DELTAK HEAT RECOVERY STEAM GENERATOR Industrial Resources, Inc. A Training Services Company  DELTAK HEAT RECOVERY STEAM GENERATOR Click to Start Program © - 2016 Industrial Resources, Inc.

DELTAK HEAT RECOVERY STEAM GENERATOR FACILITY OPERATIONS TRAINING PROGRAM  DELTAK HEAT RECOVERY STEAM GENERATOR This Training Presentation has been designed to assist you in meeting the requirements of the Facility Operations Training Program. This module contains information about the Deltak Heat Recovery Steam Generator (HRSG) used to generate steam. This includes the function and details about the HRSG components and their operation.  

General Description The Deltak HRSG is a small, D-Type Package Boiler, which uses Exhaust Gas from a Gas Turbine as a heat source. It provides steam used for heating, sterilization, and other uses.

General Description D-Type Boilers have a single Steam Drum and a single Mud Drum connected by Risers located in the path of the exhaust gases and downcomers located away from the path of the exhaust gases. These boilers circulate the water/steam mixture using natural circulation created by the difference in temperature/Pressure between the Risers and Downcomers.

General Description Steam created as the Feedwater passes through the Risers naturally will rise above the water level when entering the Steam Drum and pass through the Cyclone Separators. Cyclone Separators create a swirling motion which forces the water droplets in the steam to separate and return to the water level in the drum. As the Steam continues to rise toward the Main Steam outlet, it passes through the Chevron Driers, which remove any remaining water particles from the steam. The steam continues out through the Main Steam Valve located at the top of the Drum and on to the load via the Main Steam Line.

Boiler Feedwater Flow Path Feedwater Regulator Flow Meter Temperature Transmitter Steam Drum Pressure Gauge From Boiler Feed Pumps Economizer Feedwater Control Signal (Single Element or 3-Element) The HRSG is supplied Feedwater from the Boiler Feed Pumps. These pumps operate as necessary to supply the Boiler with Feed Water. The feedwater travels from the pumps through a Feedwater Control Valve to the Economizer. The feedwater passes through the economizer to increase the water temperature making it easier to create steam when it passes through the boiler. From the economizer, the feedwater is sent to the Upper (Steam) Drum located at the top of the Boiler. The Feedwater entry into the drum is below the water line to prevent the feedwater from interfering with the passing of steam to the Main Steam Line. The feedwater passes from the Upper Drum to the Lower (Mud) Drum via downcomers located just outside the exhaust gas flow path. From the Mud Drum, the water passes back to the Steam Drum via risers that are within the gas path to increase the temperature above the boiling point to create steam. There is now a mixture of water and steam that returns to the Upper Drum. The internal structure of the Upper Drum is such that the Steam/Water mixture passes through cyclone separators to remove the heavier water particles from the steam and then a series of chevron driers that collect any remaining water particles from the steam. The steam that leaves the Upper Drum is considered Saturated Steam as the steam is at or just above the saturation temperature for steam at the system pressure.

Flue Gas Flow Path HRSG Stack Bypass Stack Economizer Diverter Bypass Stack HRSG Stack Heat Recovery Steam Generator Economizer Steam Drum (Top Drum) Mud Drum (Bottom Drum) Inlet Duct Flue Gas from the Gas Turbine provides the heat necessary to create steam in the Boiler. As shown in this drawing, the Flue Gas flows through the diverter to the Boiler and flows through the Risers containing the water being converted to steam. Once it passes the risers, the Flue Gas continues to flow upward toward the stack. The Economizer is located at the base of the stack assembly and is used to heat the feedwater being supplied to the Boiler. The heat transfer in the economizer is designed to heat the feedwater, but not heat it to the vaporization temperature.

Diverter The Diverter channels the heat energy from the Gas Turbine to the HRSG via the Inlet Duct. The Diverter is positioned so that the proper amount of heat energy is sent to the HRSG. This would typically be fully open during normal operation to provide the maximum amount of steam to the customers. If for some reason steam was not required, and the HRSG was not operational, the Diverter is moved to the closed position and the heat energy is diverted to the bypass stack.

Diverter Controls The Diverter is remotely controlled through the HRSG screen of the Balance-of-Plant Human Machine Interface (BOP HMI). This screen indicates the position of the Diverter and allows position changes as conditions warrant. Changing the position of the Diverter plate involves selecting the diverter on the BOP HMI and pressing the one (1) of two (2) position selectors: TO HRSG BYPASS Selecting BYPASS moves the Diverter to the CLOSED position and sends the heat energy to the Bypass Stack. Selecting TO HRSG moves the Diverter to the OPEN position and sends the heat energy to the HRSG.

The Diverter operates using a Pneumatically-Controlled Actuator which is connected to an accumulator that remains charged to hold the Diverter Plate in position. A Position Indicator is mounted with the Actuator to provide a quick indication of the Diverter position during walk-around inspections. The indicator will display OPEN (Green) or CLOSED (Red). The Diverter is considered OPEN when positioned to allow Flue Gas into the HRSG.

