Hot Metal Detectors: What to Specify Before You Order

A hot metal detector looks like a simple device. It watches for the infrared signature of hot stock and gives a signal when it sees one. In practice HMD installations fail more often than almost anything else on a rolling mill, and they fail for reasons that are decided at specification, not at installation.

This guide sets out what actually needs to be established before ordering, based on the enquiries that come to us and the ones that come back six months later.

What the device is doing

Hot stock radiates. A hot metal detector is an infrared receiver aimed at a defined point in the pass line, with the sensitivity and optics arranged so that it responds to the radiation from the stock and ignores everything else in the mill.

Everything difficult about HMD selection follows from that second clause. A rolling mill is full of hot objects: adjacent stands, furnace doors left open, glowing scale on the floor, steam backlit by something hot, a neighbouring line. The specification problem is not detecting the bar. It is not detecting anything else.

1. Field of view and mounting distance

These two are a single decision, because field of view at the target is a function of the optical angle and the distance.

Mount too close and you lose tolerance — a bar that wanders laterally leaves the field and the detector drops out mid-pass. Mount too far and the field widens until it takes in the stand behind, and the detector never drops out at all because it can always see something hot.

Establish the actual mounting distance before selecting the device, and establish the lateral wander the pass line really has, not the nominal centre line. On a roughing mill that difference can be substantial.

2. The smallest target you must detect

Specify by the smallest and coolest stock the line runs, not the typical product. A detector configured around a 100 mm slab at full temperature will miss the tail end of a thin section that has cooled on a delay. The awkward end of the product range sets the requirement.

3. Response time against line speed

On a fast finishing mill the stock may be in the field of view for a very short time, and the detector output has to be present long enough for the PLC scan to catch it. Two separate things need checking: that the detector responds fast enough, and that the control system samples fast enough to see the response.

An HMD that works perfectly on the roughing side and misses intermittently on the finishing side is almost always this problem rather than a faulty device.

4. Temperature threshold

Too sensitive and the detector picks up residual heat, scale and radiated background. Too insensitive and it misses cooled tail ends. The threshold should be set from the coolest stock that must be detected, with margin, and then checked against the hottest background that must be ignored. If those two overlap, the geometry needs changing — no threshold setting will resolve it.

5. The environment the device has to survive

A rolling mill is a hostile place for electronics. Three things matter:

• Ambient temperature at the mounting point. Not the mill ambient — the temperature where the device actually sits, which near a stand can be far higher.
• Cooling and air purge. Most mill installations need cooling for the housing and a purge to keep the optical window clear. An air purge is not an optional extra; a dirty window is the most common cause of a detector that gradually stops working.
• Water, scale and mechanical damage. Descaling spray, cobbles and maintenance traffic all reach places the designer assumed were protected.

Plan the purge air supply at the same time as the detector. Retrofitting it is expensive and frequently never happens.

6. Output and integration

Confirm what the control system expects — relay contact, transistor output, or an analogue signal — and the supply voltage available at the mounting point. On older mills the panel may have no DC supply where you need it, which changes the specification.

7. Alignment and maintenance access

The detector will need aligning during commissioning, and realigning after any mechanical work nearby. A device mounted where nobody can reach it safely with the line running will drift out of alignment and stay there. Specify the mounting bracket and access at the same time as the detector.

The three mistakes we see most often

Specifying from the catalogue instead of the geometry. The model number matters far less than field of view at the actual mounting distance.

No air purge. Works for three months, degrades slowly, gets blamed on the device.

Ignoring what else is hot. The single most common cause of false triggering, and the hardest to fix afterwards because it usually requires moving the mounting position.

Applications beyond rolling mills

Hot metal detectors are specified wherever hot stock has to be tracked without contact — continuous casting, forging lines, billet handling, furnace charging and discharge, shear control and cut-to-length. The specification logic is the same in each case: geometry first, timing second, environment third.

Getting a recommendation

Send us the mounting distance, the smallest and coolest stock to be detected, the line speed, what else is hot within the likely field of view, the ambient temperature at the mounting point, whether purge air is available, and the output your control system needs. A photograph or sketch of the intended mounting position is worth more than any of the individual numbers.

For background on how HMDs are used across a mill, see our overview of hot metal detectors in rolling mills.