--- title: "BPT requirement; residual head at stand post" subject: "Water Supply Engineering" chapter: 9 key: "semester_5/water_supply_engineering:bpt-requirement-residual-head" source: https://www.bctengineer.com/q/semester_5/water_supply_engineering/bpt-requirement-residual-head site: BCT Engineering --- # BPT requirement; residual head at stand post Water Supply Engineering · Chapter 9: Valves and Fittings - Asked in: 2080 Baisakh, 2+2 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/water_supply_engineering/bpt-requirement-residual-head - Whole chapter: https://www.bctengineer.com/md/semester_5/water_supply_engineering/chapter-9 ## Question What is the requirement of installing the Break Pressure Tank (BPT) in a water supply system? Also explain why certain residual head has to be maintained at the stand post. ## Short answer A break pressure tank is installed when static head exceeds 60–70 m to reduce pressure and allow lower-class pipes. Residual head of 7–10 m at a standpost ensures adequate flow, overcomes losses, prevents back-siphoning, and maintains supply when other taps are open. ## Answer #### 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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why is a BPT required in hilly gravity schemes? A. To chlorinate B. To reduce very high static pressure that could burst pipes C. To pump water D. To raise pressure Answer: B. It limits static head and pipe class. (Key point: BPT need) 2. When is a BPT generally provided? A. At every 5 m B. Only at the source C. When static head exceeds about 60-70 m D. Never in gravity schemes Answer: C. At each break of the pipe-class pressure limit (60-70 m). (Key point: BPT limit) 3. What is residual head? A. Static head B. Pressure at the source C. Pressure remaining at the tap after all losses D. Head in the pump Answer: C. It is the pressure left at the tap after losses. (Key point: Residual head) 4. Why is a minimum residual head needed? A. To reduce flow B. To corrode pipes C. To increase tap size D. To fill a bucket quickly and prevent back-siphoning/negative pressure Answer: D. Positive head gives adequate flow and prevents contamination entering through leaks. 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