Common Process Switches and Their Symbols in P&IDs ~ Learning Instrumentation And Control Engineering Learning Instrumentation And Control Engineering

Common Process Switches and Their Symbols in P&IDs

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A discrete sensor is one that is only able to indicate whether the measured variable is above or below a specified setpoint. Discrete sensors usually take the form of switches designed and built to “trip” when the measured quantity either exceeds or falls below a specified value. They are very useful in the field of instrumentation for control and alarming purposes.Most process switches are discrete in nature in that they indicate whether a process variable is
above or below a certain setpoint . Whenever the process variable value, exceeds or falls below a specified setpoint, they trip the process and indicate an alarm or initiate a control action.Many different types of discrete sensors exist, detecting variables such as position, fluid pressure, material level, temperature, and fluid flow rate. The output of a discrete sensor is typically electrical in nature, whether it be an active voltage signal or just resistive continuity between two terminals on the device. 

Some of the common discrete sensors used as process switches in piping and instrumentation diagrams and other diagrams are:
  1. Hand switch 
  2. Limit switch 
  3. Proximity switch 
  4. Pressure switch 
  5. Level switch 
  6. Temperature switch 
  7. Flow switch 
Common symbols of process switches used in piping and instrumentation diagrams and other drawings include:

Hand switches
A hand switch is exactly what the name implies: an electrical switch actuated by a person’s hand motion. These may take the form of toggle, push button, rotary, pull-chain and many others.

Limit Switches
A limit switch detects the physical motion of an object by direct contact with that object. An example of a limit switch is the switch detecting the open position of a car’s door, automatically energizing the interior light of the car when the door opens. The symbol for a limit switch is shown below:







Proximity Switches 
A proximity switch is one detecting the proximity or closeness of an object. These switches are non-contact sensors, using magnetic, electric, or optical means to sense the proximity of objects. Most proximity switches are active in design. That is, they incorporate a powered electronic circuit to sense the proximity of an object.
The schematic diagram symbol for a proximity switch with mechanical contacts is the same as for a mechanical limit switch, except that the switch symbol is enclosed by a diamond shape, indicating a powered (active) device as shown below:






Pressure Switches 
Pressure switches are one of the most ubiquitous discrete sensors you can find in a plant. Hardly do you go through a process plant without seeing a pressure switch. A pressure switch is one that detects the presence of fluid pressure. Pressure switches often use diaphragms or bellows as the pressure-sensing element, the motion of which actuates one or more switch contacts. The schematic symbol for a pressure switch is shown below:





Level Switches
A level switch is one detecting the level of liquid or solid (granules or powder) in a vessel. Level switches often use floats as the level-sensing element, the motion of which actuates one or more switch contacts. The symbol for a level switch is shown below:






Temperature switches 
A temperature switch is one detecting the temperature of an object. Temperature switches often use bimetallic strips as the temperature-sensing element, the motion of which actuates one or more switch contacts. An alternative design uses a metal bulb filled with a fluid that expands with temperature, causing the switch mechanism to actuate based on the pressure this fluid exerts against a diaphragm or bellows. The symbol for a temperature switch is shown below:







Flow Switches
A flow switch is one detecting the flow of some fluid through a pipe. Flow switches often use paddles as the flow-sensing element, the motion of which actuates one or more switch contacts. The symbol for a flow switch is shown below: