Because isolating the supply is only half the job - the air already downstream has to go somewhere.

A ball valve closed in an air line stops further air entering. It does not release what is already in the pipework, the receiver volume of the machine, or the cylinders themselves. That stored air is at full pressure and is entirely capable of driving a cylinder through its stroke if a valve is operated or a component fails while someone is working on the machine.

A lockout valve does three things: it closes the supply, it opens the downstream side to atmosphere so the system exhausts to zero, and it accepts a padlock in that position so it cannot be restored.

That is what makes it an energy isolation device rather than a shutoff. Lockout procedures generally require the hazardous energy to be dissipated and the isolation secured, not merely interrupted.

Check that any valve intended for lockout duty exhausts downstream and will accept a lock - not all manual valves do.

It restores pressure to a machine gradually rather than instantly, and it is needed wherever a sudden repressurisation would move something.

When a machine is isolated and exhausted, its cylinders come to rest wherever gravity and the mechanism leave them. Restore full pressure in an instant and every cylinder is driven hard to whichever end its valve is currently commanding - fast, with full force, and with no warning.

That is dangerous if anyone is near the machine, and it damages the mechanism and the product.

A soft-start valve admits air through a restricted path so the system pressure rises slowly. Cylinders move gently to their commanded positions. Once pressure reaches a set threshold, the valve opens fully and the machine runs normally.

Soft-start and exhaust functions are commonly combined in one unit with the lockout function, so a single valve isolates, exhausts, locks and then re-pressurises safely - which is why these appear at the machine's air inlet as standard practice.

At the point where the machine's air supply enters it, accessible and clearly identified - not somewhere convenient for the pipefitter.

An isolation valve is used by whoever needs to make the machine safe, often in a hurry and often not by the person who installed it. It should be at the machine, reachable without climbing or opening a guard, and labelled so it is unambiguous which machine it isolates.

A valve buried at high level, behind a panel or in a group of identical valves serving several machines is a valve that will be operated wrongly at some point.

Where a machine has more than one air supply - a secondary feed to a clamp or an accumulator - every one must be isolated, and the procedure must name them all. Machines with a single obvious inlet and a hidden second supply are a recognised cause of lockout failures.

Document the isolation points on the machine and in the procedure, and verify by testing that isolating them actually removes all stored energy.

Enough that the valve does not become the restriction in the supply - sized on flow, not on the pipe thread it happens to match.

A valve's port thread tells you what will screw into it, not how much air it will pass. Two valves with the same thread can have substantially different internal flow paths and therefore different flow capacities, usually published as a flow coefficient or as a standard flow rate at a stated pressure drop.

On an isolation valve feeding a whole machine, the flow required is the machine's peak demand, not its average - a machine firing several cylinders simultaneously draws far more air for a moment than its steady consumption suggests.

An undersized isolation valve shows up as a machine that behaves sluggishly under load or cannot fire fast sequences, and it is easy to overlook because the valve is upstream of everything and seems unrelated.

Size on peak flow with an allowance, and check the pressure drop figure rather than matching thread sizes.

Manual directional valves are, but that is a different job from isolation and the two should not be conflated.

A manual directional valve - operated by a lever, knob or button - is a legitimate control element, used to drive a cylinder by hand on a jig, a clamp or a simple machine with no automation. It sits in the circuit and switches air between ports.

An isolation valve is a safety device. Its purpose is to remove energy from the machine entirely, and it should not be used as a routine control - a valve operated many times a shift as an on/off control is not being treated as an isolation point and will not be found in that state when it matters.

Where both functions are wanted, fit both: an isolation valve at the machine inlet, used for maintenance and labelled as such, and separate control valves for operation.

Mixing them tends to end with the isolation valve wired around or left permanently open.

A valve that vents trapped air in a circuit that would otherwise stay pressurised after isolation - closing a gap that a normal exhaust does not cover.

Exhausting a machine at the inlet drops the pressure in the supply pipework, but sections of the circuit can remain charged: air trapped between a closed valve and a cylinder, a volume behind a non-return valve, or an accumulator fitted to hold pressure deliberately.

That residual air can still drive a movement. Residual pressure relief valves are fitted at those points to vent them when the machine is isolated, so the whole circuit reaches zero rather than just the parts connected directly to the inlet.

Identifying where they are needed comes from examining the circuit rather than from a general rule - the trapped volumes depend on the specific valve arrangement.

This is worth doing properly on any machine where people work inside the guarding, because a circuit that reads zero at the gauge can still have a charged pocket in it.

The usual range - brass and aluminium bodies as standard, stainless and specialist materials for washdown, corrosive and food environments.

Standard industrial valves are typically brass or anodised aluminium with nitrile seals, which suits general plant air in a dry factory.

Washdown, food and pharmaceutical areas need stainless bodies, seals compatible with the cleaning chemicals used, and an ingress protection rating suited to high-pressure hosing - and the cleaning regime is often more aggressive than the process itself.

Corrosive and marine environments may need stainless or specially coated components, and outdoor installations need attention to temperature range and to water ingress at the exhaust port, which is a route in that is easy to forget.

Specify the environment including the cleaning method, not just the process. Valves are frequently chosen for the air they carry and destroyed by what is sprayed on them.