These days, many facilities use pneumatically actuated valves for both on/off (shutdown) valves and throttling control valves. If you’re trying to size an air compressor for your plant, you need a practical way to approximate air consumption without waiting for every vendor datasheet.

Below are some rule-of-thumb heuristics you can use for preliminary instrument-air sizing. Final design should always be confirmed with vendor data and your project’s design basis.

1) Air Consumption Heuristics

These values are estimated rates and can vary with valve actuator size and design.

Control valves: steady vs transient

Control valves usually regulate a process variable such as flow in pipes (or pressure/level/temperature) to match a setpoint. Once the process reaches the setpoint, the valve is mostly holding that position, so we treat it as steady state. When the loop is actively chasing a setpoint (or the process is disturbed and the valve is moving), we treat it as transient.

A valve positioner regulates the air pressure supplied to the actuator so the valve stem reaches (and holds) the required position. During transient movement, the positioner needs higher airflow because it must rapidly pressurize and vent the actuator chambers to overcome actuator, packing, spring, and process forces.

For preliminary sizing, we don’t try to predict every movement pattern. Instead, we assume a fraction of valves are “moving” at any moment, as you’ll see in the section below.

On/off valves at steady state

Unlike pneumatic control valves, which consume air continuously due to positioner bleed regardless of whether they are in steady-state or transient operation, pneumatic on/off valves typically only consume air during transient movement. This is because compressed air is only needed to pressurize or vent the actuator chamber to change valve position, after which the chamber is generally sealed and retains pressure without continuously bleeding air. “Transient” does not mean the valve consumes that rate all the time. It is only while it is moving.

Similarly, in early-stage sizing, we treat a fraction of all on/off valves as “moving” to represent expected worst-case behavior.

2) Total Instrument Air Demand

A practical way to estimate total demand is to assume:

• Most control valves are in steady state (unless your process requires frequent setpoint changes)
• A smaller fraction of control valves are transient
• A fraction of all on/off valves may stroke simultaneously during normal operation or due to an upset.

The table below outlines normal-demand and peak-demand scenarios:

These split factors are useful for early checks. Later in the project, you’ll replace them (or validate them) using credible operating scenarios (including upsets).

3) Utility Air

Utility air is the “shop air” used for general-purpose connections (maintenance, cleaning, air tools, hose stations, etc.). If you want a quick estimate, a simple rule of thumb is:

Utility air demand ≈ 0.5 scfm per utility hose / station

4) Compressor Sizing Workflow

This is the workflow I typically use at a high level during early project stages:

1. Calculate instrument air demand for valve instrumentation using either the normal or peak case (select the maximum demand case for your sizing basis).
2. Add a margin for growth/unknowns as needed (up to 25% is common at this stage).
3. Calculate utility air demand (if included in the same compressor) using the hose/station assumption.
4. Account for other plant air users if applicable (process equipment that consumes air, vessel pressurization, purges, packages, etc.). If the plant uses air directly in the process, that demand must be added explicitly.
5. Total the flows and identify the highest required delivery pressure among all users (that will drive your compressor discharge and package tie-in requirements).
6. Don’t forget to consider distribution losses.
7. Engage compressor vendors with:
   • required flow at the package tie-in point,
   • required pressure at the package tie-in point,

Vendors will then size the compressor and the package components (dryer, filters, etc.). Note that compressor packages typically have dryer-related losses, so it’s better to communicate your requirements clearly at the battery limit and let the vendor design around the package details.

Air Receiver Vessel

You’ll also want to communicate your receiver vessel requirement. The receiver provides buffering for credible scenarios like:

• multiple valves stroking simultaneously beyond your assumed peak,
• brief compressor upsets/trips,
• maintaining air long enough for critical fail-safe actions.

At a very high level, the amount of usable compressed air stored in an air receiver vessel depends on both the vessel volume and the pressure range over which the vessel can operate. A larger vessel stores more air simply because it has more physical volume, while a higher vessel pressure stores more air because the air is compressed to a greater density.

The amount of usable air in the receiver is proportional to the following equation:

V × (P_high − P_low)

Where:

• P_high = the normal maximum operating pressure of the receiver vessel
• P_low = the minimum pressure at which the downstream instruments and pneumatic devices can still operate reliably

As compressed air is withdrawn from the receiver, the vessel pressure gradually decreases. The larger the allowable pressure drop between P_high and P_low, the more usable air can be extracted from the same vessel before the pressure becomes too low for the plant instrumentation. Therefore, for a given vessel size, increasing the allowable pressure range effectively increases the usable stored air capacity of the receiver.

Receiver sizing can be a blog post on its own. Join the email list to receive updates when I release my next blog.