An automatic flush tank accumulates water and releases it intermittently. This animation shows a representative bell-siphon arrangement progressing through filling, priming, discharge and air break. Enter volume and emptying time to compare average discharge flow, a full-pipe-area velocity and ideal refill time.
Automatic Flush Sewer Tank Interactive Calculator
Compare average flush flow, full-area velocity and refill time from entered volume, time, inlet flow and pipe diameter.
Equation Used
- Receiving sewer is treated as a full circular pipe during the flush slug.
- Discharge is averaged over the selected emptying time.
- The 0.6 m/s value is an illustrative velocity reference, not a sediment-removal guarantee.
- Pipe diameter is an independent input; the model does not select a sewer size.
Refill and discharge are shown on different compressed time scales. This calculator does not establish scour performance.
Fill, prime, drain and admit air
A small inlet flow raises the water level around the bell. Water reaching the standpipe crest starts entering the outlet. With the appropriate geometry and flow conditions, the outlet becomes primed and siphonic discharge continues as the tank level falls.
When the water level exposes the lower bell opening, air can enter and interrupt the siphon. The inlet then begins the next fill. The available elevation difference supplies the flow energy; the siphon is not an energy source.
Parts in the cutaway
- Tank: stores the selected dose.
- Inlet: supplies the refill flow.
- Bell and standpipe: form the illustrated priming and siphon path.
- Outlet: carries the discharge downstream.
- Lower air-entry opening: permits the siphon to break near the end of drainage.
The animation compresses refill and discharge onto different viewing time scales. It does not calculate priming, losses, air evacuation or the instantaneous discharge curve. Specific siphon designs can have different vent and trap arrangements; this drawing is not a dimensioned Field-pattern siphon.
Intermittent discharge and downstream flow
An accumulating tank separates a slow incoming flow from a shorter discharge event. This is the principle behind siphon dosing and intermittent flushing arrangements. The calculator compares the average quantities for an entered event rather than predicting a siphon’s performance.
For sewer cleaning, sediment movement depends on the full hydraulic conditions, including water depth, slope, roughness, event duration and the material being moved. Dividing average flow by a full circular area cannot establish removal of grit, grease or other deposits.
Priming behavior, available head, downstream backwater and connections must be assessed for the actual installation. The example supplies no construction dimensions, maintenance diagnosis or adjustment procedure.
Average flow and a full-area velocity
With volume V in liters and discharge time t in seconds, Flush Flow Q = V/t liters per second. Convert sewer diameter from millimeters to meters and calculate A = πD²/4. Full-area velocity v = (Q/1000)/A meters per second.
This is average discharge divided by the full circular area. It is not peak velocity. In a partly filled sewer, the flow area changes with depth; in a transient event, flow and depth also vary with position and time.
Refill Time = V/q_in minutes, with inlet flow q_in in liters per minute. It assumes the entered volume must be replenished at a constant inlet rate. The simple discharge-volume calculation does not add water entering during the emptying interval.
The 0.6 m/s comparison is 100(v/0.6 − 1) percent. Positive means above that example reference, and negative means below it. Neither sign establishes scour performance, absence of surcharge or protection of connected traps.
Worked example: the same volume at three discharge times
Use V = 600 L, D = 200 mm and inlet flow 2 L/min. The full circular area is 0.03141593 m² and the ideal refill time is 300 minutes, or five hours.
At t = 30 s, average flow is 20 L/s and full-area velocity is 0.636620 m/s. The comparison with 0.6 m/s is +6.1033%.
At t = 50 s, flow is 12 L/s, velocity 0.381972 m/s and comparison −36.3380%. At t = 18 s, flow is 33.3333 L/s, velocity 1.061033 m/s and comparison +76.8388%. Refill time is unchanged because the selected dose and inlet rate are unchanged.
These cases isolate the effect of entered discharge time. They do not assign slow flow to corrosion or claim the faster case clears a particular sediment. Doubling pipe diameter would divide the full-area velocity by four at the same average flow.
Interpreting the inputs
| Input change | Effect with other inputs fixed |
|---|---|
| More flush volume | Higher average discharge and longer refill time. |
| Longer discharge time | Lower average flow and full-area velocity. |
| Higher inlet flow | Shorter refill time. |
| Larger receiving diameter | Lower computed velocity when the full circular area is used. |
Actual siphon discharge time is an outcome of its hydraulic design. Here it is an input, so this model cannot size the siphon from a requested emptying time.
Flush-tank calculation questions
No. It uses total entered volume divided by total discharge time and then by full pipe area.
No. It only exceeds the selected reference. Sediment transport requires more hydraulic and material information.
A 2 L/min inlet takes 300 minutes to supply 600 L, while the chosen discharge releases that volume over seconds. The display compresses those times for viewing.
No. It has no siphon geometry, air behavior, head or loss model.
Its wetted flow area differs from the full circle used here. A hydraulic model is needed to calculate depth and velocity.
No. It illustrates a representative bell-siphon sequence, without specifying a historical product’s details.
No. The tool only calculates consequences of entered time. It does not determine why a real system behaves that way.
Refill Time is simply V divided by inlet flow. It does not solve priming delays or account for incoming water during discharge.
Flow-rate reference
OpenStax, Flow Rate and Its Relation to Velocity, explains volume flow and cross-sectional average velocity.
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