Process Description
Simtronics’ Heat Recovery Steam Generator (HRSG) program represents a typical steam generator found in a Combined Heat and Power Plant (CHPP). Waste heat from gas turbine exhaust is recovered by generating high pressure steam in the HSRG. Boiler feedwater is provided from battery limits. The HRSG has coils for preheating boiler feedwater, steam generation and steam superheating. Superheated steam is routed to steam users such as a steam turbine for electric power generation.
A full range of operations can be learned and practiced on the Heat Recovery Steam Generator simulator. These include normal, startup, shutdown, and emergency shutdown procedures.
Heat Recovery Steam Generator
Hot gas turbine exhaust from battery limits enters the Heat Recovery Steam Generator (HRSG) E-201 through a plenum. E-201 consists of three heating sections: boiler feedwater heating (economizer), water boiling (evaporator) and steam superheating (superheater). The gas turbine exhausts flows through E-201 countercurrently with respect to these sections. The cooled gas turbine exhaust is routed to a stack at the end of the HRSG to vent the exhaust safely to the atmosphere.
Warm boiler feed water from battery limits flows into the economizer coils of E-201 to recover heat from the gas turbine exhaust before it is discharged to the stack. The economizer consists of three sections of coils. These coils provide a large surface area to absorb most of the available heat from the gas turbine exhaust. The large area is needed because of the low temperature difference between the cooler gas turbine exhaust leaving the evaporator section and the boiler feedwater. Preheated boiler feed water from the third coil enters the Steam Drum D-201.
Water from the steam drum D-201 circulates through the evaporator coils of E-201 by natural circulation. The coils are connected to a mud drum located near the bottom of the plenum of E-201. Relatively cooler water from the Steam Drum circulates down one-half of the coil to the mud drum. As heat is picked up from the gas turbine exhaust, the water partially vaporizes by the time it reaches the mud drum. Additional heating and vaporization occurs in the riser coils of the evaporator, resulting in a natural circulation of water through the evaporator. The riser coils return a mixture of water and steam to the Steam Drum which 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 boiler feed water and is circulated back down to the mud drum. To avoid accumulation of solids in the mud drum over time, it is continuously drained. The relatively small blowdown flow from the mud drum is sent to battery limits for disposal.
Steam produced by the Steam Drum flows to the superheater coils of the HRSG. The superheater consists of two coils. Boiler feed water is injected into the Spray Desuperheater J-201 to control the final superheat temperature of the steam. The second superheater coil is designed to withstand the hot temperatures of the gas turbine exhaust from battery limits and acts as a thermal shield for the downstream coils.
Superheated steam from the last coil is normally sent to steam users such as a steam turbine for electric power generation. Excess superheated steam is sent to the low pressure steam header at battery limits via an automatic pressure vent.
Note that for HRSG units located in combined cycle power plants, there is usually a separate intermediate pressure (IP) superheating coil to reheat steam that passes through the main steam turbine. There may also be an IP steam generation coil and steam drum in such plants. Functionally, these IP services operate just like the high pressure (HP) superheater and evaporator coils of the HRSG simulator.
Instrumentation
Heat Recovery Steam Generator
Boiler feedwater flow from battery limits to the economizer coils 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 is indicated on TI-201 and the pressure is indicated on PI-201. The temperature of the boiler feedwater leaving the 3 economizer coils of the HRSG are indicated on TI-202, TI-203 and TI-204, respectively.
HIC-202 controls the flow of blowdown water leaving the Mud Drum on the evaporating coils of the HRSG. The flow of blowdown water is indicated on FI-205.
The temperature of superheated steam leaving the first superheating coil is indicated on TI-205. The temperature of steam from the second superheating coil is controlled by TIC-206 which adjusts the flow of boiler feedwater injected into the 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 10 DEG F 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 superheated steam header pressure is controlled by PIC-203 which routes excess superheated steam to the low pressure (LP) steam header at battery limits. The flow of letdown steam is indicated on FI-204. Controller FIC-203 controls the flow of superheated steam to users; this effectively gives the operator control over the steam demand.
The flow of hot gas turbine exhaust from battery limits is controlled by using HIC-211 to control the opening of a simulated damper connecting the gas turbine to the HRSG. In real units there normally is no such damper because the plenum connecting the two units has a very large cross-sectional area. HIC-211 is provided on the simulator to effectively give the operator control over the gas turbine’s operating capacity. If interlock I-201 is tripped (low Steam Drum water level), HIC-201 will be locked in the closed position and effectively simulates a trip of the gas turbine. The temperature of gas turbine exhaust is indicated on TI-211. The pressure of the gas turbine exhaust at the inlet of the HRSG is indicated on PI-211. The temperature of cooled gas turbine exhaust leaving the HRSG is indicated on TI-212.
Steam Drum
The temperature of boiler feedwater entering the Steam Drum D-201 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 HRSG). A second, independent level instrument LAL-202 is used to sense a low level condition in the Steam Drum. When this occurs, interlock I-201 will activate. The pressure of the Steam Drum is indicated on PI-202.
The instruments for the superheated steam header are described in the previous section.
Interlock I-201
Interlock I-201 protects the HRSG from thermal damage due to low water level in the Steam Drum. I-201 activates if the level of LAL-202 is less than 10%. I-201 will remain active anytime LAL-202 is less than 10% and will close and lock the gas turbine exhaust damper HIC-211. I-201 will automatically reset when LAL-202 indicates higher than 10%. However, HIC-211 must be manually opened after the interlock resets. The interlock status is indicated on XA-201.