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Diaphragm Pneumatic
Quick Specs Spring-return (single-acting) or double-acting 100–10,000 lbf (diaphragm area × supply pressure) 40–80 psi (3–6 bar) Linear, up to 4 inches (100mm) ISO 5211 (mounting interface) ISO 5211 flange to valve bonnet/yoke; direct mount on globe, gate, and control valves Nitrile (NBR) / EPDM / Viton (FKM) / stainless steel Multiple spring cartridges; range adjustable by cartridge selection; fail-safe closing or opening Why Diaphragm Actuators Run Control Valves Walk through any petrochemical unit or power plant control room — the valves throttling flow on the process lines are almost always diaphragm-actuated. Not by accident. A globe control valve moves its plug up and down. That's linear motion. The diaphragm actuator pushes the stem down (air pressure lands on top of the diaphragm, compresses the spring, drives the stem toward closed) and releases the stem up (air removed, spring pushes toward open). That push-pull is exactly what a globe valve stem needs. The diaphragm itself is the pressure-to-force converter. A flat or convoluted rubber sheet that deflects when air pushes on one side. The math is straightforward — a 100-square-inch diaphragm at 60 psi supply delivers 6,000 lbf thrust. Because the diaphragm area is large, you get equivalent thrust at lower supply pressure. No need to crank air up to 100+ psi like piston actuators demand. Standard instrument air at 40–80 psi does the job. That lower pressure requirement matters more than most engineers realize. Instrument air systems in older plants run at 60 psi header pressure with local regulators dropping to 40 psi at the valve. A diaphragm actuator works fine at that pressure. A piston actuator would stall. Spring-Return Means You Sleep Easy Fail-safe action is the other reason diaphragm actuators dominate process control. If air supply fails — compressor trips, header ruptures, supply line gets plugged — the spring drives the valve to its safe position. Closed for fail-close. Open for fail-open. The spring doesn't depend on air, electricity, or any external energy source. It's just stored mechanical force releasing. Gravity and spring are always available. On a 400-psig steam line, you want that control valve to close on air failure. The spring does it. No battery backup. No secondary air tank. Just a compressed spring inside the actuator housing pushing the stem to the safe position. That reliability is why diaphragm actuators sit on every critical control valve in petrochemical and power applications. Diaphragm Material Matches the Service Nitrile handles general oil and gas service. EPDM takes steam and hot water — common on boiler feedwater control valves. Viton resists chemical attack on acid and solvent lines. When the process fluid is aggressive enough that even Viton degrades, stainless steel diaphragms step in. Choose based on what's inside the valve body, not what looks cheapest in the catalog. Know the Stroke Limit Standard diaphragm actuators deliver up to 4 inches of linear stroke. Beyond that, the diaphragm deflects too far and loses effective area — thrust drops. Multi-stage diaphragm designs extend stroke length, but you trade thrust for travel. If your gate valve needs 6 inches of stem travel, you're looking at a piston cylinder, not a diaphragm actuator. FAQ A: Not directly. Diaphragm actuators produce linear thrust, not rotary torque. You'd need a bracket and linkage to convert linear motion to 90° rotation — and that conversion adds complexity and lost motion. For quarter-turn valves, use rack-and-pinion or scotch yoke actuators instead. A: The diaphragm deflects and its effective pressure area shrinks as stroke increases. A diaphragm at 4 inches of deflection has less working area than at 1 inch. Multi-stage designs compensate somewhat, but the tradeoff between stroke length and thrust output is fundamental to the diaphragm geometry. A: Change the spring cartridges. Different spring sets produce different fail-safe forces. The actuator housing holds multiple springs — you swap the spring set to match the seating force your valve requires. No need to change the entire actuator. A: 60 psi is the sweet spot for most installations. It gives adequate thrust at standard diaphragm sizes and matches typical instrument air header pressure. If your air system runs lower — 40 psi on older plants — specify the larger diaphragm size to compensate. A: On modulating control service. Diaphragm actuators have negligible seal friction, which means lower hysteresis and better position accuracy. The diaphragm flexes freely; a piston O-ring drags against the bore. That friction difference matters when you're throttling at 30% open and the positioner is making small adjustments.
