LOGOS Learning · 5
Pump discharge valve throttling: closing the valve cuts the flow
Throttling the pump discharge valve adds a head loss that grows with the square of flow, so the system curve becomes steeper: TDH = H_st + hf + hf_valve. The new crossing with the pump curve sits further left, at lower flow and higher pump head.
The valve does not change the pump or the static head. It adds head loss, and the system curve gets steeper.
Quick test
Three questions about this lesson. Got one wrong? The explanation shows right away.
1. Throttling a valve hard at high ΔP can cause…
2. Closing the discharge valve moves the pump point toward…
3. Partly closing the discharge valve…
Why this happens
Arrows compare with the valve at the widest opening of the lesson. Highlighted terms have changed.
Total dynamic head
Hst runs from the suction level to the top of the discharge pipe and does not change. What changes is the loss the valve adds.
Valve loss (IEC 60534)
In the simplified liquid form, Kv is the water flow in m³/h that passes with a 1 bar drop. Closing the valve lowers the available Kv; for the same flow, the drop across it would have to grow with the square.
Operating point
With less Kv the system curve is steeper and crosses the pump curve further left: less flow. The pump climbs its own curve and delivers more head, which the valve burns.
What PIT-01 reads
The PIT sits upstream of the valve, so the valve loss is part of the downstream losses. Closing the valve raises the upstream pressure: exactly what you see in the field when someone throttles the discharge.
Teaching pump with an illustrative curve.
Formulas in plain text
- Total dynamic head with a valve
TDH = H_st + hf_suction + hf_discharge + hf_valve- TDH = total dynamic head (m) · H_st = static head (m) · hf_suction, hf_discharge = pipe friction losses (m) · hf_valve = head loss across the valve (m)
- Valve flow coefficient (IEC 60534, liquid)
Kv = Q * sqrt((rho / rho0) / dP)- Kv = flow coefficient (m3/h at 1 bar drop) · Q = flow rate (m3/h) · rho / rho0 = specific gravity relative to water (-) · dP = pressure drop across the valve (bar)
- Operating point with throttling
H_pump(Q) = H_st + hf(Q) + hf_valve(Q, Kv)- H_pump(Q) = head from the pump curve (m) · hf_valve(Q, Kv) = valve loss, larger for smaller Kv (m)
Frequently asked questions
Why does closing the discharge valve reduce pump flow?
The valve does not change the pump or the static head; it adds loss. With less Kv the system curve gets steeper and crosses the pump curve at a lower flow.
Does throttling the discharge valve increase pump pressure?
Yes. The pump climbs its own curve to a higher head at the lower flow, so the pressure upstream of the valve rises. The valve burns that extra head as loss.
What is the valve Kv?
Kv is the flow of water in m3/h that passes through the valve with a 1 bar pressure drop. For liquids, Kv = Q * sqrt(SG / dP), and the US Cv is about 1.156 * Kv.
Is it bad to throttle a centrifugal pump on the discharge?
Moderate discharge throttling is a common way to control flow, but it wastes energy as valve loss. Throttled too far, the pump runs well left of its best efficiency point and the valve itself may cavitate.