Economizer The sole purpose of the economizer is to increase the temperature of the Feedwater prior to entering the Steam Drum. This reduces the heat energy required to convert the Feedwater into Steam. The Economizer is of a multi-pass counterflow design with the coolest Feedwater encountering the coolest Flue Gas as is flows through the tubes. The temperature increase of the Feedwater is typically in the range of 75-80 degrees Fahrenheit.  

Economizer Controls There are no direct controls for the Economizer, other than a bypass that is used when there is no need for heating the feedwater or when the Economizer is not available for use. The bypass is manually controlled by closing the Feedwater valve at the inlet to the Economizer and opening the bypass valve adjacent to the Economizer near the Upper Drum. The HRSG Screen indicates Economizer Inlet and Outlet Feedwater Temperatures, as well as the Temperature of the Exhaust Gases at the bottom (inlet) and top (outlet) of the Economizer.

Feedwater Regulator The rate of flow of Feedwater into the Upper Drum must be controlled to prevent the Feedwater Level from exceeding safe levels in the Drum. The Upper Drum is divided into the steam and water sections and the water level is controlled to prevent the water from impeding the flow of steam out of the drum at the proper pressure and temperature. A Regulator Valve is placed in the Feedwater Piping to control the flow of water into the drum at all phases of operation. When the unit has been shut down for maintenance and it is necessary to fill the Drum with water, the Control Valve is fully OPEN and water flows at the maximum rate as controlled by the Boiler Feedwater Pumps. Once the water is near the Normal Water Level (NWL), the flow rate is reduced. Typically, the water level is held just below NWL at the start to accommodate any swell that may occur during the initial heating of the water. The Control Valve is set to maintain the NWL in the Drum and prevent high and low water levels.

Feedwater Regulator Control When the unit is not in operation, the Feedwater Regulator should be OPEN 100 percent as that was the last command sent during a unit shutdown. The Regulator Isolation and Bypass Valves are CLOSED. During a Unit Start-Up, The Upstream Isolation Valve is OPEN and the Downstream Valve is OPEN to begin filling the Upper Drum at a rate determined by the Feedwater and Upper Drum Temperatures. During Normal Operation, The Feedwater Regulator is part of the Three (3)-Element Control Loop that controls the Upper Drum Water Level based on Feedwater flow, Steam Flow, and Upper Drum Water Level. This configuration will automatically adjust Feedwater Flow based on the above criteria. It is possible to put the Feedwater Regulator in Manual Control and manually adjust its position in anticipation of changes to the operating conditions. During Unit Shutdown, the Feedwater Regulator is adjusted to 100 percent OPEN as part of the shutdown process. The Valve’s current position and Feedwater Flow Rate are displayed on the BOP HRSG Screen. When the valve is OPEN and controlling flow, it is shown in Green on the display. When the valve is CLOSED, it is displayed in Red.  

Three (3) Element Control The feedwater control system is a three element type, designed to monitor changes in steam flow, water flow and drum level. Steam flow is the rate of steam leaving the boiler - the demand. Water flow is the rate of feedwater flow into the boiler - the supply. Drum level reflects the amount of water in the boiler - the inventory. With changes in boiler load (steam flow), steam and water flow become unbalanced and water level consequently deviates from the normal position. In such an event, the system changes water flow to the extent necessary to restore the balance between steam flow and feed flow and return the water level to normal.

Upper (Steam) Drum The steam drum is designed to pass steam to the Main Steam Outlet while preventing water from doing the same. Chevron driers and cyclone separators provide direction and velocity changes within the steam flow path to force water droplets entrained within the steam to fall away from the steam and return to the water path to provide further heating and creation of steam. Typically, a non-return valve is included in the main steam line to prevent reverse flow of steam into the Steam Drum.

Upper Drum Controls The Upper Drum Water Level and Steam Pressure are monitored continuously via the BOP HRSG Page. There is a setpoint in Diverter Control to begin closing the Diverter if the pressure setpoint, typically 100 – 120 psig, is exceeded.

Upper Drum Pressure Control In the event of a further, uncontrolled pressure rise, two (2) Safety Valves are installed on the Upper Drum that will open when the pressure exceeds the maximum pressure allowed by the manufacturer. The two (2) safety valves will have staggered pressure settings.

Lower (Mud) Drum The Lower Drum provides a collection point for sediments and other material that do not remain in the water during the transition to steam. These sediments are usually removed from the drum by blowing down the drum at predetermined intervals or whenever the conductivity of the water in the drum exceeds a set value. The Lower Drum is also the drain point for the HRSG and the double block valves are opened fully when the unit is drained.

Lower (Mud) Drum Controls The Lower Drum does not have any specific controls or indicators for operation. The only requirement for operation is a short blowdown that is typically accomplished once per day. During this blowdown, it is important to remember the order of opening and closing the two (2) valves. The valve closest to the HRSG Drum is opened and closed first in the sequence and the Outlet Valve is opened and closed last in the sequence.

The End This concludes this presentation. If you have further questions about the Deltak Heat Recovery Steam Generator or its operation, contact your supervisor or training manager