LOGOS Learning · 2
Pump operating point: why flow drops when the tank goes up
The pump operating point is where the pump curve crosses the system curve: H_pump(Q) = H_st + hf(Q). Raise the static head and the system curve moves up, so the crossing slides left to a lower flow.
Drag the discharge tank up and down. The pump stays the same; what changes is the elevation of the top of the discharge pipe, and with it the head the pump has to overcome.
Quick test
Three questions about this lesson. Got one wrong? The explanation shows right away.
1. Raising the discharge tank, pump flow…
2. At zero flow, the system curve starts at…
3. To convert the PIT reading from meters of liquid column to bar, you need to know…
Why this happens
Arrows compare with the starting point (Hst = 15 m). Highlighted terms have changed.
Total dynamic head
Static head runs from the suction level to the highest point of the discharge pipe. The pump lifts the water to that crest continuously, for as long as it runs; from there the water falls to a free discharge open to air, and that drop gives no head back to the pump. That is why the discharge tank level does not change Hst.
Head loss (Darcy-Weisbach)
Loss grows with the square of velocity. Less flow, less velocity, much less loss.
Operating point
The pump can only run where the two curves cross. The pump curve falls with flow; if the system asks for more head, the crossing happens at a lower flow.
What PIT-01 reads
Bernoulli between the PIT tap and the top of the pipe: Δz = ztop − zPIT. Raising the tank adds more Δz than the loss removes, so the reading goes up. With fixed ρ, about 10.2 m of water column equals 1 bar.
Teaching pump with an illustrative curve.
Formulas in plain text
- Total dynamic head
TDH = H_st + hf with H_st = z_top - z_suction- TDH = total dynamic head (m) · H_st = static head (m) · hf = head loss in the piping (m) · z_top = elevation of the highest point of the discharge pipe (m) · z_suction = suction tank level (m)
- Head loss (Darcy-Weisbach)
hf = f * (L / D) * v^2 / (2 * g) with v = Q / A- f = Darcy friction factor (-) · L = pipe length (m) · D = internal diameter (m) · v = mean velocity (m/s) · Q = flow rate (m3/s) · A = pipe cross-section (m2) · g = 9.81 m/s2
- Operating point
H_pump(Q) = H_st + hf(Q)- H_pump(Q) = head from the pump curve at flow Q (m) · H_st + hf(Q) = system curve (m)
- Discharge pressure transmitter
p = rho * g * (dz + hf - v^2 / (2 * g))- p = gauge pressure at the tap (Pa) · rho = fluid density (kg/m3) · dz = z_top - z_tap (m) · hf = head loss from the tap to the pipe top (m)
Frequently asked questions
How do you find the pump operating point?
Plot the pump curve (head vs flow) and the system curve H_st + hf(Q) on the same chart. The pump runs where they cross, because only there the head it delivers equals the head the system asks for.
Why does pump flow drop when the static head increases?
A higher static head shifts the whole system curve up. Since a centrifugal pump curve falls with flow, the new crossing happens at a lower flow and a higher head.
What is the difference between static head and total dynamic head?
Static head is only the elevation the liquid must be lifted, from the suction level to the highest point of the discharge. Total dynamic head adds the friction losses: TDH = H_st + hf.
What happens if the static head equals the pump shut-off head?
The balance only holds at Q = 0. The pump keeps spinning but delivers no flow, which overheats it if it runs like that for long.