Cooling tower type: Multi-cell cooling tower
Cooling fan details:
Blade type: Twisted blades
Number of blades: 6
Speed reduction: Single-stage Gear reduction with ratio of 3.27:1
Coupling type: Twin-flex drive shaft (Bushed pin type with spacer shaft)
Motor power: 22kW
Rated speed of motor: 1770 RPM
Initial pitch angle: 5 degrees
Direction of rotation (DOR): Counter Clock Wise (CCW) when viewed from the top.
Vibration data collected using Emerson CSI 2140 Machinery Health Analyzer
Initial vibrations: 22mm/s with dominant 1x at fan and motor RPM
Introduction:
The 2-cell cooling tower was in operation for the past 20 years. A few years back, the appearance of chronic vibrations and performance reduction forced to increase the PM frequency to 6 months. Several structural modifications were carried out which reduced vibrations temporarily. Recently, after planned shutdown maintenance, the vibrations started increasing furthermore. On analyzing the vibrations, it was found that there is high 1x component at fan speed. Recommendations were provided to do a detailed inspection as the whole structure was shaking. Within a few days, the high vibrations led to the detachment of spacer shaft (coupling spacer). In-depth inspections were carried out on both cooling tower fans and structures during the available opportunity. Previous records and fan data were not available for verifying the existing arrangement.
Analysis:
Preliminary finding through vibration analysis: High 1x at the fan and motor RPM.
Prime suspects: Unbalance, structural looseness.
Preliminary analysis:
Structural inspections revealed that almost all the fasteners of the fan support structure joints and fan gearbox and motor bases were loose. Cracks were found near the weld joints at 2 locations on the structure. The structure foundation jack bolts were also found loose. A crack was found on one of the blade clamp too.
The coupling hubs were found damaged due to excessive deflection of the shaft. The hub bolt holes got enlarged due to the damage of rubber bushings. The rubber bushings were damaged by the deflections of the shaft which led to direct contact of metallic bolts with the hub holes, accelerating the damage. The equipment got misaligned while in operation due to the relative movement of fan and structure. It is called a Dynamic Misalignment as the misalignment occurred while the equipment is running and not while it is stationary.
So once the vibrations are corrected, the dynamic misalignment gets rectified automatically as long as the static alignment is good.
Preliminary corrections:
As time was limited, we went for the improvement of structural rigidity. All the loose bolts and nuts on the structure were tightened. Additional support brackets were provided under the motor base plate to reduce motor vibrations. The cracks on the frame were repaired by welding and the cracked clamp was replaced with a new one.
Damaged couplings were replaced and static alignment readings were brought to within 0.05inches (axial).
A trial run was conducted and the vibrations were still high. The overall values were brought down by 2mm/s only. The motor vibrations got reduced from 9mm/s to nearly 5mm/s.
But wait…there is something wrong with Direction of Rotation (DOR). But how to clarify?!
Secondary analysis and corrections:
During the trial run, it was observed that the fan is rotating in the reverse direction. (Why didn’t you notice it before?) As the data sheets for the fan were not available, nothing was documented related to DOR. (Then how you suspected it now?) During the inspection opportunity, the blade profile was noted, i.e. the thick edge, thin edge and twist of the blade. Always the thicker edge of the blade is supposed to be the leading edge while in the rotation. The cross-section of blades is of aerofoil shape, and to reduce the wind resistance and turbulence, the thicker edge is kept as the leading one (basic aerodynamics).
The fan was rotating in CCW direction when viewed from the top making the thinner edge the leading one. Finally, DOR is decided as per the recommendation. But this won’t solve the issue.
Again a trial run was taken after correcting the DOR. The vibrations are still high.
Now, an unbalance effect in a fan can be produced by two factors. Uneven mass distribution or uneven force distribution. Uneven mass distribution is common in rotors. That’s why we do balancing. Uneven force distribution can arise due to variation in pitch angle between the blades of the same fan.
(How?) There are two components of force acting on an aerofoil blade due to the wind, the lift, and the drag force. The CT fans are designed such that there won’t be a lift, but a push downwards. Now, these components are dependent on the pitch angle. So, as the pitch angle varies between the blades, the lift and drag vary. The components of the lift or push force and drag force acting on each blade will be different on each blade. There will be forces of different magnitude acting in the downward direction on each blade. The resultant of these forces will be away from the center and will act as the unbalance force. So maintaining a uniform pitch angle is very important.
