HPAC Series Compact Scotch Yoke Pneumatic Actuators for Ball & Butterfly Valves

Operating Principles of HPAC Series Double-Acting Scotch Yoke Pneumatic Actuator

The HPAC double-acting Scotch yoke pneumatic actuators use compressed air to power both the opening and closing strokes.

• DA Open (Counterclockwise Rotation): When air pressure is supplied to Port A, the pistons move inward, and the output shaft rotates counterclockwise to open the valve. Air comes out through Port B.

• DA Close (Clockwise Rotation): When air pressure is supplied to Port B, the pistons move outward, and the output shaft rotates clockwise to close the valve. Air comes out through Port A.

• Loss of Air (Fail State): Fail-in-Place (Stay-Put). There are no reset return springs inside the actuator. The valve will stay in its current position unless an external pneumatic circuit or a pneumatic lock valve is used to change the position.

Output Torque of Double Acting Actuators

The torques shown in the diagram are measured values. The required size of the actuator can be established by comparing the values of the actuator with the required torque of the valve. The suggested safety factor for double-acting actuators under normal working conditions is 20%-50%.

Operating Principles of HPAC Series Spring Return Scotch Yoke Pneumatic Actuator

The HPAC single-acting scotch yoke actuator uses compressed air to drive in one direction, which compresses the internal modular spring. When the air supply pressure fails, the mechanical spring energy automatically drives the actuator to a preset safe position.

Fail-Close (FC) — Air-to-Open / Spring-to-Close

• Air Supply (Valve Opening): When air pressure is supplied to Port B, the pistons move outward and compress the internal springs. The output shaft rotates counterclockwise to open the valve. Air from the spring chamber comes out through Port A.

• Loss of Air (Valve Closing): When air pressure is lost or exhausted, the compressed springs push the pistons inward. The output shaft rotates clockwise to close the valve.

Fail-Open (FO) — Air-to-Close / Spring-to-Open

• Air Supply (Valve Closing): When air pressure is supplied to Port B, the pistons move outward and compress the internal springs. The output shaft rotates clockwise to close the valve. Air from the spring chamber comes out through Port A.

• Loss of Air (Spring Opens): When air pressure is lost or exhausted, the compressed springs push the pistons inward. The output shaft rotates counterclockwise to open the valve.

Output Torque of Single-Acting Actuators

How to Select the Right HPAC Series Compact Scotch Yoke Pneumatic Actuator

Selecting the right HPAC series actuator size requires balancing process safety, available compressed air supply, mechanical mounting standards, and the specific dynamic torque requirements of quarter-turn equipment. Hearken Flow follows this general sizing approach to ensure reliable valve seat sealing, avoid sticking throughout the full stroke, and optimize the overall package size.

Step 1: Sure Action Mode And Fail-Safe Requirement

• Double-Acting (HPAC-DA):

  • Powered by compressed air for both opening and closing.

  • If the air or power supply fails, the actuator stays in its last position (fail-in-place / stays in position).

  • Recommended for: Continuous modulation or throttling control (with an electro-pneumatic positioner), flow balancing, and non-critical process or utility lines.

• Single-Acting Spring Return (HPAC-SR):

  • Powered by compressed air in one direction to compress the internal modular spring assembly.

  • The spring energy automatically returns the quarter-turn valve or device to the preset safe position when the air supply fails:

    • Fail-Close (FC / Air-to-Open): Air supply opens the valve. The mechanical springs automatically close the valve when the air supply is lost. Recommended for emergency shutdown and flammable line isolation.

    • Fail-Open (FO / Air-to-Close): Air supply closes the valve. The mechanical springs automatically open the valve when the air supply is lost. Recommended for emergency venting, exhaust valves, and thermal relief.

Step 2: Calculate Safety Margin and the Required Torque

We always calculate the required target torque by applying the appropriate safety factors for the process media and operating environment to the rated torque provided by the valve or equipment manufacturer.

