LCC Quick Notes, Steam Turbine Controls
Steam turbine controls consist of two linked control systems, the Governor and the Valve Operators. The Governor is a specialized closed loop controller which maintains an adjustable set point speed or load on the turbine. The Valve Operator accepts the Governor's output demand and close a position loop for steam admission valve as commanded by the Governor. When tasked with the replacement of an old or unreliable system, several key issues need examination before selecting any of the multitude of retrofit options. This Quick Notes will review alternate steam turbine control options and provide links to LCC recommendations.
◘ Why use a Governor at all? Why not have the plant Distributed Control System (DCS) handle the turbine speed or load control? These are not trivial questions. In some cases a DCS can perform quite satisfactorily as a steam turbine governor. A good guideline for determining when a DCS can handle a turbine control task is to compare the required application response time with the DCS functional response time. If a DCS is too slow to enact control changes to avoid upsets, it probably needs external help to close the turbine speed control loop. The application response time for turbine control is determined by calculating one-fifth the maximum acceleration rate of the turbine (RPM/second) which provides either a process upset or an over speed trip, whichever is less. For example, a typical feed pump turbine has a maximum acceleration rate of 1000 RRM/second and operates within 500 RPM of over speed trip. For over speed control, an adequate DCS response period is then 500/1000 x 1/5 = 0.1 second. Many newer DCS platforms have I/O scan cycles this quick. The process upset is next evaluated. A BFPT feeding a drum-type boiler has a very forgiving 1000 RPM upset delta, yielding a needed control response period of 1000/1000 a 1/5 = 0.2 second which can also be handled by a DCS. The same BFPT in a once-through boiler or nuclear RFPT application has a process upset of 100 RPM yielding a response period of 100/1000 x 1/5 = 0.02 seconds which disqualifies all DCS from successfully controlling the loop. For reference, LCC 200-Series and Series-2 Governors have loop closures of 15 milliseconds. In summary, the DCS can do some things but not ALL things.
◘ The selected Governor must be capable of interfacing with a DCS, usually through an Ethernet or ModBus port. Even if a DCS is not used to close the control loop, it certainly is advantageous to use it for operator input and data exchange.
◘ If a pump drive, the selected Governor must have the ability to characterize or curve-fit the application pump's load curve with droop correction. Without this feature a compromise droop must be used which can be far from optimum over the pump speed range.
◘ If a generation drive, the selected Governor needs to have On-Line Line/Load Regulation test capability to be ready for the new NERC testing standards without costly future retrofit.
◘ If a generation application benefits from splitting a multiple governor valve camshaft drive arrangement into individual valve control the selected Governor needs to be capable of controlling multiple valves, each with characterization.
◘ Any Governor should feature isolated inputs and outputs and not induce any exchange signal grounding.
◘ When the application warrants, the selected Governor should have Operator Control Stations free to locate anywhere remote from the governor enclosure.
◘ In critical applications Governors should be installed as redundant duty/standby pairs or 2-of-3 auctioned triples.
LCC Governor Links:
Governor Valve Operators were mechanical/hydraulic monsters in the old days and high pressure hydraulics in the later 1900's, but since the turn of the century roller screw electric actuators are by far the preferred type for a host of reasons:
◘ No dependence upon turbine lube oil for controlled operation. Contamination problems affecting small-orifice fed devices, temperature sensitivity, piston friction, stroke hysterisis, and leakage problems are all immediately eliminated.
◘ Unlike high pressure hydraulic systems, no fluid delivery skid with high maintenance filters is necessary.
◘ The variable response sensitivity due to change of stroke direction (dynamic hysterisis) of all hydraulic positioners is eliminated with the predictable response electric actuator.
◘ Condition Monitoring of key electric actuator running parameters provides ideal health assessment capability not found in hydraulics.
LCC Electric Actuator Links:
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