Example flowsheet · Gravity drain hydraulics

Tank Drain Valve Tailpipe
Put the head and the vacuum on the correct sides of the valve.

A vented water tank drains through a restrictive open outlet valve, then down a five-foot tailpipe to an atmospheric drain. This ChemForge example keeps the tank liquid head upstream of the valve and the tailpipe elevation effect downstream of the valve, where it can pull sub-atmospheric pressure.

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What the model separates

The open pipe end is an atmospheric boundary, so the discharge itself is about 0 psig. That does not mean the whole tailpipe is at atmospheric pressure. In a descending flowing pipe, static pressure upstream of the outlet can be below atmospheric because elevation head is being converted into velocity and friction losses.

In the example, the tank liquid level is represented as pressure on the valve inlet. The valve pressure drop is calculated across the valve. The five-foot downward pipe and the pipe-exit loss are modeled on the valve outlet side.

Element Example value Why it matters
Tank head at valve 8 ft water Positive pressure available at the upstream side of the valve.
Valve 1" Sch 40 open globe-style valve Separate K-factor loss across the valve body, standing in for the outlet valve Cv.
Tailpipe 5 ft downward, 1" Sch 40 Downstream elevation term that can make valve outlet pressure negative gauge.
Flow sweep 0-40 gpm Finds the free-drain flow where the tailpipe outlet is about 0 psig.

How to use it

  1. Open the example and run the flowsheet.
  2. Open the Sensitivity Study unit.
  3. Read the flow where the tailpipe outlet pressure crosses about 0 psig.
  4. Change the tank head, valve type, pipe size, or tailpipe length and rerun the sweep.

Interpreting the pressure

A valve manufacturer Cv or K value belongs across the valve. The head pressure from the tank is on the inlet side of that valve. The downstream tailpipe is a separate hydraulic element; because it drops to a lower elevation before reaching the atmospheric outlet, it can place the valve discharge under vacuum pressure even though the pipe end is open to air. In the supplied case, the balance occurs near the middle of the sweep: the tailpipe outlet is near 0 psig while the valve outlet is already negative gauge.

For this kind of question, a simulator is often better than a single hand equation because it keeps valve loss, pipe friction, fittings, elevation, and outlet boundary pressure in one consistent pressure balance.

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