Understanding Pneumatic Diaphragm Valves: Design, Functionality, and Applications

Introduction

In industrial and manufacturing applications, the control of fluid flow is critical for operational efficiency and safety. One of the essential components employed to regulate fluid passage is the valve, with various types designed to serve specific functions and environments. Among these, pneumatic diaphragm valves have gained prominence due to their unique design and versatility. This article thoroughly explores pneumatic diaphragm valves, including their design, functioning, advantages, applications, and considerations during selection and installation.

The diaphragm valve solves a problem no other family in the valves used in water treatment plants solves as cleanly: it separates the wetted flow path from every moving part and every packing gland with a single flexible membrane. There is no stem penetrating the process, no seat exposed to the stream when the valve is open, and therefore no leak path to atmosphere and no crevice for product to collect in. That architecture is why diaphragm valves dominate aggressive chemical feed and sanitary service, and it also fixes their limits, because the same membrane that provides the isolation is a consumable with a finite cycle life. This page serves as the category hub for diaphragm valves: it covers pneumatic actuation in depth, then maps the wider category — body geometry, material selection, and actuation options — so that the specification follows from the service.

1. Overview of Pneumatic Diaphragm Valves

Pneumatic diaphragm valves are a type of on-off valve primarily used in applications requiring precise control of fluid flow. Unlike traditional valves that rely on rotating elements, diaphragm valves use a flexible diaphragm or membrane to open and close the flow path, providing a tight seal to prevent leakage. As the name suggests, these valves are actuated by pneumatic power, using air or gas pressure to drive the diaphragm’s movement.

1.1 Components of Pneumatic Diaphragm Valves

The primary components of a pneumatic diaphragm valve include:

• Body: This is the main structure of the valve, which houses the internal components and connects to the piping system. The body can be made from various materials, including plastic, stainless steel, and alloy materials, depending on the application’s specific needs.
• Diaphragm: A flexible membrane that acts as the closure element of the valve. It is usually made from elastomers or polymer materials that can withstand chemical interactions and mechanical stress.
• Actuator: The pneumatic actuator applies pressure to one side of the diaphragm, causing it to move upwards or downwards, thereby opening or closing the flow path.
• Bonnet: This is the cover that holds the diaphragm and actuator in place. It protects them while also providing a means to secure the valve to the piping system.
• Seat: The seat is the area against which the diaphragm seals when the valve is closed. It plays a crucial role in preventing leaks and ensuring flow control.
• Fastening elements: Various bolts and screws hold the valve components together.
• Compressor: The shaped plate between the actuator stem and the diaphragm that distributes closing force evenly across the membrane and carries it back on opening. Diaphragms are usually retained to the compressor by a stud or bayonet fitting, and a diaphragm that has torn free of its compressor is a common failure mode.
• Travel Stop: An adjustable limit on the closing stroke that prevents the compressor from over-squeezing the diaphragm into the weir. This unremarkable-looking screw is the difference between a diaphragm that lasts years and one that fails in months.

2. Working Principle of Pneumatic Diaphragm Valves

The operational mechanism behind pneumatic diaphragm valves is relatively straightforward:

1. Actuation: When the pneumatic actuator receives air pressure (typically from a compressor or pneumatic control system), it generates a force that pushes the diaphragm.
2. Diaphragm Movement: As the actuator extends, it pushes the diaphragm upward, pulling it away from the seat. This action opens the flow path, allowing fluids to pass through the valve.
3. Flow Control: The extent to which the diaphragm is raised determines the flow rate. A partially open valve allows for modulated flow, making diaphragm valves suitable for precise control applications.
4. Closure: When the air pressure is released, a spring mechanism or gravity closes the diaphragm against the seat, sealing the valve and ceasing fluid flow.

2.1 Air-to-Open, Air-to-Close, and Fail Position

The sequence above describes one of two possible arrangements, and which one a valve uses is a specification decision rather than a property of the technology. In an air-to-open, spring-to-close actuator, applied air retracts the stem and compressor, lifting the diaphragm off the weir; the spring drives the valve shut when air is lost, so the valve fails closed. In an air-to-close, spring-to-open actuator the relationship is reversed and the valve fails open. Double-acting actuators use air on both sides and fail in place, holding whatever position they occupied when air was lost.

