HDPE (black high-density polyethylene) pipe is the best choice for the main line from a source to a storage tank because it bends around obstacles, tolerates sunlight and is joined with barbed fittings held by clamps. Lay it with gentle bends to avoid kinks, clamp every barbed joint, and leave slack because 100 meters of it can change length by about 0.4 meters over a 20 degree temperature swing.
Part of Community Water System
HDPE pipe is flexible black polyethylene tubing sold in coils, used for the buried main line of a community water supply because it needs few joints and survives ground movement.
Flexibility Cuts The Number Of Joints, Which Are Where Leaks And Contamination Begin
The community water system page recommends HDPE for the main line from the source to the storage tank: it is flexible, resists UV and joins with barbed fittings, with the cautions that it needs clamps and can kink. The black colour comes from carbon black, which is what protects the plastic from sunlight, so an uncolored or white pipe is not equivalent.
Because it comes in long coils, a route of several hundred meters can have only a handful of joints. A rigid pipe sold in short lengths needs many times more joints.
What You Need
- HDPE pipe of the right pressure class (Pipe Materials) and bore (Pipe Sizing)
- Barbed couplings or compression fittings, and two clamps per barbed joint
- A drinking-water-rated pipe, or one whose past is known
- A trench with a bed of sand or fine soil, free of sharp stones
- A sharp saw or pipe cutter, to cut square
Method: Uncoil, Cut, Join, Lay
Step 1: Uncoil the pipe in warm weather if possible. Cold polyethylene is stiff and springs back; warm pipe lies flat.
Step 2: Cut square and remove burrs. A ragged cut leaks at the barbs.
Step 3: Make each joint. Push the pipe end fully over the barbs of the fitting; warming the end of the pipe in hot water makes this easier. Fit two clamps, with their screws offset, past the barbs. A joint without clamps pulls off under pressure.
Step 4: Lay it with slack. Do not pull the pipe straight. Lay it in a gentle snake in the trench so that it can shorten in cold weather without tension.
Step 5: Bend gently. Round a tight corner with an elbow fitting, not by forcing the pipe, because it kinks and chokes the flow.
Step 6: Bed and bury. Cover with fine soil. Bury below the frost line where the ground freezes (Burial Depth).
Step 7: Flush before use. Run water through until it comes out clear.
Worked Example: Expansion And Flush Volume
Plastics expand more than metals. Take a linear expansion coefficient for HDPE of about 0.2 mm per meter per degree C. This is a handbook value I am recalling and it varies by grade, so use the maker’s figure if you have it.
- Length change = 100 m × 20 °C × 0.2 mm/m/°C = 400 mm = 0.4 m.
A pipe laid taut in the cool of morning and warmed by afternoon sun pushes 0.4 m of extra length per 100 m into the nearest bend or joint. That is why it is laid with slack and why exposed runs wander.
The water in 100 m of 32 mm bore pipe is π × 0.016² × 100 = 0.080 m³, about 80 L. Plan to flush at least that much through before connecting the line to the tank.
Common Mistakes
- No clamps. The joint blows off at the first pressure surge.
- Pulling the pipe tight. It strains the joints when it cools.
- Kinks. A kink halves the bore or closes it and cannot be fixed by straightening; cut it out and join.
- Laying on sharp stones. The wall is cut through over time.
- Using irrigation pipe of unknown quality for drinking water. Use pipe marked for potable use or of known past.
Safety
Pressure, heat and the water in the pipe
Do not test a plastic line with compressed air. Stay clear of the pipe and fittings when first pressurising, because a joint that lets go throws the fitting. Do not use a flame to soften pipe ends; hot water is enough. Flush every new line, and treat the water until it has been tested (Community Water Testing).
What’s Next
- Learn next: PVC Pipe — the rigid pipe used for distribution
- Combine with: Burial Depth — how deep to lay it