KEVOS® MAKEOS · Hydraulics
Tank Drain Time Check (Volume Drained, Initial Outflow, Time Through an Orifice)
How long will this vertical tank take to drain from one level to another through a bottom outlet, and what outlet size would drain it in the time I want?
Drain time of a vertical cylindrical tank through a bottom orifice or short outlet: the volume between the two levels (as the tank volume tool), the outflow at the starting level from the head as a pressure drop (as the orifice flow tool), the time to fall from the starting to the final level with the head falling as the tank empties, and the outlet diameter that would drain it in your time, against the time allowed.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Tank inside diameter | mm | Inside diameter of the tank. |
| Starting level above the outlet | mm | Liquid level above the outlet at the start. |
| Final level above the outlet | mm | Liquid level above the outlet when draining stops. |
| Outlet diameter | mm | Outlet diameter. |
| Discharge coefficient | Discharge coefficient of the outlet (about 0.6 for a sharp orifice, 0.8 for a short pipe). | |
| Liquid density | kg/m³ | Liquid density (for the outflow as a pressure drop). |
| Time allowed | min | Time the drain must take at most. |
Method and formulas
- Volume = π D² (H₁ − H₂) ÷ 4 (as the tank volume tool).
- Outflow at the start Q = C_d A √(2 Δp ÷ ρ) with Δp = ρ g H₁ (as the orifice flow tool).
- Time t = (A_tank ÷ C_d A_outlet) √(2 ÷ g) (√H₁ − √H₂); the outlet for a time t₀ is d √(t ÷ t₀).
- Example: 1,583.4 L drains in 26.258 min; a 23.389 mm outlet would meet the time allowed.
Limitations
- Unrestricted outflow to air from a tank open at the top; a vented, pumped or piped outlet changes the flow.
- Outlet losses taken in the discharge coefficient; long drain pipes need the pipe friction tool.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Find how long a tank takes to drain for cleaning
- Size a drain outlet for an emergency dump
- Estimate outflow from a tank at a given level
Who it is for
- Process engineers
- Plant operators
- Maintenance engineers
What you get
- Volume drained
- Outflow at the start
- Drain time and outlet for the time allowed
Interpreting the result
- The last part of the tank drains slowest because the head is lowest; draining fully takes far longer than draining half.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 68–69, 3024–3027. Topic reference; no source tables or recommended settings are reproduced.
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Open the Tank Drain Time Check (Volume Drained, Initial Outflow, Time Through an Orifice) calculator
