The short answer
TPH is tonnes per hour through the feed end with everything running. A 250 TPH plant on two shifts does not make 250 x 16 x 30 tonnes a month. After working days, stoppages inside the shift, product mix, weather and the pace at which the road can take material, plan on 25,000 to 30,000 tonnes a month, or roughly 1,000 to 1,150 tonnes a working day.
The rating plate on a crusher plant is a true figure about a narrow thing. It says what the primary can take in an hour when the feed is well broken, the rock is of average hardness and nothing downstream is holding it back. A programme built on that number multiplied by the calendar will be short by the end of the first quarter.
01From nameplate to dispatch
| Step | Allowance | Tonnes a month |
|---|---|---|
| Nameplate x clock | 250 TPH x 16 h x 30 days | 120,000 |
| Working days | 26 of 30 | 104,000 |
| Hours actually crushing | 11 of 16 | 71,500 |
| Feed rate on graded road products | About 60% of nameplate | 42,900 |
| Weather, breakdowns, liner changes | About 15% lost | 36,500 |
| What the site can take and stock | Laying fronts, yard space, tippers | 25,000 to 30,000 |
The allowances are planning figures from our own plants. A dry season on an easy rock with a hungry road will beat them. A monsoon month will not come close.
02What each line is
Hours actually crushing
A sixteen-hour day loses time at both ends of each shift, at meals, during the daily greasing and inspection round, and every time the quarry clears for a blast. It also loses time waiting for rock. The plant can only crush what the excavators and tippers put in the hopper, and a face that is behind on drilling starves it.
Feed rate on graded products
The nameplate assumes a coarse product. Road gradings ask for a good share of 20 mm, 10 mm and fines, which means stone going round the secondary and tertiary circuit more than once. Every tonne that recirculates takes the place of a new tonne at the feeder. Harder rock slows it further, and a tight closed side setting slows it again. The machines behind this are described in jaw, cone and VSI.
What the site can take
This is the line nobody puts in a brochure. A plant on a highway package makes stone for a road that is being built in fits and starts. Laying waits on land, on bitumen, on approvals and on weather. When the fronts are not taking material the yard fills, and a full yard stops the plant as surely as a broken belt. The plant's month is set by the road's month.
03Sizing a plant for a package
Work back from the total stone the package needs and the months in which it can really be produced. Leave out the monsoon months and the mobilisation month, then compare what is left with a realistic monthly figure.
- Total stone required
- 9,00,000 t
- Construction period
- 30 months
- Less mobilisation and two monsoons
- 7 months
- Effective producing months
- 23
- One 250 TPH plant at 27,500 t a month
- 6,32,500 t
- Required rate
- 39,100 t a month
One 250 TPH plant falls about 2.7 lakh tonnes short. The choices are a larger plant, a second plant for the peak, a third shift through the dry season, or buying in the balance. Each has a cost and a lead time, and all of them are cheaper to arrange at award than in month fourteen.
04A third shift is not fifty per cent more
Running round the clock adds eight scheduled hours. It also removes the gap in which maintenance was being done. Liners still wear, belts still need tracking, screens still need changing, and now each of those stops the plant during production instead of between shifts. A third shift is a good answer for a few dry months and a poor one as a standing plan.