It uses system air pressure acting on a piston to produce the force that shifts the main element, so the signal only has to switch that pilot.

Shifting a large valve spool against system pressure, seal friction and spring return can require a substantial force. Producing that force directly - with a solenoid, a lever or a diaphragm acted on by the signal itself - would need a large and expensive actuator.

In a pilot-operated valve, the signal is admitted to a small pilot chamber. Pressure there acts on a pilot piston with a useful area, generating a large force, and that force does the work of shifting the main element.

So the signal source only has to supply a small flow at pilot pressure - which a tiny solenoid, a whisker valve or a length of small-bore tube can do easily.

The energy comes from the compressed air system, not from the signal. That is the whole principle, and it is why very large pneumatic valves can be switched by very small devices.

Whether the pilot air is taken from the valve's own inlet or from a separate line - and it matters at low pressure and in vacuum work.

An internally piloted valve takes its pilot supply from the main inlet port inside the valve body. That is simple: one connection does everything. But it means the valve cannot operate when the inlet pressure is below its minimum pilot pressure, and it cannot work at all if the inlet is under vacuum or at zero.

An externally piloted valve has a separate pilot port fed from its own supply. That allows the main flow to be at any pressure - low, zero, or vacuum - while the pilot is held at a reliable working pressure.

External piloting is therefore required for vacuum service, for low-pressure applications, and where the main supply pressure varies widely.

Many valves can be converted between the two by moving a small plug or screw, which is worth knowing - a valve that will not shift at low pressure may simply need external piloting selected.

The lowest pressure at which the pilot can generate enough force to shift the valve - and below it the valve simply does not operate.

Every pilot-operated valve has a stated minimum pilot pressure. Above it the valve shifts reliably; below it the pilot piston cannot overcome friction and the return spring, and nothing happens.

It catches people out in three situations. During commissioning, when system pressure is being brought up slowly and valves fail to respond. On low-pressure applications, where the whole system runs below the valve's minimum. And on systems whose pressure sags under peak demand, so valves work most of the time and intermittently do not.

The last is the difficult one, because it presents as an unreliable valve rather than a supply problem.

Check the minimum pilot pressure against the lowest pressure the system will actually see, not the nominal setting - and use external piloting where the main pressure cannot be guaranteed.

Wherever the valve must be at the machine but the control should not be, or where electricity is unwelcome.

The valve handling the main flow generally needs to be close to the actuator, to keep response fast and avoid wasting air in long lines. The control point may need to be elsewhere - on a panel, at an operator position, or in a safe area.

A pilot line lets the two be separated. A small-bore tube carries the signal, and because it carries almost no flow it can be long and thin without significant delay.

The classic case is a hazardous area: the valve and the actuator sit in the hazardous zone with no electricity at all, and the pilot signal is generated in a safe area by a solenoid valve. That avoids the cost of certified electrical equipment entirely.

Other uses are control from several positions, interlocks assembled in air logic, and machines where the control panel is remote from the working head.

There is a limit: very long pilot lines do introduce delay, so response should be checked on long runs.

Pilot signals can be combined with simple elements to make logic functions, so a circuit can make decisions without any electronics.

Because a pilot-operated valve is switched by a pressure signal, those signals can be manipulated. A shuttle valve passes whichever of two inputs is present, giving an OR function. A dual-pressure valve passes an output only when both inputs are present, giving AND. A normally-open valve inverts a signal to give NOT.

Combining these with pilot-operated valves builds sequencing, interlocking and safety logic entirely in air.

That approach is far less common than it once was, since a small PLC does the same job with more flexibility. But it persists where electricity is prohibited or impractical - explosive atmospheres, deep mining, some underwater and remote applications - and it has the property that the whole system fails safe when air is removed.

Understanding it is also useful when maintaining older machines built that way, which are still in service in numbers.

From small valves to very large ones - and pilot operation is precisely what makes the large sizes practical.

Direct-acting valves are limited by the force the actuator can produce, which restricts them to small ports. Once pilot operation supplies the shifting force from system air, the main element can be as large as required, since the pilot piston is sized to suit.

So pilot-operated valves are available with very large ports and correspondingly high flow capacities, used for main air isolation, large actuator control, dust collector pulse valves and process duties.

As always, size on the flow coefficient at the working pressure drop rather than on the port thread. Large valves differ substantially in internal flow path even where the connections match.

Also check the shifting time on large valves - a big element takes a measurable time to move, and where fast response matters that figure should be compared rather than assumed.

Clean, appropriately lubricated air and attention to the pilot path - which is small enough to be blocked by contamination that the main flow passes without noticing.

The main flow path in these valves is generous and tolerates a good deal. The pilot passages are not: they are small-bore drillings and orifices, and particulate, sludge or desiccant dust from a failed dryer will block them.

The symptom is a valve that shifts slowly, intermittently or not at all, while the main air supply appears perfectly healthy - which sends people looking at the wrong thing.

So filtration ahead of the pilot supply matters more than its size suggests, and pilot lines should be blown through when a system has been disturbed or has had a filter failure.

Beyond that: check whether the valve requires lubricated air, since running a lubricated valve dry wears the seals, and confirm the exhaust is unobstructed - a blocked exhaust prevents the valve returning as surely as a blocked pilot prevents it shifting.