Pneumatic valve actuation for process plants

Three decisions that must stay separate

A control or on/off valve in a refinery, chemical plant, pharmaceutical facility, power station, water treatment works or food plant involves three linked but different engineering decisions:

• Process valve sizing decides the valve type, size and trim for the process duty. That belongs to the process and instrumentation engineers.
• Actuator selection decides the torque or thrust needed to move and seat the valve under the worst process conditions, and the actuator type and size to deliver it. That belongs to the valve and actuator supplier and the project design basis.
• Instrument-air engineering makes sure the selected actuators and their accessories always receive enough air, at the right pressure and quality, including during upsets.

Problems arise when one decision is used to shortcut another, for example sizing actuators from valve diameter, or sizing the instrument-air system from the number of valves rather than their air volumes and duty.

Start from the valve’s process function

Before counting air, record what each valve does and what happens if it fails to move:

• Function: isolation, throttling control, diversion, emergency shutdown, venting or depressurising, batch sequencing, utility control.
• Required fail position on loss of air or signal: fail-closed, fail-open or fail-in-place.
• Consequence of failure: a small shutdown valve may deserve far more protection than a large non-critical control valve.
• Required stroke time, especially for shutdown and blowdown valves.

This classification decides which valves need dedicated storage, which need redundant supply, and which can tolerate a brief pressure dip.

Actuator types and how they use air

Do not assume rotary and linear actuators, or spring-return and double-acting designs, have similar air consumption or response. Use the actuator supplier’s swept volume for each direction.

Calculating air per stroke

For an on/off actuator, the free air used per stroke is approximately:

Free air per stroke ≈ swept volume × (supply pressure + 1.013) ÷ 1.013

with pressure in bar(g) and volumes in litres. Add the volume of tubing, solenoid valve passages and any volume boosters filled each stroke. For double-acting actuators, add both directions for a full open-close cycle.

Worked example (illustrative only): a spring-return quarter-turn actuator has a swept volume of 4 litres and is supplied at 5.5 bar(g), 79.8 psig. Free air per opening stroke ≈ 4 × 6.513 ÷ 1.013 ≈ 25.7 litres. If the valve must open in 3 seconds, the peak flow during the stroke is about 8.6 l/s, roughly 0.5 m³/min (about 18 cfm). If it cycles 20 times an hour, its average demand is only 20 × 25.7 ≈ 514 litres per hour, under 0.01 m³/min.

Two lessons follow. First, stroke time decides the flow capacity of the solenoid valve, tubing and regulator. Second, average actuator demand is often small; the system is challenged by simultaneous movements, positioner bleed and leaks.

Positioners, bleed and continuous consumption

Modulating control valves have positioners, and many pneumatic and electro-pneumatic positioners bleed a small continuous flow even at steady state. On a plant with hundreds of control valves, positioner steady-state consumption plus leakage at tubing fittings can easily exceed the average stroking demand of all the on/off valves. Collect the positioner maker’s steady-state consumption figure for each type, and include it in the base load. Low-bleed positioners are worth considering on new projects and replacements.

Simultaneous movements and upset scenarios

The instrument-air system must cope with the worst credible moment, not the average hour. Identify scenarios such as:

• A batch transfer or CIP sequence opening several valves together.
• A unit start-up or shutdown sequence.
• An emergency shutdown where many valves move at once and must reach position within their required times.
• A compressor trip or power failure, during which actuators must still complete their safe action from stored air.

For each scenario, add the air volumes of the valves that move and the time allowed. This sets the receiver size and the minimum pressure the receiver can fall to while every actuator still has at least its minimum supply pressure.

Instrument-air quality

Actuators, solenoid valves and positioners are damaged by water, oil and particles. Water freezing in exposed tubing, sticky spool valves from oil varnish and blocked positioner nozzles are classic causes of valve failures. Typical instrument-air practice is:

• A pressure dew point well below the lowest temperature any instrument-air line will see. Desiccant dryers at −40 °C PDP are widely used for process-plant instrument air; refrigerated dryers (typical PDP around +3 °C) suit only indoor systems with no cold exposure.
• Coalescing filtration to remove oil aerosols and fine particles, with oil-free compressors chosen where the plant standard or process requires it.
• A specified class to ISO 8573-1 for particles, water and oil, written into the instrument-air design basis, and widely used guidance such as ISA-7.0.01 where the project adopts it.
• Stainless or other suitable tubing, and filter-regulators at each actuator where the supplier requires them.

Heatless desiccant dryers use a significant share of the dried air for regeneration, so include that purge flow when sizing compressors.

