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Solving a Problem Related to Shear Pin Failure

The 506.4 MW Peixe Angical hydroelectric plant, on the Tocantins River in central Brazil, contains three vertical 168.8 MW Kaplan turbines. On March 28, 2010, all 24 wicket gate shear pins in Unit 1 failed. Plant mechanics determined that this failure began with rupture of an oil pipe in the hydraulic circuit of intake gate 1, which caused the unit to shut down and break all the shear pins. The entire turbine shifted downstream, scratching out the chamber rim and then displacing the unit upward.

In addition to repairing the damage to this unit caused by the shut down, owner Enerpeixe, a subsidiary of utility Eletrobras Furnas, took multiple actions to prevent a similar problem from occurring again. These five actions included: installing a wicket gate monitoring system, adding a compressed air system to detect shear pin failure, changing the mechanical closing rule for the distributor, increasing the torque of the distributor's safety device and installing a stop pin in the wicket gate lever.

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Two of the bolts (shown) used to connect the hydraulic commanding pipe to the cylinder block of the servomotor system for intake gate 1 failed due to misalignment problems during assembly of the unit, ultimately resulting in shut down of the unit.

Peixe Angical plant personnel needed 135 days to complete the above modifications and repair the damage to the unit. Unit 1 was returned to service on August 10, 2010, and the other two units have been modified as well.

Event description

Unit 1 at the Peixe Angical plant began operating in June 2006. The five-bladed runner has a diameter of 8.6 meters.

On March 28, 2010, the restoring hydraulic system for the Unit 1 wicket gates failed. In less than five minutes after the first indication of a problem, the unit had stopped.

The sequence of events is as follows: At 16:06:28, an alarm indicated the turbine intake gate had fallen, emergency tripping of the unit, and a lockout relay. Three seconds later, another alarm indicated wicket gate disalignment. Three seconds after that, at 16:06:34, rotor uplift occurred, and one second later an overspeed mechanical device alarm sounded. One second after that, the unit was operating at speed no load. Seven seconds later, at 16:06:43, the wicket gates closed.

More than 2.5 minutes after this event (at 16:09:25), intake gate 1 closed, followed by intake gate 3 closing 19 seconds later and intake gate 2 just 1 second after that (at 16:09:45). The unit stopped at 16:11:16.

Assessing the situation

Immediately after the failure, Peixe Angical plant personnel began disassembling the unit and performed a visual inspection to help determine the cause of the incident and the extent of the damage to the unit.

Two bolts used to connect the hydraulic commanding pipe to the cylinder block of the servomotor system for intake gate 1 failed due to misalignment problems during initial assembly of the unit. Plant personnel determine this was the inciting incident in this failure. The hydraulic circuit began to leak, and the restoring system could not supply enough oil to keep up with the leakage. Thus, the intake gate level lowered, leading to activation of the lockout relay.

The subsequent disalignment of the wicket gates unbalanced the turbine. The oscillation values were not recorded, but the marks on the chamber rim give an indication of the extent of the imbalance. This disturbance caused the unit to rise above the 3 mm limit imposed by the axial anti-thrust bearing. As a result of the malfunction and the resulting reduction in the downstream turbine gap, the unit made contact with the chamber rim, but there was no apparent damage to the turbine runner.

In addition, the high level of oscillation caused the overspeed mechanical device to improperly operate at 130% of turbine maximum speed. The nominal level is set at about 165%.

The distributor shut down 15 seconds after the trip, and the remaining two intake gates closed within 20 seconds after closing of gate 1.

Parts of the unit suffering damage during this incident included the turbine guide bearing, intake gate, chamber rim, shear pins and wicket gate.

Four of the 24 fixing bolts for the turbine guide bearing cover were broken. The bearing is designed in two parts, and these bolts are placed near the contact surface of each half part. All 10 segments of the guide bearing were scratched.

In addition, the axial anti-thrust bearing in the lower part of the turbine guide bearing was damaged by the uplift of the unit during the accident. The nominal axial freedom (clearance) to uplift the rotating parts is 3 mm. The gap measured after the accident was 6 mm. The axial gap was raised by the plastic deformation of the inner head cover produced by the accident. The scraping signs observed at the support of the radial monitoring sensor caused by the chock of the fixing bolts of the semi-axes of the Kaplan turbine head shows that the unit raised as much as 15 mm.

