Process Description
The Thermal Power Plant consists of the following four sections:
- Boiler feedwater system (deaeration and pumping)
- High Pressure boiler
- Steam turbine with generator
- Cooling tower
Polished makeup water from battery limits is fed to Deaerator D-101 to offset the loss of boiler feedwater elsewhere in the plant. The Deaerator also receives warm steam turbine condensate from the Boiler F-201. The Deaerator allows release of any non-condensables in the feed waters through an overhead vent to atmosphere. Deaerated water is collected in the base of D-101 and serves as a reserve to handle fluctuations of water inventories elsewhere in the plant. Boiler feedwater is pumped out of the Deaerator by BFW Pumps P-101A/B and mainly supplies water to the Boiler F-201 to generate high pressure steam in Steam Drum D-201. Boiler feedwater is also used for desuperheating steam in the Boiler and in the Steam Turbine’s Intermediate Pressure (IP) header as needed. Normally, no boiler feedwater is used for desuperheating.
The Boiler F-201 generates high pressure steam in a water-wall boiler using natural gas fuel to boil and superheat the steam. The water-wall boiler uses a forced circulation system to push water through heat-absorbing panels throughout the exterior walls of the Boiler. A large flow of water from Steam Drum D-201 is distributed through these panels at the bottom of the Boiler with Boiler Circulation Pumps P-201A/B. The partially boiled water is collected from the top of the Boiler and returns to the Steam Drum. Saturated steam from the returned water is separated in the Steam Drum D-201. The produced steam is superheated in three different heat exchange coils located in the upper section of the Boiler F-201. IP Steam from the Steam Turbine JT-301 is also reheated in another heat exchange coil located between two of the HP steam superheating coils. High pressure boiler feed water from P-101A/B is preheated in the three economizer coils of the Boiler which are located in the downward flue gas plenum. Steam Turbine condensate is preheated in the lowest coil after the economizer coils before being returned back to the Deaerator D-101.
The combustion air to the Boiler is provided by Forced Draft Fans G-201A/B. The air flows through and is preheated in the Air Preheater E-201. The Air Preheater is a regenerative type of heat exchanger that employs a circulating set of metal grids which are heated as they pass through the flue gas side of the exchanger. The heated portion of the grid then returns to the combustion air side of the heat exchanger to warm up combustion air from the Forced Draft Fans. A sealing system inhibits air from passing into the flue gas side of the Air Preheater.
The heated combustion air is then routed through ducts to four burner assemblies located in the corners of the Boiler’s radiant section. Fuel gas is burned in these burner assemblies (B-201A/B/C/D) and a majority of the heat produced is transferred to the walls of the Boiler in the radiant section. The flue gas leaving the radiant section then is routed through the steam preheating coils in the upper section of the Boiler and then passes downward through the exhaust plenum to preheat water. Warm flue gas from the last coil then passes through the Air Preheater as described above.
Flue gas leaving the Air Preheater is then blown to the Stack S-201 for discharge to atmosphere by Induced Draft Fans G-202A/B. Because the fuel is clean natural gas, no special flue gas treating equipment is required to comply with environmental regulations. The pressure developed at the inlet of the Induced Draft Fans is significantly lower than atmospheric while the pressure developed at the outlet of the Forced Draft Fans is significantly above atmospheric. The pressure near the top of the radiant section of the Boiler is maintained at slightly below atmospheric pressure to guarantee no hot flue gas leaves through any gas in the walls of the Boiler. This slightly sub-atmospheric pressure also minimizes the flow of leaked air into the box.
Superheated steam is produced by the Boiler at 1,031 DEG F and is distributed from the High Pressure (HP) Steam Header to Steam Turbine JT-301. The header is maintained at 1,800 PSIG by control of the steam flow to the Steam Turbine. In case the Steam Turbine is shut down, a bypass line is provided to route HP Steam to the Condenser E-301 on a temporary basis.
The Steam Turbine JT-301 consists of three stages: High Pressure (HP), Intermediate Pressure (IP) and Low Pressure (LP). Steam exits the HP Stage at 345 PSIG and 640 DEG F and is routed to the IP Reheat coil of the Boiler to be reheated to 1031 DEG F before entering the IP Stage. The steam exiting the IP Stage is then introduced to 2 LP Stages arranged counter-opposed on the common shaft of the Steam Turbine. The shaft drives the Generator G-301 to produce additional electric power.
Steam exhausting from the 2 LP Stages of the Steam Turbine is directly routed into Surface Condenser E-301 which is cooled with circulating cooling water from Cooling Tower CT-501. The low temperature of the surface condenser results in all the steam condensing at a low (vacuum) pressure. To prevent case non-condensables from accumulating in the steam side of the Surface Condenser, a vacuum system is provided. The condensed steam is collected in Hotwell D-301 and pumped by Condensate Pumps P-301A/B to the last coil of the Boiler to be reheated before returning to Deaerator D-101.
