 Switches are commonly employed as input devices to indicate the presence or absence of a particular condition in a system or process that is being monitored and/or controlled.  When the switch makes physical contact with a moving object, a set of electrical contacts is forced either open or closed.  Limit switches have two main parts ◦ Electrical contacts usually sealed within an enclosed body ◦ Actuating mechanism  The contacts are either normally open (N.O), normally closed (N.C)

 Proximity detectors are electronic sensors that indicate the presence of an object without making the physical contacts.  The detector normally does not respond by producing a linear output signal proportional to the distance of the object to the sensor.  The two major types of proximity detectors are inductive and capacitive.

 The inductive proximity switch detects the presence of ferrous metallic materials.  An Inductive Proximity Sensor consists of an oscillator, a ferrite core with coil, a detector circuit, an output circuit, housing, and a cable or connector.  The oscillator generates a sine wave of a fixed frequency. This signal is used to drive the coil. The coil in conjunction with ferrite core induces a electromagnetic field. When the field lines are interrupted by a metal object, the oscillator voltage is reduced, proportional to the size and distance of the object from the coil.  The reduction in the oscillator voltage is caused by eddy currents induced in the metal interrupting the field lines. This reduction in voltage of the oscillator is detected by the detecting circuit. In standard sensors, when the oscillator voltage drops below a present level, an output signal is generated.

 Capacitive proximity sensors are similar to inductive proximity sensors.  The main difference between the two types is that capacitive proximity sensors produce an electrostatic field instead of an electromagnetic field.  Capacitive proximity switches will sense metal as well as nonmetallic materials such as paper, glass, liquids, and cloth.

 Control valves are used to provide a number of functions and are typically selected on the following basis:  - Application function  - Operating conditions  - Construction  - Sizing  Sliding Stem Valves: The basic body styles are:  - Globe  - Cage  - Angle body  - Y pattern  - Split body  - Three way  - Single seated  - Double seated

 The globe is one of the most common types of body style for sliding- stem valves  The globe body differs considerably depending on the trim used.

 The valve cage is another type of globe valve that has been very popular over the last few years.  Its name comes from the shutter is driven by some kind of cage immersed in the valve body.  It is used in many applications replacing the double-entry with some additional advantage such as greater capacity, low noise, good stability, easy change of the internal organs (trim), use of small trim in the same shutter and decreased problems with erosion.

 The butterfly valve consists of a cylindrical body with a disk the same size as the internal diameter of the valve body, mounted on a shaft that rotates perpendicular to the axis of the body.

 The valve is shown open, and can be closed by forcing a flexible membrane down onto a lower flexible membrane, using a pincer action.  A flexible membrane also can be forced down onto a weir.  Diaphragm valves are good low- cost choices for slurries and liquids with suspended solids, but tend to require high maintenance and have poor flow characteristics.

 The valve is a partial sphere that rotates, and the valve tends to be slow to open.  The ball valve is available in other configurations with various shaped spheres for different flow characteristics.  The valve is good for slurries and liquids with solid matter because of its self- cleaning operation.

 Solenoid valves are used to supply ON – OFF control of air to the valve actuator.  They are normally mounted on the valve actuator, but they can be mounted remotely as well.  Solenoid valves can be supplied in different voltages and configurations if required.  Solenoid manifolds are available and used when a large number of valve actuators require control or contain power to a local area reducing assembly time.