What the class number means
On PVC and polyethylene pressure pipe made to South African standards, the class is the maximum continuous working pressure in bar at 20 °C. Class 6 pipe is rated for 6 bar, Class 16 for 16 bar. A higher class has a thicker wall for the same outside diameter.
Engineers' specifications sometimes write the same thing as PN (nominal pressure): PN 10 and Class 10 describe the same rating.
Converting class to pressure and head
Pressure in water pipe is often easier to think about as head: the height of a column of water that would create that pressure. 1 bar is 100 kPa, or about 10.2 m of head. For quick sums, 10 m per bar is close enough.
| CLASS | PRESSURE | IN kPa | HEAD | PIPELINK PIPE IN THIS CLASS |
|---|---|---|---|---|
| Class 3 | 3 bar | 300 kPa | ≈ 31 m | LDPE |
| Class 4 | 4 bar | 400 kPa | ≈ 41 m | uPVC |
| Class 6 | 6 bar | 600 kPa | ≈ 61 m | uPVC · mPVC · HDPE (32 mm and up) · LDPE |
| Class 9 | 9 bar | 900 kPa | ≈ 92 m | uPVC · mPVC |
| Class 10 | 10 bar | 1000 kPa | ≈ 102 m | HDPE |
| Class 12 | 12 bar | 1200 kPa | ≈ 122 m | uPVC · mPVC · HDPE |
| Class 16 | 16 bar | 1600 kPa | ≈ 163 m | uPVC · mPVC · HDPE |
| Class 20 | 20 bar | 2000 kPa | ≈ 204 m | uPVC · mPVC · HDPE |
Maximum continuous working pressure at 20 °C. Head uses 1 bar ≈ 10.2 m of water.
How to choose a class
The class must be higher than the highest pressure the pipe will ever see, not the average. Work it out from:
- Static head: the height difference between the water source (or the pump) and the highest outlet or tank.
- Friction losses: pressure lost as water moves through the pipe. Longer, smaller and faster-flowing lines lose more.
- Surge (water hammer): pressure spikes when a pump starts or stops or a valve closes quickly. On pumped lines this can be significant.
- The pump's maximum pressure: what the pump produces against a closed valve, which is usually higher than its running pressure.
- How often the pressure cycles: lines where pumps start and stop many times a day put the pipe through repeated pressure changes. Engineers allow for this fatigue, often by choosing a higher class.
Find the highest pressure from the list above, convert it to bar, and choose a class above it with a margin. When in doubt, go up a class.
Two worked examples
Gravity supply from a tank
A tank stands on a hill 35 m above the lowest house it feeds. The static pressure at the bottom is about 3.5 bar when the taps are closed. Class 6 pipe gives a comfortable margin for a gravity line like this.
Pumped borehole line
A pump set 60 m down a borehole feeds a tank 6 m high. The static head is about 66 m, roughly 6.5 bar, before friction and surge. Class 6 is too low. Class 10 gives a margin, provided the pump's maximum pressure stays below about 100 m.
Temperature matters
Pressure ratings are given at 20 °C. Plastic pipe loses some of its pressure capacity as the water gets warmer, so lines that carry warm water, or that lie exposed in the sun, may need a higher class. Ask us if your water will run warm.
Which classes we make
- uPVC: Class 4, 6, 9, 12, 16 and 20, from 50 to 500 mm (not every class in every size).
- mPVC: Class 6, 9, 12, 16 and 20, from 50 to 500 mm.
- HDPE (certified range): Class 6, 10, 12, 16 and 20, from 20 to 110 mm (from Class 10 at 20 and 25 mm).
- LDPE: Class 3 and 6, from 20 to 50 mm, for low-pressure irrigation.
Sewer and drain pipe (SANS 791) is non-pressure pipe and has no class; it comes in heavy duty and normal duty grades.