A dramatic decrease in this level may uncover boiler tubes, allowing them to become overheated and damaged. An increase in this level may interfere with the process of separating moisture from steam within the drum, thus reducing boiler efficiency and carrying moisture into the process or turbine.
The functions of this control module can be broken down into the following:
The three main options available for drum level control are
The simplest but least effective form of drum level control.
This consists of a proportional signal or process variable (PV) coming from the drum level transmitter. This signal is compared to a setpoint and the difference is a deviation value.
This signal is acted upon by the controller which generates corrective action in the form of a proportional output. The output is then passed to the boiler feedwater valve, which then adjusts the level of feedwater flow into the boiler drum.
The two-element drum level controller can best be applied to a single drum boiler where the feedwater is at a constant pressure.The two elements are made up of the following:
Level Element: a proportional signal or process variable (PV) coming from the drum level transmitter. 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: a mass flow rate signal (corrected for density) is used to control the feedwater flow, giving immediate corrections to feedwater demand in response to load changes. Any imbalance between steam mass flow out and feedwater mass flow into the drum is corrected by the level controller. This imbalance can arise from
The three-element drum level control is ideally suited where a boiler plant consists of multiple boilers and multiple feedwater pumps or where the feedwater has variations in pressure or flow. The three-elements are made up of the following:
Level Element & Steam Flow Element: corrects for unmeasured disturbances within the system such as
Feedwater Flow Element: responds rapidly to variations in feedwater demand, either from the
In order to achieve optimum control, both steam and feedwater flow values should be corrected for density.
The enhanced three-element drum level control module incorporates the standard three element level components with the following improvements: