Case Study 1
Impulse Turbine Governor Needle Position Feedback Mechanical Design Improvements
Governor system reliability is a top priority for a power producer that operates hydro turbines on an isolated power system.
Its generating units are responsible for setting the system frequency and providing the system response necessary to maintain the reliability of the power for three communities.
An unexpected failure of either of the impulse turbines means that the communities served will go black, and their customers will have to rely on costly power from their backup diesel generators.
The existing needle feedback assemblies utilized a small hydraulic cylinder to provide constant force on the existing needle feedback rod. The mechanical design allowed the cylinder shaft to “flex,” which caused damage to the cylinder rod seals and resulted in oil leakage. Periodically, the hydraulic loading cylinders would completely fail.
This resulted in:
1) One to three power system blackouts a year since the system was installed, caused by the old retrofit needle feedback assemblies.
2) Daily cleaning of oil off turbine pit floor by operations staff. Two to three times a year, a complete failure of the load cylinder occurred, which created a dangerous environment with potential for oil to get into the water.
3) Calibration that required installing a calibration adapter into pressurized hydraulic system, which lengthened downtime.
Additionally, the operations / maintenance personnel who were called out to repair the unit faced the following challenges:
1) Needle feedback was difficult to repair because the entire assembly had to be removed to replace the loading cylinder.
2) The assembly was complicated to calibrate; calibration required that the proportional valve be replaced with a manually operated valve.
3) Operations staff had to decide whether to concentrate on repairing the unit and working in an oily mess or cleaning up the oil turbine pit before the oil mess contaminated the water in the tail race.
The power producer contacted the equipment OEM to see if it would redesign the problematic needle feedback assembly. The OEM responded with an expensive alternative that utilized a complex spring arrangement to replace the hydraulic loading cylinder. The new design did not address the challenges presented by oil leakage or calibration.
SEGRITY was engaged to provide recommendations to improve the reliability of the needle feedback assemblies. Upon review of the existing design and the original system requirements, SEGRITY’s engineers developed an upgrade kit that allowed the power producer to retain part of the existing design and provide a 100% O-ring sealed design that was based on a mechanical spring instead of a hydraulic load cylinder.
The mechanical upgrade kit was designed to utilize the existing hydraulic components and bolt into the same footprint as the existing design. A key advantage of the upgraded assembly was that it could contain full system pressure. This eliminated any potential leakage of governor hydraulic oil into the turbine pit.
SEGRITY supplied a total of 14 sets of feedback kits. Each of the six needle impulse turbines required six upgrade kits. Installation of the six kits on each turbine was completed in less than an eight-hour work day. This included removing the existing feedback assemblies, installing the new kits, and calibrating all six needle feedbacks. This was roughly the same amount of time it took to repair one of the original assemblies and clean up the resulting oil mess after the load cylinder had failed.
The calibration of each unit was simplified by design. The preliminary calibration was designed to be robust to calibration drift due to changes in the alignment of the assembly.
The adjustment was as simple as setting the height of the black magnet holder and locking it into place with the white lock nut. This was a five-minute process during the installation of each kit.
Both units have been operating for over two years with no forced outages of the units due to the needle feedback. The operations staff has been able to focus on operating and maintaining the plant instead of cleaning up oil and repairing failed feedback assemblies!
- Zero forced outages since installation.
- Zero oil leakage since installation.
- Calibration time has been reduced to 1-2 hours compared to a full day that it used to take.