Event Sequence:
On 20th February plant was operating normally until 4:30 AM after which rapid rise of temperature in turbine guide bearing pad (TGB) 2 was noted which was later followed by rapid temperature rise of turbine guide bearing pad (TGB) 1. Noting this active power was reduced to 10 MW from 12 MW. After inspection active power was set to 6 MW. Then “high temperature accident signal” tripped the generator circuit breaker at 4:45 AM.
Inspection:
After noting the temperature rise, significant vibration in turbine housing assembly was observed and temperature of bearing housing assembly and heat exchanger was higher than usual. All the shear pins were intact and initially color of turbine oil was also not altered. This may be due to the mild chipping of TGB at start. Temperature of water baffle was normal. But TGB pad 2 was hotter than pad 1. After accident shut down sequence became active, the turbine oil color started to turn black.
Potential Roots of Problem:
From inspection major cause of temperature rise was observed to be the large vibration of runner which led shaft to strike the TGB thereby chipping the TGB and increasing the temperature. The major known cause of large vibration in the Francis runner are:
- Misalignment: Misalignment of the shaft and other components can cause the turbine to vibrate excessively. This can occur due to improper installation or due to wear and tear over time.
- Unbalanced or damaged runner: A damaged or unbalanced runner can cause vibrations in the turbine. Damage to the runner can occur due to cavitation, erosion, or foreign object damage.
- Resonance: Resonance occurs when the natural frequency of the turbine matches the frequency of the vibrations. This can cause the vibrations to become amplified and lead to damage to the bearings and other components.
- Mechanical looseness: Looseness in the bearings or other components of the turbine can cause it to vibrate excessively. This can occur due to wear and tear, improper installation or maintenance, or damage to the components.
- Cavitation: Cavitation is a phenomenon where bubbles of vapor form in the water due to low pressure areas. When these bubbles collapse, they create shock waves that can damage the runner and cause vibrations. Cavitation can be caused by several factors, including high water flow rates, rough surfaces in the runner, and improper design of the turbine.
While evaluating our case, misalignment and mechanical looseness can be rule out as the plant is running smoothly before and after the incidence of this problem and vibration is normal after maintenance i.e., about 0.08 – 0.09 mm peak-peak.
Uneven water flow due to the foreign objects may have caused unbalanced runner leading to the vibration. It is one of the strong candidates to be the root of the problem but as the shear pins were intact so there is some doubt in incidence of foreign object in runner.
Cavitation however is the best possible cause for the vibration in the runner. But as the vibration was persistent for longer duration, only the pressure wave of cavitation cannot be accounted for longer duration of vibration. So, plausible root cause of the problem in our scenario can be cavitation-induced resonance.
Cavitation Induced Resonance:
When cavitation occurs, bubbles of water vapor form and then collapse, creating pressure waves that can cause vibration and damage to the turbine. If the natural frequency of the turbine matches the frequency of these pressure waves, it can cause the turbine to resonate.
Resonance occurs when the natural frequency of a system matches the frequency of an external force or vibration. In the case of a turbine, this can occur if the frequency of the cavitation pressure waves matches the natural frequency of the turbine’s components. When resonance occurs, the vibration can become amplified, leading to damage of bearing.
There are no clear solutions to eliminate this problem. Some control methods like air or water injection, runner core extension, mounting of stabilizer fins on the draft tube wall, etc., are often proposed to alleviate this problem. But they are not fool-proof and have their inherent limitations.
Maintenance Duration: 2079/05/30 from 6:00 AM to 9:00 PM
DISMANTLING OF TURBINE BEARING COMPONENTS
- Removed the sensor terminal housing assembly.
- Removed the oil level sensor and the oil-water mixture transmitter from the circuit.
- Removed the Oil Tank Cover, oil inlet-outlet pipe, and pitot tube from the oil tank.
- Disconnected the RTD sensor and labeled the terminal wires.
- The clearance between the turbine guide bearing pad and the main shaft was measured using feeler gauge.
- Removed the bearing bush assembly and jacked it up.
Component Maintenance:
After overhauling the turbine guide bearing the nature of chipping is noted and spare turbine guide bearing was assembled.
Assembly of Turbine Bearing Components:
- Assembled the guide bearing’s bush assembly.
- Measured the clearance gap of turbine guide bearing pad to the main shaft using feeler gauge.
- Attached the RTD sensor terminal wires which was marked respectively.
- Plugged the oil level sensor, oil-water mixture transmitter.
- Attached the terminal housing assembly on the bearing top cover.
Clearance Data After Assembly:
Turbine Guide Bearing Assembly
Turbine Guide Bearing As-Built Records S.N. Location Pad serial No. Clearance value (mm) Remarks 1 (+Y) 2 0.05 Technical Requirement from installation record is 0.125-0.175 mm (mono-clearance) 2 (+X) 3 0.20 3 (-Y) 4 0.15 4 (-X) 1 0.05 Note: The turbine guide bearing pad in the +Y direction is marked 2 and others are numbered clockwise, viewing from the top.
Testing:
To test the final assembly first of all the shaft was rotated by small guide vane opening manually from the governor and inspected for abnormal noise and rise in heat. Then the opening was slowly increased and machine was operated in standby mode from 17:25 PM to 19:00 PM, in no-load mode from 19:00 PM to 20:15 PM. During the test the temperature of turbine guide bearing pad 1 (TGB Pd 1), turbine guide bearing pad 2 (TGB Pd 2) and turbine guide bearing oil temperature (TGB Oil) were kept under scrutiny. The temperature rising rate was following its natural saturation curve. Then unit-1 was tested by synchronizing to the gird from 20:15 PM to 23:45 PM and bearing temperatures were normal as well as the operation was smooth throughout the test. The temperature data during test is tabulated below:
Temperature Data (in °C) During the Test:
Time TGB Pd1 (°C) TGB Pd 2 (°C) TGB Oil (°C) Remarks 17:25 20 18 22 Standby Test 17:30 23 21 24 17:45 31 29 28 18:00 36 34 30 18:15 38 37 32 18:30 41 40 34 18:45 42 42 35 19:00 42 42 35 19:15 43 42 36 No Load Test 19:30 43 43 37 19:45 43 43 37 20:00 44 43 37 20:15 44 43 37 20:30 45 42 37 Load Test 20:45 45 41 37 21:15 45 41 37 21:45 45 40 36 22:15 45 40 36 22:45 46 40 35 23:15 46 41 36 23:45 46 40 36