Two points follow. First, the closing force in a spring-return valve comes entirely from the spring, not from gravity — a diaphragm valve mounted horizontally or inverted closes exactly as well as one mounted upright, because gravity plays no meaningful role against spring preload and line pressure. Second, the fail position should be chosen from the consequence of losing air, not from what is convenient in normal operation. Chemical feed almost always fails closed; cooling and seal water frequently fails open; a valve whose sudden movement would create a hazard may warrant fail-in-place with a dedicated air receiver.

2.2 Why Actuator Size Grows Quickly

The actuator must generate enough force to seat the diaphragm against line pressure acting over the diaphragm’s effective area, plus the compression needed to make the seal. Consider a valve whose diaphragm presents an effective area of about 12 in² on a line at 75 psi: the pressure component alone is roughly 900 lbf before any seating force is added. If the actuator diaphragm is 40 in² and instrument air is available at 60 psi, the actuator develops about 2,400 lbf, less spring preload in an air-to-close arrangement — adequate, but the margin narrows quickly if header pressure sags. Because required force scales with the square of valve size while available actuator force scales with actuator area, pneumatic diaphragm valves become physically large and expensive above roughly mid-range diameters, which is the practical ceiling on the technology rather than any limitation of the sealing principle.

3. Diaphragm Valve Category Overview

Three subtopics organize the wider category, and they correspond to the three decisions that define any diaphragm valve: the shape of the flow path, what the wetted parts are made of, and how the valve is driven.

Diaphragm Valve Types

Body geometry is the first decision, and diaphragm valve types covers the weir, straightway, and multi-port configurations along with the trade-offs among them. A weir body raises a saddle in the middle of the flow path so the diaphragm has only a short distance to travel to seal; this gives long diaphragm life, modest actuator force, and usable throttling, at the cost of a raised obstruction that adds headloss and prevents the body from draining fully unless the valve is sloped. A straightway or full-bore body has no weir, so the passage is unobstructed and drains completely and solids pass without lodging, but the diaphragm must flex through a far greater distance and consequently wears out sooner and demands more actuating force. Three-way and multi-port bodies combine diaphragms in a single block to divert or select flow, which eliminates fittings and dead legs in tightly packed skids.

Diaphragm Valve Materials

Material selection carries unusual weight here because the diaphragm, not the body, sets the operating envelope, and diaphragm valve materials covers body and membrane options together. Bodies range from PVC, CPVC, and polypropylene for chemical service through lined ductile iron to solid stainless steel for sanitary and high-purity duty. Membranes divide broadly into homogeneous elastomers — EPDM, nitrile, butyl, and similar compounds offering long flex life and good abrasion resistance — and two-piece constructions in which a PTFE face contacts the process while an elastomer backing provides resilience. PTFE-faced diaphragms deliver near-universal chemical compatibility but flex less readily, so their cycle life and pressure and temperature ratings are lower than the elastomer alternatives. Whatever the body, the membrane is the component that decides what the valve can handle and how long it will last.

Diaphragm Valve Actuation

How the valve is driven is a separate axis again, and diaphragm valve actuation covers manual, pneumatic, and electric options. Manual handwheel operation remains entirely appropriate for infrequently operated isolation and for point-of-use service, and it is the least expensive and most fault-tolerant arrangement available. Pneumatic actuation, the subject of this article, provides fast stroking, defined fail-safe behavior through spring return, and safe operation in classified areas, at the cost of a dependency on clean dry instrument air. Electric actuation suits sites without an air header and applications needing precise intermediate positioning or long position holds, though stroke speed is slower and fail-safe behavior requires a spring pack or stored-energy device. The choice usually follows from which utilities the site already distributes and from what must happen when they fail.

4. Advantages of Pneumatic Diaphragm Valves

Pneumatic diaphragm valves offer several advantages over other valve types, making them a preferred choice in various applications:

4.1 Sealing Capability

The primary benefit of diaphragm valves is their exceptional sealing capacity. The design eliminates any direct contact between the valve’s fluid and moving parts, which prevents wear and minimizes the risk of leaks.

4.2 Chemical Resistance

The diaphragm can be made from various elastomers and polymers, providing excellent resistance to corrosive and abrasive materials. This quality makes diaphragm valves ideal for use in chemical industries.

4.3 Low Maintenance

Pneumatic diaphragm valves typically require less maintenance due to fewer movable parts that directly interact with the fluid. This trait leads to lower operational costs over time.

