LOGOS Learning · 3
Static head with free discharge: why the discharge enters from the top
With a free discharge from the top, static head is fixed by the pipe crest: H_st = z_top - z_suction, so the discharge tank level drops out and only the suction level moves the flow. With a submerged bottom inlet, H_st = z_disch - z_suction and flow keeps falling as the tank fills.
Change both tank levels and switch the inlet type. With the pipe arriving from the top and discharging freely, flow depends only on the suction side.
Quick test
Three questions about this lesson. Got one wrong? The explanation shows right away.
1. When static head increases, the system curve…
2. As the suction tank empties, pump flow…
3. With a fuller suction tank, TDH on the chart drops but discharge PIT-01 rises. Why?
Why this happens
Arrows compare with a full suction tank and an empty discharge tank, same inlet type.
Static head with free discharge
The elevation of the pipe top is fixed. The discharge tank level does not appear in the formula; only the suction level does.
Operating point
Same pump and same piping as the single-pump lesson. Only the static head changes with the levels.
Operating range
The shaded band on the chart gathers every possible system curve as the levels vary. With a top inlet it is half as wide: the pump stays close to the same point.
Discharge pressure = suction + pump
The chart shows TDH: what the pump adds. Discharge pressure is the head already arriving through the suction plus that TDH. Fill the suction tank: TDH drops (the point moves down the chart, at higher flow), but the inlet rises even more, so PIT-01 goes up. With a bottom inlet, filling the discharge tank also raises the reading, because the column up to its level grows.
Flow depends only on the suction side
The pipe rises to a high point above the roof and drops to a free discharge. That crest has a fixed elevation, the pump lifts the water to it all the time, and it is what sets H_st. The discharge level drops out of the equation: filling or emptying, flow does not change.
No backflow when the pump stops
With a bottom inlet, the tank column pushes water back through the pump as soon as it stops, so the line needs a check valve. With the outlet above the level there is no column to flow back. If the pipe dips into the liquid this is lost and a vacuum-breaker hole is needed.
The price: a little more energy
With a top inlet, the pump always lifts the water to the top of the pipe, even with the tank empty. A little efficiency is traded for predictability. Bottom inlets are used when splashing, foaming or aeration of the product must be avoided.
Teaching pump with an illustrative curve.
Formulas in plain text
- Static head, free discharge (top inlet)
H_st = z_top - z_suction- H_st = static head (m) · z_top = elevation of the pipe crest (m) · z_suction = suction tank level (m)
- Static head, submerged inlet (bottom)
H_st = z_disch - z_suction- z_disch = discharge tank level (m)
- Operating point and range
H_pump(Q) = H_st + hf(Q) -> Q_min <= Q <= Q_max- hf(Q) = friction loss at flow Q (m) · Q_min, Q_max = flows at the highest and lowest static head (m3/h) · dH_st = H_st,max - H_st,min (m)
- Discharge pressure = suction + pump
H_disch = H_inlet + TDH with H_inlet = z_suction - hf_suction- H_disch = head at the pump discharge (m) · H_inlet = head arriving at the pump inlet (m) · TDH = total dynamic head (m) · p_PIT = rho * g * (H_disch - z_PIT - v^2/(2*g))
Frequently asked questions
Does the discharge tank level affect pump flow?
Only if the pipe enters below the liquid level. With a free discharge from the top, the static head is set by the pipe crest, so filling or emptying the discharge tank does not change the flow.
Why do discharge pipes enter tanks from the top?
It makes the flow predictable, since only the suction level changes the static head, and it prevents backflow through the pump when it stops. The price is a little extra energy, because the pump always lifts to the top.
Does a bottom tank inlet need a check valve?
Yes. When the pump stops, the liquid column in the tank pushes water back through the pump. A top inlet above the liquid has no column to flow back, unless the pipe dips into the liquid, in which case a vacuum-breaker hole is needed.
Why does discharge pressure rise when the suction tank fills?
Discharge pressure is the head arriving through the suction plus the pump TDH. Filling the suction tank lowers TDH a little, but raises the inlet head more, so the discharge gauge goes up.