Pump curves have sections with positives slope at very low f

Pump curves have sections with positives slope at very low flow rates. Why would this be so?

Solution

The shape of the curve in pump indicates its flow conditions under particular pressure. The intersection of a system and pump cure determines the operating point of a pump. Any operating problem in a pump can be determined with its operating curve. A slope is change in differential upon change in flow which is indicated as a tangent to a particular point on the curve. If the tendency of a pump differential is to decrease, then it indicates a easy to handle situation of a pump. A negative slope of a pump indicates its free of problem operation under most conditions.

When there is a sudden change in the curve, it indicates a flow change and a positive slope. At that section of the curve, the pump operation is unstable. The unstability of the pump is due to the two flow rates for a particular differential set. The system tries to switch between both the flow rates continuously. Due to this unstability, there is a noise caused in the system which is known as surging. This switching between the flow rates cause a pressure drop in the system.

To reduce the surging, one can recirculate the fluid bypassing from its line or simply put a throttle to increase the resistance in the flow.