BPT requirement; residual head at stand post

Requirement of a break pressure tank (BPT)

In a gravity-flow scheme in hilly country, the difference in level between the source and the lower parts of the line is large, and the static pressure becomes very high (1 kg/cm² per 10 m). Such pressures can burst pipes, joints and taps, or require costly high-class pipes. A BPT is a small open tank in which the water is brought to atmospheric pressure. The next section then starts from zero, so the static head is limited to the drop below the tank, and the pipe class and cost can be reduced. A BPT is usually provided when the static head is more than about 60–70 m (the limit of the pipe class), and at each break of this height. It also removes the air from the line and a small storage gives control of the flow.

Why residual head must be maintained at the stand post

Residual head is the pressure remaining at the tap after all losses.

1. It is needed to push water out of the tap at a reasonable flow so that a bucket is filled in a short time (a minimum of about 7–10 m is provided in the design).
2. It overcomes the local losses in the tap fittings and the riser height.
3. It prevents negative pressure and contamination from entering the pipe by back-siphoning at leaks.
4. It lets water reach the taps at the end of the line even when other taps are open together.
5. Too high a pressure, however, damages the taps and wastes water, so it is also limited at the upper side.