Understanding fan vibration and imbalance
Detecting imbalance early reduces damage and lowers repair costs
Imbalance and vibration are not the same
Dirt, temperature, and loose fits can cause imbalance.
Fans play a key role in most manufacturing processes by recirculating air, ventilating hazardous gases, and cooling machinery. One problem that can develop in all of these applications is imbalance. Since imbalance is a potentially dangerous condition and can result in breakdowns and costly plant shutdowns, it is imperative that plant engineers understand what it is, how to detect it, what causes it, and how it can be addressed.
Detecting imbalance early can save large amounts of money. The less damage, the less the cost of repair. In some cases, imbalance results from an improper manufacturing process.
It is important to clarify the difference between “imbalance” and “vibration.” A fan rotor generally consists of a welded, riveted, or cast fan impeller mounted on a shaft. Even if the manufacturer takes care in locating blades and weighing component parts, the weight center is separated from the axis of rotation. This difference between the weight center and axis of rotation is referred to as “imbalance.”
Imbalance is not a function of rotational speed and therefore can be assessed and measured when the fan is not in operation. Imbalance can be quantified by multiplying the weight of the fan rotor by the radial distance between the weight center and axis of rotation (Fig. 1).
Vibration occurs during fan operation and may have many causes, one of which could be imbalance. Other causes of vibration include mechanical looseness, coupling misalignment, defective bearings, insufficient flatness of bearing mounting surfaces, rotor cracks, driver vibration, and V-belt slippage.
Fig. 1. A rotor with the center of mass off slightly from the axis of rotation creats imbalance.
How to calculate imbalance:
Imbalance = Weight×weight offset
Where: Weight, oz
Imbalance = 300 lb×16 oz/lb x 0.0052 in.
Buildup on blades
Imbalance may be the result of a manufacturing process or operating conditions. For example, many fan rotors are used on wet scrubber systems where wet and sticky particulate matter sometimes passes through and adheres to the surfaces of the fan impeller. Usually this buildup of particulate matter is evenly distributed over all surfaces and the resulting imbalance is minimal. However, if a piece of the built-up material flies off due to centrifugal force, then significant imbalance can occur.
In some cases, backward-curved fan blades have proven effective in controlling buildup, particularly on preheater ID fans in the cement industry, where buildup can be a regular problem. The design of the backward-curved fan must be carefully selected. If there is too much curvature of the blade, buildup can develop in the hollow pocket on the backside. Backward-curved fan designs are available with steeply sloped blades preventing this sort of buildup.
Another common cause of imbalance is nonuniform temperature. If a fan rotor is left at rest during an outage, a differential temperature may develop between the top and bottom of the housing (Fig. 2). A similar, though less pronounced, temperature differential may develop in the shaft, resulting in differential thermal expansion. Bowing can result from as little as a 1-deg F temperature difference between the top and bottom of the shaft (Fig. 3).
Fig. 2. A difference in temperature between the top and bottom of a fan housing during outage or shutdown can be significant.
Bowing in the shaft causes vibration on startup. The vibration is quite high at first and then decreases slowly as rotor temperature becomes uniform. If correction weights are applied during startup, then vibration is minimal during startup, but quite severe once the temperature differential is corrected. The solution is an auxiliary drive that rotates the fan rotor slowly during shutdown periods, ensuring uniform fan temperature.
Dirt or fluid in hollow blades
Imbalance may also occur because of the accumulation of dirt or fluid inside hollow sections of rotor blades. Some centrifugal fans have hollow airfoil blades, which offer maximum efficiency in clean operating conditions. However, during extended operation in wet or dirty environments, pinholes can develop in the blade skins and dirt or fluid builds up in one or more of the blades. Since the accumulated material shifts in the hollow blade during each start, the fan rotor is nearly impossible to balance. Solid blade shapes (backward-curved, backward-inclined or radial-blade fan designs) are usually selected for centrifugal fans in extremely dirty or wet environments.
Fig. 3. Bowing in a shaft can result from just a 1-deg F temperature difference between the top and bottom of the shaft.
Loose hub-to-shaft fit
During initial start-up, the fan hub may be securely held in place by setscrews. After a period of time, however, the setscrews may loosen due to fretting or corrosion. This loosening of the setscrews allows the hub and fan impeller to become displaced relative to the axis of rotation. The result is extreme imbalance. For this reason, hub-to-shaft connections with an interference fit or some type of tapered bushing are usually preferred to setscrews.
Where there is rapid temperature change, the fan impeller and hub may heat up faster than the shaft, which could cause looseness in the hub-to-shaft fit and imbalance. In such cases, an extreme interference fit or an integral hub/shaft arrangement is preferred.
Tolerances for vibration
Tolerances for vibration vary widely according to industry and application. The Air Movement and Control Association Intl., Inc.’s (AMCA) Standard 204, Balance Quality and Vibration Levels for Fans ,” recognizes five different fan application categories (BV-1, BV-2, BV-3, BV-4, and BV-5) for the required balancing grade during manufacturing of the fan (see table). These balance categories are ordered from a less sensitive group (HVAC) to the most sensitive fans, which include those for petrochemical processes and computer chip manufacturing. (Note the table describes fans not only with regard to their application, but also their driver.)
