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Understanding Vibration and Imbalance in Industrial Fans

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. However, any number of…

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. However, any number of plant situations can lead to imbalance, including: (1) build-up of particulate matter on fan blades; (2) differential temperatures between the top and bottom of the fan housing; (3) accumulations of dirt and particulate matter in hollow fan blades; (4) a loose hub-to-shaft fit; and (5) improper or makeshift balancing procedures.

It is important first of all to clarify the difference between “imbalance” and “vibration.” A fan rotor is generally comprised of a welded, riveted or cast fan impeller mounted on a shaft. Even if the manufacturer takes care in locating the blades and weighing the component parts, one can be sure that the weight centre will be separated from the axis of rotation by a small amount at least. This differential between the weight centre and the axis of rotation is referred to as “imbalance.” Imbalance is not a function of rotating 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 centre and the axis of rotation. See Figure 1.

Vibration occurs during fan operation and may have many causes, one of which would be imbalance. Other causes of vibration would include mechanical looseness, coupling misalignment, defective bearings, insufficient flatness of bearing mounting surfaces, rotor cracks, driver vibration and v-belt slippage.

Build-up on blades

Imbalance may be the result of a manufacturing process. It may also be the result of operating conditions. For example, in some applications wet or sticky particulate may adhere to the surfaces of the fan impeller. Usually this build-up 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 will occur.

In some cases, backward-curved fan blades have proven effective in controlling build-up. The design of the backward-curved fan must be carefully selected. If there is too much curvature of the blade, build-up can develop in the hollow pocket on the back side of the blade. Backward-curved fan designs are available with steeply sloped blades so build-up of this sort may be prevented.

Temperature differentials

Another common cause of imbalance is non-uniform temperature. If a fan rotor is left at rest during an outage, a differential temperature may develop between the top and bottom of the fan housing. See Figure 2. A similar, though less pronounced temperature differential may develop in the shaft, resulting in differential thermal expansion. For typical steel shafting, the coefficient of thermal expansion is approximately 6.5E-6 in./in.-*F. Bowing in the shaft can result from as little as a one-degree F temperature difference between the bottom and top of the shaft. See Figure 3.

Bowing in the shaft will cause vibration upon start-up. The vibration will be quite high at first and then will decrease slowly as rotor temperature becomes uniform. If correction weights are applied during start-up, then vibration will be minimal during start-up 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 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, for instance, 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, with the result that dirt or fluid builds up in one or more of the blades. Since the accumulation of material shifts in the hollow blade during each start, the fan rotor is nearly impossible to balance. Therefore, solid blade shapes (backward-curved, backward-inclined or radial-blade fan designs) are usually selected for centrifugal fans in extremely dirty or wet environments.

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 possibly to fretting or corrosion. This loosening of the setscrews may allow the hub and entire fan impeller to become displaced relative to the axis of rotation. The result would be extreme imbalance. For this reason, hub-to-shaft connections with an interference fit or some type of tapered bushing are usually preferred.

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 would be preferred.

Tolerances for vibration

Tolerances for vibration vary widely according to industry and application. The Air Movement and Control Association International’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, BV-5) for the required balancing grade during manufacturing of the fan. See Table 1. These balance categories are ordered from the least sensitive group (residential ceiling fans, attic fans), which operate with low power requirements, to the most sensitive fans, which include fans for petrochemical processes and computer chip manufacturing. Note that Table 1 describes the fans not only with regard to their application but also with regard to their driver power.

Table 2 shows the appropriate balance quality grade for each of the fan application categories. These values vary from balance quality grade G-16 (the least stringent requirement) to balance quality grade G-1.0 (the 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 will be 2.5 millimetres per second (approximately 0.10 inches per second). In other words, each balance quality grade refers to the expected vibration velocity in free space measured in millimetres per second.

Assessing vibration in 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 will offer some degree of stiffness, vibration levels will generally be lower when the bearing system is considered. Consider an extreme case: a relatively light rotor supported by a massive bearing system. In this case, the force of imbalance generated as the unit rotates probably will not be significant enough to cause much movement in the bearing housing and structural supports. Such a system is said to have low vibration sensitivity.

On a stiff-support system with low vibration sensitivity, the health of the machine can be difficult to monitor 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.

So how can one more effectively monitor the health of a fan? 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 w
here it directly measures the radial movement of the shaft. With a proximity probe, high levels of vibration can be detected before any damage occurs. Proximity probes are not practical, however, with roller bearings. Vibration can be measured effectively on the housing when rolling element bearings are used.

Occasionally, it may be necessary to mount a fan on structural steel instead of a concrete foundation at grade level. In such applications, one must be careful to guard against the possibility of a correspondence in vibration frequency between the fan excitation and the natural frequency of the steel supports. When the vibration frequency of the two corresponds, the total vibration amplitude will be significantly higher than it would be if the fan were mounted on a concrete foundation at grade level. A system such as this is said to have 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) supported by a spring isolation system. For most applications, static deflection of the spring isolators should be on the order of 1.0 inch. 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. Note that 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 better tolerated for flexible mounted systems than for rigid mounted systems.

Conclusions

Those in charge of industrial fans must be vigilant. Regular maintenance and inspection of fans prevents 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. Furthermore, most balance and vibration problems can be corrected through adjustments or repairs. In general, repairing fans as opposed to buying new replacements is highly economical and efficient. The sooner the problem is detected, the lower the cost of repair or correction.

Parts of 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 International, Inc., 30 West University Drive, Arlington Heights, Illinois 60004-1893. Phone (847) 394-0150, fax (847) 253-0088. Web site: www.amca.org.

Les Gutzwiller is the president and Thomas J. Kuli is the chief engineer for Robinson Industries, Inc., Zelienople, Pa.

Table 1 and 2 in PDF format only. Click here to download the file.(44KB PDF)