Final Control Element - Pumps
Pumps are used extensively in liquid flow control system application because they are simple to operate,are energy efficient and can provide both flow and pressure control in a single piece of equipment.
Types of Pump Used in Process Control Systems
1.Centrifugal Pump 2.Positive Displacement 3.Reciprocating
Centrifugal pump operates at a fixed speed to recirculate or transfer process fluid. If control fluid flow is required,a control valve is generally installed in the discharge line. In some application VFD is used to control the speed of pump because control valve consumes energy.This energy loss cannot be tolerated in some application. Centrifugal Pump is the most widely used process fluid-handling device.
Positive Displacement pumps can be divided in two types-
Rotary pumps function by continuously producing reduced pressure cavities on the suction side,which fills with fluid.The fluid is moved to discharge side of the pump,where it is compressed and then discharged from the pump.In most case,flow is directly proportional to pump speed. Therefore,the control system only needs to control the pump speed to control the process fluid flow. This type of pump provides accurate,uniform flow and a minimized power requirement.
Gear Pump - Used to pump high viscosity fluid or generate moderately high differential pressure.
Circumferential Pump - widely used in sanitary and food application.
Progressive Cavity Pump - A single helical rotor turns eccentrically in a stationary stator. As the rotor turns cavities are produced,which are filled at the inlet by the process fluid and move through the stator to the discharge side of the pump.
Reciprocating Pumps use a linear reciprocal stroke in combination with check valves to pump fluid.
Reciprocating pumps are commonly use to control the rate at which a volume of fluid is injected into a process stream or vessel.These pumps are also called metering pumps in some application because they are highly accurate and consistent in the volume of fluid discharged per cycle.Large reciprocating pumps generally have variable speed drives.
Plunger Pump - A packed plunger draws in and then expels fluid through a one way check valve.
Diaphragm - Reciprocating Pump - mechanically driven,directly coupled to a plunger. It draws in a precise amount of fluid and discharges it Cyclically