A drain point is a valve at a low point of a pipeline that lets a section be emptied for winter or repair, so fit it at every low point and no water is left to freeze. 100 m of 50 mm pipe holds about 196 L, and it drains only if air can enter at the high end, so open a vent or air valve before the drain.
Part of Water Distribution
A drain point, also called a washout or blow-off, is a branch and valve at the lowest point of a pipe run through which water and sediment can be let out. It does the opposite job to an air valve, which sits at the high points.
Water Collects At The Low Points, So That Is Where The Drain Goes
When a pipe is closed off from the tank, the water in it runs downhill and collects in every dip. If a dip is not drained, that water stays and freezes. The water-distribution page calls for drain valves at every low point so that exposed sections can be emptied. Sediment settles in the same dips, so the drain also flushes the line.
Draining needs air. If the top of the section is closed, the water leaving the bottom creates a vacuum above it and stops flowing. Open an air valve or vent at the crest first.
What You Need
- A tee and valve at each low point, of the same bore as the pipe or smaller
- A pipe or channel to take the discharge to a safe place, such as a soakaway
- A map marking every drain point, with the order to open them
- A marked, reachable valve box
- A key or spanner
Method: Isolate, Vent, Drain, Close
Step 1: Close the isolation valve at the top of the section (Isolation Valves).
Step 2: Open the vent or air valve at the highest point (Air Valves).
Step 3: Open the drain valve slowly. The jet at a low point is under the full static head of the section above it.
Step 4: Leave it open until it stops running, then check each low point in turn.
Step 5: In spring, close the drains, open the vents, and refill slowly.
Worked Example
Volume per 100 m of pipe: π × (d ÷ 2)² × 100.
- 32 mm: π × 0.016² × 100 = 0.0804 m³ = 80 L
- 50 mm: π × 0.025² × 100 = 0.196 m³ = 196 L
Draining a 300 m run of 50 mm pipe therefore releases about 3 × 196 = 590 L. The pit or channel it goes to has to take that, so run it to a soakaway or ditch, not to the foot of a house wall. The pressure at the drain valve when opened depends on the height of the section above it; at 40 m of head it is 3.9 bar.
Common Mistakes
- No vent at the top. The section will not drain.
- Opening the drain wide at once. The jet scours the ground and can injure.
- A drain valve below ground in a pit that fills with water. The valve can leak dirty water back into the line.
- Forgetting the riser to the tap. Water stays in it.
- Leaving the drains open after refilling. The line runs to waste.
Safety
A drain jet is under full pressure
A drain at the bottom of a long run discharges at the full static head of the section above it. Keep clear of the jet, direct it away from people and foundations, and open the valve gradually. Never put your hand over a drain opening to feel the flow.
What’s Next
- Learn next: Winterization — when to use them
- Combine with: Burial Depth — the depth below which they are less needed