--- title: "Break pressure tank: need and construction" subject: "Water Supply Engineering" chapter: 9 key: "semester_5/water_supply_engineering:break-pressure-tank" source: https://www.bctengineer.com/q/semester_5/water_supply_engineering/break-pressure-tank site: BCT Engineering --- # Break pressure tank: need and construction Water Supply Engineering · Chapter 9: Valves and Fittings - Asked in: 2078 Bhadra, 4 marks; 2078 Kartik, 1+2 marks; 2070 Chaitra, 4 marks; 2069 Asar, 4 marks; 2081 Bhadra, 5 marks; 2072 Chaitra; 2067 Asar (old course); 2066 Jestha (old course); 2066 Bhadra (old course) (asked 9 times) - Video: [014 Break Tank](https://www.youtube.com/watch?v=WdppHrBAilA) — ÉlanSyr (Riyadh An-najjar) - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/water_supply_engineering/break-pressure-tank - Whole chapter: https://www.bctengineer.com/md/semester_5/water_supply_engineering/chapter-9 ## Question Why is a break pressure tank (BPT) necessary in a water supply scheme? Describe/explain its construction with a neat sketch. ## Short answer In steep gravity schemes, high static pressure can burst pipes; a break pressure tank reduces pressure to atmospheric, limiting the head in the next section so lower-class pipes can be used. It is a small masonry/RCC tank with an inlet float valve, outlet, overflow, washout, vent, and cover. ## Answer #### Why a break pressure tank is needed In a gravity scheme laid on steep ground, the static pressure at the lower part of the pipe becomes very large (it increases by 1 kg/cm² for every 10 m drop). This can burst pipes, joints and taps, and needs expensive high-class pipes. A **break pressure tank (BPT)** is a small open tank placed along the pipeline that makes the water pressure zero (atmospheric) at that point, so the static head of the next section is reduced to the height below the tank. Thus lower-class pipes can be used, and pressure at taps stays within safe limits (generally below 60–70 m). It is also used to remove air and reduce the energy of the incoming water. #### Construction ```text inlet pipe (from above) | float valve ___|_____ /___ vent/cover | ~~~~~~~~~ |-- overflow | water | |_______________|-- outlet to next section washout ``` - The tank is built of stone/brick masonry or RCC with cement plaster, usually 1 m × 1 m to 1.5 m × 1.5 m in plan and about 1–1.5 m deep (volume is just enough to hold a few minutes' flow). - **Inlet** pipe with a ball (float) valve closes the supply when the tank is full. - **Outlet** pipe to the next section, at 100–150 mm above the floor, with a strainer. - **Overflow** pipe, **washout** pipe for cleaning, a vent, and an RCC cover with a manhole for inspection. - Free board about 0.3 m. A fence is provided around it and the site is drained. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How does static pressure grow on steep ground? A. It is independent of drop B. 1 kg/cm2 per 10 m drop C. 1 kg/cm2 per 1 m drop D. 10 kg/cm2 per 10 m Answer: B. Static head rises with level drop: 10 m equals about 1 kg/cm2. (Key point: Static pressure) 2. What does a BPT do to the water pressure? A. Doubles it B. Keeps it constant C. Makes it zero (atmospheric) at that point so the next section starts afresh D. Raises it above 70 m Answer: C. The open tank resets pressure to atmospheric, limiting static head to the drop below the tank. (Key point: BPT function) 3. What is the economic benefit of a BPT? A. Pumps are eliminated B. Larger pipes are needed C. Higher-class pipes are needed D. Lower-class pipes can be used Answer: D. Reduced static head allows cheaper low-class pipe. (Key point: BPT benefit) 4. What is the role of the float (ball) valve at the inlet? A. Admits air B. Allows overflow C. Washes out silt D. Closes the supply when the tank is full Answer: D. The float valve shuts the inlet when the tank is full. 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