Cooling water from the basin of Cooling Tower CT-501 is circulated by Cooling Water Pumps P-501A/B to the Surface Condenser E-301. The warm cooling water is returned to the top of the Cooling Tower to be cooled with air flowing up through the Cooling Tower. The air flow through the Cooling Tower is produced by a fan at the top. Cooled water falls into the basin. Makeup water is continually added to the basin to replace water which is evaporated in the air flowing through the Cooling Tower.
Boiler Feedwater System
Polished makeup water from battery limits is sent to the top of the stripping section of Deaerator D-101 where it combines with warm steam condensate returned from the HRSG E-201. The stripping section of D-101 is filled with structured packing to ensure good mixing of any added deaeration steam and water. Intermediate Pressure (IP) steam from the IP header on Steam Turbine JT-301 can be injected to the base of the stripping section of D-101 to ensure the Deaerator produces a small flow of steam from the top of D-101 through restriction orifice RO-101. Normally, the returning condensate is warm enough to generate enough top steam by flashing so that no stripping steam is required.
Stripping steam is added at the base of the stripping section and warming steam, if required, is sparged into to the water in the reserve base of D-101. A flow of steam through RO-101 ensures any dissolved oxygen and other gases are removed from the makeup water and recirculated steam condensate and vented from the system. Oxygen is particularly undesirable in high pressure boiler service because it leads to corrosion of the steam generating equipment.
The Deaerator normally operates slightly above atmospheric pressure. Water low in dissolved oxygen falls from the stripping section into the bottom section of the Deaerator which serves as a reserve volume of boiler water for the system.
Boiler Feed Water Pumps P-101A/B are high head, multi-stage centrifugal pumps and take suction from the bottom of D-101. Normally only one pump is in operation. The pumps supply deaerated boiler feedwater to the users in the plant as follows:
- Economizer coils of HRSG E-201 for supply of Steam Drum D-201
- Desuperheater J-201 between the superheating coils on HRSG E-201
- Desuperheater J-202 on the bypass line from steam header of HRSG E-201 to Steam Turbine Condenser E-301
- Desuperheater J-301 on the line from IP header of Steam Turbine JT-301 to the IP reheater coil of HRSG E-201
To protect the pumps against being run blocked-in, a minimum flow line from the discharge line of the pumps back to D-101 is provided. In case the Deaerator is overfilled, a manual drain line to the offsite water storage tank is provided off the discharge line from the pumps.
Boiler Process Description
The Boiler F-201 generates high pressure steam in a water-wall boiler using natural gas fuel to boil and superheat the steam. Water is supplied from Steam Drum D-201 to the Boiler Water Circulation Pumps P-201A/B and then to wall panels via a manifold at the bottom of the Boiler. The Boiler Circulation Pumps are motor driven. Only one pump is normally in operation. The circulation flow of water is roughly 3.5 times the flow of steam generated by F-201. Fuel gas fired in the four corners of the radiant section of the Boiler produces a high temperature flue gas that transfers a large portion of its heat to the exposed wall panels. Circulating water boils inside channels embedded within the panels. The partially boiled water is collected from the wall panels via a manifold at the top of the Boiler and returns to the Steam Drum.
The Steam Drum D-201 is fitted with separators to disengage the steam from the risers and route it to the top of the Steam Drum. Separated water combines with preheated boiler feed water from the economizer coils and is circulated back down to the Boiler Circulation Pumps via a tall standpipe. To avoid accumulation of solids in the Steam Drum over time, it is continuously drained. The relatively small blowdown flow from the Steam Drum is sent to battery limits for disposal.
The hot flue gas leaving the radiant section flows into the superheating coils located along the top above the radiant section of the boiler. These superheating coils are specially designed to withstand the high temperatures possible from being exposed to the radiant section of the Boiler. The following heating sections are provided in this section:
- HP Steam Divisional Superheater Coil
- HP Steam Platen Superheater Coil
- IP Steam Reheater Coil
- HP Steam Low Temperature (LT) Superheater Coil
The produced steam is superheated in the three HP Superheating coils. Boiler feed water is injected into the Spray Desuperheater J-201 to control the final superheat temperature of the steam from the Platen Superheater Coil. J-201 is located between the Divisional and Platen Superheater Coils.
Superheated steam from the Platen Superheater Coil is sent to the HP Steam Header for distribution to the Steam Turbine J-301. The HP Steam can also bypass the Steam Turbine under transient conditions. Any HP Steam bypassing the Steam Turbine is first desuperheated in Bypass Desuperheater J-202 before continuing to the Surface Condenser E-301. Desuperheating of the bypass steam is required to avoid extreme thermal stresses and performance issues when high temperature steam is directly passed to the Surface Condenser. In case of severe upsets, HP Steam can be directly vented to the atmosphere if the pressure of the HP Steam Header becomes too high.