The energy involved in this accident drew all wicket gates to torsional plastic deformation caused by its impact with the closing stops placed in the outer headcover. The shear pin and friction system of all the wicket gates failed. All 24 shear pins were found to be broken. And all the closing stops were marked by the impact.

The two slotted bushings in each wicket gate responsible for torque transmission between the wicket gate and its gate lever were also deformed.

Analyzing the data gathered

An analysis did not point to any one specific element as the main cause of the failure because the oil leakage was not the root cause.

Plant personnel selected two shear pins to undergo nondestructive examination of the microstructure of the fracture. The pins chosen were working on opposite sides of the unit (positions 5 and 12) and had oxidation signs that could be microscopically analyzed to preview the cause of the fracture. The results of this analysis indicated that the pins did not show fatigue but rather suffered a sudden fragile rupture.

Another important fact is the closing desynchronization of the intake gates. One of the three intake gates (number 1) started closing earlier. However, the manufacturer determined that this fact would not geometrically influence the further instability.

The data collected by the monitoring systems were synchronized with the data from the SDSC (supervision system), leading to the following conclusions: The unit changed to a load rejection condition activated by the emergency actuation. The gap between the rotor blade and chamber rim began its reduction 3 seconds after the wicket gate disalignment occurred.

Furthermore, 6 seconds after load rejection, the rotor reached an upward displacement level of 8.5 mm. During the incident the blade of the Kaplan turbine touched the chamber rim at maximum overspeed. At that moment, the rotor blades, which had just been opened, closed rapidly and then returned to maximum open position a few seconds later.

Repair work

Voith Hydro Brasil completed the repair work on this unit. This involved, among other things, replacing the damaged turbine guide bearing. The axial anti-thrust bearing was replaced, and the inner head cover welds were tested using die checking. All thrust and guide segments were submitted to NDE using an ultrasound.

Repairs to the hydraulic circuit were limited to correction of the misalignment problems. The chamber rim of this unit did not require any repair.

In addition, all of the slotted bushings in the wicket gates were replaced, with an increase of 2 mm in diameter (from 70 to 72 mm) because of deformations in the bushing holes.

Improvements

Plant personnel determined that five actions were necessary to be undertaken for all three units to prevent future similar accidents at the Peixe Angical hydro installation:

— Install an online wicket gate monitoring system. Thus, a programmable logic controller now is responsible for identifying broken shear pins.

— Install a compressed air system to detect shear pin failure, even with no misalignment among the levers. A pneumatic series circuit connected to all shear pins supervises the occurrence of any cracking in a shear pin by monitoring the air pressure.

— Change the mechanical closing rule of the distributor to a dual rate system to reduce the tendency of the unit to move upward during emergency closure or load rejection. Under this system, the distributor will decrease from 100% to 20% of speed in 8.4 seconds and from 20% of speed to full stop in 12.7 seconds. The initial closing time was about 9.5 seconds. This modification promotes a smoother closing with consequent decrease in lifting of the unit during closing procedures.

— Increase the torque of the distributor's safety device. The friction torque was increased to 95 kNm from 75 kNm, the shear torque to 138 kNm from 115 kNm and the total torque to 233 kNm from 190 kNm. The increase in these values was limited by the yielding limits of the materials involved.

— Install a stop pin in the wicket gate lever to avoid its closing movement in case of a shear pin rupture. This is one more safety system in case of a possible loosening of the original one, which consists of the shear pin and friction device. This stop pin is strong enough to support a resistant safety equal to the original system torque. The total equivalent resistant torque results in 466 kNm.

Performance improved

Repairs were completed on this unit in August 2010. Since that time, the unit has operated reliably.

In addition, the five improvement actions discussed above have since been implemented for all three units. Commissioning tests were repeated when each unit was returned to service, with particular emphasis on the dual rate system mentioned earlier. Implementing this system resulted in a significant reduction (53%) in the axial displacement of the units.

— By Amauri Alvarez, formerly operations director for Enerpeixe, now with Santo Antonio Energia; Jose Henrique Vilela, operations director with Enerpeixe; and Fernando A. Blanco Resende, maintenance engineer with Eletrobras Furnas

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