1, valve throttling
Changing the centrifugal pump the most simple way is to adjust the water pump outlet valve opening, horizontal multistage centrifugal pump speed remains the same (usually rated speed), its essence is to change the location of pipeline characteristic curve to change the operating point of the pump. Pump characteristic curve Q-H and line of intersection of the curve Q-σ h valve open limit of pump operating point. Turn down the valves, piping local resistance increase pump operating point moves to the left, the corresponding flows. When the valve closed, the equivalent resistance is infinite, flow is zero, at which point pipeline characteristics curve coincides with the ordinate. Thus, to turn down the valve to control the flow, multi-stage centrifugal pump water supply capacity will not change itself, lift characteristics constant, tube characteristics will change with the changes of the valve opening. This method is simple, continuous flow, you can adjust between 0 and a maximum flow, and no additional investment in a wide application. But throttling is to burn the excess energy of a centrifugal pump (shaded area in the figure) to maintain a certain level of supply of centrifugal pump's efficiency will be decreased, not economically reasonable.
2, frequency adjusting speed
Operating point deviation from essential conditions for effective area is the water pump need timing. When the speed changes of the multistage centrifugal pump, valve opening remains constant (usually the maximum opening), system characteristic constant water supply capacity and head characteristics to be changed. In the case of the desired flow rate is less than the rated flow, head of variable-frequency adjustable-speed smaller than the valve throttle, variable-frequency adjustable-speed water power required is smaller than the valves ' throttling. Apparently, compared with valve throttling, energy saving effect of variable frequency speed is outstanding, horizontal multistage centrifugal pump more efficiently. In addition, the variable frequency variable speed, not only helps to reduce the possibility of cavitation in centrifugal pumps, but also through the presets to speed up/slow down time to extend the start/shutdown process, dynamic torque is greatly reduced, which to a great extent eliminates the destructive effect of water hammer, greatly extend the service life of the pump and piping system.