4.4 Versatility

These valves can be adapted for various applications, from water treatment to food processing and pharmaceutical manufacturing. They are available in various sizes and materials to meet diverse operational requirements.

4.5 Safety

Pneumatic diaphragm valves can operate in hazardous environments since they do not require electricity and can minimize the risk of electrostatic discharge or ignition, which is critical in volatile settings.

4.6 Limitations and Honest Trade-Offs

Four constraints bound all of the above. The first is that the diaphragm is a consumable with a finite flex life, so “low maintenance” means few unscheduled failures rather than no scheduled work — the membrane will need replacing on a predictable interval, and planning for it is part of owning the valve. The second is the operating envelope: pressure and temperature ratings are set by the membrane rather than the body, so a stainless body good for several hundred psi may be limited to a fraction of that by a PTFE-faced diaphragm, and ratings derate sharply as temperature rises.

Third, size and actuator force scale unfavorably, which puts a practical ceiling on the technology at mid-range diameters and makes large diaphragm valves expensive and heavy compared with a butterfly or plug valve of the same line size. Fourth, weir bodies do not self-drain unless deliberately sloped, and a horizontal weir valve retains a small volume of process fluid on the upstream side of the weir — an irrelevance on plant water and a genuine problem on chemical or sanitary service where dead legs are unacceptable.

5. Applications of Pneumatic Diaphragm Valves

Pneumatic diaphragm valves are used in a wide range of industries and applications including, but not limited to:

5.1 Chemical Processing

In the chemical industry, diaphragm valves are used to control the flow of corrosive or hazardous chemicals in pipelines. Due to their non-contact sealing mechanism, they prevent contamination and ensure safe handling.

5.2 Water and Wastewater Treatment

These valves are commonly used in water treatment stations and sewage treatment plants to manage the flow of water effectively. They help in controlling processes such as disinfection, filtration, and sludge management.

5.3 Food and Beverage Industry

In food processing applications, diaphragm valves provide sanitary benefits because they can be designed to meet cleanliness standards and regulations. They can be used in processes like ingredient dosing and product transfer.

5.4 Pharmaceutical Manufacturing

Pharmaceutical applications benefit from diaphragm valves’ ability to prevent contamination, allowing for sterile processing when handling active pharmaceutical ingredients (APIs) and solutions.

5.5 Power Generation

Within power plants, these valves control fluid flow rates during the cooling process and manage the steam used for electrical generation.

5.6 Where They Sit Inside a Treatment Plant

Municipal placement clusters around chemical service, where the diaphragm valve’s hermetic construction is worth paying for. Sodium hypochlorite, ferric chloride, sodium hydroxide, sulfuric acid, polymer, and fluoride feed systems all use diaphragm valves for isolation at pumps, injection points, day tanks, and calibration columns, typically in plastic or lined bodies with EPDM or PTFE-faced membranes matched to the specific chemistry. Metering pump suction and discharge isolation is a particularly common placement, since the alternative valve families all present a packing gland to a chemical that must not reach the operator.

Beyond chemical feed, diaphragm valves appear on lime slurry and other abrasive dosing lines where a straightway body passes solids that would score a ball valve, on sampling and analyzer lines where cleanliness matters, and occasionally on small sludge transfer. They are not economical for general plant isolation on large clean-water lines, where butterfly and gate valves do the same job at a fraction of the cost, and they are not suited to high-pressure duty of any kind.

6. Comparison Tables

7. Selecting Pneumatic Diaphragm Valves

Choosing the suitable pneumatic diaphragm valve for a specific application involves several considerations and factors:

7.1 Material Compatibility

Evaluate the materials used for the valve’s body and diaphragm. Ensure they are compatible with the fluids being controlled. Refer to compatibility charts to avoid corrosive interactions.

7.2 Temperature and Pressure Ratings

Different applications will have varying operational temperatures and pressures. Choose a diaphragm valve rated for the required conditions to ensure reliability and operational safety.

7.3 Size and Flow Requirements

Determine the appropriate valve size based on the system’s flow rate and pressure requirements. Oversized or undersized valves can lead to inefficiencies and operational issues.

7.4 Actuation Type

Pneumatic valves can be actuated in various ways. Consider whether you require normally open or normally closed operation based on your automation needs and control systems.

