If used properly, the different approaches to root cause failure analysis (RCFA) all achieve the goal of finding the reason for failure. The differentiator is the process or program used to reach this goal. When choosing between programs, most people lean toward the one that is easiest to understand and use. RCFA programs are no exception.
One motor company had an opportunity to use this method when a plant contacted the company about pricing for replacement circulating water pump motors. After asking a few questions, it became clear that the problem was larger than a motor issue. This prompted a series of key questions to help the motor company fully understand the situation.
Asking the Right Questions
After further discussion with plant personnel, the motor company established the history of the problem and had a thorough understanding of the current situation. The failure mode the plant experienced is a common issue in power plant circulating water pump applications. In this case, the company asked for the following information:
- Motor repair history mean time between repairs (MTBR), which established when the plant began experiencing failure
- Pump MTBR
- Catastrophic events that occurred with the motor and pump or times when equipment was in failure mode
- Plant operation, especially starts and stops
- Pump start and stop procedure
- Changes in the plant or system
- Pump and motor installation procedure
- Plant load swings or whether the envelope changed over time
- Pump system behavior during high condenser back pressure
History & Current Situation
The coal-fired power plant was built in 1985 and has two coal-fired boilers, referred to as Units 1 and 2. Each unit has a natural-draft cooling tower to cool the circulating water with two 50 percent capacity vertical circulating water pumps. The circulating water is pumped from each cooling tower basin through the condenser and back to the cooling tower by the circulating water pumps.
The circulating water pumps have a mixed flow design. Each pump produces 113,750 gallons per minute at 76 feet of head. The pumps are direct coupled to 3,000 horsepower, 514 rpm induction motors (see Figure 1).
One of the circulating water pumps in Unit 2 (2B) experienced most of the motor-related failures. Typical motor issues included the bottom bearing being wiped or the rotor coming into contact with the stator. Plant maintenance records indicated that all four pumps had begun to experience wiped bearings during the past few years when the plant began swinging load.
During a walk-down of the entire cooling water system, the motor company reached the following conclusions about the circulating water pumps:
- Intake structures were badly degraded, concrete was crumbled and the discharge pipe support had broken loose from the concrete mount.
- Unit 2B had a replacement motor because of catastrophic failure.
- Significant shimming was used between the motor and motor mounting surface.
- The Unit 2B motor stand was 0.060 inches out of level.
After reviewing the entire system and interviewing plant maintenance, engineering, operations and management personnel, the motor company obtained the following answers to their key questions:
- Unit 2B began experiencing issues upon commissioning. The plant does not have a hard specification for repair.
- Pump MTBR increased over time.
- Catastrophic events occurred with both the motor and pump in Unit 2B.
- The plant remains on line, and the circulating water pump operates 24 hours per day, seven days per week until an outage occurs.
- The pumps are started and stopped against closed discharge.
- Load swings increased over time.
- Pump and motor installation procedures are inconsistent because of high plant personnel turnover.
- The plant swings load across a wide range, especially during the past five years.
- Pump and motor vibration increases during high condenser back pressure. This is typical when catastrophic failure occurs.
The root causes were determined to be alignment, pump installation, degraded intake and support structure, net positive suction head (NPSH) and motor repair specification.
Reliability in a vertical pump application begins with the installation. For a circulating water pump application, this would include the intake structure, sole plate, motor stand and suction discharge piping (see Figure 2).
The intake or support structure must be able to tolerate the corrosive effects of the environment as well as stresses from starts and stops and handle the weight of the pump and motor.
Vertical pump installation can best be explained by using the motor thrust bearing as a pivot point (see Figure 2). A line or plane is considered plumb when it is exactly vertical. In the alignment of vertical pumps, plumb is essentially the reference point for all measurements. A common error when aligning vertical pumps is that the primary goal is to make the shaft itself plumb. This is also the reason for most motor radial bearing failures.
Pump bushings, placed along the shafting, are not true bearings. Instead, they serve as rotor guides with ample running clearance compared with a typical hydrodynamic bearing found in the motor. Bearings have about one thousand per inch of shaft of clearance, while a guide bushing may have three to four times that amount, about 0.02 inches. The guide bushings in the pump lack rotor stabilizing capability.
This is why proper restoration of the fits and concentricity of the stationary parts during overhauls, installation and alignment are important. Line shaft bushings, unable to withstand any substantial side loads, will quickly wear or fail if the shaft comes in close proximity because of incorrect eccentricity of the line piping fits. True hydrodynamic bearings or motor radial bearings need to be well-aligned and centered.
During the investigation, personnel noted that the plant was unable to achieve full load during high condenser back pressures. Plant digital control system historical data indicated reduced flow to the condenser and a drop in motor amperage. This was traced to insufficient NPSH available at the circulating water pumps. The short-term solution was to throttle the discharge slightly until amperage stabilized and flow to the condenser increased. Long-term corrective action will require adjustments to the cooling tower basin level control, stationary screens, and make-up and blow-down pump controls.
To fix some of the plant’s problems, motor company personnel checked mounting rabbit fit and motor mounting face run-out as noted in Figure 3. In theory, the motor should sit metal-to-metal on the motor stand without shims with the rabbit fit allowing for off-set alignment. This common mistake is amplified when the intake structure has degraded, sole plate is out of level, and pipe stress and subsequent motor radial bearing fail.
Look Beyond the Motor
To solve frequent failures, operators should look beyond the failed component. Tricky component failures may be a symptom of a system-related issue. It may be a recipient of the issues related to the driven component.
Operators should also ask the right questions. Electrical, mechanical, operations, maintenance and I/C personnel should work as a team.