On measurement, pitch angle variation of up to 1 degree was found between the blades. This created additional unbalance effect creating high 1x peaks. Uniform pitch angles were maintained in all blades.
Even this didn’t completely solve the issue. The vibrations were further reduced by 4mm/s. But still not enough. Vibrations are higher than the acceptable limits as per ISO 10816. So the only thing left was balancing and the whole fan hub assembly (blades, hub, blade clamps, and nuts) were taken to the balancing workshop during an available opportunity.
Final analysis and correction:
The hubs were dynamically balanced as per ISO 1940 balancing standards with grade G 1.6. The initial unbalance was found to be 37 times the tolerance value and 29 times for the second fan hub. Balancing weights were added on the hubs to bring these within the tolerance limit.
Now static balancing was done for the hub-blade assembly. For static balancing, the first step was to measure the weights of blades individually. The measure weights are mentioned in the table below.
A-Fan
|
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Blade #
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Weight (kg)
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Blade #
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Weight (kg)
|
A1
|
9.6
|
A4
|
9.6
|
A2
|
9.4
|
A5
|
9.6
|
A3
|
9.6
|
A6
|
9.4
|
B-Fan
|
|||
Blade #
|
Weight (kg)
|
Blade #
|
Weight (kg)
|
B1
|
9.6 Kg
|
B4
|
9.6 Kg
|
B2
|
9.6 Kg
|
B5
|
9.8 kg
|
B3
|
9.8 Kg
|
B6
|
9.6kg
|
The blades of approximately equal weights were installed opposite to each other on the hub. Each hub assembly was mounted on mandrels which were supported by two bearings placed on V-blocks to facilitate free rotation of the rotor. The blades should be pulled out as far as possible (depending upon the type of blade) to ensure repeatability. For repeatability, the blades were pulled out because the centrifugal forces will act in the same direction and since the blades are fixed after pulling out to the maximum extent, the centrifugal forces will not change the position of blades. But the effect is feeble as the U-bolts or clamps provided will be already holding the blades tightly.
After balancing, the fans were installed and the vibrations came down to less than 1.5mm/s. Now the next task was to set the pitch angle as per the efficiency requirement. The initial pitch angle of 5 degrees did not provide the required efficiency in the new/original DOR.
The restricting criteria for setting the pitch angle are based on the maximum motor amperage (if the design data are not available). The actual pitch angle should be calculated from the fan curve at the design point. The operation of the fan at the design point is at most important to avoid the stall. The fan is designed to operate outside of the stall region and generally a margin of 2O pitch is kept from the stall region for the safe operation. The pitch angle determines the airflow. The pitch angle was increased in steps until motor amperage was near to the maximum rated amperage. This method cannot be applied if the motor is oversized, because each blade has an HP restriction. And also the fan performance is maximum at the optimum pitch angle above which the performance again decreases.
A pitch angle of 35O was used and the best efficiency was obtained at this pitch angle. The vibrations were observed to be 2mm/s at fan speed and 5mm/s at motor speed. The high vibration at the motor is due to misalignment. The misalignment was corrected and overall vibrations were observed to be 2mm/s.
The total reduction in vibration is 20mm/s, which is a great achievement.
How it was solved:
1. Correction of DOR.
2. Balancing of the rotor.
3. Fastening the loose structural components and fan and motor bases.
4. Maintaining proper alignment.
5. Correct setting of pitch angle and maintaining uniformity among the blades.
Things to look out if cooling tower is having high vibration (Before going for balancing):
1. DOR: DOR is utmost important in CT fan. Most of the blade profile restricts its operation to one direction only. But in certain cases, the CT fans are operated in reverse direction such as in extreme winter/icing conditions to prevent ice formation. As negative pressure leads to ice formation in induced draft fans, it will be operated in reverse direction to act as a forced draft fan reducing the ice formation and thus choking of cooling tower fills. The DOR will be mentioned in the manual and if not provided, the blade profile can be checked and the thick edge should be kept as the leading edge. This is to maintain the aerodynamic stability by reducing the turbulence across the blades. The wrong DOR will produce extremely high forces on blades which might lead to an increase in vibrations.
As explained in the figure, if the blade is tilted in such a way that the tip assists in air flow in the upward direction, more forces will act on the blade tip which will cause the blade to bend and will produce additional stress on the clamps as explained in the figure below.
