Whether the coil moves the valve itself or merely switches a pilot that does.

In a direct-acting valve the solenoid armature is connected to the main valve element and shifts it directly. The force available is whatever the coil produces, which limits the valve to small ports - but it means the valve works at any inlet pressure, including zero and vacuum, and responds very quickly.

In a pilot-operated valve the solenoid switches a tiny internal pilot, and system air pressure acting on a pilot piston shifts the main element. That allows much larger valves for the same small coil, which is why most solenoid valves above the smallest sizes are built this way.

The cost is a minimum operating pressure. Below it there is not enough force to shift the valve, and it simply does not respond.

So: small, any pressure, vacuum, fastest response points to direct-acting; larger flows at normal working pressure point to pilot-operated.

Almost always because it is internally pilot-operated and the pressure is below its minimum - and it is the first thing to check.

An internally piloted valve draws its pilot supply from its own inlet. If that inlet pressure is below the valve's stated minimum, the pilot piston cannot generate the force to shift the main element, and the valve does not move even though the coil is energised and warm.

It presents as a failed valve or a wiring fault, and people replace coils and check circuits before considering pressure.

The situations where it arises are commissioning at partial pressure, genuinely low-pressure applications, vacuum service, and systems whose pressure sags under peak demand.

The fixes are to select a direct-acting valve, or to select an externally piloted version and feed its pilot port from a reliable supply. Many valves can be converted by relocating a plug.

Check the data sheet's minimum operating pressure before assuming the valve is faulty.

It states the number of ports and the number of switching positions - the shorthand that defines what the valve can do in a circuit.

A valve described with two numbers gives ports first and positions second. A three-port, two-position valve has an inlet, an outlet and an exhaust, and switches between two states - the arrangement used to drive a single-acting cylinder or to generate a signal. A five-port, two-position valve has an inlet, two outlets and two exhausts, which is what a double-acting cylinder needs: one side pressurised while the other exhausts, and vice versa.

Three-position valves add a centre condition, and what happens in that centre matters: closed centre holds the cylinder in place, exhausted centre lets it float free, and pressured centre applies pressure to both sides.

Choose the centre condition deliberately - it defines what the machine does when the valve is de-energised mid-stroke, which is a safety question as much as a functional one.

Spring return wherever a defined state on power loss matters, which is most safety-related circuits.

A single-solenoid, spring-return valve has one coil and a return spring. Energise the coil and it shifts; de-energise it and the spring returns it to a known position. Lose power and it goes to that position too - so the circuit has a defined, predictable de-energised state.

A double-solenoid valve has a coil at each end and no spring. A pulse to one coil shifts it and it stays there; a pulse to the other shifts it back. It holds its last position when power is lost, which is useful where a cylinder must not move on power failure - but it also means the machine's state after a power loss depends on what it was doing at the time.

That is the safety question. If a defined safe state is required, spring return provides it by construction. If holding position is the safe outcome, a double solenoid does - but the risk assessment has to say which.

Operating the valve without electrical power - for commissioning, fault-finding and recovery.

Most solenoid valves have a small override, operated with a screwdriver or a fingertip, that shifts the valve mechanically. It lets a fitter move a cylinder during setup before the control system is running, confirm during fault-finding whether a problem is electrical or pneumatic, and free a machine that has stopped in an awkward position.

Overrides come in momentary types, which return when released, and detented types, which stay where they are put.

Detented overrides deserve caution. A machine with an override left latched behaves unpredictably, because that valve no longer responds to the control system - and it is not obvious from a panel. It is a recognised cause of confusing faults and of unexpected movement.

Make it a habit to check all overrides are released before returning a machine to service, and prefer momentary types where the risk of one being left on is significant.

Voltage and current, the connector type, ingress protection, and whether the coil is rated for continuous duty.

Coils are made in a range of AC and DC voltages, with low-voltage DC dominating modern machine building. The current drawn matters when many valves are driven from one PLC output card or when they are wired in parallel - the total can exceed what the output can supply.

Inductive coils produce voltage spikes when switched off, which can damage electronic outputs, so suppression is usually built into the connector or the valve and should be confirmed rather than assumed.

Connectors are largely standardised, and the type determines how quickly a valve can be replaced and how well it resists water ingress. Ingress protection rating should match the environment including washdown.

Most machine valves are energised for long periods, so confirm the coil is rated for continuous duty - and note that coils run warm in normal operation, which is not a fault.

From the valve's own shifting time plus the time to fill and exhaust the actuator - and the second usually dominates.

Manufacturers publish a valve response time, typically a few milliseconds for a small direct-acting valve and longer for a large pilot-operated one. That is the time for the valve element to move.

What the machine experiences is much longer, because after the valve shifts, air has to travel down the tubing and fill the cylinder before it moves. Tube length and bore, fitting restrictions and cylinder volume all add to it, and on a long run they add far more than the valve does.

So where speed matters, mounting the valve close to the actuator is usually more effective than buying a faster valve. Valve terminals mounted on the machine, or valves mounted directly on the cylinder, exist for exactly this reason.

Measure the actual cycle rather than calculating it, since fittings and tube behaviour are hard to predict.