Diaphragm Pneumatic Actuator with Handwheel
Quick Specs Spring-return (single-acting) or double-acting 100–10,000 lbf (diaphragm area × supply pressure) 40–80 psi (3–6 bar) Linear, up to 4 inches (100mm) ISO 5211 (mounting interface) ISO 5211 flange to valve bonnet/yoke; handwheel on side or top of actuator yoke Nitrile (NBR) / EPDM / Viton (FKM) / stainless steel Multiple spring cartridges; range adjustable by cartridge selection Declutchable manual override; clutch engages handwheel to stem when turned, disengages when actuator resumes pneumatic control The Handwheel Is Your Backup Plan Same diaphragm actuator. Same thrust output. Same spring-return fail-safe. The handwheel adds one thing — manual override when air isn't available. That sounds simple, and it is mechanically. But the operational scenarios where it matters are worth understanding. Plant startup. Compressed air systems take days to commission. Pressure fluctuations, system leaks, compressor problems — the instrument air header isn't stable yet. Operators need to position control valves manually until the air system settles. The handwheel lets them do that. Turn the wheel, the clutch engages, the valve stem responds. No air required. Air supply failure mid-run. The compressor trips. The air header ruptures. The supply line to a specific valve gets blocked. The spring-return drives the valve to fail-safe position — full open or full closed. That protects against process hazard. But what if the process can continue running at reduced rate during the outage? Full-open or full-closed might be too aggressive. The handwheel lets you set the valve at 30% open, keep production going at reduced throughput, and wait for the air system to come back. How the Declutchable Mechanism Works The handwheel doesn't fight the actuator during normal operation. That's the declutchable design. When pneumatic control is active, the clutch disengages the handwheel from the stem — the handwheel spins freely, and the stem responds only to actuator thrust. When you turn the handwheel, the clutch engages and the wheel drives the stem directly. When the actuator takes over again, the clutch slips back out. No manual disengagement required. The mechanism handles it automatically. That auto-disengage feature prevents a common field mistake — operators leaving the handwheel engaged after manual override, which causes the actuator to fight the handwheel position during normal operation. The declutchable design eliminates that problem. When to Specify the Handwheel Option Add the handwheel on control valves that serve critical process functions — feedwater control on boilers, reactor temperature control, main process line throttling valves. Any valve where continued operation at reduced capacity during air failure is preferable to full shutdown. Also on valves in remote locations where maintenance access to the air system takes hours — offshore platforms, remote well sites, distributed pipeline stations. The handwheel gives operators local control regardless of air supply status. The handwheel adds cost and weight. It takes up space on the yoke. On non-critical service valves where full shutdown on air failure is acceptable, skip it. But on valves that keep the plant running, the handwheel option is cheap insurance compared to a unplanned shutdown. FAQ A: Yes, when you engage it. The handwheel drives the stem to whatever position you set — including intermediate positions the spring can't reach. When you disengage the handwheel and air supply is still absent, the spring drives the valve back to fail-safe position. A: Mechanically yes, but you shouldn't. The actuator is controlling the valve position based on the process signal. Manually overriding that position defeats the control loop. Use the handwheel only during air supply outages or maintenance. A: Side-mount on the actuator yoke is standard — it keeps the overall height down and fits in tight vertical installations. Top-mount is available when side clearance is limited. The mounting location depends on valve type and pipe rack layout. A: It adds weight to the actuator assembly — the handwheel, clutch mechanism, and mechanical linkage. On small actuator sizes (diaphragm areas under 50 square inches), the weight addition is noticeable. On larger actuators, the handwheel weight is proportionally small. Check the total assembly weight against the valve body mounting capacity. A: Yes. Double-acting diaphragm actuators have no spring-return, so air failure leaves the valve in its last position. The handwheel provides manual positioning during air outages — same function, same declutchable mechanism. The handwheel is arguably more important on double-acting actuators because there's no spring fail-safe to rely on.
