Preventing Fan Failure
- Published: Thursday, 01 May 2008 08:00
Mining and aggregate processing plants throughout the world rely on critical process fans for operation. So what happens when a fan rotor, with a wheel rotating at a peripheral speed of 450 mph, breaks down?
The scene looks something like this: Pieces of the wheel (as large as a coffee table) rip away from the rotor, tear through the ?-inch-thick scroll of the fan housing and fly 500 feet into the air. The 12-ton steel shaft ó eight inches in diameter at the bearings and 24 inches diameter at the hub of the fan wheel ó bends and twists like a pretzel. The cast-iron bearing housings break apart. The 4,000-hp motor separates from its concrete pedestal and lands on the foundation. The entire plant is shut down. It takes weeks to clean up the mess, rebuild the fan housing and install a spare rotor (assuming the plant has a spare rotor on hand).
This scenario has been played out over and over again in mining operations and processing plants worldwide. Supply- and exhaust-ventilation fans are at risk. Process fans may be at even greater risk (due to potential corrosion and erosion of the structural fan rotor materials).
What are the causes of this kind of catastrophic failure? What steps can be taken to minimize the chances of such a failure? Before looking at possible prevention measures, consider the real-world examples presented here.
EXAMPLE NUMBER ONE
A 96-inch-diameter centrifugal fan had been in service for three years at a coal dryer application in West Virginia. The fan operated at 1,180 rpm with a peripheral speed of 337 mph. Although an upstream cyclone separator provided gas cleaning, some coal dust continually passed through the rotor. Erosion-resistant liners protected the fan wheel, but small particles of coal dust (entrained in the high-velocity gas stream) slowly eroded the unprotected welds at the edges of those liners. There was no planned inspection of the rotor and no routine monitoring of the bearing vibration levels.
One morning, minutes after fan startup, the rotor ripped apart, completely destroying the fan and damaging nearby equipment. Fortunately, no operators were in the area. The plant was shut down for nearly three weeks while a new fan was fabricated and installed on an emergency basis. (See Figures 1 and 2.)
EXAMPLE NUMBER TWO
An 88-inch diameter, double-inlet centrifugal fan, a variable speed driven unit, was operating at 350∞ F. The operators noticed some increase in noise and vibration levels at certain speeds but once the fan changed speeds, the vibration seemed to subside, easing their concerns. The fan had no bearing temperature or vibration-monitoring equipment installed and predictive maintenance vibration monitoring was spotty. In this application, the blade generates a pressure pulse when it passes the cutoff of the scroll-type centrifugal fan housing.
The operators didn't realize that, at certain speeds, the fan's blade-passage frequency (and the resulting frequency of pressure pulsations) caused excitations that exactly matched the natural frequencies of some of the fan rotor components. The result was higher than expected deflection and stress levels in those components. This caused high-cycle fatigue cracking of the rotor material near the ìtoeî of several fillet welds. Left undetected, the cracks grew until they reached a critical crack length. Without warning, the crack growth rate increased dramatically and the fan wheel flew apart. (See Figure 3.)
These two examples demonstrate the significant effects that axial- and centrifugal-fan-rotor failure can have on plant operation and worker safety. In addition to the obvious damage and plant downtime, similar situations have resulted in severe injury and even death to people working near the fan when it failed. In underground mining operations, a shutdown of the supply and/or exhaust fans means an emergency closure and evacuation of the mine. So what can plant managers do to protect their operations and their workers from catastrophic failures?
CONTINUOUS PROCESS MONITORING IS CRUCIAL
Preventative and predictive maintenance based on frequent visual inspections, non-destructive testing of welds and periodic measurement of bearing vibration helps; but this technique alone cannot prevent sudden and catastrophic failures such as those described here. The best approach is continuous monitoring of vibration and bearing temperatures with the instrument signals linked to quick-response, automatic shutdown of the system. Here's why:
* If erosion is a possibility, thinning structural components subjected to the high stress that results from centrifugal forces can suddenly reach a failure point. *
If fatigue cracks have been initiated, they can begin to propagate faster and faster until there is a sudden, unpredicted failure.
While routine visual inspections, including non-destructive testing aids such as magnetic-particle, dye-penetrant or fluorescent-particle techniques, are helpful in establishing the presence of cracks, these inspections cannot be performed continuously during operation.
Similarly, predictive-maintenance vibration monitoring is effective in establishing a baseline and determining dangerous trends (or step-changes) in vibration levels; however, this method cannot always protect against the risk of sudden and catastrophic failure. Continuous monitoring offers the best protection.
Many mining and material processing fans are designed to use rolling element bearings, either single-row-deep-groove ball bearings or spherical roller bearings. (See Figure 4.)
MONITORING FANS WITH ROLLING ELEMENT BEARINGS
With these bearings, there is a direct mechanical link from the shaft, through the rolling elements and the races, to the bearing housing. Therefore, it's best to mount seismic probes to the bearing base (or better, mount directly to the fan bearing housing) in order to monitor the bearing housing vibration level at all times. These seismic vibration pickups are available as:
* Accelerometers, which measure vibration in acceleration units (typically G's), or, *
Velocity pickups, which measure vibration in velocity units (typically inches/second or millimeters/second).
