Scotch Yoke Pneumatic Piston Actuators

Purchase Guide

Who this is for

Engineers and procurement teams specifying a scotch-yoke pneumatic actuator for quarter-turn valve automation — particularly where a heavy-duty scotch-yoke unit is oversized for the application, or where a scotch-yoke design is specifically required.

Step 1 — Get the valve torque requirement

Obtain the maximum required torque (MRT) from the valve manufacturer. For ball valves and butterfly valves, peak demand occurs at breakaway (0°) and end of stroke (90°) — not mid-stroke.

If the valve supplier provides only a single torque value, treat it as the breakaway figure. Apply a safety factor of 1.25–1.5 before actuator selection to account for pressure variation, temperature, and seat wear over service life.

If the valve supplier provides only new-condition torque, add a further aging factor of 1.2–1.3 on top — seat wear and media deposits increase required torque over time.

Step 2 — Confirm supply pressure at the actuator port

HPAC(B) torque tables are published at 3, 4, 4.5, 5, 5.5, and 6 bar. Use the minimum guaranteed pressure at the actuator port — not the upstream line pressure. Pressure drop across solenoid valves, tubing, and fittings is commonly 0.5–1 bar. If supply stability is uncertain, size at one pressure step below the nominal figure.

Step 3 — Choose acting type

Double acting (DA): both open and close strokes are powered by air. Suitable when no fail-safe position is required, or fail-safe is managed at the system level.

Spring return (SR): one stroke is powered by air, the other by the internal spring. Required when a defined fail-safe position (fail-close or fail-open) is needed on loss of air supply — including SIL 3 safety instrumented systems and ATEX installations.

For SR models, verify that the spring torque output at the fail-safe position exceeds the valve’s required torque at that angle with margin. Also check the spring output at mid-stroke (45°) — if the valve running torque exceeds spring output here, the actuator will stall before completing the stroke.

Step 4 — Select the model from the torque table

At the supply pressure confirmed in Step 2:

1. Find the row where Close torque at 0° ≥ valve MRT × safety factor
2. Confirm Open torque at 0° also meets requirement (DA close and open torques differ)
3. For SR: confirm Spring torque (FC or FO column) ≥ valve MRT at the fail-safe angle
4. Check torque at 90° — seating and unseating loads apply here too

If the required torque falls between two model sizes, select the larger.

Quick reference — DA, 5 bar, Close torque at 0°:

Full torque tables at 3–6 bar for all sizes and SR configurations are in the product datasheet.

Step 5 — Confirm mounting and environmental requirements

Mounting interface: all HPAC(B) spring return air actuator models use ISO 5211 bottom flange and NAMUR side port. Verify the valve’s ISO 5211 pad size matches the actuator flange designation (F03–F16 depending on model), and that shaft bore dimensions match the valve stem.

Seals: standard is NBR. Specify EPDM for steam or hot water service; FKM for hydrocarbon or chemical exposure.

Temperature: standard build is −20°C to +80°C. Specify low-temperature build for −40°C to +80°C service.

Certifications: ATEX / Ex for hazardous area installations; SIL 3 applies to SR configuration in safety instrumented systems.

FAQ

Q: What is the cycle life of the HPAC(B) Scotch Yoke Pneumatic Piston Actuators?

The actuator has passed a seal leakage test of 1,300,000 cycles under laboratory conditions with no leakage recorded.

Q: What is the visual difference between single acting and double acting models?

Single acting (spring return) scotch yoke pneumatic piston actuators  models have one longer end cover than double acting models. The extended end cover houses the return spring.

Q: Can the spring return air actuator end cover color be customized?

Yes, custom colors are supported. Confirm the requirement at order.

Q: Are custom labels available on standard models?

Yes, custom labels are available across the full Hearken Flow actuator range including standard HPAC(B) models.

Q: Can the spring return air actuator port color be changed?

Yes.

Q: Can the rotation angle be set to something other than 90°?

Yes. The range is adjustable to 90° ±10°, fixable at any angle within the full 100° travel.

Q: What seal materials are available?

Standard is NBR. EPDM and FKM are available across all sizes — specify at order.

Q: Does HPAC(B) support direct mounting of solenoid valves and limit switch boxes?

Yes. The NAMUR top interface accepts solenoid valves, limit switch boxes, and positioners without additional brackets. Pneumatic connections are PT1/4.