A break pressure tank is an open, vented tank placed in a gravity pipeline so that the water pressure falls to zero at its water surface, which keeps every pipe section below the 60 m (about 6 bar) static head the system allows. On steep ground, with a source 80 m above the taps, one tank placed about 40 m down the slope turns one 8 bar section into two sections of about 4 bar, and a tank of 100–200 L with a float valve on its inlet is enough.
Part of Water Distribution
A break pressure tank limits static pressure in a gravity pipeline by letting the water flow into an open tank and out again at a lower level. It is the standard answer when a source sits so far above the settlement that the pipe at the bottom would carry more pressure than its joints and taps can take, as in the steep sites described in Community Water System.
Static Head Does The Damage, And A Tank Throws It Away
Pressure at the bottom of a still column of water is density × gravity × height. Taps closed, a pipe has no flow and no friction loss, so it carries the full static head of everything above it:
- 80 m: 1,000 kg/m³ × 9.81 m/s² × 80 m = 784,800 Pa, about 7.9 bar (the usual rule of 10 m per bar rounds this to 8 bar)
- 60 m: 1,000 × 9.81 × 60 = 588,600 Pa, about 5.9 bar
- 40 m: 1,000 × 9.81 × 40 = 392,400 Pa, about 3.9 bar
Water Distribution sets the ceiling at 6 bar (60 m) in any section, and Community Water System calls for a tank when the source is more than 60 m above the distribution point. Beyond that, joints blow and cheap pipe splits.
A break pressure tank works because the pipe discharges into open air. The water surface in the tank is at atmospheric pressure, so the pressure at that point is zero, and the pipe leaving the tank starts a fresh column measured from the tank’s own water level. The height lost is not recovered: the tank cannot feed a tap above its own water level.
Steep Terrain: Count The Sections, Then Place The Tanks
Divide the total drop by the allowed section head, round up, and subtract one for the number of tanks. For the 80 m source: 80 ÷ 60 = 1.3, rounded up to 2 sections, so 1 tank. For a 200 m drop: 200 ÷ 60 = 3.3, so 4 sections and 3 tanks.
Place them so no section exceeds the limit, and prefer roughly equal sections over a full 60 m followed by a short one. For the 80 m case a tank at 40 m gives two sections of 3.9 bar static, about 4 bar each. A tank at 60 m would leave a section at 5.9 bar, with no margin at all. If your pipe is a lower class than that, or you are unsure of its rating, use more tanks and shorter sections.
Every tank must itself sit at least 5 m above the highest tap it feeds, to keep the 0.5 bar minimum.
What You Need
- A tank of 100–200 L — ferrocement, masonry, or a food-grade drum with a lid
- A float valve on the inlet (see Float Valves)
- An overflow pipe at least as large as the inlet, and an outlet drawn from near the bottom
- A lid with a screened vent
- A drain valve and an isolation valve on the outlet
Method: Build The Tank Into The Line
Step 1: Mark the site at the elevation your section count gives, on stable ground.
Step 2: Set the tank level on a firm base. Inlet enters above the high-water line, the float valve closes at full level, and the overflow pipe sits just above that.
Step 3: Take the outlet from near the bottom, with a short stand-pipe so sediment does not enter, as with a cistern outlet.
Step 4: Cover and vent. The tank is open to the atmosphere, so it is the weak point for contamination; keep out insects, birds and rain-splash with a lid and fine mesh.
Step 5: Route the overflow to a rock-lined drain or channel. On a steep slope, an overflow dribbling onto bare soil cuts a gully quickly.
Step 6: Fill the line slowly from the top, venting air at high points (see Air Valves).
Worked Example: Sizing The Volume
The tank is not storage. Its volume only keeps the float valve stable and rides out the moment when outflow briefly exceeds inflow. With a flow of 0.5 L/s, a 100 L tank holds 100 ÷ 0.5 = 200 s, or 3.3 min; a 200 L tank holds 200 ÷ 0.5 = 400 s, or 6.7 min. If the outflow pipe can pass more than the inlet can supply, the tank drains, air enters the outlet and the line airlocks, so keep the outlet pipe no larger than the inlet can fill.
Common Mistakes
- Putting a tank too late. The section above it already carries more than the pipe can bear.
- A bypass round the tank “to get more flow”. It puts the full static head back on the lower pipe.
- No overflow, or an undersized one. A stuck float valve then pressurises or floods the tank.
- Leaving the tank uncovered. Contamination enters at this one open point.
Safety
The pressure is still there until the tank is built
Do not run the upper section at full static head to “see if it holds”: it bursts at the weakest joint and throws fragments. Test with water, bleed air first, and never test with compressed air. Do not fit a bypass or closed valve that carries the upper pressure past the tank into the lower pipe. If you build the tank as a chamber large enough to climb into, it is a confined space: no entry without ventilation, a harness and line, and an attendant who never follows the entrant in.
What’s Next
- Learn next: Pressure Zones — handling elevation change across a whole settlement
- Combine with: Pipe Sizing — friction loss between tanks