1 Cooing tower introduce
SEGC cooling tower is largest cooling tower in Canada which provide 507 MW steam turbine condenser. The cooling tower length is 231.04meter, width is 18.28 meters, to fill the cooling tower needs 7492.12 m3 water. Two circulation pumps installed, each one has 24100 m3/hr flow, cooling includes 14 cooling tower fans. Plant has three gas turbines and 1 steam turbine the total install capacity is 1050 MW, after the commissioning at 2018 up to four years operation, the cooling system has serial problems which this paper will provide the summary of problems.
2 Gear box damage
the cooling tower design and constructed by GEA POWER COOLING, INC GEC. The fans provide supplemental cooling that depends on demand, which is typically higher during summer. The system incorporates a carbon woven drive shaft, several of which also failed
catastrophically. In March 2011, two of 14 cooling tower gearboxes failed catastrophically (figure 1)
The 300 hp rated gearboxes, designed specifically for CT drive systems, have been in service since the plant was commissioned in 2008.
Figure 2 shows the component locations in the gearbox. The characteristics of the gearbox are as follows:
Make: Amarillo Model no 1713A
Type: Right-angle, double-reduction
Service factor: 2.0
Service rating: 300 hp
Overall reduction ratio: 15.5:1
Drive motor: AC – 300 hp @ 1800 rpm
Figure 3 cooling tower fan gear box
The cooling tower gearboxes suffered what could be described as a complete system
failure, involving multiple, progressive damage mechanisms. The rate of damage
accelerated over time, as accumulating wear compounded the problems.
2) The major damage mechanisms found were: tooth breakage by bending fatigue;
macropitting; and micropitting.
3) The common factor in the damage mechanisms is repetitive, excessively high loads on
the gears and the gear teeth.
4) Severe fretting corrosion was also found where the pinion gear was mounted on the
input shaft, and various bearing locations. It was probably a consequence of the
deteriorating condition of the components and not a primary cause.
5) The bearing failures are believed to have followed the gear failures and a consequence
of them.
6) The electrical characteristics of the AC drive motor, in particular high start-up torque, is
the primary cause of failure. The intermittent nature of CT fan operation gave the
occasions for such extreme high-load events.
7) A secondary cause was occasional snow and ice build-up on the fan blades, which
increased the system inertia and exacerbated the load at start-up. Additionally, long
idle periods (more than one week) may have compromised gear lubrication on start-up.
8) Gearbox time-to-failure is predicted by the frequency of start-ups. Higher frequency of
CT fan start-ups in some units led to earlier failures of those units.
9) The progress of gear wear is reflected in increasing in iron content in the oil analyses.
Viscosity of the oil increases concurrently.
10) The gearbox service rating is satisfactory for steady-state operating conditions.
11) The manufacturing quality and metallurgy of the gears was acceptable, as indicated by
composition, microstructure, case hardness and depth, and core hardness.
2. Gear box shaft damage
Cooling tower shaft broken due to frequency start up and shutdown, the cooling tower motor was not installed the VFD device, so when the cooling tower motor start, the specialty at winter time the fan accumulated the snow and ice cause start up torque higher than summer time, so winter time shaft more easy fatigue broken, anther factor winter time need less cooling tower operation.
Figure 4 cooling tower fan shaft broken
Figure 5 shaft broken also damage the couplings
Figure 6 New purchase shaft on site ready to repair the damaged shaft
3 Cell 7, 8 distribution pipes broken
Cooling tower has 14 fans and each cell has 54 inch raiser to guide the flow to the individually cell than connect to 8 inch distribution pipe, but when night time or weekend the plant stop then, it normally requests to stop the circulation water, condenser chamber will accumulated air inside if the air not proper release which will bring back the distribution pipe to cause the water hammer to damage the glue connector and pipe clips bolts and also broken the 8 inch pipe, the root cause the condenser water box air release valve was sticked and no one working good (each side of condenser has two air release valves) can release the air, replace all four of the condenser air release valves the problem was solved
Figure 7 cooling tower side aluminum wall deformed due to water hammer
Figure 8 cooling tower water hammer push out the wall and lose the clips bolts
Figure 9 circulation water start up water hammer case the 58 inch pipe move to south side
Figure 10 8 inch distribution pipe broken
Figure 11 8 inch distribution pipe broken
4 anti freeze pipe support damage
Cooling tower has a 108 inch pipe there was two function as recycle pipe and winter time can be anti-freeze pipe to prevent the cooling tower freeze, the situation was happen some air from the condenser to entire the pipe and accumulated at the end side of the pipe, to cause the pipe float and damaged the anchor at the bottom of the cooling tower, installed 3 inch pipe to release air and set it down to bottom and in the meaning to entre the cooling tower to fix the anchor bolts when the pipe in the place.
Figure 12 cooling tower anti freeze pipe float and damage the anchor bolts
Figure 13 install a vent valve to release the air
5 under ground pipe connector leaking
One day floor operator water found there were water come out from ground, the water gradually increase, look at the under ground the pipe line and found there was the cooling tower feed line to auxiliary cooling water line, and a connector with the isolation valve, after closed the valve the leaking become smaller, so to dig up the ground and to replace the valve seal was challenge due to the amount of soil need to remove, the problem was fixed after replace the valve seal ring and refill the ground.
Figure 14 underground pipe branch to ACW water connect leaking
Figure 15 underground pipe branch to ACW water connection leaking
6 Circulation water pump motor damage
One day the circulation water pump trip, and big bump noise also come out and black smoke also was observed in the air, open the junction box found the water is entire the junction box and cause the pump short and trip, circulation water was installed outside of the building so if the terminal junction box was not proper sealed the water go through then it would cause the short and damage, damaged pump was sent to shop for repair, and plant purchased a new motor to installed for normal service
Figure 16 circulation water pump damaged shaft
Figure 17 the circulation water pump winding damaged
Figure 18 Rain go through to short the circuit
Figure 19 Rain water entry the junction box to short the lead winding and damage the motor
7 Conclusion
For the two-shift operation power plant, start up and shutdown frequency was a challenge for the large scale cooling tower plus design fault which was requested plant to spent a lot of money to correct it.