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It is the Part II Design Considerations for Heated Gas Fuel, continued from Design Considerations for Heated Gas Fuel Part I

Disclaimer: This document is owned by GE, being printed here for the reference only for readers. This is being printed in parts for easy reading on screen

System Flowpath
As the incoming gas fuel supply enters the plant facility, it first passes through one of two 100% coalescing filters. These filters are required to remove both liquids and particulate from the customer’s gas supply. The filters may not be required if similar equipment is installed upstream by the gas supplier. Liquids collecting in the Coalescing Filter Sump are automatically drained into the common Drain Tank. A differential pressure switch installed across the filters monitors pressure differential and alarms when cleaning or cartridge replacement is required.

Downstream of the Coalescing Filter, the gas fuel supply enters the Electric Startup Superheater. This startup heater is required when the gas supply does not meet the minimum superheat requirement. The electric heater is turned off and then bypassed at the point when the Performance Gas Fuel Heater is capable of maintaining gas temperatures above the minimum superheat requirement.

The electric heater is turned off and then bypassed at the point when the Performance Gas Fuel Heater is capable of maintaining gas temperatures above the minimum superheat requirement.

 

As fuel exits the superheater, it enters the Performance Gas Fuel Heater. This system
incorporates a stacked two-shell heater arrangement with the gas on the shell side and the feedwater on the tube side. Each of the heat exchanger shells is furnished with low point collection sumps. These sumps house level instrumentation that provide early indication of a heat exchanger tube leak or rupture and automatically control the sump drain valves.

Activation of a single high level switch indicates detection of a Level 1 leak, while triple redundant high-high level switches indicate a Level 2 leak (See Figure 5.) A full bypass/bypass valve is provided around the Gas Fuel Heater to allow for certain modes of operation when the heat exchanger is not in service. Dependent on combustion type and frame size, these “cold” modes of operation may be load and/or emissions limited. (Refer to the Combustion Specific Requirements.)

The gas fuel exiting the Gas Fuel Heater Skid enters the gas fuel scrubber. This “dry” scrubber performs two functions in that it a) provides the final level of particulate filtration upstream of the gas turbine, and b) removes gasentrained water droplets present as the result of a minor tube leak (i.e., pinhole). Two levels of instrumentation within the scrubber monitor for the presence of liquids. A high level switch will generate an alarm and automatically open the scrubber drain valve that drains collected fluids to the drain tank. Two out of three high-high level switches indicate detection of a Level 3 leak, thus initiating a signal to trip the gas turbine. (See Figure 5.)

Downstream of the gas fuel scrubber, the gas fuel supply enters the gas fuel metering tube. The metering tube houses a flow orifice, two differential pressure transducers, three temperature elements and a pressure transducer. The gas turbine control systems read signals provided by these instruments to calculate a pressure and temperature-compensated fuel flow.

The typical Gas Fuel Heating System uses intermediate pressure feedwater as the heating medium. The feedwater enters the Gas Fuel Heater Skid and passes through a double blockand-bleed valve arrangement to the tube side of the heat exchanger. These automated blockand-bleed valves prevent gas from backflowing into the feedwater systems during unit shutdown if a tube leak is present. A similar threevalve block-and-bleed configuration is provided at the heat exchanger feedwater outlet. The gas temperature control valve is located directly downstream of the second isolation valve.

Component Description
The following section provides a detailed description of the hardware components within the typical Gas Fuel Heating System. Unit specific components may differ based on incoming gas conditions, heating requirements and over-all plant configuration. The component out-line drawings may differ depending on the equipment supplier:

Coalescing Filter Skid — The Coalescing Filter Skid is designed to protect the downstream gas fuel system against the entry of both liquid phase fuel and particulate contaminants. (See Figure 6.) At rated flow, the efficiency of the filter is 100% for solid and liquid particulate larger than 0.3 microns at rated flow. This skid is not designed to remove large quantities (i.e., “slugs”) of liquids.

The skid, as shown, consists of two 100% gas flow coalescing filters. Each filter is designed for performing maintenance without removing the gas turbine from service. Peaking units may use a simplex arrangement, where the filter can be cleaned or maintained during unit down time.

Each filter house contains a liquid collection sump. The sump is furnished with a drain system that automatically removes liquids from the vessel. A high level switch is provided to monitor the sump level. (See Coalescing Filter Skid Controls.) (Note: If large quantities of gas entrained liquids are expected, a scrubber may be required upstream of the coalescing filter.)

Electric Startup Superheater — The Electric Startup Superheater is needed at ignition when the fuel supply does not meet the minimum required superheat level as defined in GEI-41040. (See Figure 7.) The heater’s capacity is sized to provide this temperature rise for fuel flows up to the point where the performance heater can maintain the temperature. The heater’s capacity will not maintain the super-heat level at fuel flows in excess of this value.

The heater is an industrial unit designed for natural gas application. A Silicon Controlled Rectifier (SCR) controls the heater. The SCR controller maintains a constant differential across the heater and over the entire range of gas fuel flows where superheating is necessary. (Note: Non-electric heat exchanger designs, i.e., gas-fired or oil-fired, may be used for this application. The startup superheater requires a heat source available at gas turbine ignition.)

Gas Fuel Performance Heater Skid- The Gas Fuel Performance Heater Skid consists of two stacked shell and tube heat exchangers in series, gas and water side isolation valves, vent and drain valves, and instrumentation required to support the operation of the gas fuel heater. (See Figure 8.) The heat exchangers are single pass, fixed tubesheet type, and include expansion bellows on the shell. The heat exchangers are mounted on a common base. The heat exchanger is designed for the intermediate pressure feedwater to flow within the tubes and the lower pressure gas fuel to flow through the shell.

