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turbine is in combined cycle operation, the second gas turbine can be started, again using IP steam for combustor cooling. The second gas turbine is synchronized at 100MW. Loading on the train continues at 11MW/minute up to full rating. Case 3: The use of low BTU “waste liquid” fuel.6 Deregulation is now a major feature in the power production industry’s development. The incentive for “small” power users, such as process and petrochemical plants, to produce their own power (become small power producers or SPPs), increases. Thailand provides an excellent illustration of this. Thailand has difficulty producing all the power the country needs with just the efforts of their national power company. For several years now, she has allowed bids from large independent power producers (IPPs) to better match her power demand curve growth. What she has also done is provided incentives for process plants to produce their own power, and sell the excess back to the national grid. The amount that can be sold back is often limited by distribution line size, which is as small as 15 kV, in the case of the grid adjacent to Esso’s Sriracha refinery for instance, but nevertheless the scheme is in place. Most countries in SE Asia are “a work in progress” in terms of their power supply and tariff infrastructure. The Petrochemical Corporation of Singapore (PCS) decided to take advantage of “pool rules for small generators” which covered generators of less than 10 MW and industrial in-house generators (“auto-generators”), which were instituted in Singapore as of April 1, 1998. An SPP such as PCS does not have the luxury of a known steady load for its power needs. Also, the quality, type and heating value of their fuels will vary. This is because they use process gases and fluids for fuel whenever they can, especially if that is the most cost effective use for what would otherwise be a waste process fluid. Due to the variations in the different characteristics of these fuels which are in essence different process streams, two things are required: - A gas turbine design that will accommodate fuels with a wide range of heating values. Such a turbine generally also has a more conservative design with turbine inlet temperatures (TITs) that will not be the highest for that turbine’s power range. - A very fast response valve (for cut-off of the fuel supply) is required. Without such a valve the exhaust gas thermocouples on the gas turbine would note large swings in turbine exhaust temperature. The key to PCS’s successful use of process fluids - which it didn’t have much other use for - as fuel, is valve response time and actuation characteristics. An ideal valve for this type of application is a “stepper” valve or its equivalent. The “stepper” valve and functional equivalents: The stepper valve is a fast response electrically operated valve which was pioneered by Vosper Thornycroft, UK (aka HSDE, UK) in the mid 1960’s. The term “stepper” actually refers to the motor type that drives the valve as opposed to the valve itself. The motor is a stepper motor, as opposed to a torque or AC or DC motor. Its self-integrating function ensures that the valve will proceed to a desired position and then the motor will stop. With other motors, the motor has to continue to run in order to keep the valve in that position - such valves need signals to cue them: run, stop running, then start running again, and so forth. If something were to happen causing the valve to fail, the stepper-type valve position would still lock and the system would continue running. The valve then makes the system fault tolerant, which is critical in applications such as emergency power supply generators. It also provides the fast response required by aeroderivative and some industrial gas turbines. This is useful for both power generation and mechanical drive service. Before the stepper valve was introduced in the mid 1960s, hydraulic and pneumatic actuation valves were used to provide the required response time. This increased the overall complexity of the fuel system. As always with instances where system complexity is heightened, system cost rose, but mean time between failures (MTBF) and availability decreased. The valve takes up very little space on the installation and service people unused to this new design spend frustrated time looking for the extensive “old” equivalent control system. Development of valves that could compete with HSDE’s original stepper arose from competition with that early design. As a result, there are now many manufacturers who produce functional equivalents on the market, for use in gas turbine fuel systems, high resolution controls for robots, automatic machining controls and so forth. In PCS’s application, they use a Moog (German manufacturer) valve which uses a DC motor. To get the same “stay in position” feature as a stepper type valve would have, manufacturers typically use a spring to hold a position. Design aims of fast response valves: The original design aims of the stepper type valve generally include the following safety considerations: • A fail freeze or fail closed option, depending on whether the operator is a power generation facility (“freezing” at the last power setting is then required) or a pipeline (in which case turbine shut down on valve failure is required). • The liquid fuel version of the valve incorporates a pressure relief valve protecting the system against over pressure and the fuel pump running on empty or “deadheading”, caused by closure of valves downstream of the fuel valve during system operation. 45PDF Image | GAS TURBINES IN SIMPLE CYCLE COMBINED CYCLE APPLICATIONS
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