Two Hours of Labour per Tanker: The Cost Line That Never Gets Quoted

The spreadsheet said 46 dollars per cubic metre. The year-end accounts said 61. Between the two numbers sat a line that does not appear on any quotation: the time two operators spend making a batch, testing it, cleaning the mixer and waiting for the tanker. At one site that came to 4.2 man-hours for a 20 cubic metre batch, which at a loaded rate of 38 dollars an hour is 160 dollars, or 8 dollars per cubic metre, before supervision and before any re-work.

Labour is the line that decides whether a small campaign is worth running at all, and it is almost always estimated from the plant’s nameplate rather than from a stopwatch.

Where the Hours Actually Go

Time a batch from the moment the operator walks to the shed to the moment he signs the sheet, on three separate occasions, and the shape of the day becomes uncomfortable.

• Getting the sump valve set and the feed running: 15 to 25 minutes, worse if the valve is manual and the sump is 200 metres away.
• Bag handling and dosing: 20 to 30 minutes for 500 kilograms, more if the bags are 25 kilograms and stored at the far end of the yard.
• Mixing: 10 to 16 minutes, plus the top-up.
• Testing and sampling: 15 minutes, plus the wait if the test has a holding time.
• Discharge to the tanker: 25 to 40 minutes, and this is where the estimate usually fails — a firm gel discharges slower than a thin one.
• Clean-down: 20 to 30 minutes. A gel batch leaves a film on everything, and a mixer that is not cleaned before the next batch will carry set lumps into it.

Add it up and the honest figure is between three and five man-hours per 20 cubic metre batch, depending on how far apart the sump, the mixer and the tanker stand.

The Counter-Intuitive Result

The site that cut its polymer dose by 8 percent to save money added 40 minutes to every batch. The batches needed longer mixing, a top-up, and a second test because the first one was marginal. The polymer saving was 3.50 dollars per cubic metre; the labour added was 4.20, and the re-work on the marginal batches added more on top.

Over-dosing slightly is often cheaper than dosing exactly, once labour and testing are counted. Not by a lot, and not indefinitely — the 15 percent over-dose that everyone worries about is a real cost — but the optimum for a site with expensive labour sits higher than the optimum for the chemical alone.

Cutting the Hours Without Cutting Corners

The big wins are layout, not effort. Put the polymer store within 20 metres of the mixer and on the same level. Put the sump valve on a float or a timer so nobody walks to it. Use 500 kilogram bulk bags with a hoist rather than 25 kilogram sacks, and you remove about 15 minutes of lifting per batch along with a good deal of the dust.

Second win: batch size. Doubling from 10 to 20 cubic metres does not double the labour. The fixed costs — setup, testing, clean-down — stay roughly constant, so the labour per cubic metre falls by around 30 percent. Sites running small batches for no better reason than tank size are paying for that choice twice.

Third: test in parallel with discharge rather than before it, where the acceptance arrangement allows. Fifteen minutes saved per batch is an hour a day at four batches.

What to Put in the Model

A cost model that survives contact with the accounts has five lines: polymer, other chemicals, haulage and gate fee, labour, and re-work. The first three are quotable. The fourth comes from a stopwatch. The fifth comes from your rejection rate, which at one site was 6 percent of loads in the first season and 1 percent by the third, once the sampling and mixing habits settled.

Re-work is the line that hurts most, because a rejected load costs the haul both ways, the gate fee, the labour to re-process, and a phone call. At 400 dollars a rejection and six rejections a month, that is more than a full-time operator’s worth of cost per year, and it is invisible in every quotation.

For sites trying to put a defensible number in front of a finance team, the grade and dose data gives the chemical side with enough precision that the labour line becomes the dominant uncertainty — which is where the effort should go.