How to Determine Whether Grate Bars Need Replacement?

Whether grate bars need replacing can be judged from four aspects: appearance, operating signals, dimensional accuracy and service intervals. Bars whose wall thickness is worn beyond 1/3 of the original thickness, or that show through cracks, should be replaced at once.

Appearance and dimension checks

1. Damage, cracks and breakage: through cracks, large losses at the edges, missing teeth, holes, burn-through, fracture or missing sections leave the bar unable to engage and overlap normally. Dense thermal fatigue crazing and repeated cracking and repair mean the strength is no longer sufficient, so continued use easily leads to grate jams, lost bars, seizure and shutdown, even stalling and chain breakage. Where the pin holes of a grate bar are badly worn, the bore enlarges, the pin sits loose, shifts, wobbles and may even drop out.
2. Deformation, warping, elongation and arching: overall twist, wavy deformation, arching, unevenness, obvious thinning and creep elongation leave the bar out of line with the neighbouring bars, with abnormally wide gaps. This leads to heavy coal leakage, local burn-through, jamming and drifting, and also scrapes the furnace wall and the coking box, with harsh noise during operation. A chain grate has a fixed pitch, and permanent elongation disrupts the overall grate pitch and engagement accuracy, so the bar cannot be kept in service.
3. Wear, corrosion and oxidation burning: marked thinning of wall thickness, honeycomb corrosion pits, large areas of oxide scaling, and pitting from sulphur attack and molten slag. Wear beyond 1/3 of the original thickness generally calls for replacement.
4. Surface material degradation: stubborn clinker penetrating the matrix, brittle material that crumbles at a touch, and embrittlement ageing show that the material has seriously deteriorated. After repeated high-temperature ageing the internal microstructure degrades and strength drops sharply.

Abnormal operating signals

A clear abnormal increase in coal leakage means the gaps have widened and the overlap has failed, so a large amount of coal leaks into the wind box, which both wastes fuel and may burn out the lower parts and cause secondary combustion. Noise, jamming, drifting and shaking during grate operation usually come from obstruction and rubbing that raise the chain drive load abnormally, tripping motor overload alarms and frequent breaker trips, and can even stop the grate.

Unstable combustion, local air leakage, one-sided burning, flame spitting out, and damage to the side walls and frame come from uneven gaps, excessive local air leakage or flame passage that upsets normal air distribution, raising furnace temperature and worsening clinker. Frequent breakdowns and repeated repair welding mostly follow frequent lost bars, jams and damaged pins, which disrupt continuous boiler operation and create safety hazards. Abnormal local temperature and overheating may mean a broken bar is passing flame onto the grate supports and the wind box structure.

When measuring thickness, pitch and fit dimensions, thickness worn beyond the limit, pitch stretched beyond the limit, and failed engagement teeth and slots that no longer mesh with the neighbouring bars or the sprocket call for replacement. Where the drive teeth of a driving bar are worn and cracks appear at the tooth root, affecting the whole drive, replacement should be given priority.

Service intervals and replacement rules

Routine inspection is a visual check of the combustion zone, both side edges and the slag return section every shift or every day. Shutdown maintenance combined with the annual inspection and intermediate overhaul covers full strip-down, thickness measurement and flaw detection, focused on thermal fatigue cracks and internal hidden cracks. Repair welding is limited to slight surface cracks and small local loss, while through cracks, severe creep deformation, major thinning of wall thickness and overall embrittlement must never be welded repeatedly and should be replaced directly. An aged batch of bars should not be replaced only a few pieces at a time, to avoid mismatched thickness and pitch between old and new bars causing jams across the whole grate.

Medium-silicon nodular iron RQTSi5 grate bars normally last about 1~3 years depending on duty, and at the end of that period they should be sampled and assessed even if they still look sound. High-sulphur and high-temperature duty shortens life markedly, so inspection intervals must be shortened. Bars must be replaced immediately where there are through cracks, fractures, large burn-through areas or severe thinning of wall thickness, severe overall deformation, abnormal pitch, jamming and drifting with the risk of chain breakage and stalling at any time. The same applies where the material has become brittle and extensive hidden cracks affect safe boiler operation.

A replacement log is recommended, recording replacement dates, duty and failure types, and a preventive replacement cycle is built on that record. This avoids running a sick grate into a major outage.