IP Steam from the exhaust of the HP Stage of Steam Turbine JT-301 is reheated in the IP Steam Reheater Coil. The steam returns to the IP Stage of the Steam Turbine. This reheating increases the power output of the Steam Turbine which, in turn, produces more electric power from the Generator.
The LT Superheater Coil is located in the upper corner section of the Boiler and connects with the flue gas plenum. HP Steam from the LT Superheater Coil continues on to the Divisional Superheater Coil.
The flue gas from the LT Superheater Coil continues to the three Economizer Coils located in the flue has plenum. Warm boiler feed water from BFW Pumps P-101A/B flows into these three economizer coils to recover heat from flue gas before it passes to the condensate reheating coil. The Economizer Coils have a fairly large surface area to absorb most of the available heat from the flue gas. Preheated boiler feed water from the third Economizer Coil combines with water in the standpipe from the Steam Drum D-201 to the Boiler Circulation Pumps P-201A/B.
Cold condensate from Condensate Pumps P-301A/B flows into the Condensate Reheat Coil of F-201 to recover heat from the flue gas before it is discharged to the Air Preheater E-201. The warmed condensate is sent to the Deaerator D-101.
The combustion air to the Boiler is provided by Forced Draft Fans G-201A/B. These fans are electric motor-driven and are outfitted with jalousie-style dampers the close when the motor is stopped to prevent recirculation from the other running fan. Normally both fans are running so that a loss of one will not require a shutdown of the boiler. The outlet dampers automatically open when the motor of the fan is started. The fans have blades with variable pitch in order to control the amount of pressure (head) buildup by the fan. This allows combustion air flow to be controlled without wasting electric power that would occur by using a damper on the inlet to the fan to control air flow.
The combustion air flows through and is preheated in Air Preheater E-201. The Air Preheater is a regenerative type of heat exchanger that employs a circulating set of metal grids which are heated as they pass through the flue gas side of the exchanger. The heated portion of the grid then returns to the combustion air side of the heat exchanger to warm up combustion air from the Forced Draft Fans. A sealing system inhibits air from passing into the flue gas side of the Air Preheater. An electric motor turns the grids. In case the electric motor stops, an air motor continues to rotate the metal grids of the Air Preheater but at a much slower speed. This helps keep the rotating grids from seizing up.
The heated combustion air is then equally routed through ducts to four burner assemblies (also known as wind boxes) located in the corners of the Boiler’s radiant section. Fuel gas is burned in these burner assemblies (B-201A/B/C/D) and a majority of the heat produced is transferred to the walls of the Boiler in the radiant section. A pilot system is provided to ensure the burners are re-lighted in case there is a loss of flame due to excessive air flow, especially during upsets. Warm flue gas from the Condensate Reheating Coil then passes through the Air Preheater as described above.
Flue gas leaving the Air Preheater is then blown to the Stack S-201 for discharge to atmosphere by Induced Draft Fans G-202A/B which are motor driven. These fans have the same operational features as the Forced Draft Fans G-201A/B as described above. The pressure developed at the inlet of the Induced Draft Fans is significantly lower than atmospheric while the pressure developed at the outlet of the Forced Draft Fans is significantly above atmospheric. The pressure near the top of the radiant section of the Boiler is maintained at slightly below atmospheric pressure to guarantee no hot flue gas leaves through any gas in the walls of the Boiler. This slightly sub-atmospheric pressure also minimizes the flow of leaked air into the box. The pressure of the radiant section of Boiler F-201 is controlled by adjusting the pitch angle of the blades of Induced Draft Fans G-202A/B. Normally, both fans are running so that a loss of one will not require a trip of the boiler.
Steam Turbine Process
Superheated high pressure (HP) steam from the HP Steam Header of the HRSG enters the Steam Turbine JT-301 under control of the turbine control system which regulates the inlet throttle valve SV-301. JT-301 is a three-stage turbine; the first stage exhausts intermediate pressure (IP) steam which is routed to IP Desuperheater J-301 prior to passing through the IP reheating coil of HRSG E-201. Increasing the temperature of the IP steam before using it in the second stage increases the power availability of the steam and makes the power generation cycle more efficient. Reheated IP steam from the HRSG is readmitted to JT-301 and passes through the IP stage of the steam turbine and then to the low pressure (LP) stages within the same casing. Steam exiting the LP stages is exhausted directly to the shell side of Surface Condenser E-301 which uses cooling water to condense the exhausted steam from JT-301.
E-301 normally condenses all the steam from JT-301. The condensate from E-301 drains directly into the Hotwell D-301. The pressure of E-301 is essentially determined by the vapor pressure of the condensate leaving E-301 so it normally operates at vacuum conditions. To ensure vacuum conditions are maintained, the Vacuum Ejector EJ-302 pulls a small flow of low pressure vapor from E-301 using IP Steam taken off from JT-301. The motive steam and vapor from EJ-302 are condensed in Vacuum Condenser E-302 using cooling water. The condensate from E-302 drains into the Hotwell D-301.
