pressure switch a familiar device used in process control applications .we know they sense pressure and changes the status of contact output about a setpoint . Usually it is used in pressure applications like compressor control, fluid pressure control and alarm initiation,trip functions on high pressure and low pressure setpoints etc .
Can we use it as level switch????
Yes ……A.properly calibrated pressure switch can be used as level switch . In this article we will learn how it can be used as level switch . Before this let us learn the concept .
How a pressure switch can be used as level switch???
We know pressure transmitters and differential pressure transmitters are broadly used as level transmitter in process control applications .They actually sense hydrostatic pressure .And this hydrostatic pressure can be interpreted as level .The same principle can be applied for pressure switch .If a pressure transmitter can be used as a level transmitter,then a pressure switch can be used as a level switch .
In hydrostatic level measurement the pressure acting due to a height of liquid column is directly proportional to the height of liquid column above that point . Provided density of the liquid is invariant .
P = hρg
p =h* S.G
See figure .
The pressure reading on the pressure gauges changes with level in the tank provided density is invariant ..If we know the height and specific gravity of the liquid we can calculate the pressure reading on the pressure gauge .
The same principle can be applied for the pressure switch to use it as a level switch
The below diagram shows a tank in a process stream .we want to generate a High alarm when the level in the tank is 6 metre .A pressure switch is fitted in a impulse tapping taken from the tank.see figure .
Let us calculate the setpoint to which the pressure switch has to be calibrated .
We only need to found the head pressure acting on the pressure switch when the level is 6 metres .And calibrate the pressure switch for that amount of pressure .
So what is the pressure acting in the pressure switch when level equal to 6 metre ????
Here we only need to consider only the height above the pressure switch tapping .This is because only that amount of head is acting on the pressure switch .
So when level in the tank equals 6 metre,
Height of liquid surface from pressure switch tapping
= 5 metre ( see figure)
Or 5000 mm
Pressure acting on the pressure switch at this level.
P = Height × specific gravity
=5000× 1.25
= 6250 mmwc
So the pressure switch has to be calibrated to a setpoint of 6250 mmwc to use it as a level switch for the installation shown in figure .Upon reaching a level of 6 metres the pressure switch actuates and changes its contact output which will be interpreted as ‘HIGH’ Alarm