7.5 Regulatory Compliance

In industries such as pharmaceuticals and food processing, it is essential to comply with industry standards and regulations. Ensure the valves selected meet required certifications for cleanliness, safety, and operation.

7.6 Working Through the Decisions in Order

Those criteria resolve more reliably as a sequence. Begin by confirming that a diaphragm valve is the right family: it earns its cost where a stem leak path is unacceptable, where the fluid is aggressive or abrasive, or where full drainage and cleanability are required. For plain isolation on clean water at any substantial size, it is the wrong choice and a butterfly or ball valve will serve better for less. Where solids are heavy enough to challenge even a straightway body — lime slurry, grit, thickened sludge — pinch valves apply the same crevice-free principle with a full-bore sleeve and no weir at all, and are usually the more durable answer.

With the family settled, pick the body geometry from the drainage and solids requirements, then choose the membrane from the chemistry and cycle frequency, checking that its pressure and temperature ratings at the operating condition still cover the duty. Size on flow coefficient rather than line size, remembering that a weir body has a noticeably lower coefficient than a straightway body of the same diameter. Fix the fail position from the consequence of air loss. Finally, confirm actuator force against line pressure at the minimum available instrument air pressure, not the nominal header value, since a sagging air header and a high line pressure combine to leave a valve unable to seat.

8. Installation Considerations

Proper installation of pneumatic diaphragm valves is critical for optimal performance:

8.1 Positioning

Pneumatic diaphragm valves can be installed in various orientations; however, check the manufacturer’s guidelines. Typically, the valve should be installed in a position consistent with the fluid’s flow direction (indicated by arrows on the valve body).

8.2 Piping Alignment

Ensure that the piping is accurately aligned to prevent excessive stress on the valve body and connections. Misalignment can lead to failure and leaks.

8.3 Air Supply for Actuation

Verify that the pneumatic actuator has access to a clean, dry air supply without contaminants. Moisture and particulates can cause malfunction and wear. In practice the air reaches the actuator through a small solenoid valve mounted on or near the valve, which is what the control system actually commands, so the solenoid’s coil rating, enclosure, and manual override belong in the same specification as the valve itself. Confirm minimum available header pressure at the far end of the run rather than at the compressor, and provide a filter-regulator at each valve.

8.4 Testing and Commissioning

After installation, conduct testing to confirm proper function, sealing, and actuation. This step is vital for ensuring that the system is safe and operates according to specifications.

8.5 Orientation for Drainage and Dead Legs

Weir-bodied valves retain liquid upstream of the weir when mounted with the bonnet vertical in a horizontal line. Where full drainage matters — chemical service that must be flushed between batches, sanitary and high-purity systems, anything that will be cleaned in place — the conventional remedy is to rotate the valve so the stem is inclined and the weir slopes toward the outlet, following the manufacturer’s specified angle. Straightway bodies drain without this measure. Deciding drainage at layout time costs nothing; discovering it after the pipe is hung means cutting and rewelding.

9. Maintenance of Pneumatic Diaphragm Valves

Routine maintenance of pneumatic diaphragm valves is advisable to prolong lifespan and efficiency:

9.1 Routine Inspections

Regularly inspect the valve for corrosion, leaks, and aesthetic damage. Checking for signs of wear on the diaphragm is especially important, as it’s typically the first component that may need replacing.

9.2 Diaphragm Replacement

Diaphragms should be replaced based on signs of wear or during scheduled maintenance. A good practice is to keep replacement diaphragms on hand for quick change-outs.

9.3 Lubrication

Pneumatic actuators may require periodic lubrication to maintain smooth and reliable motion.

9.4 Cleaning

Keep the external surfaces of the valve clean from dirt and debris to avoid contamination and ensure operability.

9.5 Planning Diaphragm Life and Avoiding Over-Compression

Diaphragm replacement is better planned than triggered. Manufacturers publish cycle life for each membrane construction, and a straightforward calculation turns that into an interval. A valve cycling four times an hour completes 96 cycles a day, or roughly 35,000 a year; against a PTFE-faced membrane rated near 100,000 cycles, that implies replacement at about a three-year interval, well before failure. The same arithmetic on a valve cycling once a minute gives a very different answer, and it is worth doing before the valve is installed rather than after the first unplanned release.

