Proximity Sensors for Steel Plants and Rolling Mills: What Survives

A steel plant destroys sensors in a small number of very predictable ways. Once you know what they are, specification becomes a matter of designing around each one rather than hoping a high IP rating covers everything.

These are the five failure mechanisms we see, in roughly the order they cause problems.

1. Radiant heat, which is not the same as ambient temperature

The mistake is specifying against the plant ambient. A sensor mounted two metres from hot stock is not at ambient — it is receiving radiant heat continuously, and its case temperature can be far above the air around it.

Three responses, in order of preference:

• Move it. Distance is the cheapest heat shield there is, and a mounting position half a metre further back often solves the problem permanently.
• Shield it. A simple steel plate between the sensor and the radiating source blocks the line of sight. Effective and cheap.
• Cool it. Air purge or water cooling where the first two are not possible. Reliable, but now you have a service that can fail.

Measure the actual case temperature at the intended position during normal production before ordering. It is a ten minute job and it prevents a recurring replacement.

2. Scale

Mill scale is conductive, magnetic and abrasive, and it accumulates on everything. On an inductive sensor face a scale build-up can trigger the sensor permanently, which presents as a sensor that is stuck on and gets replaced unnecessarily.

Mount sensors facing downward or vertically where possible so scale falls away rather than collecting. Where the face must point up, plan for cleaning access, because it will need it.

3. Descaling water and cooling water

High-pressure descaling spray is closer to an IP69K condition than an IP67 one, and it comes with thermal shock: a hot sensor hit by cold water contracts rapidly and draws water past marginal seals. See our note on IP67, IP68 and IP69K for what each rating actually tests.

Specify for the spray, and point cable entries downward. An upward-facing cable gland in a wet area collects standing water and fails eventually regardless of its rating.

4. Vibration and mechanical damage

Mills vibrate, and cobbles happen. Two consequences worth designing for:

Mountings loosen. A sensor that drifts out of position reads intermittently and gets blamed on electronics. Use locknuts on both sides, and check alignment as part of routine maintenance rather than after failure.

Things get hit. A sensor in an exposed position will eventually be struck by stock, by scale, or by a maintenance trolley. Where you cannot move it out of the way, a simple guard bracket costs very little and saves the sensor.

5. The cable, which is what usually actually fails

In our experience of mill installations, the sensor is rarely the first thing to go. The cable is — chafed on a sharp edge, crushed under something, cooked against a hot surface, or pulled out of the gland by thermal movement.

Specify the cable properly. Route it away from heat and away from sharp edges. Support it so the gland carries no weight. Leave a service loop. And where the run is long, consider specifying a made-to-length cable so the first joint is outside the hostile zone rather than inside it.

What to specify where

Position and limit sensing on cold or warm sections — D30C inductive switches, sized with generous sensing distance so the standoff can be increased for heat and scale clearance. M30 and above rather than M12 where space allows.

Hot stock detection and tracking — hot metal detectors, which sense radiated infrared and therefore need no proximity to the stock at all. Specification is a geometry problem; see what to specify before ordering an HMD.

Long-distance presence detection across a pass line — D31C photoelectric in through-beam mode, specified with substantially more range than the gap so there is margin as the lenses foul.

Roll and shaft speed, and direction — magnetic speed sensors from the D33C range, which detect through non-ferrous material and tolerate the environment better than optical encoders.

Screwdown, roll gap and hydraulic cylinder position — this is not proximity switching but absolute position measurement, and it wants a Novotechnik transducer. For high-cycle or wet positions the touchless rotary series are the appropriate choice.

Hazardous areas in the gas plant or by-products section — NAMUR switches with a suitable amplifier. See specifying intrinsically safe NAMUR loops.

Standardise the spares holding

A practical point that matters more than any individual specification. Mills accumulate sensor variety — a dozen manufacturers, forty part numbers, and a stores holding that cannot cover any of them properly.

Rationalising to a smaller number of variants across the plant reduces stockholding, makes substitution possible during a breakdown, and means the maintenance team knows the parts. If you want to do that exercise, send us the current sensor list and we will map it to the smallest set of equivalents that covers it.