When using velocity output units, the recommended ìalarmî and ìshutdownî vibration levels are independent of the shaft operating speed. Therefore, regardless of which type of seismic probe is used, the output is usually converted to velocity units for monitoring and recording vibration levels. (Suggested vibration limits are specified in ANSI/AMCA Standard 204, ìBalance Quality and Vibration Levels for Fans.î)
In this standard, fans are categorized over a wide range of applications and motor sizes. For most industrial applications, Balance-Vibration categories BV-3 and BV-4 apply. The maximum recommended vibration levels (peak velocity, in/sec) in these categories are shown in the accompanying chart.
MONITORING FANS WITH SLEEVE-TYPE BEARINGS
Many larger mine and cement process fans are designed to use hydrodynamic sleeve-type bearings, such as plane cylindrical bore, elliptical bore and tilting pad units. (See Figure 5.)
The rotating shaft rides on a thin film of oil inside the bearing liners. Typically, the bearing housings are very robust with high rigidity and are mounted on equally rigid steel pedestals and/or concrete piers. Significant vibration of the bearing housings requires a tremendous amount of vibratory force.
Therefore, when these bearings are used, the shaft vibration should be monitored, as opposed to the bearing housing movement. This can be accomplished by means of eddy-current proximity probe systems. The probes are mounted into the bearing housings, a ìcleanî environment that protects the proximity probes against dust and corrosion.
These systems monitor the proximity of the rotating shaft surface to the tip of the probe; the output is the relative motion between the shaft surface and the bearing housing. Of course, the shaft surface quality, diameter and eccentricity must be closely controlled during fabrication, so that the output accurately reflects the shaft vibration (not the shaft ìout-of-roundî condition). Typically, the output is in displacement units (mils peak-to-peak, or mm peak-to-peak). ANSI/AMCA Standard 204 includes the recommended maximum displacement of the shaft relative to the available clearance to the bearing liner. (See diagram on this page.)
A SYSTEM THAT WORKS
This article has provided examples of what can go wrong in the absence of preventative and predictive maintenance. Here's an example of what can go right when plants use vibration monitoring:
A 7,500-hp kiln induced draft fan at a cement manufacturer's plant was subject to erosion due to particulate passing through the fan. After welds at the edges of the liners began to erode, a portion of a blade liner (weighing 50 pounds) loosened, then broke off while the rotor tip-speed was running 430 mph. One might expect the tremendous out-of-balance forces (that would normally have resulted) to cause severe damage.
Although the force of the 50-pound piece did cause a failure of the fan housing inspection door, no damage occurred to the fan housing, shaft, bearings, coupling or motor.
The reason: The vibration monitoring system (installed on the fan bearings) sensed the higher vibration levels developing as the blade liner began to loosen. The system, which was interlocked to the fan motor, shut down the fan and system immediately. No plant operator would have been able to react to this situation as quickly.
This is an excellent example of the value of continuous-vibration monitoring that is interlocked to the main fan motor-driver and set up for instant response if the vibration exceeds a pre-set ìshutdownî level.
Mining and aggregate processing fans that are subject to erosion, corrosion or elevated temperatures are often at risk to catastrophic failure. All rotating equipment, even ìclean airî fans (such as mine ìsupplyî fans), is still subject to fatigue-related failures. A visual inspection of the fan rotor combined with non-destructive testing of all rotor welds is an important step to minimize the chance of a sudden failure. Periodic measurement of vibration levels (with historical trend records reviewed and analyzed) also can help prevent such failures. While no system can eliminate the risk of failure absolutely, continuous-vibration monitoring combined with a fast-response shutdown system offers the best protection.
Les Gutzwiller is vice president of technical services for Robinson Industries Inc., an industrial fan manufacturer based in Zelienople, Pa.
|Factory Mechanical Run Testing||Normal Operation in-situ|
|Rigidly Mounted||Flexibly Mounted||Rigidly Mounted||Flexibly Mounted|
|Shop TestÖBV-3||0.15 in/s||0.20 in/s|
|Shop TestÖBV-4||0.10 in/s||0.15 in/s|
|Start-up Condition BV-3||-||-||0.25 in/s||0.35 in/s|
|Start-up Condition BV-4||-||-||0.16 in/s||0.25 in/s|
|Alarm Condition BV-3||-||-||0.40 in/s||0.65 in/s|
|Alarm Condition BV-4||-||-||0.25 in/s||0.40 in/s|
|Emergency Shutdown BV-3||-||-||0.50 in/s||0.70 in/s|
|Emergency Shutdown BV-4||-||-||0.40 in/s||0.60 in/s|
ALARM: Action should be taken immediately to determine the cause of the high vibration and to correct it.
SHUTDOWN: The fan should be shut down immediately.
RIGIDLY MOUNTED: Fan mounted by means of anchor bolts and epoxy grout on a heavy concrete foundation in a properly prepared soil or bedrock, letting the fan operate well below the system-first natural frequency.
FLEXIBLY MOUNTED: Fan mounted on elevated structural steel or on a spring-isolation system, letting the fan operate well above the system-first natural frequency.
|Operating Condition||Maximum recommended shaft displacement as a percent of the available diametral clearance (any axis).|