During transient operation at startup, shutdown and upsets, the HP Steam Header will bypass some or all of the flow produced by the HRSG around the Steam Turbine JT-301 directly to the Surface Condenser E-301. The steam is desuperheated before entering E-301 so as to minimize thermal stresses on the Condenser and to avoid heat transfer problems due to very high steam temperatures.
At startup or in case of leaks of air into E-301 or in case of non-condensables in the HP steam, the pressure in E-301 may build due to pocketing of the non-condensable vapor. If this occurs, the Startup Ejector EJ-301 can be placed in service to exhaust the vapor directly to atmosphere via a vent. Refer to Schematic #15 in the “Process Schematics” section below for a diagram of EJ-301.
The Hotwell D-301 is a vertical drum directly connected to outlet of Surface Condenser E-301 and collects steam condensate from E-301 and E-302. The condensate is pumped by Condensate Return Pumps P-301A/B to the condensate reheat coil of the HRSG E-201 and then to the Deaerator D-101 in the BFW treatment section of the plant. Either pump can be set to auto-start in case of high level in D-301.
Cooling Tower, CT-501
Water/Air Contacting in Cells
Warm cooling water is returned from the Surface Condenser E-301 and Ejector Condenser E-302 via the return header and is routed to the top of the Cooling Tower CT-501 via a hand valve controlled by HIC-503. The warm water is split equally and sent to opposite sides of the Cooling Tower and distributed evenly into the two opposing cells.
Within each cell, falling warm water is contacted with air drawn into the cell by a motor-driven fan located at the top of the tower. Air is drawn in along the outside of each cell the contacting method is known as “cross-flow”. In a cross-flow cooling tower, adjustable louvers are installed to allow control of the air flow into the tower. A hand controller, HIC-508, allows adjustment of the louvers’ position. Only one cell is simulated.
Intimate mixing of the warm water and ambient air is accomplished by engineered packing material (also known as “fill”) installed inside the framework of the cell structure of the Cooling Tower.
Heat Transfer
As warm water contacts ambient air, heat is transferred from the warm water to the cooling tower at the surfaces of the fill material. The more surface area, the more heat transfer will occur until the temperature of the air and the water are the same. In practice, the equilibration of the air and water temperatures does not normally occur in a cooling tower operating at normal loads.
Evaporation of Water
In addition to heat transfer, some of the cooling water will evaporate into the air. Evaporation results in a loss of heat that can produces temperatures below the ambient air temperature. The rate of evaporation of cooling water into the air will depend on the temperature of the air and its moisture content. At the same temperature, air with lower moisture content can evaporate more water than air with higher moisture content.
The moisture content of ambient air is represented by its dewpoint. The dewpoint is the temperature at which the moisture in ambient air will begin to produce dew if it is cooled any further.
The maximum evaporative capacity of ambient air is essentially the difference between the saturated moisture content of the air leaving the cooling tower and the actual moisture content of the air supplying the cooling tower. So, a cooling tower whose air leaves at 80 DEG F will be able to theoretically increase its moisture content by 3.45 – 0.83 = 2.62% with ambient air at a dewpoint of 40 DEG F but only increase it by 3.45 – 2.46 = 0.99% with ambient air at a dewpoint of 70 DEG F.
Makeup water must be added to the cooling system to offset evaporation losses. Water can also be lost due to carryover of water mist in the air drawn by the fan and by leaks.
Cold Weather Operation
In cold weather, the Cooling Tower may overcool the water at low process rates even with the louvers at their minimum opening. To raise the cooling water supply temperature in this condition, a portion of the return water can be routed directly to the basin as needed with the manual valve controlled by HIC-502.
Cooling Water Pumps, P-501A/B
The Cooling Water Pumps P-501A/B are identical electric motor-driven centrifugal pumps with a design capacity of 43,000 GPM at 55 PSIG. Normally only one pump is in operation. The pumps take suction from the basin of the Cooling Tower CT-501 and deliver cooling water to Surface Condenser E-301 and Ejector Condenser E-302 via the cooling water supply header. The flow of the cooling water to E-301 is adjustable using hand controller HIC-305 and the flow to E-302 is adjusted with HIC-307.
Note that in high capacity cooling systems, there is normally more than one cooling water pump in operation to improve service reliability and for purchase cost and capacity capability reasons. On the simulator, only one pump is in service. This improves training since the loss of or trouble with the operating pump will have immediate and stronger consequences on the operation of the Surface Condenser and Steam Turbine compared to the loss of one pump out of two or three operating pumps.
