Makeup boiler feedwater is preheated in a heat exchanger called an Economizer before entering the steam drum. The Economizer uses hot combustion gases to preheat the boiler feed water in order to minimize the amount of heat lost to the stack.
Preheated boiler feedwater from the Economizer enters the Steam Drum situated atop the boiler’s structure. The Steam Drum is a long horizontal vessel and supplies water to the Downcomers. The Steam Drum also serves to separate the water and steam entering the vessel from the Risers. Arrays of specially engineered moisture separators separate saturated steam from the boiler water. The separated steam is saturated and is routed to the Superheater.
The Downcomers are a string of tubes that pass outside the firebox to supply water to the Mud Drum by gravity. The water from the Downcomers collects in the Mud Drum.
The Mud Drum is a long horizontal vessel that sits at the base of the boiler firebox and collects water and a small amount of steam from the Downcomers. The Mud Drum supplies the Risers with water. The Mud Drum also serves to collect, as its name implies, solids formed in the boiler system. These solids (sediment) will settle at the bottom of the Mud Drum. Regular purging of water from the Mud Drum (known as blowdown) helps keep these solids from accumulating. Blowdown also helps prevent accumulation of undesirable dissolved chemicals that can increase scaling and corrosion of the boiler’s tubes.
The Risers are also a string of tubes that pass through the main volume of the firebox to produce steam from pickup of heat from hot combustion gases. The string of tubes is commonly referred to as a waterwall.
The upward flow of fluid in the Risers is induced from a physical phenomenon commonly referred to as the Thermosiphon Effect or the Bernoulli Effect. This effect is that lower density fluids will rise in a gravitational field compared to higher density fluids (e.g., a hot air balloon in cool air). As steam is produced from heat pickup in the Risers, it lowers the net density of the fluid in the Risers. As the fluid rises higher in the Risers, it picks up even more heat and more vaporization occurs. This decreases the fluid’s density even more, resulting in a vigorous upward flow from the Mud Drum back to the Steam Drum. Thus, the Thermosiphon Effect will cause the Risers to pull water (and steam) from the Mud Drum which, in turn, receives water by gravity from the Steam Drum via the Downcomers.
Saturated steam is generated in the Risers of the boiler. Saturated steam readily forms condensate in piping as the steam cools. Therefore, saturated steam needs to be heated well above its dew point (superheating) to avoid excessive condensation in the utility piping network the Boiler serves. Superheating also improves the performance of equipment such as steam turbines.
Before going to the Steam Header, the saturated steam leaves the Steam Drum and passes through the Superheater where it reenters the firebox of the boiler and is superheated.
The final temperature of the superheated steam from the boiler is controlled by injecting a small flow of boiler feedwater into the steam line. A special mixer known as an Attemperator is used for injecting the boiler water. The boiler water normally vaporizes completely. This system prevents excessively superheated steam from reaching the Steam Header. High steam temperatures can damage piping and equipment that the Steam Header serves.
In addition to the steam produced by the Boiler, two other boilers of similar design and capacity as the main boiler provide superheated steam to the steam header. These other boilers are not simulated in detail. Steam users then draw the superheated steam from the Steam Header.
Fuel and air enter the firebox of the boiler where they are ignited and burned. The superheater, the downcomers, and risers are heated both by radiant and convective heat transfer. The hot combustion gases then preheat the boiler feedwater in the economizer before passing out the stack. The rate of steam generated will depend on the rate of fuel fired.
Three boilers provide approximately 250 KPPH (500 GPM of BFW) of superheated steam each to the steam header. The pressure in the steam drum is 735 PSIG. The saturated steam is then superheated to 761 DEG F. The pressure in the steam header is maintained at 700 PSIG.
Each boiler requires approximately 12,675 PPH of fuel to accomplish this task. 3,815 MSCFH of air is required to maintain a 25 % excess of air in the boiler, whereby the stack O2 is maintained at 4.25 VOL %. The stack gases leave the boiler at approximately 300 DEG F.
The boiler consists of a firebox where the fuel and air mixture is burned, a radiant heat transfer section, and convective heat transfer section, superheater, downcomers, risers, steam drum, mud drum, economizer, and stack. The boiler consumes a total of approximately 96 MW of energy. Each boiler operates at approximately 60% to 75% of its maximum capacity.
In the event of a flame out, and during startup, there is burner logic that requires that the furnace is purged with air for 60 seconds before the flame can be ignited.
Boiler feed water to the Steam Drum is controlled by flow controller FIC-101. Boiler feedwater temperature is indicated by TI-101. Boiler feedwater temperature entering the Steam Drum is indicated by TI-102. Blowdown from the Mud Drum is controlled by flow controller FIC-103.
