To ensure smooth operations and maximize productivity, every process industry needs an efficient steam boiler. Every steam boiler has its own peculiarities. Two boilers that were designed at the same time by the boiler manufacturer have different qualities and tuning issues. For a smooth process, it is important to monitor every component in the steam boiler. The various steam boiler controls that lead to optimal productivity can be used to explain the controls of steam boilers. Learn about steam boiler controls in detail.
Operator Control:
In the control system, an operator’s desk is also known as a control station or a control station. Station is an operator interface that controls a particular control loop. It can be found on the control panel of older plants, or it can be accessed from the operator station when equipped with all-digital controls. The control station is designed to allow the operator to switch between manual and automated modes of operation. The control loops that have been discussed are combined to form a set of controls for managing the main steam boiler functions.
In manual mode, the control loop allows direct control by the operator. In automatic mode, the output is modulated by the proportional-integral-derivative (PID) controller. In most cases, the operator can control the operating point or setpoint of the process either directly or by using a signal. In certain instances, like primary airflow controls, the setpoint can be seen on the graphic display on the computer or controller. The cascade mode allows the operator to hand over the control of the setpoint from the master to internal logic.
Furnace Pressure Control:
The control of furnace pressure is not only simple, but it also has significant safety implications. Steam boilers control boiler pressure using induced draft (ID), fans, or inlet dampers. A controller compares the pressure difference between the boiler and the setpoint pressure using a signal that is usually based on the forced draft fan master output. The output of the controller is typically fed through an ID Fan Master Control Station.
A logic for high and low furnace temperatures is essential to prevent the ID fan from increasing its speed or decreasing it, depending on what is required. A negative furnace pressure signal requires an override to close the ID inlet dam or reduce ID fan speed. These signals are set by the boiler and fans supplier when designing the system. The system must be tuned to maintain a furnace pressure of -0.5 inches H2O.
Steam boiler tuners make a common mistake when they use the quick integral action on the furnace pressure control. Furnace pressure changes quickly, but not immediately. It is important to take into account the size of the furnace and the amount ductwork that connects the furnace with the fans. The air is compressible.
Airflow Trim and Oxygen Trim:
Normally, the FD Fan Master only controls the airflow. Some steam boilers have secondary airflow dampers that control the airflow. The control of air and oxygen is essential for the safe, efficient and effective operation of a steam boiler. Airflow signals are usually measured as a percentage, and not in mass or volumetric units. The amount of oxygen in the flue gases, also known as O2, is measured to ensure proper combustion. This is due to variations in coal heat, air temperature and combustion conditions within a boiler.
Drum level and Feedwater control:
The cascaded controller system for drum level control involves an outer and inner controller. Steam flow is used to indicate the rate of water removal from the drum. Steam flow is fed forward to the outer controller. The outer controller is used to operate the fault in the drum’s level. This controller outputs the setpoint for the flow of feedwater, and the output from this controller is used to modulate feedwater flow control.
The feedwater is fed to the drum by a set of valves, which are connected in parallel to a set of pumps that deliver constant pressure. The water in the drum can entangle in the steam that passes through the turbine if the level of feedwater is too high. The drum can overheat if the feedwater level in the drum is too low.
Superheat Temperature Control:
The control of superheated steam is easy. Steam exits the drum, passes through the primary superheater and then enters the desuperheater. The steam is heated with attemperation before entering the next section of superheaters. The outlet temperature of the superheater is measured once the steam has passed through it.
Deaerator Level Control:
Deaerator levels are often controlled by a three element controller. The three-element deaerator controller uses the same controls as the drum level control, which use the steam flow and feedwater flows.