Scotch Yoke Pneumatic Actuator
Quick Specs Spring-return (single-acting) or double-acting 500–100,000 Nm across model sizes 40–120 psi (3–8 bar) 90° quarter-turn rotation ISO 5211 (mounting), NAMUR (solenoid mounting) Single cylinder, dual piston; scotch yoke mechanism converts linear piston motion to 90° rotary output Spring cartridges in cylinder (spring-return models); multiple spring sets available; or double-acting with no springs ISO 5211 flange direct to valve body or bracket; NAMUR interface for solenoid and accessories Variable Torque Is the Scotch Yoke's Real Advantage Rack-and-pinion actuators produce constant torque across the 90° stroke. Scotch yoke actuators produce variable torque — and that variability is the reason they exist. Quarter-turn valves need more torque to unseat the ball or disc than to keep it rotating. The breakout torque at the seated position exceeds the running torque at mid-stroke by 30–50% on typical butterfly and ball valves. On large, high-pressure valves, that differential gets bigger. A 24-inch Class 300 ball valve might need 15,000 Nm to break out of the seat but only 8,000 Nm running torque at mid-stroke. The scotch yoke geometry addresses this. The yoke arm converts piston force into rotational moment — and that conversion ratio changes across the stroke. At the initial breakout position (0–10° rotation), the yoke arm geometry produces a higher torque multiplier. The piston force gets amplified into more rotational force at the start of the stroke, exactly where the valve needs it most. At mid-stroke, the multiplier drops, matching the lower running torque requirement. The torque profile fits the valve's torque demand curve. Smaller Actuators on Big Valves Because the scotch yoke delivers higher breakout torque per unit of air supply, you can specify a smaller actuator for the same valve. Or run lower supply pressure. Or both. On a 36-inch butterfly valve in water service, a scotch yoke actuator at 80 psi might produce adequate breakout torque where a rack-and-pinion actuator would need 100 psi to deliver the same initial force. That pressure difference matters on plants where instrument air headers run at 80 psi and boosting to 100 psi requires a dedicated booster regulator at each valve. Smoother Motion Extends Seat Life Rack-and-pinion actuators apply force suddenly when the valve pops off the seat. The constant-torque output means the actuator delivers full torque the instant the valve begins to move — mechanical shock that hammeres the seat every cycle. The scotch yoke starts slow. Piston acceleration builds through mid-stroke. The valve disc lifts off the seat gradually rather than snapping free. On large, high-pressure quarter-turn valves, that smoother unseating motion reduces impact force on the seat ring. Seat life extends. Maintenance intervals stretch. On valves that cycle frequently — automated isolation valves on batch processes — that seat life difference adds up over thousands of cycles. Spec Scotch Yoke When Torque Demands Are High Scotch yoke actuators are the right choice on large quarter-turn valves (8–48 inch butterfly, 8–24 inch ball) where seating torque exceeds what rack-and-pinion delivers at reasonable supply pressure. On smaller valves where torque requirements are moderate, rack-and-pinion is simpler and cheaper. Don't over-specify — the scotch yoke's complexity and cost are justified only when the variable torque characteristic solves a real problem. FAQ A: Cost and complexity. The scotch yoke mechanism has more moving parts than rack-and-pinion — the yoke arm, slot, and pin add machining and assembly cost. On valves where constant torque is adequate, rack-and-pinion does the job for less money. Scotch yoke earns its premium on large valves with high breakout torque. A: It changes the sizing method. You size based on breakout torque at the start of the stroke, not constant torque. Calculate the valve's seating/unseating torque requirement, then verify that the scotch yoke's torque at the breakout position exceeds that requirement by your safety margin. The torque curve is published in the manufacturer's datasheet for each model size. A: 80 psi is typical. Higher than diaphragm actuators because the piston bore is smaller than a diaphragm area. Scotch yoke actuators tolerate up to 120 psi on high-torque models — the piston seal handles the pressure. If your plant air header runs at 60 psi, specify a larger actuator size to compensate. A: They can, but they're not ideal for precise throttling. The variable torque output means the positioner sees different force conditions at different stem angles. For modulating quarter-turn valves, rack-and-pinion's constant torque gives more predictable positioner response. Scotch yoke shines on on/off and severe-service isolation. A: Spring cartridges sit behind the pistons in the cylinder. When air is removed, the springs drive the pistons back, which rotates the output shaft to the fail-safe position (0° or 90°). Different spring cartridge sets produce different fail-safe torque values. You select the spring set based on the valve's seating torque at the fail-safe end position.
