. New PTFE packing on a 3-inch globe valve may contribute 100-300 lbs of friction. Graphite packing on the same valve can contribute 300-800 lbs, and over-tightened graphite packing can exceed 1,500 lbs. Live-loaded packing maintains consistent but elevated friction. Packing friction also increases with temperature, age, and exposure to certain process fluids. Conservative actuator sizing uses the maximum expected packing friction, not the nominal value. Under-estimating packing friction is the leading cause of actuator undersizing.
A minimum force margin of 25% above the calculated required force is standard practice for industrial control valves. This margin accounts for packing friction increases between maintenance intervals, spring relaxation over time, air supply pressure variations, and process pressure variations not captured in the sizing data. For safety-critical (SIL-rated) applications, many companies specify 50% margin or higher. The cost of a slightly larger actuator is trivial compared to the cost of a valve that cannot close during an emergency or a maintenance call to re-pack a valve that has lost controllability.
Disclaimer: This tool provides actuator force balance estimates for reference purposes. Actual actuator sizing must account for all process conditions, safety requirements, and environmental factors. Critical and safety-instrumented valve applications require engineering review per IEC 61511 and manufacturer verification. ToolGrit is not responsible for actuator sizing, valve performance, or safety system outcomes.

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