• Clean fluids and air (water, lubricating oil, clean gas, light-duty dampers): Multiply the rated torque by 1.20 to 1.25 (20% to 25% safety margin).

• Dry gas, steam, and non-lubricating media: Multiply the rated torque by 1.30 to 1.35 (30% to 35% safety margin).

• Slurries, viscous media, particles, and heavy-duty industrial dampers: Multiply the rated torque by 1.40 to 1.50 (40% to 50% safety margin).

• Ball Valve Standard (Hearken Flow Sizing Rule): Due to the high seat breakaway friction inherent to ball valve designs, always apply a minimum +50% safety margin (Multiplier: 1.50) for all ball valves, regardless of the media type. 

Next, match the safety-adjusted torque with the HPAC dynamic output torque table based on the quarter-turn resistance curve:

Type A: Valve with Higher End Torque (Butterfly Valves, Ball Valves, Segment Valves) 

Suitable for valves where resistance peaks at the seat position (0°) and End of Stroke (90°) due to seat interference and pipeline pressure differences.

Butterfly Valves

• Double-Acting (DA):

  • Check the air supply torque at 0° (BTO – Break to Open) is ≥ the target safety torque.

• Single-Acting Fail-Close (SR-FC):

  • Check the spring output torque at 0° (ETC – End to Close) is ≥ the target safety torque.

  • Check the air supply torque at 0° (BTO – Break to Open) is ≥ the target safety torque.

• Single-Acting Fail-Open (SA-FO):

  • Check the air supply torque at 90° (ETC – End to Close) is ≥ the target safety torque.

  • Check the spring output torque at 90° (BTO – Break to Open) is ≥ the target safety torque.

Ball Valves & Segmented Ball Valves (Minimum +50% Safety Factor Applied)

• Double-Acting (DA):

  • Check the air supply torque at 90° (BTC – Break to Close) is ≥ the target safety torque.

• Single-Acting Fail-Close (SR-FC):

  • Check the spring output torque at 0° (ETC – End to Close) is ≥ the target safety torque.

  • Check the air supply torque at 0° (BTO – Break to Open) is ≥ the target safety torque.

  • Check the air supply torque at 90° (ETO – End to Open) is ≥ 70% of the target safety torque.

• Single-Acting Fail-Open (SR-FO):

  • Check the spring output torque at 90° (BTO – Break to Open) is ≥ the target safety torque.

  • Check the air supply torque at 90° (ETC – End to Close) is ≥ the target safety torque.

  • Check the spring output torque at 0° (ETO – End to Open) is ≥ 70% of the target safety torque.

Type B: Continuous Torque Throughout the Full Stroke (Plug Valves, Dampers, Louvers, Diverter Gates) 

Suitable for quarter-turn equipment where mechanical friction, packing resistance, or airflow resistance is present throughout the full 90° stroke. In addition to the 0° and 90° end positions, verify the 45° mid-stroke running torque. The 45° running torque must be ≥ 75% of the equipment's target safety torque to prevent stalling during mid-travel.

Plug Valves & Heavy Dampers

• Double-Acting (DA):

  • Check the air supply torque at 0° (BTO) and 90° (BTC) is ≥ the target safety torque.

  • Check the air supply torque at 45° (RTO / RTC – Running Torque) is ≥ 75% of the target safety torque.

• Single-Acting Fail-Close (SR-FC / Air to Open, Spring to Close):

  • Pneumatic Stroke (Opening): Check the air supply torque at 90° (ETO – End to Open) is ≥ the target safety torque, AND the air supply torque at 45° (RTO – Running Torque to Open) is ≥ 75% of the target safety torque.

  • Spring Stroke (Closing): Check the spring output torque at 0° (ETC – End to Close) is ≥ the target safety torque, AND the spring output torque at 45° (RTC – Running Torque to Close) is ≥ 75% of the target safety torque.