The most common cause of premature failure is over-compression on closing. Torquing the actuator or handwheel down beyond the point of sealing forces the membrane into the weir, cold-flows the PTFE face, and tears it — often within weeks. Set the travel stop so the valve seals without excess squeeze, and after any diaphragm change, retorque the bonnet bolts in a crossing pattern to the manufacturer’s specified values, since the elastomer relaxes and an initially tight bonnet will weep after a few thermal cycles.

10. Common Misconceptions

10.1 Pneumatic Diaphragm Valves are Only for Gases

While pneumatic diaphragm valves are actuated by gases, they are widely used for controlling liquid flows, including corrosive liquids.

10.2 They are High Maintenance

In reality, pneumatic diaphragm valves are often lower maintenance than mechanical or electric counterparts due to fewer moving parts in direct fluid contact.

10.3 They are Inefficient for Modulation

Although diaphragm valves are typically used as on-off valves, they can provide effective flow modulation when designed appropriately.

10.4 Any Diaphragm Will Fit Any Body

Diaphragms are matched to a specific body size, geometry, and compressor design, and substituting a similar-looking membrane from another manufacturer is a reliable way to produce a valve that neither seals nor lasts. Membrane, compressor, and body are a matched set, and rebuild kits should be ordered against the valve’s model and serial designation rather than by size alone.

11. Field Notes

Commissioning Considerations

Flush the line before the valve is placed in service, since construction debris lodged on a weir will hole a diaphragm on the first close. Stroke the valve through its full travel with the actuator installed, confirm the fail position by cutting the air supply and watching what the valve does, and set the travel stop with the line pressurized rather than dry — a stop set on an empty line will usually be too tight once pressure is behind the membrane. Record the bonnet bolt torque and the date, because a diaphragm’s clock starts at installation.

Common Specification Mistakes

The recurring errors are rating the valve on the body rather than on the membrane, specifying a weir body where full drainage is required, choosing the fail position for operating convenience instead of failure consequence, sizing the actuator against nominal rather than minimum air header pressure, omitting the filter-regulator at the valve, and treating the diaphragm as a spare rather than as a scheduled replacement item. A quieter error is specifying a diaphragm valve for general isolation on large clean-water lines, where the cost and actuator size are difficult to justify.

Operations and Maintenance Comparison

Manual diaphragm valves are the simplest to own and the easiest to over-tighten, since nothing limits an operator’s arm but the travel stop. Pneumatic valves apply a repeatable force every cycle, which is better for diaphragm life, but add air quality and solenoid reliability to the list of things that can stop the valve. Electric actuators remove the air dependency and add their own duty-cycle and fail-safe considerations. Across all three the consumable is the same, and the maintenance program that matters is the one that replaces membranes on a calculated interval.

12. Design Details and Standards

12.1 Applicable Standards

Diaphragm valve design and testing are addressed by MSS SP-88 for diaphragm valve construction and by ASME B16.34 for pressure-temperature ratings of metallic bodies where applicable. Plastic-bodied valves follow the relevant ASTM material and pressure-rating standards for PVC, CPVC, and polypropylene, and thermoplastic valve pressure ratings derate substantially with temperature. In sanitary and bioprocess service, ASME BPE governs surface finish, drainability, and dead-leg criteria, and 3-A Sanitary Standards apply in food and dairy applications. Compressed air supplied to the actuator should meet a defined purity class under ISO 8573-1, since moisture and particulate carryover are a leading cause of actuator and solenoid failure. All wetted materials in potable water service must comply with NSF/ANSI/CAN 61 and NSF/ANSI 372, and valves handling treatment chemicals should carry documentation of elastomer compatibility with the specific solution and concentration in use.

12.2 Key Parameters That Differ by Configuration

Flow coefficient differs markedly between weir and straightway bodies of the same nominal size, so the geometry decision changes the sizing calculation and must precede it. Diaphragm cycle life differs by roughly an order of magnitude between homogeneous elastomer and PTFE-faced constructions, which changes the maintenance model rather than just a number on a datasheet. Pressure and temperature ratings are governed by the membrane and derate with temperature. Required actuator thrust scales with the diaphragm’s effective area and line pressure, which is what makes large valves expensive. Face-to-face dimension and bonnet height vary between manufacturers and between weir and straightway bodies, and bonnet height in particular is easy to overlook in a congested chemical room.