The cooling water supply header pressure is controlled to 50 PSIG by returning a portion of the cooling water back to the Cooling Tower CT-501. Keeping the supply pressure constant minimizes cooling water flow disturbances to the process heat exchangers. Also, a line is provided off the supply header for sending blowdown water to disposal facilities as needed.
Cooling Water Filter Circuit
The cooling water filter circuit consists of the Cooling Water Filter Pump P-502 and the Cooling Water Filter F-501. The circuit circulates water from the basin of the Cooling Tower through the filter to remove any solids that are suspended in the cooling water. The filtered water is returned to the basin. This ensures that fouling of the Cooling Tower and the process heat exchangers is minimized. P-502 normally circulates 250 GPM of water. The pressure drop across F-101 is around 5 PSIG when clean.
Instrumentation
Deaerator
Polished water from battery limits is controlled by LIC-101 to maintain the level in the base of Deaerator D-101. The flow of polished water is indicated on FI-101. The pressure and temperature of the polished water are indicated on PI-101 and TI-101, respectively.
Steam flow from the IP header of Steam Turbine J-301 to the sparger of the stripping section of the Deaerator is controlled by FIC-104. The flow of IP steam to the sparger in the base of the Deaerator is controlled by FIC-105.
The pressure of the Deaerator is indicated by PI-103. Steam flow from the overhead of the Deaerator is indicated on FI-106 before passing through restriction orifice RO-101.
The flow of steam condensate returning from the HRSG is indicated on FI-107. The temperature of the returning condensate is indicated on TI-107.
The level of water in the base of the Deaerator is also indicated on LAH-102 which is used by Interlock I-101 (see below) to protect the Boiler Feedwater Pumps against operating when there is excessively low water level.
Boiler Feedwater Pumps
Switch HS-101A operates the motor of Boiler Feedwater Pump P-101A and switch HS-101B operates P-101B. These switches are locked in the STOP if interlock I-101 is tripped as indicated on switch XA-101. XA-101 is an indicate-only switch.
FIC-103 controls the flow of boiler feedwater to the boilers at battery limits. FIC-102 controls the amount of boiler feedwater passing through pumps P-101A/B to a minimum flow in case the demand for water to the boilers is low. This helps protect the high-head pumps against damage at low-flow conditions due to high temperatures and cavitation/vibration. The pressure of the discharge header of the pumps is indicated on PI-102.
Boiler Firing
TI-210 indicates the temperature of preheated combustion air from Air Preheater E-201. TI-211 indicates the temperature of the flue gas leaving F-201’s flue gas plenum (after the Condensate Reheating Coil).
FIC-206 controls the flow of low-pressure natural gas fuel to the burners of F-201. The pressure and temperature of the natural gas supply are indicated on PI-201 and TI-200, respectively. HS-206 controls the status of the pilot system for B-201A/B/C/D. The pilot system must be working in order for fuel gas to be burned in F-201.
The motors of Circulation Pumps P-201A/B are controlled by switches HS-201A and HS-201B. XA-201 alarms when no motors are operating and is used as a trip input for I-202.
PIC-211 controls the pressure of the radiant section of Boiler F-201 by adjusting the setpoints of the fan blade pitch hand controllers HIC-212A/B of Induced Draft Fans G-202A/B (see Schematic #8). PAH-211 is an independent pressure indication and will trip I-202 when it reaches the high alarm condition. PAL-211 is an independent pressure indication and will trip I-202 when it reaches the low alarm condition.
HS-202 is a switch to trip or reset Boiling Firing Interlock I-202. XA-202 alarms when I-202 is tripped. Refer to the section on interlocks below for more details. Note that the auto-synch controller may not successfully work if there are large fluctuations in the fuel gas supply or in the grid frequency during synchronization.
Air Preheater and Fans
TI-213 indicates the temperature of ambient air flowing to the Induced Draft Fans G-201A/B. The motors of G-201A/B are controlled by switches HS-210A and HS-210B. When a fan motor is started, the discharge louver will automatically be opened. When a fan motor is stopped, the louver will automatically be closed to prevent the other operating fan from recirculating air through the stopped fan. XA-210 alarms if the motors of both Forced Draft Fans are stopped. XA-210 also causes a trip of I-202.
The pitch of the fan blades of G-201A/B are adjusted by hand controllers HIC-210A/B. In order to avoid excessive mechanical wear on the blade control mechanisms, HIC-210A/B should be operated in either cascade mode (setpoint coming from air flow controller FIC-210 output) or in automatic mode. HIC-210A/B are tuned to make gradual changes to the pitch of the blades. A 100% indication on HIC-210A/B will provide maximum blade angle and, therefore, maximum air flow while a 0% indication will put the blades at a minimum angle and produce a minimum air flow. Both fans are normally in operation and it is most efficient to operate with identical fan blade pitch positions. Otherwise, the fan with the higher pitch angle will suppress flow from the other fan. This wastes electric power.
