💧 Tank Flushing Calculator

• 🏷️ As shown on the tank label, e.g. 28.3 Sq.m × 3.57 m (H). Volume = Floor Area × EWL.
• 📏 EWL is the tank height minus about 500 mm as a general rule — do not use the full height.
• 👁️ Site tanks often differ from theory, so always replace the preset EWL with the level you actually see on the indicator tube when you arrive.
• ✅ Picking a tank fills in its size and estimated EWL — check it against the tube.

• 📐 EWL = overflow level − 75 mm: the water level is 25 mm below the warning alarm (7.1.7), which is 50 mm below the overflow pipe (7.1.6).
• 📏 EWL is set by the overflow/alarm arrangement, not by the tank height — so the '500 mm rule' (EWL = height − 0.50 m) is only a rough preset estimate.
• 👁️ Always check the preset EWL against the indicator tube on site, or the label EFF. CAP. ÷ floor area (7.2.4).
• 💧 Outlet is 75–100 mm above the bottom; wash-out pipe is flush with the bottom (7.3.2).
• ✅ The 150 mm dead water sits above the outlet, so gravity draining works down to dead-water level; the wash-out handles the final dead water during cleaning.

• 🏷️ Type the EFF. CAP. from the tank label and it is used as the Effective Volume.
• 🧮 Leave it blank and the volume is calculated from the size × EWL.
• 💧 The flushing time also needs the water level — enter the EWL from the indicator tube, or the tank size, so the level can be worked out from this capacity.

• 🔧 On site the valve is usually already set (by the supervisor / client) — measure the drop time at that opening and leave this on “Valve at current setting”: the time is used as measured (×1), so it cannot be counted twice.
• 📊 Only pick “Valve fully open” when the drop time was measured with the valve fully open and you want to predict a slower, partly-open flush.

• ⚠️ Only used when the drop time / flow was measured with the valve fully open (see above).
• 🎚️ Pick the opening you will flush with: Three-quarter open takes about 1.33× longer than Fully open, Half open about 2×, Quarter open about 4×.
• 🔒 Keep the valve at the same opening while you flush.

• ⏱️ Watch the Water Level Indicator tube. Start the timer at a level mark, stop it when it has dropped 100 mm.
• 🔒 Measure at the valve opening you will flush with, and keep that opening the same for the whole flush.
• ✅ The drop-rate method is best on site when there is no flow meter.

• 📉 Gravity draining slows as the level falls, so the second drop is usually slower.
• 🔁 Measure again around the middle of the tank — the calculator averages the two times.

• 🪣 From a flow meter, or a timed bucket: flow = bucket volume ÷ fill time. E.g. a 20 L bucket in 10 sec = 120 L/min = 7.2 m³/hr.
• 🔒 Measure at the valve opening you will flush with, and keep that opening the same for the whole flush.

• 🛡️ 15% is the recommended default for a fully open gravity drain.
• 🌊 With the valve partly open the flow is steadier, so 10% is usually enough.
• 🔒 Keep the valve opening the same for the whole flush — changing it changes the rate.

• 🕐 Shows the current time with seconds (HH:MM:SS) and ticks live.
• 📌 Tap “Set to now” to capture the current time — it then stays fixed, so the finish time does not keep moving.
• 👆 Tap the time itself to pick a different time with the clock (this also stops the live ticking).
• ▶️ Tap “Live” to resume the live clock.

The “Estimated Flushing Cleaning Finish” covers both flushing and cleaning: the cleaning is carried out during the flushing — the walls are scrubbed as the water drops, and the tank is scrubbed from the inside when the water is low. If in doubt, use a 20% safety buffer — that covers the cleaning time as well.

• ⚡ Refilling is faster than draining: the water comes in under PUB direct supply or transfer-pump pressure, so the flow is high and steady — unlike gravity draining, which slows as the level falls.
• 💧 The tank is empty after washing, so the water to refill is the Effective Volume.
• ⏱️ Measure the refill rate the same way as the drop rate — time how long 100 mm takes to rise on the indicator tube, or use a flow rate in m³/hr (or L/min).
• 🛡️ The same buffer % is added.
• ✅ The cleaning is done during the flushing, so no extra cleaning time is added between flushing and refilling.

• 📊 Turns from fully closed — 2" SS: 7 turns (50%: 3.5, 70%: 4.5–5); 4" DI: 13 turns (50%: 6.5, 70%: 9); 6" DI: 18 turns (50%: 9, 70%: 12.5); 8" DI: 23 turns (50%: 11.5, 70%: 16).
• ⚠️ Turns vary by maker and design — the exact total is on the valve nameplate/datasheet or by counting on site (at Police Cantonment the 6" was 13 turns and the 8" was 36–37 turns, not 18/23).
• 🔄 How to count 1 full turn: start from fully closed, mark the handwheel at 12 o'clock, turn counter-clockwise; when the mark returns to 12 o'clock that is 1 full turn.
• 🛡️ Safety: tanks of 70 m³ or more — limit the first opening to 50%; open 100% only when the tank is below 10% capacity; check the siphonic roof drain pit is free of debris before opening; re-secure the vortex grating (aluminium cover) after draining.
• 💡 Why throttle: opening the valve gives very high flow from hydrostatic pressure, and with the vortex breaker removed the roof drain works by gravity only — the tailpipe reduction chokes and can cause air entrainment and flooding.

• 🔗 After flushing & cleaning, disinfect the tank — 💧 Water Tank Disinfection Calculator 🌐︎.
• ⏳ Standard: 50 ppm free chlorine, kept 24 h (SS 636 A.1.2.3 & A.1.2.5).
• 🚨 Urgent / single tank: 200 ppm, kept 2 h (A.1.2.12 & A.1.2.13).
• ⚠️ Powder must be pre-dissolved in water and strained — never pour dry powder into the tank (A.1.2.2).
• 💧 After disinfection, drain completely, refill with fresh water, and take samples — residual chlorine must be below 2.0 ppm.