Hostel Shower Trickle and a Short-Cycling Pump
A 24-bed girls' hostel is fed from a 3,000 L underground tank and a 1,000 L rooftop tank. GWCL water enters the underground tank through a float valve only two days a week, and a 0.75 kW pump lifts water to the roof. The rooftop tank sits on a 1.2 m stand, so its water level is 4.5 m above the first-floor shower outlets. Students complain that ground-floor taps are strong but first-floor showers are a trickle. The 20 mm galvanised riser reduces to 15 mm at each shower, and the pump switches on and off every few seconds when no tap is open. The matron wants a bigger rooftop tank as the solution. You are asked to diagnose the problem and quote for a reliable upgrade before the next term.
1. Calculate the static pressure at the first-floor shower outlet when the rooftop tank is full, and explain why this does not guarantee a good shower. Use 1 m head = 9.81 kPa.
Read the worked answer
Static pressure = height × 9.81 kPa/m = 4.5 × 9.81 = 44.1 kPa, or about 0.44 bar, at the outlet when the tank is full and no water is flowing.
That is the pressure before friction and before the outlet itself. A conventional shower rose usually needs roughly 50–100 kPa to work well. The 20 mm riser reduced to 15 mm at each shower adds friction, especially at peak use when several students shower at once. The 4.5 m head is therefore marginal at best, and the trickle is explained by low head plus a restrictive branch rather than by the tank being too small. A bigger tank would only prolong the supply; it would not raise the pressure.
2. Diagnose the pump short cycling and state the checks and parts you would inspect before recommending replacement.
Read the worked answer
Short cycling with no tap open usually means the pump is reaching its cut-out pressure and the pressure falls quickly back to cut-in, or the pump is protected by a pressure switch with too narrow a differential. Likely causes are: a leaking non-return valve allowing water to run back to the underground tank; a pressure vessel with lost air charge or a punctured diaphragm; a pressure switch set too close; a leaking float valve or pipe; or a pump oversized for the small draw-off.
I would check the non-return valve for backflow, test the pressure vessel pre-charge, inspect the pressure switch setting and contacts, and look for leaks. If there is no pressure vessel, fitting a correctly sized diaphragm tank will stop rapid cycling. Replacement of the pump should only follow those checks, because a new pump on a leaking check valve will still cycle.
3. Recommend an upgrade with reasons, including a storage check and backflow protection.
Read the worked answer
Daily demand at, say, 50 L per student is 24 × 50 = 1,200 L/day. If GWCL supplies only two days a week, the gap is about two days, needing roughly 2,400 L of usable storage. The 3,000 L underground tank is therefore just adequate when full; it should not be reduced. The 1,000 L rooftop tank is a buffer, not two days' storage.
The better upgrade is to keep the underground tank and install a small booster set with a pressure vessel and a pressure-reducing valve, serving a dedicated 25 mm riser with 15 mm branches kept as short as possible. This gives reliable shower pressure without relying on a very high tank stand, which would be structurally difficult. The GWCL main must not be directly connected to the pump suction; fill the underground tank through a float valve with a visible air gap, and fit an overflow/warning pipe, insect screen and locked lid. Add a servicing schedule for tank cleaning and check the non-return valve and pressure vessel at the same time.