FIC-210 controls the air flow to the Air Preheater E-201 by adjusting the setpoint of both HIC-210A and HIC-210B. HIC-210A/B are normally operating in cascade mode. When engaging the air flow cascade control at startup, it is important to first initialize the output of FIC-210 to the position of HIC-210A/B. This way, when cascade mode is selected on HIC-210A or HIC-210B the air flow from the fans will not bump and cause an upset of the Boiler F-201. The discharge pressure of the Induced Draft Fans is indicated on PI-210.
The motor of Air Preheater E-201 is controlled by switch HS-211. If the motor stops, XA-211 will alarm. HIC-211 controls the position of the Air Preheater bypass louver. Normally the louver is closed. HIC-211 can be opened in case the flue gas outlet temperature of E-201 as indicated on TI-212 becomes too low. Low flue gas temperature from E-201 can cause water condensation leading to corrosion of E-201 and erosion of the internals of Induced Draft Fans G-202A/B. TI-210 indicates the temperature of preheated combustion air from Air Preheater E-201.
TI-211 indicates the temperature of the flue gas leaving F-201’s flue gas plenum (after the Condensate Reheating Coil). AI-203 indicates the oxygen concentration of the flue gas leaving Boiler F-201.
The motors of Induced Draft Fans G-202A/B are controlled by switches HS-212A and HS-212B. When a fan motor is started, the discharge louver will automatically be opened. When a fan motor is stopped, the louver will automatically be closed to prevent the other operating fan from recirculating flue gas through the stopped fan. XA-212 alarms if the motors of both Induced Draft Fans are stopped. XA-212 also causes a trip of I-202. In order to avoid excessive mechanical wear on the blade control mechanisms, HIC-212A/B should be operated in either cascade mode (setpoint coming from F-201 radiant section pressure controller PIC-211 output) or in automatic mode. HIC-212A/B are tuned to make gradual changes to the pitch of the blades. A 100% indication on HIC-212A/B will provide maximum blade angle and, therefore, flue gas flow while a 0% indication will put the blades at a minimum angle. PI-212 indicates the pressure at the inlet of the Induced Draft Fans.
Boiler Coils
Boiler feedwater flow from P-101A/B is controlled by FIC-201. The setpoint of FIC-201 is adjusted by LIC-201 to maintain the level of water in the Steam Drum D-201. The temperature of the boiler feedwater leaving the 3 economizer coils of the Boiler are indicated on TI-202, TI-203 and TI-204, respectively.
The temperature of reheated condensate leaving the Boiler for the Deaerator D-101 is indicated on TIC-207. TIC-207 can control be used to the condensate return temperature by opening the bypass valve TV-207 around the Condensate Reheating Coil of F-201 during unusual operating conditions. Normally, TIC-207 is in manual and TV-207 is closed.
The temperature of saturated steam to the LT Superheater Coil is indicated on TI-201 and the outlet temperature of steam from the coil is indicated on TI-209. The temperature of superheated steam leaving the Divisional Superheater Coil is indicated on TI-205. The temperature of steam from the Platen Superheating Coil is controlled by TIC-206 which adjusts the flow of boiler feedwater injected into Spray Desuperheater J-201. The flow of injected boiler feedwater is indicated on FI-202. Normally, there is no flow of boiler feedwater to J-201 because the setpoint of TIC-206 is set a few degrees higher than the design outlet temperature from the second desuperheating coil. TIC-206 helps protect the desuperheating coils from extremely high temperatures which can occur during upsets, startup, shutdown and off-design operation. The flow of superheated HP steam from F-201 is indicated on FI-203.
The temperature of IP steam from the Steam Turbine’s HP Stage is indicated on TI-302. The reheated IP Steam from the IP Reheater Coil is indicated on TI-208.
Steam Drum and Heater
HIC-202 controls the flow of blowdown water drawn from the Steam Drum D-101. The flow of blowdown water is indicated on FI-205.
The temperature of boiler feedwater entering the standpipe that supplies water from the Steam Drum to the Circulation Pumps P-201A/B is indicated on TI-204. The level of water in the Steam Drum is controlled by LIC-201 which adjusts the setpoint of FIC-201 (boiler feedwater flow to the economizer section of the Boiler). A second, independent level instrument LAL-202 is used to sense a low level condition in the Steam Drum. When this occurs, interlock I-202 will activate. The pressure of the Steam Drum is indicated on PI-202.
The flow of superheated steam from the Boiler to the HP Steam Header is indicated on FI-203. The header pressure is normally controlled by PIC-203A which adjusts the output of Steam Turbine flow controller SIC-301 (see Steam Turbine instrumentation section for details). PIC-203B routes excess superheated steam to the Bypass Steam Desuperheater J-202 and on to the Surface Condenser E-301. The outlet temperature from J-202 is controlled by TIC-221 which adjusts the flow of boiler feedwater from P-101A/B to the Desuperheater.
