BOILER WATER LEVEL CONTROL SYSTEM

Steam boilers are very common in industry, principally because steam power is so useful. Common
uses for steam in industry include doing mechanical work (e.g. a steam engine moving some sort
of machine), heating, producing vacuums (through the use of “steam ejectors”), and augmenting
chemical processes (e.g. reforming of natural gas into hydrogen and carbon dioxide).
The process of converting water into steam is quite simple: heat up the water until it boils.
Anyone who has ever boiled a pot of water for cooking knows how this process works. Making steam
continuously, however, is a little more complicated. An important variable to measure and control
in a continuous boiler is the level of water in the “steam drum” (the upper vessel in a water-tube
boiler). In order to safely and efficiently produce a continuous flow of steam, we must ensure the
steam drum never runs too low on water, or too high.
 If there is not enough water in the drum the water tubes may run dry and burn through from the heat of the fire.
If there is too much water in the drum, liquid water may be carried along with the flow of steam, causing problems downstream

LT (Level transmetter) The purpose of this device is to sense the water level in the steam drum and report (“transmit”) that measurement to the controller in the form of electrical signal 4-20mA
LIC Level controller
 The purpose of this instrument is to compare the level transmitter’s signal
against a setpoint value entered by a human operator representing the desired water level in the
steam drum. The controller then generates an output signal telling the control valve to either
introduce more or less water into the boiler to maintain the steam drum water level at setpoint

When the controller is placed in the “automatic” mode, it will move the control valve to whatever
position necessary to maintain a constant steam drum water level. The phrase “whatever position
necessary” suggests the relationship between the controller output signal, the process variable signal
(PV), and the setpoint (SP) is complex. If the controller senses a water level above setpoint, it will
close off the valve as far as necessary to decrease the water level down to setpoint. Conversely, if the
controller senses a water level below setpoint, it will open up the valve as far as necessary to raise
the water level up to setpoint.