A radial-flow centrifugal pump has its maximum head and minimum brake power at zero flow (shutoff). Starting against a closed discharge valve therefore presents the motor with the lightest load, so it runs up to speed quickly, draws high inrush current for the shortest time, and never sees an overload. Once it is up to speed, you open the discharge valve to walk the pump out to its duty point — within seconds, because no centrifugal pump should run dead-headed for long.
The exception: axial-flow (propeller) pumps draw their maximum power at shutoff. They are started with the discharge valve open. Switch the pump type in the simulator below and watch the rule flip.
The rule, and why it isn't arbitrary
Ask an operator why the discharge valve is shut before a centrifugal pump is started and you will often hear "that's just how we do it." It is, in fact, one of the most defensible procedures in rotating-equipment operation — but only because of the shape of one curve.
A centrifugal pump does not move a fixed volume per revolution the way a positive-displacement pump does. It adds energy to the liquid as head, and how much liquid actually flows depends on what the downstream system will accept. Three curves govern everything:
- The pump head curve — head produced as a function of flow. For a radial pump it is highest at zero flow (the shutoff head) and falls as flow rises.
- The system curve — the head the piping demands as a function of flow: static lift plus friction that grows with the square of flow. Throttling the discharge valve steepens this curve; closing it fully makes it vertical, forcing flow to zero.
- The power curve — the brake power the shaft must deliver. For a radial pump this rises with flow, so it is lowest at shutoff.
Energise the motor with the valve shut and the operating point sits at the far left of the pump curve: maximum head, zero flow, minimum power. The motor only has to overcome its own inertia and a light hydraulic load, so it accelerates fast. That is the whole argument.
The clean way to say it: a radial centrifugal pump is "soft to start" — its torque demand is lowest exactly when the motor is weakest (at standstill, pulling locked-rotor current). Closing the valve keeps those two facts aligned.
The arrangement
Here is the equipment the procedure is talking about — a single-stage end-suction pump taking suction from a tank and discharging through an isolation/throttle valve. The pressure gauge sits on the discharge nozzle, downstream of the pump and upstream of the valve, which is why it reads shutoff head when the valve is closed.
Interactive model — prove it yourself
Change the inputs and watch flow, discharge pressure, brake power and motor current respond in real time. The two charts show where the pump is operating: the left chart is head vs flow (pump curve meeting system curve), the right chart is the power curve that makes the whole argument. Start by dragging the discharge valve from open to closed and watch the power and current fall.
Centrifugal Pump & Motor Simulator
Live modelHead vs Flow
Brake power vs Flow
Direct-on-line start — current vs time
H = s²·H₀ − k·Q²; system curve H = Hstat + R(valve)·Q²; speed via the affinity laws. Brake power uses a type-specific characteristic shape (radial rising, axial falling) scaled to a 16 kW BEP / 18.5 kW motor. Current is approximated from load plus a no-load magnetising component. Figures are representative of a mid-size process pump for teaching — not a substitute for the manufacturer's certified curves or your NPSH and minimum-flow calculations.
Reading the power curve
The right-hand chart is the one that matters. For a radial pump, drag the valve closed and the operating point slides left and down the power curve — brake power drops toward its shutoff value, typically 40–60% of the best-efficiency-point power. Open the valve and the point climbs back up.
This is why the motor is happiest starting closed. A direct-on-line motor pulls roughly 6× its full-load current at the instant of energisation regardless of load — that inrush is set by the motor's electrical characteristic, not by the pump. What the closed valve changes is everything after that instant:
- Run-up time is shorter. Less load torque means the motor reaches full speed faster, so it spends less time drawing that high inrush current — and inrush is a thermal stress on the windings measured in amp-seconds.
- The settled running current is lower. At shutoff the motor draws perhaps 60% FLA instead of 90%+ — headroom against nuisance overload trips.
- The voltage dip on the bus is smaller and briefer, which matters on weak networks and when several large motors share a switchboard.
- No hydraulic transient. The discharge line fills and pressurises in a controlled way when you crack the valve open, instead of a slug of liquid being accelerated the instant the pump starts (water hammer).
Run the Simulate start button with the valve closed, then again with it open. The inrush spike has the same height both times — but closed, it collapses to a lower running current sooner. That is the protection you are buying.
The correct start-up sequence
Closing the valve is one step in a sequence. For a standard radial process pump:
- Confirm the pump is primed and the suction valve is fully open. A centrifugal pump cannot start dry, and a throttled suction is the fastest route to cavitation and seal damage. The suction valve is never the one you throttle.
- Close (or nearly close) the discharge valve. This puts the operating point at minimum power for the start. Some installations crack it slightly open to guarantee a trickle of cooling flow.
- Energise the motor and confirm it reaches full speed. Watch for correct rotation and that current settles. With the valve shut this happens quickly.
- Open the discharge valve smoothly to walk the pump out along its curve to the duty point. Watch discharge pressure fall from shutoff head toward the duty head as flow establishes.
- Do not leave it dead-headed. Open the valve within seconds. All the liquid energy is becoming heat in a closed casing — temperature rises fast, and the pump can be damaged in minutes.
Dead-head is for starting, not for running. A radial pump churning against a closed valve converts its entire shaft power into heat in a few litres of trapped liquid. Small pumps can flash the liquid to vapour and wreck the seal and bearings within minutes. This is exactly why a minimum-flow recirculation line exists on critical pumps — and why the start procedure ends with "open the valve."
The exception: axial-flow pumps
Now switch the simulator to Axial flow and look at the power curve again. It is inverted: power is highest at shutoff and falls as flow increases. Close the valve on an axial (propeller) pump and you load the motor to its worst case — the simulator will throw an overload at you.
The reason is geometry. A radial impeller flings liquid outward and develops head from centrifugal action; at zero flow it is just spinning trapped liquid, which is cheap. An axial impeller is a propeller generating head by lift; at zero flow each blade is fully stalled at maximum angle of attack, and that takes enormous torque.
| Pump family | Specific speed | Power at shutoff | Start the valve… |
|---|---|---|---|
| Radial flow | Low | Minimum (~40–60% BEP) | Closed |
| Mixed flow | Medium | Roughly flat (~85–100% BEP) | Part-open / either |
| Axial flow | High | Maximum (up to ~2× BEP) | Open |
The rule is "minimise starting power," not "close the valve." For 90%+ of the pumps you meet — process, water, cooling, transfer — that means a closed discharge. But always check the power curve for the specific machine. The simulator's mixed-flow setting shows the flat middle ground where it genuinely doesn't matter much.
A note on the affinity laws
Drag the speed slider and watch all three readouts move together. The affinity laws say that for a given pump:
- Flow scales with speed:
Q₂/Q₁ = N₂/N₁ - Head scales with the square:
H₂/H₁ = (N₂/N₁)² - Power scales with the cube:
P₂/P₁ = (N₂/N₁)³
That cube on power is why variable-speed drives save so much energy, and why a small over-speed costs so much. Drop the speed to 80% in the model and brake power falls to roughly half — the same physics that makes throttling wasteful makes speed control efficient.
Key takeaways
- Radial centrifugal pumps draw least power at zero flow — so you start them against a closed discharge valve to give the motor the lightest load.
- The closed valve doesn't lower peak inrush current — that is electrical — but it shortens its duration, lowers the settled current, and avoids water hammer and overload trips.
- Open the valve promptly after start. Dead-head running turns shaft power into heat and damages the pump.
- Axial-flow pumps reverse the rule — they draw most power at shutoff and are started open. Always read the power curve for the actual machine.
- The suction valve stays open at all times — never throttle suction.