Steam Turbine Controls and Instruments
The HP steam flow to JT-301 is indicated on FI-301. HP steam passes through trip valve XV-301 which is controlled by HIC-301. HP steam flow can also pass through a smaller warm-up line at startup. HIC-302 controls the warm-up valve HV-302 on this line. Speed controller SIC-301 regulates the opening of speed control valve SV-301 inside the turbine casing. SV-301 controls the flow of HP steam to the first stage of JT-301.
The speed of the shaft of JT-301 is measured by SI-301. This instrument is also used by the speed control system for JT-301. The speed of JT-301, expressed as % of design speed (3,600 RPM), is indicated on SIC-301. SIC-301 normally operates in steam pressure control mode when Generator G-301 is connected to the electric power grid. This control mode is entered by placing pressure control switch HS-304 into the P_CTL state. In this mode, the output of PIC-203A on the HP Steam Header directly adjusts the output of SIC-301. SIC-301 is locked in manual while in pressure control mode. In this mode, the power produced by Generator G-301 will essentially be changed in proportion to the steam produced by the Boiler.
Steam Turbine JT-301 may also be placed in droop mode. This control mode is entered by placing droop control switch HS-303 into the DROOP state. Droop control is explained in the next section. SIC-301 can be taken out of droop/cascade control and placed in automatic or manual mode. Automatic mode is used only at startup when the generator is not connected to the grid. In this case, SIC-301 directly controls the shaft speed. In manual mode, SIC-301 is used to manually adjust the HP steam flow to JT-301. Manual mode of SIC-301 is available any time the Steam Turbine is not tripped. Manual mode is entered any time the droop control switch HS-303 is changed from the DROOP to the OFF state or the pressure control switch HS-304 is changed from P_CTL to OFF.
Important: Note that whenever HS-304 is in the OFF state, steam header pressure controller PIC-203A will also be locked in manual mode and its output will track the output of SIC-301. In this situation, the pressure of the HP Steam Header will float. If the header pressure becomes too high or too low, it is up to the operator to make adjustments to the Boiler fuel firing or to the Steam Turbine JT-301 operation to balance steam generation with steam consumption. Venting of the steam header to atmosphere with HIC-201 or relying on the Steam Turbine bypass controller PIC-203B should only be used as short term measures as these constitute a large economic penalty for plant operation (i.e. a portion of the fuel consumed by the plant is not used to generate electricity).
The pressure of IP steam exhaust from JT-301 is indicated on PI-302 and its temperature is indicated on TI-302. The IP steam is reheated in the Boiler F-201. The outlet temperature of reheated steam is controlled by TIC-303 which controls the rate of injection of HP boiler feedwater through TV-303 to the IP steam line upstream of the IP reheat coil of the Boiler. The outlet pressure of steam from E-303 is indicated and controlled by PIC-303. An IP steam admission valve PV-303 is provided on JT-301. Normally this valve is wide open at design operation. However, PIC-303 will regulate this valve to make sure the IP steam does not drop below 290 PSIG at the outlet of E-303. Too low an IP pressure can lead to condensate in the exhaust from the 1st stage turbine of JT-301 which will quickly erode the turbine blades.
The flow of cooling water to E-301 is controlled by HIC-305 which adjusts the valve opening of HV-305. The temperature of steam condensate leaving E-301 is indicated on TI-304 and its pressure is indicated on PI-304. The temperature of cooling water leaving E-301 is indicated on TI-306.
The shaft speed of Generator G-301 is indicated on SI-302. The power output of G-301 is indicated on JI-320.
Generator G-301 is provided with a synchroscope in order to visually see the difference of the frequency and phase between electricity produced by G-301 and the electric grid at startup. Generator G-301 is connected to the electric power grid using switch HS-322. SI-320 indicates the frequency of electricity at the terminals of G-301. SI-321 indicates the frequency of electricity of the electric grid after the breaker switch. SI-322 indicates the phase difference between electricity generated at the terminals of G-301 and the electric grid. Before connecting the Generator to the grid with breaker switch HS-322, the frequencies of the Generator must be the same and the phase difference must be nearly zero. Otherwise, the Generator may suffer major damage when the breaker switch is closed.
The synchroscope also includes an auto-synch controller for connecting the Generator with the grid. XI-320 will indicate when it is permissible to activate the auto-synch controller. The permissive signal is given under the following conditions:
- The breaker switch HS-322 is OPEN
- The absolute value of the frequency difference between the Generator (SI-320) and the electrical grid (SI-321) has been less than 0.5 Hz for more than 20 seconds continuously
Placing auto-synch switch HS-320 into the ON state will activate the auto-synch controller XIC-320 (not shown on schematic 14). While auto-synching, XIC-320 will lock speed controller SIC-301 in manual mode and adjust its output until the phase difference as indicated on SI-322 is within 2 degrees of 0 and the frequency difference has been less 0.01 Hz for more than 20 seconds continuously. If closure of the breaker switch HS-322 cannot be made within 10 minutes, the auto-synch controller will stop (HS-320 will be placed back in the OFF state) and SIC-301 will remain in manual mode. Note that the auto-synch controller may not successfully work if there are large fluctuations in the steam header pressure or in the grid frequency during synchronization.