Steam Drum pressure is indicated by PI-101, and the superheated steam temperature is indicated by TI-103 and tempered by TIC-104. Steam Drum level is indicated by LIC-101. Steam rate is indicated by FI-102. The Boiler may be isolated from the steam header by HIC-102. An atmospheric vent of superheated steam is controlled by HIC-103.
Fuel rate to the Boiler is indicated by FI-301 and air to the Boiler is controlled by FIC-201. The switch “BURNER” acts as both an ignitor and a flame indicator. Fuel and air temperature are indicated by TI-301 and TI-201 respectively. Stack temperature is indicated by TI-202 and stack O2 is indicated by AIC-201.
Steam Header pressure is controlled by PIC-501. The steam generated by the other 2 boilers is indicated by FI-401. The percentage of steam generated by the boilers is indicated by HIC-301 and HIC-401. Steam demand is controlled by FIC-601.
The boiler master control consists of the steam header pressure controller (PIC-501) and the firing bias controllers (HIC-301 and HIC-401). The output of the pressure controller (PIC-501) is multiplied by the setpoint of the bias controllers (HIC-301 and HIC-401). This signal is the output of the bias controllers (HIC-301 and HIC-401) which in turn adjusts the fuel valves.
To get maximum firing from the boilers, the setpoints of the bias controllers (HIC-301 and HIC-401) should be 100%. To change the ratio of firing amongst the boilers the setpoints of the bias controllers should be changed. To increase the amount of steam generated by the main boiler, the setpoint of the bias controller for the other two boilers (HIC-401) should be decreased. To increase the amount of steam generated by the other two boilers, the setpoint of the main boiler’s bias control (HIC-301) should be decreased. The bias controllers (HIC-301 and HIC-401) indicate the percentage of total steam that each boiler generates.
Please keep in mind that when you are manipulating the bias controller for the other two boilers (HIC-401), you are in fact changing the bias for those two boilers simultaneously. Since all three boilers are of similar design, the ratio of steam generated by each boiler should be approximately one-third each. However, keep in mind that under loads vastly different from design, the setpoints of the bias controllers (HIC-301 and HIC-401) may have to be trimmed to maintain a steam generation rate of one-third each.
The air rate to the boiler is controlled by FIC-201. This controller gets its setpoint from the fuel rate indicator process variable (FI-301). This process variable is multiplied by a ratio parameter to maintain a 25% excess of air. This ratio is valid only for a particular fuel. When the stack oxygen controller is in automatic mode, it adjusts the ratio of FIC-201 to obtain the desired oxygen content in case the fuel characteristics change. Note that output and setpoint of AIC-201 are initialized in manual mode so that the controller does not bump the ratio of FIC-201 when AIC-201 is placed in automatic mode.
Three-Element boiler Feedwater Control
When steam vapor is generated in the risers of the boiler, some of the volume in the tubes is displaced by steam bubbles. These steam bubbles have a smaller density than the boiler feed water. The density of these steam bubbles is also a function of temperature and pressure. As more and more of these steam bubbles are generated, the level indicator does not give a true indication of the inventory of boiler feed water in the system. As steam demand is increased, this effect becomes increasingly more pronounced, and the level in the steam drum rises rather than decreases, as one would expect.
This phenomenon is known as “Shrink and Swell”. Under these circumstances, a simple level controller would cut back rather than increase the amount of boiler feed water to the steam drum. Consequently, a more sophisticated control scheme is required. This control scheme is known as a Three-Element Boiler Feed Water Control. Three inputs are used to control the drum level. The three elements are drum level (LIC-101), feedwater flow (FIC-101), and steam flow (FI-102). When operating, the steam flow signal acts as a feedforward signal. This permits the feedwater control valve (FIC-101) to respond to steam flow changes without having to wait for a change in drum level.
The Logic system trips off the burners if any of the following conditions occur:
- FIC-201 air flow rate is zero
- FI-301 fuel flow rate is zero
- LIC-101 steam drum level is below its low alarm limit
Similarly, the Logic system allows the burners to start only if all of the following conditions are met:
- FIC-201 air flow rate is greater than zero
- FIC-201 air purging exceeds 60 seconds
- FI-301 fuel flow rate is greater than zero
- LIC-101 steam drum level exceeds its low alarm limit
Note that fuel cannot be introduced until the air purge is completed. The air purge timer starts when the fuel flow FI-301 is zero and the air rate FIC-201 is greater than zero.