• Single-Acting Fail-Open (SR-FO / Air to Close, Spring to Open):

  • Pneumatic Stroke (Closing): Check the air supply torque at 90° (ETC – End to Close) is ≥ the target safety torque, AND the air supply torque at 45° (RTC – Running Torque to Close) is ≥ 75% of the target safety torque.

  • Spring Stroke (Opening): Check the spring output torque at 0° (ETO – End to Open) is ≥ the target safety torque, AND the spring output torque at 45° (RTO – Spring Running Torque to Open) is ≥ 75% of the target safety torque.

Quarter-Turn Equipment Sizing Reference Table (at 5.0 bar Air Supply)

Step 3: Verify Mounting and Drive Shaft Interface

• Valve Mounting Flanges (ISO Direct Mount and Custom Brackets): The bottom mounting pad meets ISO 5211 / DIN 3337 standards, with sizes from F03 to F25; our HPAC actuator can be mounted directly on valves and dampers without custom brackets. For non-standard or older valves without an ISO mounting pad, or for high-temperature applications that require thermal insulation, custom mounting brackets and machined drive couplings (adapters) can be used for easy actuator installation.

• Drive Shaft Connection: Confirm the valve stem uses a double-square drive (9 × 9 mm to 46 × 46 mm) or a keyed round drive. Our HPAC double-square output shaft supports both parallel (0°) and diagonal (45°) valve stem positions and can easily connect to custom-machined valve stem couplings.

• Top Automation Interface: The top shaft and accessory mounting dimensions fully comply with VDI/VDE 3845 (NAMUR) standards, enabling quick, direct installation of limit switch boxes, pneumatic positioners, and visual dome indicators.

Step 4: Select Operating Temperature and Manual Override Options

• Standard (ST): -20°C to +80°C (Standard NBR seals, synthetic grease) — General industrial plants, water treatment, HVAC, utility pipelines.

• Low-Temp (LT): -50°C to +60°C (Low-temp silicone rubber seals, Arctic grease) — Sub-zero climates, Arctic oil & gas pipelines, cryogenic refrigeration plants.

• High-Temp (HT): -15°C to +150°C (FKM fluororubber seals, high-temp grease) — Steam headers, thermal power plants, furnace exhaust lines, high-temperature flue dampers.

• Manual Override Selection:

  • Standard: Direct drive without manual override.

  • HW Series (Integrated Side Handwheel): Select HPAC-HW when overhead vertical clearance is constrained, pipeline/damper weight must be minimized, or valve stem side-loading must be eliminated.

HPAC Series Model Number Ordering Code Structure

[Series] - [Size] - [Action Mode] - [Spring Set] - [Manual Override] - [Temp Build]

• 1. Series: HPAC (Compact Scotch Yoke Pneumatic Actuator)

• 2. Cylinder Size: 040 | 055 | 070 | 090 | 115 | 140 | 165 | 180 | 205 | 220 | 260

• 3. Action Mode: DA (Double Acting) | SR-FC (Single Acting Fail-Close) | SR-FO (Single Acting Fail-Open)

• 4. Manual Override: Blank (Standard drive) | HW (Integrated inline side handwheel)

• 5. Temperature: Blank (Standard -20°C to +80°C) | LT (Low-Temp -50°C to +60°C) | HT (High-Temp -15°C to +150°C)

Ordering Example: HPAC090SR-FC-K22-HW-LT specifies an HPAC Size 090 Single-Acting actuator, Fail-Close mode, balanced K22 spring set for 5.0 bar air supply, integrated side handwheel, and low-temperature (-50°C).

FAQs for the HPAC Series Compact Scotch Yoke Pneumatic Actuator

Q1: If Scotch Yoke actuators cost more than standard Rack and Pinion actuators, why should I choose Scotch Yoke?

A: It is true that Scotch Yoke actuators have a higher unit cost because they require more complex machining and use hardened alloy steel components. However, our HPAC compact Scotch Yoke pneumatic actuator's real advantage is the total cost of ownership (TCO).

