The process performance of a centrifugal pump is described by a curve called the head versus flow characteristic. See Figure 7. Centrifugal pumps are constant-head devices. This means that they provide a nearly constant head, or pressure differential, even though the flow rate changes. As Figure 7 shows, the head produced by the pump does increase somewhat as the flow rate decreases from the design point. Conversely, the head decreases at flow rates above the design point. However, over the normal operating range of the pump, the head is relatively constant or, as we say, the curve is relatively flat. Normally, the head developed at zero flow is no more than 110 to 120% of the head at the design point. This is called the shutoff point, or shutoff head.
Note that shutoff means that the flow is shut off, for example by closing a valve at the discharge of the pump. The pump itself continues to rotate and develop differential pressure. However, a pump should not be operated this way except for a short period. After a minute or two, the pump may overheat and damage will occur.
A system curve can be constructed by calculating the system pressure drop with a fully open control valve at several rates and plotting them on the pump curve as shown in Figure 8. This will define the available control valve pressure drop and the maximum flow for the system.