Because it is an electrical component running warm and continuously, while the valve is a mechanical one running in clean air.

A coil is a winding of fine wire with insulation between the turns. Current through it produces heat, which is inherent rather than a fault, and that heat gradually degrades the insulation. Add ambient temperature from a warm machine, and the coil runs hotter still.

It is also exposed to voltage transients from switching, to vibration that can fatigue connections, and to moisture from washdown or condensation.

The valve body meanwhile is a spool in a bore, moving in filtered air, with no electrical stress at all. Rated cycle lives run into the tens of millions.

So the mismatch is structural: the two components have very different life expectancies and the electrical one governs. That is why coils are designed to be replaced, and why holding spare coils rather than spare valves is the sensible stocking policy.

From the valve model, the coil voltage and the connector type - and the voltage is the one that catches people out.

Coils for the same valve family in different voltages are physically identical: same diameter, same length, same mounting. Nothing about a coil's appearance reveals whether it is a low-voltage DC type or a mains AC one.

Fitting the wrong voltage produces one of two failures. Too low and the coil cannot generate enough force to shift the valve, so it hums or does nothing. Too high and it draws excessive current, overheats and burns out - sometimes within minutes.

The voltage is printed on the coil body, along with the part number and often the power consumption. Record both while the machine is running.

Also confirm the connector style, since several standard form factors exist and they are not interchangeable, and check whether the original included integral suppression.

On most designs yes - the coil is outside the pressure boundary - but the electrical supply must be isolated and the machine's state considered.

The coil sits over the armature tube, which is a sealed component containing the moving armature. The coil itself never contacts the compressed air, so removing it does not open the pneumatic circuit.

So mechanically it is possible to change a coil with the air on. Whether it is safe is a different question: removing the coil de-energises the valve, so a spring-return valve will shift to its rest position and whatever it controls will move. On a machine with a load in a raised or clamped position, that matters.

Electrically, the supply must be isolated - the coil terminals are live when energised, and working on them otherwise is both a shock risk and a way of shorting a PLC output.

The safe procedure is to bring the machine to a known state, isolate electrically, and then decide whether the pneumatics also need isolating for the specific valve.

Whether the coil is designed to be energised continuously or only intermittently - and most machine valves need continuous rating.

A coil rated for continuous duty can be held energised indefinitely without exceeding its temperature limit. One rated for intermittent duty is designed to be energised for a proportion of the time and to cool between operations.

In machine automation, valves are frequently held energised for long periods - a clamp that stays closed through a process, a valve holding a cylinder extended for a whole shift - so continuous rating is what is generally required.

Fitting an intermittent-rated coil in a continuously energised position produces overheating and early failure, and the cause is easily overlooked because the coil is otherwise correct.

Coils normally run warm to the touch in normal operation, which is not a fault. What matters is whether the temperature rise is within the coil's rating for the way it is actually used, including the ambient temperature at the machine.

Applying a higher voltage briefly at switch-on and then reducing it - which gives fast operation with low heat.

A coil needs considerably more force to pull the armature across the air gap than to hold it once closed. A coil sized for the pulling force therefore dissipates far more power than necessary while holding.

An over-excitation circuit applies a high current briefly to shift the valve quickly, then drops to a much lower holding current. The valve operates faster than a normally-driven coil and runs considerably cooler, so it can be held energised indefinitely and its life is extended.

The circuit may be built into the coil, into the connector, or provided externally.

It is used where fast response matters, where valves are held energised for long periods, and where many valves in an enclosure would otherwise generate too much heat.

When replacing a coil in such a system, confirm whether the electronics are in the coil or the connector - fitting a plain coil where an electronic one was will change the behaviour.

Matched to the environment including the cleaning method, which is frequently more demanding than the process.

A coil in a dry assembly cell needs little protection. The same valve in a food plant is washed down with hot water and detergent at pressure, and that is a far harder test than anything the process presents.

Ingress protection ratings cover dust and water, and the second digit matters here: resistance to jets and to high-pressure jets are different ratings and different constructions.

The connector is usually the weak point rather than the coil, since it is a separable joint. Correctly fitted with its seal and gasket, a rated connector achieves the coil's rating; fitted carelessly or with the seal omitted, it does not - and water tracking into a coil is a common washdown failure.

Specify the rating for the actual cleaning regime, and make correct connector assembly part of the maintenance procedure rather than assuming it.

Not necessarily - coils are designed to run warm - but a change in temperature is worth investigating.

A solenoid coil converts electrical energy into a magnetic field and heat, continuously, whenever it is energised. Being warm or hot to the touch is normal, and manufacturers specify a permissible temperature rise above ambient rather than an absolute limit.

What matters is whether it is hotter than it should be. Causes of genuine overheating include the wrong voltage coil fitted, an intermittent-rated coil in continuous service, a valve that is not shifting so the armature never closes the magnetic circuit, high ambient temperature, or a supply voltage above nominal.

A valve that fails to shift is the case worth knowing: with the armature held open by an obstruction or insufficient pressure, the coil draws more current than normal and overheats - so a pneumatic problem destroys the coil.

If a replacement coil also burns out, look for the underlying cause rather than fitting a third.