12.3 Specification Checklist

1. State media, concentration, temperature range, solids content, and design pressure.
2. Confirm whether full drainage or cleanability is required.
3. Select body geometry — weir, straightway, or multi-port — and state why.
4. Specify body material and lining separately from the membrane.
5. Specify diaphragm construction and confirm its pressure-temperature curve covers the duty.
6. State expected cycle frequency and the resulting replacement interval.
7. Size on flow coefficient for the chosen geometry, not on line size.
8. Specify actuator type and fail position from the consequence of air loss.
9. Confirm actuator thrust against line pressure at minimum available air pressure.
10. Specify the pilot solenoid, its coil rating, enclosure, and manual override.
11. Require a filter-regulator at each valve and state the air purity class.
12. State the installation angle where drainage is required.
13. Confirm face-to-face dimension, bonnet height, and removal clearance.
14. Specify rebuild kits by model designation and state the required stocking level.

13. Frequently Asked Questions

What is the difference between a weir and a straightway diaphragm valve?

A weir body raises a saddle in the middle of the flow path, so the diaphragm travels only a short distance to seal. That means longer membrane life, less actuator force, and usable throttling, but the weir adds headloss and traps a small volume of liquid unless the valve is installed sloped. A straightway body has no weir: the passage is unobstructed, it drains completely, and solids pass freely, but the diaphragm must flex much further and wears out sooner.

How long does a diaphragm last?

It depends on the membrane construction, the chemistry, the temperature, and above all the cycle frequency. Manufacturers publish cycle ratings, and the practical approach is to convert that rating into a calendar interval using the valve’s actual duty — a valve cycling four times an hour reaches roughly 35,000 cycles a year — then schedule replacement well inside it. Homogeneous elastomer membranes generally flex far more times than PTFE-faced ones.

Why did my new diaphragm fail within weeks?

Over-compression is the most likely cause. Closing the valve harder than needed to seal forces the membrane down into the weir, cold-flows a PTFE face, and tears it. Set the travel stop so the valve just seals, with the line pressurized. The other common causes are debris left in the line after construction, a mismatched or generic replacement membrane, and a chemistry the membrane was never rated for.

Can diaphragm valves throttle?

Weir bodies throttle acceptably over a mid-travel range and are commonly used for coarse flow balancing. Sustained operation near the closed position concentrates velocity and wear at one point on the membrane and shortens its life considerably, so continuous fine modulation is better served by a valve designed for control duty. Straightway bodies are poor throttling devices and should be treated as on-off.

Which is better for slurry, a diaphragm valve or a pinch valve?

Both isolate the process behind an elastomer with no stem leak path, which is why they compete. For light to moderate solids a straightway diaphragm valve works well and offers more body material options. For heavy abrasives — lime slurry, grit, thickened sludge — a pinch valve’s full-bore sleeve with no weir and no crevices generally lasts longer and clogs less, and it is the more common municipal choice on those services.

What pressure can a pneumatic diaphragm valve handle?

Less than the body suggests. The membrane sets the ceiling, and PTFE-faced constructions in particular are rated well below the metallic body they sit in, with ratings falling as temperature rises. Diaphragm valves are low-to-moderate pressure devices by nature; high-pressure duty belongs to ball, globe, or gate valves. Always read the pressure-temperature curve for the specific membrane, at the maximum operating temperature.

14. Conclusion

Pneumatic diaphragm valves are integral components in multiple industries, providing excellent performance in regulating fluid flow. With their robust design, impressive sealing capabilities, and versatility, they are the valve of choice in environments that require reliable and contamination-free operation. Understanding the components, working principles, advantages, and considerations for selection can empower engineers, operators, and procurement professionals in making informed decisions when implementing pneumatic diaphragm valves in their applications.

As industries continue to evolve and regulatory standards intensify, pneumatic diaphragm valves will maintain their importance in facilitating safe, efficient, and clean operations across various sectors. Proper selection, installation, and maintenance of these valves are paramount to optimizing performance and achieving long-term success in fluid control applications.

Reduced to a sequence, the selection logic runs: confirm a diaphragm valve is the right family, pick the body geometry from drainage and solids requirements, choose the membrane from chemistry and cycle frequency and verify its pressure-temperature curve, size on flow coefficient for that geometry, fix the fail position from the consequence of air loss, and confirm actuator thrust at minimum air pressure. In that order, the valve follows from the service rather than from the line size.