STEAM DRUM CONTROL ELEMENTS

The purpose of the drum level controller is to bring drum level upto a level during the startup and to maintain the level at the constant load.The decrease in the level will uncover the boiler tubes and expose them to high temperature and leads them to failure. Increase in the level may interfere the steam and moisture separation and will lead to moisture carry over to turbine and reduce boiler efficiency.

Steam flow is the rate of steam leaving the boiler- the demand. water flow is the rate of feed water flow in to the drum- the supply. Drum level reflects the amount of water in the boiler - the invetory

when there is change in the boiler load , suppose boiler load increases, the steam flow demand will be higher than the supply feed water and so the drum water level reduces. so in such a event the controller opens the feed control valve and increases the drum level.

There are types of drum level control.

single element drum level control
Two Element drum level control
Three element drum level control

The use of one of these method will depend upon boiler size and the load changes.

Single element drum level control:

This is the simplest and least effective form of control

Two Element Drum level Control

The two element drum level control can be best applied to single drum boiler where the feed water is at constant pressure. It gets rwo proportional signal from drum trasmitter and mainsteam transmitter.

The steam flow from the boiler transmitts the output signal which acts as a set point for the feed water controller. so that there will be balance between the steam flow and feed water flow. If the steam flow increases due to the load increases, The controller compares the setpoint of the steam flow with the feed flow and to match the set point it opens the feed control valve to increase the feed flow this is called feed forward flow.

The drum level transmitter sends signals to the drum controller which further trims the setpoint of the feed water flow set by the steam flow. Because the drum controller can compensate for the flow measurement errors and unmeasured load disturbances like blow down or steam circuit leakage. this is called cascade controll.

Three Element Drum level control

The three element drum level control maintains the drum level by monitoring the drum level, feed water flow  and steam flow.It provides tighter control for drum level with fluctuating steam load and ideal for a system which suffers from fluctuating feed water pressure or flow.

The three level controller uses three transmitter or process variables to maintain the drum level.

Three Element Drum level control is used in plant where multiple boiler feed water pumps run in parallel to meet the pressure rating of the steam drum, so that it can fill the water in the drum.. 

When the steam flow from the boiler increases , it sets a setpoint to the feed water controller so that the feed water control valve opens according to it. further the drum level controller further trims the set point as mentioned earlier in the two element drum control. If any of the pump pressure reduces and as the feed water valve opens to maintain the water in the drum , the feed water pressure reduces as the valve opens so in order to avois this the valve must be opened gradually for decreasing the feed water pressure this is achieved by taking the feed water pressure transmitter and comparing with the drum and feed water controller set point