- Actuator undersizing is one of the most common causes of valve failure in the field — a valve that cannot fully close against process conditions doesn’t provide isolation, and a valve that cannot fully open creates unnecessary pressure drop and reduced capacity; both result from the same error: not calculating the required operating torque or thrust correctly.
- Torque requirements for rotary valves and thrust requirements for linear valves have distinctly different calculation methods — confusing them or using inappropriate simplified assumptions produces wrong answers; the correct calculation must account for differential pressure, fluid density, stem friction, and packing friction simultaneously.
- Safety margins in actuator sizing are not arbitrary — the standard 25% minimum margin on the valve’s maximum operating torque/thrust accounts for manufacturing tolerances, wear over time, and deviations from nominal operating conditions; reducing this margin to save on actuator cost creates reliability risks that invariably cost more to resolve.
Actuator sizing is one of those calculations that looks simple and isn’t. The valve manufacturer’s torque data sheet gives you a single number, you multiply by 1.25, and you pick an actuator. That’s the simplified version — and it’s adequate for many straightforward applications. But for anything with high differential pressure, high-density fluids, demanding packing specifications, or safety-critical service, the calculation needs to be done properly. Here’s how.
Linear Valve Thrust (Globe, Gate, Diaphragm Valves)
For linear valves where the stem moves in a straight line, the required actuator output is expressed as thrust (force in Newtons or lbf). The total required thrust is the sum of four components:
1. Seating/Unseating Force
For metal-seated globe valves, a specific seating force is needed to achieve the required seat leakage class. The seating load creates the contact stress at the seating line that deforms the seat material slightly, creating a tight seal. The required seating load is defined by the seat leakage class (ANSI/FCI 70-2) and the valve manufacturer’s data — typically expressed as N/mm of seat circumference or total load in N for the specific valve size and trim design.
For soft-seated valves (PTFE seats), the seating load required to achieve Class VI shutoff is lower — the PTFE deforms elastically around any seat surface imperfections. Standard actuator sizing for soft-seated valves includes the manufacturer’s recommended seating load plus friction margins.
2. Differential Pressure Unbalanced Force
When the valve is closed and there’s differential pressure across it, the fluid pressure acts on the plug or disc cross-sectional area, creating an unbalanced force that the actuator must overcome during opening. For a simple globe valve plug with no pressure balancing:
F_DP = ΔP × A_plug
Where ΔP is the differential pressure across the closed valve and A_plug is the cross-sectional area of the plug (approximately π/4 × d²_plug). For a 4″ globe valve with 75mm plug diameter at 100 bar ΔP: F_DP = 100 × 10⁵ × π/4 × 0.075² = 10 MPa × 0.00442 m² = 44,200 N (44.2 kN). This is a very large force that requires a substantial actuator.
This is why cage-guided globe valves use balanced plug designs — holes drilled through the plug allow downstream pressure to act on the top of the plug as well as the bottom, largely cancelling the unbalanced pressure force. A balanced plug reduces the unbalanced force to approximately 5–10% of the unbalanced value, dramatically reducing actuator sizing requirements.
3. Packing Friction
The packing creates friction against the valve stem that must be overcome by the actuator during stem movement. Packing friction depends on: packing material (PTFE = low friction, graphite = moderate, live-loaded high-performance = depends on design); gland bolt load (higher load = more friction); number of packing rings; stem surface finish (rougher stem = more friction); and stem diameter.
Packing friction for PTFE packing: approximately 0.1 × packing gland load. For graphite packing with higher gland load: approximately 0.15–0.25 × gland load. The gland load is calculated from the required packing stress to achieve fugitive emissions performance × packing cross-section area. Most valve manufacturers publish nominal packing friction values for standard packing configurations.
4. Spring Load (for Spring-Return Actuators)
For spring-return actuators, the spring provides the fail-safe force. At the end of stroke in the spring direction, the air must work against maximum spring load. At the end of stroke in the air direction (opening against spring), air must overcome maximum spring compression. The actuator force output must exceed this spring force plus all friction and unbalanced forces at minimum supply pressure.
