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
- Boiler blowdown
- Boiler and superheater tube leaks
Feedwater Flow Element: responds rapidly to variations in feedwater demand, either from the
- Steam flow rate feedforward signal
- Feedwater pressure or flow fluctuations
In order to achieve optimum control, both steam and feedwater flow values should be corrected for density.
- The three-element system provides tighter control for drum level with fluctuating steam load. Ideal where a system suffers from fluctuating feedwater pressure or flow
- More sophisticated level of control required
- Additional input for feedwater flow required
Enhanced three element drum level control
The enhanced three-element drum level control module incorporates the standard three element level components with the following improvements:
- The three-element mode is used during high steam demand. The two-element mode is used if the steam flow measurement fails and the module falls back to single element level control if the feedwater flow measurement should fail or if there is a low steam demand.
- The drum level can be derived from up to three independent transmitters and is density compensated for pressure within the boiler drum.
- Tighter control through a choice of control schemes. Drum level maintained on failure of steam or feedwater flow measurements.
- This module introduces an additional level control loop Boiler drum level control
Reference : eurotherm.com