The table also shows the appropriate balance quality grade for each of the fan application categories. These values vary from balance quality grade G-16 (least stringent requirement) to balance quality grade G-1.0 (most stringent requirement).
What do these balance grades actually mean? Take balance grade G-2.5, for example. Given a rotor operating in free space without bearings or any support system, the expected vibration velocity for this rotor balanced to grade G-2.5 is approximately 0.1 in./sec. Each balance quality grade refers to the expected vibration velocity in free space measured in in./sec.
Assessing vibrationin real applications
How do these measurements in free space relate to actual applications? What are the real expected values when the rotor is supported by a bearing system? What are the acceptable vibration levels?
Since a bearing system offers some degree of stiffness, vibration levels are generally lower when the bearing system is included. Consider an extreme case with a relatively light rotor supported by a massive bearing system. In this case, the force of imbalance generated as the unit rotates is probably not significant enough to cause much movement in the bearing housing and structural supports. Such a system is said to have low vibration sensitivity.
A stiff-support system with low vibration sensitivity can make it difficult to monitor the health of the machine using the bearing housing as the point of reference. It would be possible for rotor cracks to cause great centrifugal force without significant vibration registering in the bearing housing. In other words, in a stiff-support system there is stable operation, but with a false sense of security. Catastrophic failure could occur without any prior warning.
How can the health of a fan be effectively monitored? One solution is a proximity probe, which is applicable in sleeve-type bearing installations. A proximity probe reaches down through the bearing housing and into the bearing liner where it directly measures the radial movement of the shaft. High levels of vibration can be detected before any damage occurs. Proximity probes are not practical with roller bearings. Vibration can be measured effectively on the housing instead.
Occasionally, it may be necessary to mount a fan on structural steel instead of a concrete foundation at grade level. In such applications, guard against the possibility of a correspondence in vibration frequency of fan excitation and the natural frequency of the steel supports.
When the vibration frequency of the two correspond, the total vibration amplitude is significantly higher than it would be if the fan were mounted on a concrete foundation at grade level. A system such as this has high vibration sensitivity. Very small changes in the residual imbalance of the rotor cause very large responses in the vibration levels.
When fans are mounted directly on structural steel platforms, the potential for a natural frequency excitation exists. A flexible fan mounting provides one solution to this problem.
A flexible mounting consists of a rigid sub-base under the fan, often filled with concrete, and supported by a spring isolation system. For most applications, static deflection of the spring isolators should be on the order of 1 in.
Since most fans operate at 880 rpm or higher, the natural frequency of the spring isolation system would be sufficiently removed from the operating speed of the fan to ensure minimal transmission of energy. The fan housing should be mechanically separated from the inlet and discharge ductwork so it is free to float on the spring isolation system.
The result of the flexible mounting system is lower stiffness for the bearing housing and bearing pedestal. As a result, slightly higher vibration levels, as measured on the bearing housing, can be tolerated for flexible-mounted systems than for rigid-mounted systems.
Those in charge of industrial fans must be vigilant. Regular maintenance and inspection of fans prevent costly shutdowns and catastrophic failures, which could result in injury or damage to other equipment. Most balance and vibration problems can be detected by a fan service professional. Many balance and vibration problems can be corrected through adjustments or repairs. In general, repairing fans as opposed to buying replacements is highly economical and efficient. The sooner the problem is detected, the lower the cost of repair or correction.
Edited by Joseph L. Foszcz, Senior Editor, 630-320-7135, email@example.com
Information for this article came from AMCA Standard 204, “Balance Quality and Vibration Levels for Fans.” For a complete copy of the standard, contact the Air Movement and Control Association Intl., Inc., 30 W. University Dr., Arlington Heights, IL 6000 4-1893; 847-394-0150; fax 847-253-0088. The association’s web site is amca.org.
The authors are available to answer questions about fan vibration and imbalance. They can be reached at 724-452-6121.
How to calculate imbalance
Imbalance = Weight × weight offset
Where: Weight, oz Offset, in.
Imbalance = 300 lb × 16 oz/lb x 0.0052 in. = 25-oz-in.
Application categories and required balancing grade*
Application Examples Driver power limits, kW (hp) Fan application category, BV Balance quality grade for rigid rotors/impeller HVAC Building ventilation and air conditioning & 3.7 (5.0) > 3.7 (5.0) BV-2 BV-3 G 16 G 6.3 Industrial processes power generation Baghouse, scrubber, conveying, boilers, combustion air, pollution control & 300 (400) > 300 (400) BV-3 BV-4 G 6.3 G 2.5 Petrochemical processes Hazardous gases, process fans & 37 (50) > 37 (50) BV-3 BV-4 G 6.3 G 2.5 Computer chip manufacturer Clean room Any BV-5 G 1.0 * Information adapted from AMCA Standard 204, “Balance Quality and Vibration Levels for Fans.”
Reasons for imbalance
Buildup of particulate matter on fan blades or in hollow fan blades
Differential temperatures between the top and bottom of the fan housing
Accumulations of dirt and particulate matter
Loose hub-to-shaft fit
Improper or makeshift balancing procedures