Linear Stroke Cylinder
Quick Specs Spring-return (single-acting) or double-acting 500–50,000 lbf (bore area × supply pressure) 40–150 psi (3–10 bar) 1–12 inches (25–300mm) linear ISO 5211 (mounting interface) Double-acting piston in cylinder barrel; O-ring or lip seal; air pressure drives piston in both directions Internal spring cartridges (spring-return models); range adjustable by cartridge selection ISO 5211 flange or clevis; bracket on valve yoke; direct stem coupling via coupling nut or yoke connection Piston Power Where Diaphragm Falls Short Linear stroke cylinders serve the same valve types as diaphragm actuators — globe, gate, control valves that need linear thrust. But they solve two problems that diaphragm actuators can't. Higher thrust in a smaller package. The cylinder bore replaces the diaphragm area. The bore diameter is smaller than a diaphragm for equivalent thrust, because piston cylinders tolerate higher supply pressure — up to 150 psi versus 80 psi for diaphragm. More pressure × smaller area = same thrust with less actuator bulk. On vertical installations and tight pipe racks where a diaphragm actuator won't fit between adjacent pipe runs, the piston cylinder slides in and delivers the force you need. Longer stroke without thrust penalty. Diaphragm actuators max out at about 4 inches because the diaphragm deflects and loses effective area at longer travel. Piston cylinders just extend the barrel. 6, 8, 12-inch strokes are standard for gate valves with long stem travel, and the O-ring seal stays effective at any position along the bore. Thrust output doesn't degrade with stroke length — the piston area is constant whether the piston sits at 1 inch or 10 inches from the end cap. That consistency matters on gate valves where full-open requires long stem extension and the seating force at full-close must match the unseating force at full-open. The Friction Tradeoff Piston cylinders pay for their compact size with seal friction. The O-ring on the piston drags against the cylinder bore. That drag force reduces net thrust at the valve stem — you need higher supply pressure to deliver the same effective force a diaphragm actuator produces at lower pressure. On on/off valves where the stem travels full stroke each cycle, that friction loss is acceptable. The valve seats or unseats regardless of a few hundred pounds of friction. On modulating control valves, friction creates a different problem — hysteresis. The positioner commands a small adjustment, say 2% stem travel. The friction absorbs the adjustment force before the stem moves. The positioner overshoots on the next adjustment. The stem hunts around the target position. Control loop performance degrades. The valve oscillates. The process variable swings. Diaphragm actuators have negligible seal friction. The diaphragm flexes freely — no O-ring drag against a bore surface. For modulating service where precise positioning matters, diaphragm actuators deliver better position accuracy and smoother control loop response. Choose Based on the Application Use piston cylinders on gate valves with long stroke requirements, on installations where space doesn't accommodate a diaphragm actuator, and on on/off service where hysteresis doesn't affect performance. Use diaphragm actuators on modulating control valves where position accuracy is the priority. The two actuator types overlap on the same valve types but serve different operational needs. FAQ A: When stroke exceeds 4 inches, when installation space can't fit a diaphragm actuator, or when supply pressure above 80 psi is available and you want compact actuator size. Gate valves with 6+ inch stem travel are the most common cylinder application. A: No, within the rated range. Piston O-rings handle 150 psi without fatigue issues. The cylinder bore and end caps are rated for the maximum supply pressure. Diaphragm actuators can't tolerate that pressure because the diaphragm material fatigues under repeated high-pressure deflection — the piston seal doesn't deflect, it slides. A: Depends on bore diameter, seal type, and cylinder condition. O-ring seals on standard cylinders create 50–200 lbf friction drag depending on bore size. Lip seals reduce that to 20–80 lbf but cost more. On a 5,000 lbf thrust output, that drag is 1–4% — acceptable for on/off service. On a 500 lbf output for small control valves, the same drag percentage becomes significant. A: You can, and many plants do. But expect higher hysteresis than a diaphragm actuator. A positioner with feedback can compensate somewhat, but the dead band between commanded and actual position will be wider. If the control loop tolerance is tight, specify a diaphragm actuator instead. A: Match the cylinder stroke to the valve's rated stem travel. Globe valves typically need 1–3 inches. Gate valves need 4–12 inches depending on valve size and pressure class. The manufacturer's valve datasheet lists stem travel — buy the cylinder stroke that matches, not longer. Extra stroke adds cylinder length and cost without improving performance.
