Top-Floor Pressure Failure in a G+3 Building
Shreeji Apartments, a G+3 building in Nashik with 12 flats (4 per floor), receives municipal water for about 90 minutes every alternate day at a road-level pressure of 7 m. Each flat has 4 occupants. The existing 5,000 L overhead tank sits on the terrace with its bottom 12 m above ground; the third-floor shower rose is 9.6 m above ground. Water is lifted by a 0.5 hp pump from a 3,000 L underground sump. Third-floor residents complain that the shower is a trickle and the 10 L WC cistern takes about 4 minutes to refill; the second floor is acceptable and the first floor is fine. The 25 mm service connection from the main to the sump has a partly blocked strainer, and the branch to each flat is 20 mm CPVC. The secretary proposes a 1 hp booster pump on the terrace; a plumber suggests raising the tank by 2 m. Assume 135 lpcd and one day's storage in the overhead tank plus one day's in the sump.
1. Compute the daily demand, check whether the existing storage is adequate, and recommend a storage split.
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Daily demand = 12 flats × 4 persons × 135 lpcd = 6,480 L/day. Add about 10% for common-area washing and minor losses, giving about 7,130 L/day, say 7,200 L/day. Because supply comes only every alternate day, the building must store two days: 14,400 L. Existing storage is 5,000 + 3,000 = 8,000 L, short by about 6,400 L and not even enough for one no-supply day. Recommended split: an underground sump of 9,000–10,000 L (two days' raw storage) and an overhead tank of 5,000–7,000 L (one day). Before enlarging the overhead tank, check the terrace load: 7,000 L is about 7 tonnes of water plus the tank; over a 2.5 m × 2 m platform that is about 1,400 kg/m², which needs a structural check. If the slab cannot take it, keep 5,000 L overhead and pump twice a day from a 9,000 L sump.
2. Calculate the available static head at the third-floor shower and explain the symptoms with numbers.
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Static head = 12.0 − 9.6 = 2.4 m, which is about 0.24 bar (1 bar ≈ 10.2 m). After friction in the 20 mm branch and losses through the meter and partly blocked strainer, the residual at the rose is roughly 0.10–0.15 bar. A shower rose needs about 0.5 bar, and a float valve works well at 0.5–1 bar. At 0.15 bar a 15 mm float valve passes only about 0.05 L/s, so 10 L takes about 200 s, or 3.3 minutes - exactly the complaint. The second floor has 3.2 m static (0.31 bar) and the first floor 6.2 m (0.6 bar), which is why only the top floor is bad. Diagnosis: inadequate static head plus an undersized 20 mm branch plus a restricted service connection, not a faulty shower. Cleaning the strainer alone will improve filling but will not fix the top-floor pressure.
3. The secretary wants a booster pump; the plumber wants to raise the tank by 2 m. Compare the options and specify the pump duty if a pump is chosen.
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Raising the tank by 2 m gives 4.4 m, about 0.43 bar at the top floor - still below the 0.5 bar wanted at the shower and marginal for the cistern; it also adds about 2 tonnes of load and may not be structurally possible. A booster set is more controllable. Duty: the top-floor group needs about 0.3 L/s at 0.5 bar (5 m) boost, so select a small pump giving about 12 m head at 0.3 L/s (roughly 0.5 hp) with a pressure switch, a 24 L pressure vessel and dry-run protection. Set the cut-out below 2 bar, and fit a pressure-reducing valve on the lower floors if the static pressure exceeds 2 bar. First clean the 25 mm strainer and re-measure; if the pump then cycles too often, increase the service to 32 mm subject to the main's capacity. Running cost for 2 h/day is small, but noise and maintenance must be accepted.