Supply architecture and reliability

For process plants, instrument air is a critical utility. A typical robust architecture includes:

• At least one standby compressor, sized so that the plant can run with the largest compressor out of service.
• Duplicate dryers and filters, or bypass arrangements that do not compromise dryness.
• A dedicated instrument-air receiver sized for the safe-shutdown scenario, separated from plant air by a priority or check valve so plant air users cannot drain it.
• Low-pressure alarms set above the actuators’ minimum supply pressure, with time to act.
• Power supply to compressors and dryers from a reliable source; where needed, standby generation. Our team across Reddy Generators and Reddy Compressors, under the same management, can support both.

Keep the header pressure no higher than necessary: each extra bar costs about 7% more compressor power and raises leakage across hundreds of fittings.

Building a process actuator duty register

1. List each valve tag, service, process unit and function.
2. Record fail position and criticality.
3. Record actuator type, size, swept volume per direction and minimum supply pressure.
4. Record required stroke time and operating frequency.
5. Record accessories: solenoid valves, positioners, boosters, quick exhausts, limit switches.
6. Add steady-state positioner consumption.
7. Group valves into simultaneous-movement scenarios.
8. Calculate average demand, scenario peak volume and scenario duration.
9. Add leakage, dryer purge and other instrument-air users.
10. Size compressors, dryers, receivers and distribution, and agree them with the process and safety teams.

How Reddy Compressors helps

Reddy Compressors, Tarnaka, Secunderabad, has worked on industrial compressed air since 2002. We are an independent multi-brand specialist, not an authorised dealer, and we sell, rent and service ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic and other compressors, including oil-free machines. We have tie-ups with all major and reputed brands. For instrument air we can:

• Review your instrument-air demand, receiver and standby arrangement.
• Supply compressors, desiccant and refrigerated dryers, filters and receivers, new or used/refurbished.
• Carry out air audits by the ISO 11011 method, including dew-point and pressure measurements and leak surveys (we do not issue conformity certificates).
• Provide rental compressors from our own fleet for shutdowns, commissioning or breakdown cover, including oil-free units.
• Service compressors and dryers under AMC and supply spares, through more than 20 service engineers and technicians covering Telangana and Andhra Pradesh, with supply across India.

Valve sizing, actuator selection and safety-instrumented functions stay with your process, instrumentation and safety engineers.

What to send us

• Number and types of actuated valves, or your actuator list.
• Minimum actuator supply pressure and present header pressure.
• Required dew point and air-quality class.
• Present compressors, dryers and receivers, with nameplate photos.
• Any shutdown or simultaneous-movement requirements.
• Problems seen: sticking valves, water in lines, low-pressure alarms.

Frequently asked questions

How much instrument air does a pneumatic actuator use?

Multiply the actuator’s swept volume by the absolute supply pressure and divide by atmospheric pressure to get free air per stroke. Add tubing and accessory volumes, and count both directions for double-acting designs. Multiply by strokes per hour for average demand, and divide by the required stroke time to find the peak flow the supply components must handle.

What dew point is needed for instrument air?

The dew point must stay well below the lowest temperature any instrument-air line will reach, so water never condenses or freezes. Process plants commonly specify desiccant-dried air at about −40 °C pressure dew point. Refrigerated dryers, typically around +3 °C, are suitable only for indoor systems that are never exposed to colder temperatures. Follow your plant’s design basis.

Should spring-return or double-acting actuators be used?

That is a valve and safety decision, not an air-supply decision. Spring-return actuators move to a defined fail position on air loss, which suits shutdown duties. Double-acting actuators give more output for their size but need reserve storage and logic to achieve a defined fail action. The choice changes air consumption, porting and reserve needs, so record it in the duty register.

How big should an instrument air receiver be?

Size it for the worst credible scenario, such as a compressor trip during which several valves must still complete their safe action. Add the free air those valves need, then calculate the receiver volume that supplies it while pressure falls from its normal level to the actuators’ minimum supply pressure. Use a check or priority valve so plant air cannot drain it.

Why do pneumatic valves stick in process plants?

Common causes are water in the air, oil varnish from carry-over, particles blocking positioners or solenoid valves, and low supply pressure at the actuator. Process-side causes such as packing friction or deposits also matter. Check the instrument-air dew point and filters first, then the actual pressure at the actuator during stroking, before changing the actuator.

Can instrument air and plant air come from the same compressors?

Yes, many plants do this, but instrument air needs its own dryers, filters and receiver, and priority over plant air. A priority or pressure-sustaining valve on the plant-air branch closes when pressure falls, so tools and cleaning hoses cannot starve the actuators. Keep standby compressor capacity so instrument air survives the loss of one machine.

Related pages

• Instrument air distribution to control devices
• Desiccant and adsorption compressed-air dryers
• Compressed-air receivers and storage engineering
• Refinery utility air system integration
• Power station service and instrument air
• Pneumatic application engineering centre