Pumb control in process control instrumentation


#1

Centrifugal pumps are an important instrument used in the process industries. The pumps are used to flow any kind of fluid to the process tank or other instruments as desired. So the control of the pump is an important job in the control of the process.

Centrifugal pump capacity control/pump characteristics:

Small motor-driven pumps often employ a control method in which a control valve incorporated on the discharge side can be manipulated:

Here pump is used to fill a tank with liquid from another tank. The flow rate is controlled by changing pump characteristics, which is achieved by altering the pump speed without a control valve.

Capacity control of this pump can be illustrated by the curves shown above, in which one type of curve indicates pump characteristics (flow versus discharge pressure) while the other indicates total pipe pressure loss including that of the control valve. Thus, if we consider the flow rates at two points where these curves intersect, we can see that as we increase the pressure loss in the control valve from points 1 to 2, the flow rate decreases.

Pump characteristics are in general given by the following expression:

where P is the discharge pressure, n the pump speed, F the flow rate, and K a constant.

Capacity control of a constant-volume pump:

Capacity control of rotary pumps using gears or vanes can be performed by the speed adjustment technique shown in the picture above or the bypass- valve adjustment technique.