The torque characteristics of a Scotch Yoke mechanism closely match the “U-shaped” torque curve of quarter-turn valves. This means a smaller HPAC actuator can often provide the same valve seat sealing force.

The more compact HPAC actuator size can reduce overhung load on the valve and piping, lower shipping weight, and reduce unnecessary side loads during the stroke. This can also help extend valve service life.

Q2: As a distributor, my customers have different air supply pressures, such as 4.0, 5.0, and 6.0 bar, and may require both Double-Acting (DA) and Single-Acting (SR) modes. How does the HPAC Series help me optimize my inventory?

A: The HPAC Series uses a common actuator housing design that can greatly reduce the number of SKUs you need to manage for different pressure requirements or operating modes.

• Single-Acting (SR) (DA-to-SR Conversion): You only need to stock standard Double-Acting (DA) actuators. If you require a Single-Acting (SR) fail-safe actuator for a specific air supply pressure, you do not need to stock a separate SR actuator. You can simply remove the DA end caps, install the modular spring set (K11 to K33) matched to the site pressure, and install the extended SR end caps.

• Double-Acting (DA): A single HPAC DA actuator can adapt to different air supply pressures. During sizing, you only need to calculate based on the minimum guaranteed pressure, such as 4.0 bar, to ensure reliable valve sealing. You also need to confirm that the peak output torque at the maximum pressure, such as 6.0 bar, does not exceed the valve's Maximum Allowable Stem Torque (MAST).

This modular solution lets you handle a wide range of operating conditions while reducing inventory capital tied up in stock.

Q3: Does the HPAC actuator require lubricated instrument air?

A: No. The HPAC Series is pre-lubricated with high-performance synthetic grease and is designed to operate smoothly with clean, dry, oil-free instrument air that meets ISO 8573-1 Class 3 or higher.

There is no need to install a line lubricator. In fact, we do not suggest using one, because unapproved lubricants may gradually wash out the pre-applied permanent grease over time.

Q4: Can the HPAC actuator be installed vertically or upside-down on the pipeline? 

A: Yes. The HPAC Series uses a multi-point polymer guide system and a robust rolling-friction mechanism to keep the pistons and scotch yoke properly aligned, regardless of the installation angle.

The HPAC actuator can be installed horizontally, vertically, or upside down without causing a loss of output torque, increased wear, or piston tilting.

Q5: What is the expected cycle life and maintenance requirement for the HPAC series? 

A: The HPAC Series uses a hardened alloy steel rolling-friction mechanism (hardness >50 HRC) and ultra-high-molecular-weight polymer guide pads; internal wear is minimized.

Tested cycle life can reach 1 million to 3 million cycles, depending on the operating conditions. Routine maintenance requirements are minimal, and our independent shaft seal design allows the O-rings to be replaced quickly without completely disassembling the drive shaft.

Q6: How does the compact design of the HPAC series benefit my international shipping and logistics? 

A: For international OEMs and distributors, freight costs are a major consideration. Because the HPAC series removes unnecessary torque during the mid-stroke, its extruded aluminum alloy housing is more compact than constant-torque actuators of equivalent specifications. This can reduce total weight and packaging volume by up to 35%, resulting in significant direct savings on air freight and less-than-container-load (LCL) shipping costs.

Q7: We are a valve manufacturer/distributor. Do you offer OEM and custom painting services? 

A: Of course. We fully support our B2B partners by providing comprehensive OEM services. We can provide custom nameplates with your company logo, specific epoxy coating colors to match your brand identity or EPC project requirements (including custom RAL colors), and customized packaging solutions.

Q8: Can I control the opening and closing speed of the HPAC actuator? 

A: Yes. You can easily adjust the operating speed by installing pneumatic flow control valves (throttle valves/speed control valves) at the exhaust ports of the NAMUR solenoid valve or directly at Ports A and B of the actuator.

Q9: What is the standard warranty period? Are HPAC actuators suitable for Emergency Shutdown (ESD) systems?

