Every process industry requires an efficient steam boiler to ensure seamless operations and maximum productivity in the plant. However, every steam boiler comes with a set of peculiarities. Two boilers designed by the boiler manufacturer at the same time have unique qualities and special tuning issues. Therefore, regular monitoring of every component of the steam boiler is imperative for hassle-free process operations. Controls in steam boilers can be explained with the various controls in steam boilers leading to optimal productivity from the fuels fed in the system. Let us learn about the controls of the steam boilers in detail,
The operator’s station in the control system is known as a master or as a hand/auto station, control station, or operator station. The station is the operator interface for a specific control loop and usually, a switch located on the control panel in outdated plants or accessible from the operator’s station with the equipped, all-digital controls. Generally, the control station allows the operator to function interchangeably between manual and automatic modes of operation. All the control loops discussed combine to form the set of controls to manage the key steam boiler operating functions.
As a control loop is in manual mode, it allows the operator direct control of the output. In automatic mode, the output is modulated by the proportional-integral-derivative (PID) controller. The operator usually has control over the setpoint or operating point of the process, either directly or through using a signal. In some instances, such as in primary airflow control, the setpoint is shown on the controller located on the control panel or the computer screen graphic display. The cascade mode is a subset of the automatic mode, allowing the operator to turn over control of the setpoint to the master, with internal logic generating the set point.
Furnace Pressure Control:
Furnace pressure control is simple but also one that has important safety implications. Steam boilers use induced draft (ID) fans or inlet dampers to control boiler furnace pressure. The general control system consists of one controller that compares the difference between the furnace pressure and the furnace pressure setpoint using a signal usually based on forced draft (FD) fan master output. The output from the controller is usually fed through an ID fan master control station.
It is also essential that there should be high and low furnace pressure logic to stop the ID fan from increasing or decreasing speed, as required. On a negative furnace pressure signal, an override closes the ID inlet damper or decreases ID fan speed is required. The settings of these signals are decided by the boiler and fan supplier while designing the system. A well-tuned system can maintain furnace pressure to – 0.5 inches H2O.
One of the common mistakes performed by steam boilers tuners is using the quick integral action to the furnace pressure controller. Furnace pressure changes quickly, but not immediately. Therefore, it is crucial to consider the furnace size, the amount of ductwork between the furnace, and the fans as a condenser in the system as the air is compressible.
Airflow and Oxygen Trim:
Usually, the FD fan master only controls the airflow. However, some steam boilers are designed with secondary airflow dampers to control the airflow. Air and O2 control is crucial for the safe and efficient operation of the steam boiler. The airflow signal is measured in terms of a percentage and is usually not available in volumetric or mass flow units. With variations in coal heat content, air temperature, and combustion conditions inside a boiler, proper burning is ensured by measuring the amount of oxygen content in the flue gas, commonly referred to simply as O2.
Drum Level and Feedwater Control:
Drum level control uses a cascaded controller scheme involving an outer and an inner controller. Steam flow indicates the rate at which water is removed from the drum. Steam flow is used as a feedforward to the outer controller. The fault in the drum level is operated by the outer controller. The output of this controller is the feedwater flow set point with the output from the controller used to modulate the feedwater flow control valve.
Feedwater is fed into the drum via a series of valves in parallel with a series of constant-pressure feedwater pumps. If the feedwater level in the drum is excessive, the water becomes entrained in the steam passing the turbine, causing hazardous results. If the drum feedwater level is too low, the drum itself becomes overheated, resulting in damaging consequences.
Superheat Temperature Control:
Superheated steam temperature control is simple, with steam exiting the drum and passing through a primary superheater before it enters the desuperheater. Here, the attemperation water is mixed with the steam to adjust its temperature before it enters the next superheater section. Once the steam passes through that superheater, the outlet temperature is measured.
Deaerator Level Control:
Often, a three-element controller is used for deaerator level control. As the drum level controls use drum level, steam flow, and feedwater flow, the three-element controller for the deaerator uses deaerator level, feedwater flow, and condensate flow.
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