2. Pitch angle: Pitch angle is the angle of the blade with the horizontal. The pitch angle determines the airflow through the fan. The flow rate is proportional to the pitch angle to a certain point beyond which the increase in pitch angle will reduce the flow rate and all the fan would be doing is churning (moving air in a circular motion). The more the pitch angle, the more is the resistance offered by wind on the blades and hence vibrations will be more. But the required pitch angle for best effect should be maintained and cooling tower fan should be checked for other defects if the vibrations are high at optimum pitch angle. The pitch angle can be set up to the maximum rated motor amperage if the motor is not oversized for the fan. One method of checking the optimum pitch angle is by noting the linearity of motor amperage. i.e.; with a small increase in pitch angle, the motor amperes will not rise. This can be understood from the fact that the pitch angle vs flow rate curve approximately flattens out at the top or at optimum pitch angle. Since power is proportional to the cube of flow rate (fan laws), the power will remain the same for a certain increase in pitch angle and thus the motor current. Uniformity of pitch angle across the blades is very important for maintaining equal air flow through each blade and thus the forces also. If the forces vary, it will create unbalance.
3. If match marking on blades-hub and coupling hubs are followed: Generally, after balancing, the blades will be match marked with the installing location on the hub and the coupling halves will also be marked similarly. If the match marking is not followed, the balancing will be disturbed and will lead to high vibrations. After every balancing activity, care should be taken to put marks on the blades and at respective positions of the blades on the hub. The maintenance activities should be carried out with extreme caution and care to be taken not to mix up any bolts or nuts on the hub. Each blade, nut, and bolt should be reinstalled at the exact same location from where it was removed during maintenance. Similarly before dismantling the coupling, check if match marking is faded, put new match markings on the hub halves before removal and install it in the same way by matching those marks.
4. A
5. Structural rigidity
Things to check for reduced efficiency:
1. Pitch angle: Pitch angle is the angle of the blade with the horizontal. The pitch angle determines the airflow through the fan. The flow rate is proportional to the pitch angle to a certain point beyond which the increase in pitch angle will reduce the flow rate and all the fan would be doing is churning (moving air in a circular motion). The more the pitch angle, the more is the resistance offered by wind on the blades and hence vibrations will be more. But the required pitch angle for best efficiency should be maintained and cooling tower fan should be checked for other defects if the vibrations are high at optimum pitch angle. The pitch angle can be set up to the maximum rated motor amperage if the motor is not oversized for the fan. One method of checking the optimum pitch angle is by noting the linearity of motor amperage. i.e.; with a small increase in pitch angle, the motor amperes will not rise. This can be understood from the fact that the pitch angle vs flow rate curve approximately flattens out at the top or at optimum pitch angle. Since power is proportional to the cube of flow rate (fan laws), the power will remain the same for a certain increase in pitch angle and thus the motor current. Uniformity of pitch angle across the blades is very important for maintaining equal air flow through each blade and thus the forces also. If the forces vary, it will create unbalance.
2. Tip clearance: Tip clearance is the distance between the blade tip and draft tube. Maintaining proper tip clearance is very important. The tip clearance should be as low as possible for efficient operation. If tip clearance is large, it will lead to the recirculation of air through the peripheral area which affects the efficiency of the cooling tower.
3. Restriction to air leakages: For the best efficiency, the air should enter only through the fills and after cooling, should go out of the tower. But in most cases, air leakages occur. The air might enter through the least resistance path in the tower and any gaps between the fills or open spaces in covers will assist in this process. So care to be given in proper sealing of cooling tower covers and tight packing of the fills.
4. Proper working of nozzles: As we know, more the surface area of contact between water and air, better the cooling. For this purpose, the water should be broken into very fine droplets. The nozzles assist in this purpose. The improper functioning of nozzles will increase the size of droplets. This will reduce the efficiency. Moreover, in counterflow cooling towers, nozzles are attached to branch pipelines in a series manner. So a damaged nozzle in one row might cause the reduction in pressure in the header (Large quantity of water will pass through the broken nozzle). Hence, during any available opportunity, the nozzle integrity and leakages should be checked.
5. Choking of filter screens: Choking of filter screens at air inlet will reduce the air flow through the cooling tower thus reducing the efficiency. The filter screens should be cleaned periodically to avoid the choking.
6. Choking of drift eliminators: Choked drift eliminators reduces the differential head across the fills thus reducing the airflow and efficiency. Periodic cleaning is necessary to attain the best efficiency.
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