How to control boiler drum level
Drum Level Control Systems are used extensively throughout the process industries and the Utilities to control the level of boiling water contained in boiler drums on process plant and help provide a constant supply of steam.
If the level becomes too low then its level control very critical. In the process industries, boiling water to make steam is a very important procedure .The control of water level is a major function in this process and it is achieved through a water steam interface established in a cylindrical vessel called the drum which is usually lying on its side and located near the top of the boiler.
Components Affecting Boiler Drum Water Level
Steam supporting field products such as bubbles exist below the water/steam level interface. These bubbles have volume and therefore displace water to create a misrepresentation of the true water level in the drum. Another effect upon drum level is pressure in the drum. A higher steam demand will cause the drum pressure to drop, and the steam bubbles to expand to give the appearance of a water level higher than it truly is. This fictitious higher water level causes the feed water input to be shut down at a time when more
Water is really required. A surge in water level as a result of the drum pressure decreasing is called ‘swell‘. A water level decrease due to drum pressure increase
Is called ‘shrink’.
If the steam output from the boiler is reduced suddenly, the pressure will increase in the boiler circuit. This increase of pressure will cause the drum level to shrink initially and then increase due to the higher inflow than outflow. The inverse is true when the steam output from the boiler is increased suddenly. Unstable energy input to the boiler can also cause pressure changes that result in the same swell and shrink reactions, making control of the steam drum level extremely difficult.
Singal Element Control
The steam drum level signal is used as the process variable and the output of the controller is used to control the feedwater flow to the steam drum. The drum level controller compares the drum level measurement to the setpoint and modulates the feedwater control valves to keep the water level in the drum as close to setpoint as possible.
Feed pumps are sometimes used to control the level instead of valves. The simple feedback control design described above is called single-element control, because it uses only a single feedback element for control – the drum level measurement. The output of the electronic DP transmitter (level transmitter) is the process Input for the Controller (LC), and the output is then compared to a drum level set-point. Any discrepancy between set point and drum level causes an output from the controller in compensation. Because controller action is reverse, as the drum level Increases, a resultant output signal will decrease to close the feed water control valve. The output of the Controller is fed to the feed water control valve (FCV). If the feed water valve is pneumatic, an l/P (current-to-pressure) converter is required to change the Controller current output to accommodate the pneumatic valve.
Drum level is affected by changes in feed water and steam flow rate. But because of the very slow response of the feedback control loop, changes in feed flow or steam flow can cause very large deviations in boiler drum level. Single-element drum level control can work well only if the residence time of the drum is very large to accommodate the large deviations, but this is seldom the case – especially in the power industry. For this reason, the control strategy is normally expanded to also include feed water and steam flow.
Two Element Control
If a feed pump is started up or shut down, the total feed water flow rate changes. This causes a deviation in drum level, upon which the drum level controller will act and change the feed water control valve position to compensate. As explained above, the level controller’s response is likely very slow, so switching feed pumps on and off can result in large deviations in drum level.
A faster control action is needed for dealing with changes in feed water flow rate. This faster action is obtained by controlling the feed water flow rate itself, in addition to the drum level.
To control both drum level and feed water flow rate, cascade control is used. The drum level controller becomes the primary controller and its output drives the set point of the feed water flow controller, the secondary control loop. This arrangement is also called two element control, because both drum level and feed water flow rate are measured and used for control.
Three Element Control
The combination of feed forward and cascade control loop is used in three element control. Feed flow, changes in steam flow can also cause large deviations in drum level, and could possibly trip the boiler. Changes in steam flow rate are measurable and this measurement can be used to improve level control very successfully by using a feed forward control strategy. The performance of the three-element control system during transient conditions makes it very useful for general industrial and utility boiler applications. It handles loads exhibiting wide and rapid rates of change.
For the feed forward control strategy, steam flow rate is measured and used as the set point of the feed water flow controller. In this way the feed water flow rate is adjusted to match the steam flow. Changes in steam flow rate will almost immediately be counteracted by similar changes in feed water flow rate. To ensure that deviations in drum level are also used for control, the output of the drum level controller is added to the feed forward from steam flow.
The combination of drum level measurement, steam flow measurement, and feed flow measurement to control boiler drum level is called three-element control.