LOGOS Learning · 13
Measured pump head: TDH from the suction and discharge pressure transmitters
With both transmitters at the pump centerline, the head the pump adds is TDH = (p1 - p2)/(rho*g) + (v_disch^2 - v_suct^2)/(2*g). The discharge PIT alone shows line pressure, not pump head: in the example, 33.32 m at the discharge minus 0.23 m at the suction plus 0.11 m of velocity head gives 33.2 m, which is compared with the catalog curve.
The discharge PIT shows line pressure, not what the pump adds. Change the suction level and the plant condition and compare the readings with the pump curve.
Quick test
Three questions about this lesson. Got one wrong? The explanation shows right away.
1. Without a suction transmitter, how is the pump inlet pressure estimated?
2. Same pump and discharge line: filling the suction tank makes the discharge PIT…
3. Falling flow and rising discharge PIT indicate…
Diagnosis from the readings
Compare the trend of the readings with the pump’s own history. PIT-02 separates what comes from the suction from what comes from the pump or the line. The row for the chosen situation is highlighted.
| What happened | Flow (FIT) | PIT-01 (discharge) | PIT-02 (suction) |
|---|---|---|---|
| In the suction | |||
| Suction tank level rose | ↑ rises | ↑ rises | ↑ rises (almost as much as the level) |
| Suction tank level dropped | ↓ falls | ↓ falls | ↓ falls (almost as much as the level) |
| Suction restriction: dirty suction strainer, suction valve partly closed | ↓ falls | ↓ falls | ↓ falls (cavitation risk) |
| In the pump | |||
| Worn impeller, open wear ring, lower speed | ↓ falls | ↓ falls | ≈ almost the same |
| In the discharge | |||
| Downstream valve opened further, leak | ↑ rises | ↓ falls | ≈ almost the same |
| Dirty strainer or blockage in the discharge, valve closing | ↓ falls | ↑ rises | ≈ almost the same |
To state that the pump is off its curve, use the measured TDH. Without a suction measurement, compare with history at the same level range. Classic field test: close the discharge for a few seconds and compare the shutoff head with the catalog, always briefly, because the pump heats up at zero flow.
The arithmetic behind the readings
TDH measured by the two PITs
Both transmitters sit at the nozzles, at the same elevation, so there is no height correction. Subtracting PIT-02 and adding the difference in velocity heads (the nozzles have different diameters) leaves what the pump added. That is the value compared with the manufacturer’s curve.
Differential transmitter (PDIT)
Connected to both nozzles, it measures the pump pressure difference directly. If the taps are at different elevations, impulse lines full of the process liquid already compensate. Only the velocity term is missing when diameters differ.
No suction PIT: estimate it from the level
With an open tank and a measured level (LIT), suction pressure comes from elevation: level minus centerline, minus the suction loss and velocity head. With it, PIT-01 and the flow, the control system rebuilds the TDH.
Teaching pump with an illustrative curve.
Formulas in plain text
- Measured total dynamic head
TDH = (p1 - p2) / (rho * g) + (v_disch^2 - v_suct^2) / (2 * g)- TDH = head added by the pump (m) · p1 = discharge nozzle pressure, PIT-01 (Pa) · p2 = suction nozzle pressure, PIT-02 (Pa) · rho = density (kg/m3) · v_disch, v_suct = nozzle velocities (m/s) · g = 9.81 m/s2
- Differential transmitter
PDIT = (p_disch - p_suct) / (rho * g) ; TDH = PDIT + (v_disch^2 - v_suct^2) / (2 * g)- PDIT = differential pressure across the pump (m) · p_disch, p_suct = nozzle pressures (Pa)
- Suction pressure estimated from tank level
p_suct / (rho * g) = z_level - z_axis - hf_suct - v_suct^2 / (2 * g)- z_level = suction tank level (m) · z_axis = pump centerline elevation (m) · hf_suct = suction line losses (m) · v_suct = suction velocity (m/s)
Frequently asked questions
Why does the discharge pressure rise when the suction tank fills?
More head arrives through the suction than the pump stops adding. Flow rises and the pump head drops a little along its curve, but the suction gain is larger, so the discharge PIT goes up even though TDH falls.
How do I tell a worn impeller from a suction restriction?
Both lower the flow and the discharge pressure. The suction transmitter separates them: with a worn impeller PIT-02 stays almost the same, with a suction restriction it falls, which also brings the pump closer to cavitation.
What do rising flow and falling discharge pressure mean?
The system got easier: a downstream valve opened further or there is a leak. The measured point stays on the pump curve, so the pump itself is fine.
How do I check the pump curve without a suction transmitter?
Estimate the suction pressure from the tank level, minus the centerline elevation and the suction losses, and combine it with PIT-01 and the flow. A short shutoff test, with the discharge closed for a few seconds, compares the zero-flow head with the catalog.