Boiler Drum Level Control

Problem in Boiler Drum Level Control:

• In the YS170 and SLPC (Discontinued models), single-element control needed to be configured within the user program.
• The YS1700, however, offers the primary direct mode so that the user program can switch on and off single-element control by changing a parameter.

Three-element Control:

• Disturbances in drum level are caused by fluctuations in main steam flow.
• Figure 1 shows a typical drum level controller, which measures the drum level (1), main steam flow (2), and feedwater flow (3) and manipulates the feedwater control valve to regulate the drum level. This is three-element control.
• This three-element controller is a so-called cascade controller in which a drum level control unit is cascaded into a feedwater flow control unit.

Single-element Control:

• During startup of a boiler, both the feedwater and main steam flow rates remain below the measurable ranges, and this makes the three-element control with a level controller and flow controller unstable.
• In such cases, it is effective to use single-element control in which the feedwater controller is bypassed and the drum level controller directly controls the feedwater control valve.