Compressor hire in South Africa: how many cfm and what pressure your tools need, and the paperwork to ask for

Total the air use of the tools that run at the same time, allow for how much of the time each one runs, add a reserve, and hire a compressor whose free air delivery covers the result. One compressor maker's sizing guide works the same way, estimating each tool's utilisation factor "by experience" and sizing "the compressors' free output flow rate" to cover the total.^[4]

Count grinders and sandblasting machines as continuous: the same guide notes they run for 3 to 10 minutes at a stretch despite a low overall utilisation factor.^[4]

In practice, size a tool that works for long stretches on its under-load figure, not its average: the 1/2 in impact wrench in the table uses 680 L/min under load and 170 L/min on average.^[10]

Make the reserve cover leakage, wear and future changes, as the same guide advises. For blasting, one blast-equipment maker's estimate adds 20 cfm for the operator's helmet to the nozzle's air, then a 50% reserve on that total, so a No. 6 (3/8 in) nozzle needs at least 324 cfm (9.0 m³/min). As the nozzle wears, each step up in bore takes 29% to 69% more air.^[4][13][14]

Worked example: two 42 kg breakers working together need 2 × 40 = 80 L/s (about 170 cfm) at 6 bar before any reserve. One South African hire fleet lists its portable diesel units at 190 to 600 cfm, so two breakers fit its smallest unit with little to spare, while a single No. 6 blast nozzle needs a unit of 324 cfm or more.^[7][17][13]

Set it to the highest pressure any of your tools needs, plus the losses between compressor and tool. One compressor maker's method starts from the user with the highest minimum pressure, then adds the drop across a refrigerant dryer (typically 0.2 to 0.5 bar), a standard filter (0.1 to 0.3 bar) and the friction of long or undersized lines.^[5]

In practice, the hose bore matters as well as its length: one blast-equipment maker's minimum air line is 1.5 in (38 mm) for a No. 6 nozzle and 2 in (50 mm) for a No. 8, and the impact wrench's maker gives a 3/8 in (10 mm) minimum hose.^[15][10]

Yes. The Pressure Equipment Regulations (PER) cover equipment with a design pressure of 50 kPa or more (note (j) to regulation 2), and the Department of Labour's guidance notes say "mobile air compressors or transportable pressure vessels containing air are deemed not to be transportable gas containers but pressure vessels" (note (e) to the definition of transportable gas container in regulation 1).^[1]

Every pressure vessel in use must have at least one pressure measuring device and at least one safety valve, "kept locked, sealed or otherwise rendered inaccessible to any unauthorized person" (regulation 10(3) and (4)).^[1]

In service, a pressure vessel gets an internal and external inspection and a hydraulic test at intervals not exceeding 36 months, by an approved inspection authority (AIA) appointed by the user in writing; Category I equipment, as categorised under SANS 347, may be inspected and tested by the user (regulation 11(1)(d)). SEP-class equipment is regulated but "need not meet any other regulatory requirements" beyond being made to SEP rules (note (h) to regulation 2).^[1]

For hired equipment the notes put the work on the owner: "the user is not responsible for the inspection activities for rented pressure equipment, this responsibility remains with the owner, unless otherwise agreed by the contracting parties" (note (v) to regulation 11), and the certificate of manufacture stays with the owner (note (d) to regulation 6).^[1]

The user still has to make sure the records exist: the notes make the user "responsible to ensure that all records are available" for all pressure equipment other than transportable gas containers it does not own (note (f) to regulation 14). Ask for copies of the certificate of manufacture and the last inspection report with the unit.^[1]

1. Every tool that will run on the compressor, with the air use and pressure from its plate or datasheet, and how many run at the same time.^[4]
2. Blasting: nozzle number or bore (No. 4 to No. 8), pressure wanted at the nozzle, and helmet air.^[12][13]
3. Free air delivery needed, in L/s or cfm at the working pressure, and a quote that states the unit's FAD, not its tank size or displacement.^[3][16]
4. Hose run from compressor to tool, and the line size the tools need: for example 38 mm for a No. 6 blast nozzle, or 10 mm minimum for a 1/2 in impact wrench.^[15][10]
5. Power: diesel for an off-grid site, or the electricity supply available for an electric unit.^[18][17]
6. Air quality: whether the air must be oil-free, and any dryer or filters the process needs.^[17][5]
7. Hire period in days, weeks or months, delivery address, and who refuels the unit and pays for the diesel.^[17][18]
8. Copies of the receiver's certificate of manufacture and last inspection report, who is responsible for inspection under the hire contract, and the compressor's noise and vibration emission data.^[1][2]