AT Series Cylinder
Quick Specs Double-acting (standard) or spring-return (with spring cartridges) 10–5,000 Nm (AT05 through AT350 model sizes) 40–80 psi (2.5–6 bar) 90° quarter-turn rotation ISO 5211 (mounting), NAMUR (solenoid/accessories) Dual opposing pistons driving rack gears; central pinion gear produces 90° output rotation; constant torque across full stroke Spring cartridges behind each piston (spring-return models); fail-safe positioning on air loss Extruded aluminum body; hard anodized bore surface; bolted or threaded end caps; compact and lightweight The Default Actuator for Quarter-Turn Valves AT series is what you reach for when a butterfly valve or ball valve needs automation at moderate size. HVAC chilled water butterfly valves. Water treatment flow control ball valves. General process utility isolation. The AT series handles all of it — compact, light, affordable, and standardized enough that every accessory manufacturer builds NAMUR-mount hardware for it. The rack-and-pinion mechanism is the core. Two pistons in a single cylinder barrel. Each piston drives a rack gear. The rack gears engage a central pinion. Air drives both pistons toward the center, the racks rotate the pinion 90°. Constant torque across the full stroke — rack-and-pinion geometry produces uniform rotational force regardless of pinion position. That constant output simplifies sizing. You pick the model that delivers torque above your valve's seating requirement, add a safety margin, and you're done. No torque curves to analyze, no breakout position calculations, no variable output to account for. Aluminum Keeps the Weight Down The extruded aluminum body is lighter than cast iron or steel scotch yoke housings by 40–60%. On a 12-inch butterfly valve mounted between flanges, actuator weight matters — a heavy actuator cantilevered off the valve body can distort the flange connection over time. The AT series keeps the total assembly weight within what the valve body and pipe supports handle without additional bracing. The hard anodized bore surface protects the cylinder interior without adding weight. Piston O-rings ride on the anodized surface with low friction and long seal life. The aluminum housing corrodes less than painted steel in humid and mildly corrosive environments — water treatment plants, coastal installations, outdoor HVAC equipment. For aggressive chemical environments, specify optional coating or stainless steel variants. NAMUR Means No Custom Brackets The NAMUR interface on the actuator body allows direct mounting of solenoid valves, limit switches, and positioners. Standardized bolt patterns and air passage locations. No custom brackets, no adapter plates, no drilling holes in the actuator housing to fit a non-standard accessory. Every major accessory manufacturer builds NAMUR-compatible products. That interchangeability reduces procurement complexity and maintenance inventory — one solenoid valve design fits every AT series actuator in the plant. Where AT Series Stops and Scotch Yoke Starts AT series covers torque up to 5,000 Nm. That's adequate for butterfly valves up to about 12 inches and ball valves up to about 8 inches in moderate-pressure service. Beyond that size and pressure, seating torque exceeds what the AT series delivers at standard supply pressure. The scotch yoke takes over — its variable torque output produces higher breakout force for large valves. Don't stretch the AT series beyond its range. The actuator stalls on high-torque valves, and you're back to manual operation. FAQ A: AT series for moderate-size quarter-turn valves where torque is under 5,000 Nm and seating/unseating differential isn't extreme. Scotch yoke for large valves (12+ inch butterfly, 8+ inch ball) with high breakout torque. If your valve datasheet shows seating torque that exceeds the AT series maximum for your supply pressure, you need scotch yoke. A: Simplicity. You size for one torque value. The positioner sees consistent force conditions at every stem angle. Modulating control response is predictable. No torque curve to analyze. For most quarter-turn applications at moderate size, constant torque is all you need. A: Yes, with the standard hard anodized finish. Aluminum anodizing resists UV, humidity, and mild corrosion. In salt-air coastal environments or chemical plant atmospheres with acid vapor, specify optional epoxy coating or the stainless steel variant. Don't assume bare aluminum survives aggressive exposure. A: Yes. The constant torque output gives positioners consistent force conditions, which simplifies tuning and improves control loop stability. Pair the AT series with a NAMUR-mount pneumatic or electropneumatic positioner for throttling service. The compact accessory stack keeps the total assembly neat. A: Spring cartridges sit behind each piston inside the cylinder barrel. When air supply is removed, the springs push the pistons away from center, rotating the pinion to the fail-safe position (0° or 90°). You select the spring cartridge set based on the torque your valve needs at the fail-safe end position — the spring force must seat the valve or hold it open against process pressure.