Droop Control
When any synchronous electric generator is connected to a large grid in parallel with many other synchronous machines such as generators and electric motors, a single generator cannot easily or reliably control the frequency of the electric power of the grid because it is only generating a small fraction of the total power being consumed from the grid. In this case, the generator will run at the grid speed or frequency. Therefore, the speed of the power turbine that drives the generator cannot be controlled when the generator is connected to a large grid.
The grid frequency dynamically depends directly on the balance of power generation and consumption across the grid. If generators are producing more power than the power consumers on the grid, the grid frequency will increase, causing all synchronous motors connected to the grid to speed up. As they speed up, they will consume more power until the power consumption comes into balance with power generation. In order for many generators to supply electricity to a large grid, they cooperatively adjust their power output using what is known as droop control.
Droop control simply proportions a generator’s power output to the deviation between of the actual grid frequency and its setpoint frequency (60 Hz). If the actual grid frequency is at the setpoint, the generator will put out its design power. When the grid frequency is higher than the setpoint, the generator will decrease its power output in proportion to the deviation. Each generator system with droop control is configured with a characteristic droop control constant, expressed as % of setpoint speed. For JT-301/G-301 this constant is 4%. At 4% overspeed of the grid the droop controller will adjust the power output to the minimum stable power operation for JT-301/G-301.
When SIC-301 is placed into droop mode using switch HS-303, the PV of SIC-301 is computed as follows:
PV = [SI-321.PV * 60 + (SIC-301.OP – 13.0) * 2.542373] * 100/3,600
The setpoint of SIC-301 is locked at 104.0 when in droop mode. Any deviation of the grid frequency (SI-321.PV) will cause SIC-301 to move its output such that the PV is restored back to the setpoint of 104.0. The integral action of controller SIC-301 will cause the output (and power) to move gradually.
Vacuum System Controls and Instruments
The flow of IP steam to the vacuum system is indicated on FI-307.
The flow of steam to Startup Ejector EJ-301 is controlled by HIC-306 which adjusts the opening of valve HV-306. The flow of steam to Startup Ejector EJ-301 is controlled by HIC-306 which adjusts the opening of valve HV-306.
The flow of cooling water to Ejector Condenser E-302 is controlled by HIC-308 which adjusts the valve opening of HV-308.
Hotwell Controls and Instruments
The level of condensate in Hotwell D-301 is indicated and controlled by LIC-301 which adjusts the position of LV-301 on the discharge of the Condensate Return Pumps P-301A/B. The flow of condensate taken to the Deaerator is indicated on FI-309.
The motors of Condensate Return Pumps P-301A/B are operated by switches HS-301A and HS-301B, respectively. LAH-301 also indicates the level of condensate in D-301 and its high alarm signal (80% setpoint) is used to auto-start P-301A or P-301B by selecting the AUTO state of switch HS-302A for P-301A or by selecting the AUTO state of HS-302B for P-301B. Normally one pump is in service with its auto-start switch in the MAN state and the other pump is on standby with its auto-start switch in the AUTO position.
Cooling Tower Controls and Instruments
The flow rate of makeup water to the Cooling Tower’s basin is controlled by FIC-504. The temperature of the makeup water is assumed to be the same as ambient conditions. The level of the Cooling Tower’s basin is indicated on LI-501.
The filtration system flow is indicated on FI-502 and the filter pressure drop is indicated on PDI-503.
The supply header pressure is controlled by PIC-501 which sends excess cooling water pumped by P-501A/B back to the Cooling Tower. The setpoint is normally 50 PSIG. The total flow of cooling water sent to the supply header is indicated on FI-501 and the temperature of the supply water is indicated on TI-501.
Blowdown is taken off the discharge of the Cooling Water Pumps P-501A/B and controlled by FIC-507.
The return header pressure is indicated on PI-502 and the return water temperature is indicated on TI-502. Return water is directed to the top of the Cooling Tower with HIC-503 or to the basin with HIC-502.
The opening position of the louvers of the Cooling Tower is adjusted with HIC-508. Maximum air flow is at 100% output. At 0% output, air flow will be at a minimum but will be non-zero to avoid starving the Cooling Tower’s fan. HIC-508 is used to adjust the cooling water supply temperature. However, in very cold weather regulation of the supply temperature to a desired value may not be possible. In this case, some of the return cooling water can be bypassed around the Cooling Tower by opening HIC-502.