Operating Characteristics of a Hydraulic Loop
When we put together the frictional characteristics (system head) of a hydraulic loop and the performance curve the result will describe the characteristics of entire system (e.g. one loop of primary circuit). The following figure shows typical performance curve for a centrifugal pump related to the system frictional head.
Pump head, on the vertical axis, is the difference between system back pressure and the inlet pressure of the pump (ΔPpump). Volumetric flow rate (Q ), on the horizontal axis, is the rate at which fluid is flowing through the pump. As can be seen, the head is approximately constant at low discharge and then drops to zero at Qmax. At low discharge the characteristics can be unstable (with positive slope of pump head). This is undesirable characteristics, because an unstable pump may start to oscillate between the two possible combinations of flow rate and the pipeline can vibrate.
At flow rate Q1 the pump gains more head than consumes the frictional losses, therefore the flow rate through the system will increase. The flow rate will stabilize itself at the point, where the frictional losses intersect the pump characteristics.