Technical Overview
Pneumatic actuators convert compressed air energy into valve motion — linear thrust for globe, gate, and control valves, or rotary torque for ball, butterfly, and plug valves. The actuator type matches the valve motion type: linear actuators push valve stems up and down, rotary actuators turn valve shafts 90°. FLOWKS manufactures five pneumatic actuator configurations spanning both motion types and three force-generation mechanisms. Diaphragm pneumatic actuators produce linear thrust through a flexible diaphragm that deflects under air pressure — low supply pressure (40-80 psi), large diaphragm area compensates for the lower pressure, spring-return fail-safe, and the standard actuator for process control valves where low hysteresis and precise stem positioning matter. Diaphragm pneumatic actuators with handwheel add a declutchable manual override for plant startup and air-supply-outage operation — same actuator performance, plus handwheel positioning when air isn't available. Scotch yoke pneumatic actuators produce variable rotary torque through a piston-driven yoke mechanism — torque peaks at the initial breakout position to unseat quarter-turn valves, and the geometry delivers higher torque at large valve sizes (8"-48" butterfly, 8"-24" ball) where rack-and-pinion actuators can't generate enough force. Linear stroke cylinders use a piston-in-cylinder design for linear thrust — higher supply pressure (up to 150 psi), longer stroke capability (up to 12"), more compact than diaphragm actuators at equivalent thrust, specified for gate valves with long stem travel and tight-installation spaces. AT series cylinders are rack-and-pinion rotary actuators in extruded aluminum bodies — constant torque across 90° stroke, NAMUR accessory mounting, compact and lightweight for small-to-medium quarter-turn valve automation up to 12" butterfly and 8" ball sizes.
FLOWKS pneumatic actuators automate valve operation across both linear-stroke and quarter-turn valve types. Diaphragm pneumatic actuators deliver linear thrust for globe, gate, and control valves — low supply pressure, spring-return fail-safe, low hysteresis for precise modulating control, thrust range 100-10,000 lbf. Diaphragm pneumatic actuators with handwheel add declutchable manual override for startup commissioning and air-supply-outage operation — same actuator plus manual positioning capability when compressed air is unavailable. Scotch yoke pneumatic actuators produce variable rotary torque for large quarter-turn valves — torque peaks at initial breakout for reliable ball and butterfly valve unseating, output range 500-100,000 Nm across model sizes, spring-return or double-acting. Linear stroke cylinders provide piston-driven linear thrust for long-stroke gate and globe valves — higher supply pressure up to 150 psi, stroke range 1"-12", compact installation where diaphragm actuator size doesn't fit. AT series cylinders are rack-and-pinion quarter-turn actuators in extruded aluminum — constant torque output, NAMUR solenoid and accessory mounting, compact lightweight design for automated ball and butterfly valves up to moderate sizes, torque range 10-5,000 Nm.
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Ball Valves
FLOWKS offers floating ball valve, trunnion mounted ball valve and top entry ball valve for oil & gas, petrochemical and power generation. Designed per API 6D, API 608 and ASME B16.34.
DBB Valves
FLOWKS Double Block & Bleed (DBB) valves provide dual isolation with bleed verification in a single compact body. Replacing traditional multi-valve installations, FLOWKS DBB valves reduce weight, space and potential leak paths. Available in bolted bonnet, all-welded and expanding gate designs per API 6D and API 607.
Gate Valves
FLOWKS gate valves for isolation service in piping systems. Flexible wedge, solid wedge, slab and expanding gate designs per API 600, API 602 and ASME B16.34.
Globe Valves
FLOWKS globe valves for throttling and isolation. Standard, angle, Y-pattern and bellows seal configurations per API 602 and BS 1868.