A: We provide a standard 18-month warranty from the date of shipment.

For critical Emergency Shutdown (ESD) and Blowdown systems, we can offer the following two solutions:

Single-Acting (SR): This is the most common choice for ESD applications because the built-in mechanical springs automatically drive the valve to its safe position (fail-safe position) when the air supply is lost, without requiring backup power.

Double-Acting (DA): A HPAC DA actuator can also be fully integrated into an ESD system when equipped with an external air reservoir (volume tank) and a pneumatic trip valve. These components can provide the stored pneumatic energy required for the emergency stroke.

Q10: What should I do if the plant instrument air supply is lost but I still need to operate the valve?

A: Whether you choose a DA or SR actuator, you can add our integrated side-mounted handwheel for manual operation during a plant-wide power outage or loss of air supply.

This compact, built-in manual override is suitable for both Double Acting and Spring Return Actuators and avoids the need for a bulky sandwich-type declutch gearbox, allowing you to operate the valve safely and easily by hand.

Cross-Reference Guide: Direct Equivalent & Drop-In Alternative for Rotork®, Omal®, Actuatech®, Ebro®, and Kosaplus®

This guide offers the best cross-reference from the Rotork®, Omal®, Actuatech®, Ebro®, and Kosaplus® range of compact scotch-yoke actuators to the Hearken Flow equivalent HPAC series. You may need to add options to the Hearken Flow HPAC series (such as the HW-style manual override or specific temperature builds) to perfectly match your original actuator's configurations. If you need assistance with multi-brand model cross-referencing, please contact Hearken Flow at [email protected].

(Note: The output torque values below are based on Hearken Flow HPAC Double-Acting (DA) models at a standard 5.6 bar / 80 psi air supply. Always verify your valve's safety torque factor before finalizing the HPAC model selection.)

Rotork®, Omal®, Actuatech®, Ebro®, and Kosaplus® to Hearken Flow Cross-Reference Table (Double Acting)

⚠️ A Note on Single-Acting (Spring-Return) Equivalents

Are you looking to replace a Rotork® RC200-SR, Omal® SRNV, Actuatech® GS, Kosaplus® AS, or Ebro® EB-SYS series actuator?

While Double-Acting models can be cross-referenced using a straightforward torque output comparison at a given pressure, Spring-Return (Fail-Safe) actuators require a much more complex evaluation. Because the output involves a delicate balance of pneumatic force and mechanical spring energy, the correct replacement must meticulously match your specific Air Start/End and Spring Start/End torque requirements to prevent catastrophic valve failure during a power or air loss.

Instead of relying on generic sizing charts, send us your valve's torque and minimum site air pressure. We will calculate the exact modular spring configuration (K11 to K33) for you, ensuring 100% fail-safe reliability and MAST compliance.

Why the HPAC Series Wins the "Drop-In" Replacement Test:

• The Torque Advantage: Omal's DAN series and Actuatech's GD series model numbers directly indicate their breakaway torque at 5.6 bar (e.g., the DAN 60 outputs exactly 60 Nm). At the same 5.6 bar pressure, our equivalent HPAC060DA safely outputs 83 Nm, giving you a massive buffer against unexpected valve friction.

• Zero Mounting Hassle: No need for the transition plates. The HPAC series conforms to EN ISO 5211 / DIN 3337 valve interface standards, ensuring a direct bolt-on match for legacy European and Korean setups without modifying the valve stem.

• Universal Accessory Transfer: Upgrading the actuator doesn't mean replacing the accessories. With our standardized VDI/VDE 3845 (NAMUR) top and side interfaces, you can perfectly remount the limit switch boxes, E/P positioners, and solenoid valves.

Disclaimer: Rotork, Omal, Actuatech, Ebro, and Kosaplus are trademarks of their respective owners. Hearken Flow is an independent manufacturer. Competitor model numbers (e.g., RC200, DAN, GD, EB, AD) are used solely for cross-reference, dimensional comparison, and educational purposes.