Boiler drum level control is necessary to feed water into the boiler as steam is output into the header and to the associated process equipment. For a given volume of steam and blowdown leaving the steam drum, an equal amount of water should replace into the boiler. There are three main control options available for drum level control, Single Element Control, Two Element Control and Three Element Control.
Boiler Swelling & Shrinking Effect
The downstream process controls steam flow from the boiler. A sudden increase or decrease demand for steam flow will change the pressure in the steam drum and boiler piping. The change in drum pressure will cause a change in both the density and boiling point of the steam and water.
The sudden steam demand increase, the steam bubbles expand under the water surface, increasing density and boiling point of steam and water. Hence causes an increase in steam drum level even without the top-up of feedwater. This increase in water level proportional to an increased steam flow rate and decreased drum pressure is called Swelling Effect.
In the other way, as the steam load decreases, the steam bubbles in the steam and water mixture decrease in density. Hence causes a decrease in drum level, while the mass of water and steam has not changed. This is called Shrinking Effect.
This unstable pressure changes that result in the same swell and shrink reactions, making control of the steam drum level extremely difficult.
What is Single Element Control?
A single element control only uses the drum level signal as the control process variable. After performing a Proportional (P) and Integral (I) computation on the level deviation from setpoint, the output of the controller is then passed to the boiler feed water valve, which then controls the level of feedwater flow into the boiler drum. While Single Element Control possible ineffective control option because of the swelling effect.
Single Element Control recommended on small boilers with a relatively large water volume and steady loads.
What is Two Element Control?
Two-element control is made up of Level Element and Steam Flow Element.
Level Element: The drum level transmitter as a process variable (PV). This signal is compared to a setpoint, and the resultant is a deviation value. This signal is acted upon by the controller, which generates corrective action in the form of a proportional value.
Steam Flow Element: The feedwater flow is control by using steam flow rate, giving instant corrections to feedwater demand in response to load changes. Steam flow acts as a feed-forward signal to allow faster level control.
Two Element Control recommended on small to medium boilers with moderate changes in steam demand.
What is Three Element Control?
Three Element Control is by forming the cascading between drum level and feed water controllers with the addition of steam flow as a feed-forward signal. The steam flow signal is added to the drum level controllers output. This creates two control loops in a controller, the level control loop and feed water control loop.
The principle of the three-element configuration is to control the mass balance of the steam drum. If steam is removed from the boiler circuit, the loss of steam must be replaced with an equal mass of feed water. This to prevent the water level decreased rapidly. The slow-reacting level control loop is usually tuned with a moderate proportional band setting.
Single to Three Element Bumpless Transfer
Unlike the single element configurations, three-element not suitable be used from start-up due to the steam flow being unreliable. When sufficient steam pressure is achieved after start-up and stable steam flow is maintained, the single element drum level control can be switch to three-element. Both Three Element and Single Element are utilizing the same controller to achieve the bumpless transfer of the level control.