What is a centrifugal pump?
A centrifugal pump moves liquid by spinning it. Inside the casing a bladed impeller turns fast, throwing the liquid outward from the centre towards the rim. A spiral chamber called the volute collects that fast stream and guides it to the outlet.
The trick is that the impeller gives the liquid speed, and the widening volute slows it down again. Slower liquid at the same energy means higher pressure, so the pump leaves you with flow under pressure.
Why look at the curve?
A pump is not a fixed source of pressure or flow. It follows one curve: the more liquid it delivers, the less pressure it can push with. Your valve setting decides which point on that curve you actually get.
Drag the slider and watch the trade-off. Open the valve and Q climbs while P falls; close it and the flow nearly stops while the gauge climbs towards shut-off head.
1. Liquid enters the centre eye
The inlet pipe feeds liquid to the centre of the impeller, called the eye. At this point the liquid is at low pressure, ready to be picked up by the spinning blades.
2. The impeller flings liquid outward
The impeller spins fast and its curved blades push the liquid towards the rim. This adds kinetic energy — the liquid leaves the impeller tips travelling quickly.
3. The volute turns speed into pressure
The spiral volute around the impeller is gently widening, so the fast liquid slows as it travels. That loss of speed becomes a rise in pressure, and the liquid heads for the outlet.
4. The outlet valve sets the flow
The slider sets how far the outlet valve is open. Wide open, the liquid is free to leave and the flow rate Q is high; nearly shut, almost nothing moves.
5. Pressure and flow trade off
Because the pump follows one curve, you cannot have both maximum flow and maximum pressure. Moving the valve slides the duty point along the curve, trading one for the other.
What does the slider actually change?
It sets how far the outlet valve on the discharge line is open, from fully shut to wide open. That one setting decides how much liquid can leave the pump.
Why does pressure fall when flow rises?
A centrifugal pump has a fixed curve: at zero flow it develops its highest head, and as flow rises the head falls. Opening the valve moves you to the right along that curve, so more flow means less pressure.
What is shut-off head?
Shut-off head is the pressure a pump reaches when the outlet is fully closed and no liquid moves. With the slider at zero the flow stops and the gauge sits at its maximum reading.
Does opening the valve make the pump work harder?
Up to a point, yes. More flow usually means more power drawn, because the pump is moving more liquid each second even though each drop leaves at lower pressure.
What is head, and how is it different from pressure?
Head is the height a pump could lift the liquid to, expressed in metres. Pressure is the force per unit area at the outlet. They describe the same energy in different units.
Does the impeller push liquid like a piston?
No. The spinning impeller flings liquid outward from the centre eye. The spiral volute then slows that fast-moving liquid so its speed energy becomes pressure.