Figure 6. Standard Coalescing Filter Skid

With water pressure being higher than gas pressure, this configuration insures that gas will not enter the feedwater system following tube leak or rupture. The design of the system incorporates various safeguards designed to prevent water entering the gas from being admitted to the gas turbine combustion system. Each heat exchanger is furnished with a drain pot at one end of the shell. These drain pots house level instrumentation that provide early indication of tube leak/rupture prior to and during gas turbine operation.

The physical configuration of the heat exchanger has the gas inlet at the side of the first stage heat exchanger and the outlet at the top of the second stage heat exchanger. The nozzles oriented in this manner prevent water from collecting in the inlet or outlet piping following a tube rupture event.

Each heat exchanger is furnished with a flow restrictive orifice plate located at the inlet and outlet tube sheets of each shell. This orifice plate controls the amount of water that exits as a result of catastrophic tube rupture. This design is required to both minimize the effect on the feedwater system and to limit the quantity of water entering the gas stream. The downstream orifices are non-concentric with the tubes to allow draining during shutdown. The Gas Fuel Heater is sized to accommodate temperature downstream of the heat exchanger and will be able to supply the desired temperature for all operating conditions.

Figure 7. Standard Electric Startup Superheater

It may be necessary to provide an automated bypass system around the Gas Fuel Heater in order to satisfy the Combustion Specific Requirements defined in this document. The need for this by-pass will depend highly on the actual heater system applied to a unit.

 

Figure 8. Standard Gas Fuel Performance Heater Skid

Gas Fuel Scrubber – The Gas Fuel Scrubber provides the final level of filtration directly upstream of the turbine. (See Figure 9.) The scrubber also removes water droplets from the gas stream following the event of a heater tube leak or rupture. For particulate 8 microns or larger, removal is 100% efficient at the design flow rate. The performance of the scrubber insures that the outlet gas will contain no more than 0.10 gallons of entrained liquid per million standard cubic feet of gas, at the rated flow. The scrubber is furnished with an automatic drain system that discharges to the Drain Tank.

The Gas Fuel Scrubber is a vertical, multicyclone, high-efficiency dry-type separator. The scrubber vessel is manufactured of carbon steel and is designed to satisfy the requirements of Section VIII of the ASME Boiler and Pressure Vessel Code (Reference 3). The outlet flange of the scrubber serves as the carbon-to-stainless steel interface point for the Gas Fuel Heating System. In other words, the piping and valves between the scrubber and gas turbine connection shall be stainless steel.

Drain Tank — The Drain Tank is an atmospheric horizontal tank constructed of carbon steel. The Drain Tank collects and stores liquids discharged from the Coalescing Filter Skid, the Performance Heater drain pots, and the Gas Fuel Scrubber. The vents from the performance heater also discharge to the Drain Tank. Due to the potential for collecting both gaseous and liquid hydrocarbons, a flame arrestor is mounted on the Drain Tank vent. The tank is mounted within a containment dike in order to protect the environment from hazardous discharges.

The Drain Tank is furnished with a local level gauge and a high level switch. Manual draining of the tank is required when the level reaches a specified setpoint. If excessive amounts of liquids collect in the drain tank, they should beanalyzed and their origins determined.

System Controls
This section provides a detailed description of the controls hardware and software associated with the typical Gas Fuel Heating System. Unit specific controls may deviate from the following descriptions based upon the specific plant configuration.

Figure 9. Standard Gas Fuel Scrubber
Coalescing Filter Skid Controls— Each of the two full capacity coalescing filters is furnished with a level controller and integral drain valve. The controller maintains a minimal level in the sump by continuously opening and closing the drain valve. Collected liquids are discharged to the Drain Tank. A single high level switch monitors sump level. An alarm within the plant’s control system will initiate upon activation of this switch. Each filter is also furnished with a local level gage.

A high differential pressure switch monitors the pressure drop across the coalescing filter that is in use. Activation of this switch generates an alarm in the plant controls indicating that a switch over to the clean filter is required. The gas outlet of each filter is furnished with a local pressure gage.

Electric Startup Superheater Controls — The Electric Startup Superheater controls are configured configured to achieve the desired gas fuel temperature at the heater outlet based on the temperature differential across the heater. The controls are set to maintain a constant differential temperature with a maximum temperature limit.

The constant differential is the difference between the minimum supply gas temperature and the minimum superheat level above the fuel’s dewpoint. All control functions are per-formed locally by a dedicated SCR controller.

Gas Fuel Heater Skid Controls — The gas temperature controls regulate and monitor temperature of the gas fuel supply to the turbine. Temperature elements and transmitters are furnished at the gas side and waterside inlets to the gas fuel heater and on the gas side outlet. Signals provided by these instruments are sent to the control system. These signals are used to modulate the flow control valve located at the waterside outlet of the heater in order to attain
the desired gas fuel temperature.

Summary

This publication was developed to (a) identifythe requirements of the gas turbine with respect to the gas fuel heating systems, and (b) provide a descriptive overview of GE’s standard Gas Fuel Heating System. This standard system has been developed to meet these requirements, while insuring safe and reliable gas turbine and power plant operation.

Due to the nature of this system, it is imperative that the detailed system incorporates means of personnel protection. This includes, but is not limited to, the discharge direction of pressure safety relief valves, the inclusion of personnel protection insulation and the prevention of gas fuel from entering and “hiding” in the plant’s steam and feedwater system.

Categories: gas turbines

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