Rotary Valve Torque (Ball, Butterfly, Plug Valves)
For rotary valves, the required actuator output is expressed as torque (N·m or lbf·in). The total required torque is the sum of:
1. Breakaway Torque
The torque required to unseat the closure element from the fully closed (or fully open) position. This is typically the highest torque requirement in the valve stroke. For ball valves, breakaway torque must overcome both seat contact pressure and stem seal friction simultaneously. For butterfly valves with resilient seats, the seat compression force at full closure must be overcome. Breakaway torque depends significantly on differential pressure (which adds seat contact force for floating ball valves) and on how long the valve has been closed (seated soft-seat valves can cold-flow slightly under sustained load, increasing breakaway torque above initial assembly values).
2. Running Torque
The torque required to rotate the valve through its stroke between fully open and fully closed. For ball and plug valves, running torque is primarily stem seal friction. For butterfly valves with resilient seats, the seat is in contact throughout the stroke and running torque includes seat friction in addition to stem seal friction. For triple-offset butterfly valves, running torque is lower than concentric designs because the disc lifts clear of the seat during most of the stroke.
3. Seating Torque
The torque required to achieve the specified shutoff class at the fully closed position. For metal-seated ball and butterfly valves, a defined seating force is needed — this translates to a torque requirement through the geometry of the seating cone or ball-seat interface.
4. Hydrodynamic (Dynamic) Torque
When the valve is partially open, the flowing fluid creates aerodynamic or hydrodynamic forces on the closure element. For butterfly valve discs and ball valves, this dynamic torque can be significant — particularly in the 40–70% open range where the disc or ball presents a significant projected area to the flow while remaining partially blocking.
Dynamic torque can act in either the opening or closing direction depending on valve position. For butterfly valves, dynamic torque is closing (flow tends to push the disc toward closed) below approximately 70% open, and opening (flow tends to push the disc toward open) above 70%. This is important for actuator sizing — if the actuator must control at 60% open in high-velocity service, it must overcome dynamic closing torque from the fluid flow simultaneously with its modulating function.
Required Actuator Output vs. Valve Requirement
The fundamental sizing criterion: Actuator output (at minimum supply pressure) ≥ Maximum valve requirement × Safety factor
The safety factor accounts for: manufacturing tolerance on valve torque data (±15–20%), variation in supply pressure (actuator sized at minimum supply, not nominal), wear over service life (packing friction increases), and installation variables. Standard minimum safety factor: 1.25 (25%) for standard applications; 1.5 (50%) for safety-critical or difficult-service applications.
For pneumatic actuators, “at minimum supply pressure” is critical — the actuator air spring operates at lower pressure at minimum supply. Manufacturers publish torque/thrust vs. supply pressure curves; use these at the minimum supply pressure, not the nominal.
Worked Example: Ball Valve Actuator Sizing
Trunnion ball valve, NPS 6″, Class 300, full bore, metal seats, natural gas service, P1 = 50 barg, P2 = 3 barg (upstream isolating valve). Maximum seat contact force from manufacturer’s data: 45 kN. Stem diameter 40mm, PTFE + graphite stem seal assembly. Valve breakaway torque from manufacturer’s data sheet: 950 N·m at 50 bar DP. Running torque: 650 N·m. Seating torque: 1,100 N·m. Dynamic torque at maximum flow: 320 N·m (closing).
Maximum valve torque = 1,100 N·m (seating torque governs over breakaway). Required actuator output = 1,100 × 1.35 (35% margin for gas service) = 1,485 N·m at minimum supply pressure. Check available actuator: a Rotork IQ3 or equivalent scotch-yoke actuator rated at 2,000 N·m at 5.5 barg supply would provide 1,667 N·m at 4.5 barg minimum supply — adequate with 12% additional margin.