A variety of factors, including tolerances in fabrication (or assembly), variation within materials (such as voids, porosity or the like), any non-symmetry, distortion, deflection, dimensional changes and degradation, can cause unbalance. Machine manufacturing processes are the major source of unbalance. However, improper shipment, assembly, installation and commissioning also could result in rotating assembly unbalance.
Most often, on-site field balancing is required during commissioning or startup. Failure to recognize the danger of vibration from unbalance can lead, after only a short period of operation, to costly damage — e.g., bearing destruction, ruined seals, cracks in various components, foundation deterioration and mounting system problems.
With a simply supported rotor assembly, i.e., one have bearings at both ends, the vibration due to unbalance mainly will be in the radial plane. In the case of an over-hung rotor, high axial vibrations also may occur (axial vibration amplitude may equal those measured radially).
An unbalanced rotor assembly can cause high stresses in the rotor itself, in its support structures and the entire machine/foundation system. Rotor balancing therefore is necessary to increase bearing and seal life and safety, as well as to minimize vibration (and stresses), noise, fatigue and power losses.
There are four basic types of unbalance: static, couple, quasi-static and dynamic.
Static unbalance exists when the principal axis of inertia is displaced parallel to the shaft axis. With a statically unbalanced rotor, the amplitude and phase of the vibration at both ends of the rotor is the same. This type of unbalance primarily is found in narrow disc-shaped rotating parts such as flywheels or machine wheels. It can be corrected by a single mass correction. Static balance is satisfactory only for disc-shaped components that revolve relatively slowly or for parts that subsequently are assembled onto a larger rotor that then is balanced dynamically as an assembly.
Couple unbalance arises when two equal unbalance masses are positioned at opposite ends of a rotor and spaced 180° from each other. A couple unbalance needs another couple to correct it.
Quasi-static unbalance represents the specific combination of static and couple unbalance where the angular position of one couple component coincides with the angular position of the static unbalance.
Dynamic unbalance occurs most frequently. In it, the central principal axis of inertia is neither parallel to nor intersects the shaft axis. The unbalance only can be addressed by mass correction in at least two planes perpendicular to the shaft axis.
When a shaft rotates in its bearings, an unbalance could cause a periodic vibration. A rigid rotor support system usually prompts a larger force than a flexible one (except at resonance). In practice, machinery rotor support structures are neither entirely rigid nor entirely flexible but somewhere in between.
A field balancing package usually provides sensing and monitoring instrumentation for necessary measurements for balancing of a rotor while it runs inside an installed machine (in its own bearings and under its own power). Basically, such a system consists of a combination of proper transducers and measurement devices that provides an unbalance indication proportional to the vibration magnitude. A suitable calculation module then converts the readings (usually vibration, in several runs with test masses) into the magnitude and phase angle of the required correction masses. Unbalance vibration at one end of a machine could affect vibration measurements at the other end. Accurately determining the size and phase angle of needed correction masses requires at least three runs. One is the "as is" condition, the second with a test mass in one plane, and the third with a test mass in anther correction plane.
Electric machine problems appear in two major forms:
1. mechanical, particularly rotor or bearing difficulties; and
2. electrical, mainly winding or electrical system problems.
Usually, bearing problems account for a large portion of reported electric machine issues. Other common difficulties involve misalignment, lubrication, excessive shaft loadings, and environmental issues (ice, snow or dirt).
Problems result from the gravitational center of the rotor assembly differing from the magnetic center. In this condition, the rotor continually hunts for the position it wants to run in. This can manifest itself in high axial vibrations. The phase and frequency of these vibrations may or may not prove to be synchronous. Solving this issue requires proper magnetic alignment.
If dampness in insulation is suspected as the result of shipment, storage or installation, measure the insulation resistance of the windings or other electrical components before the site commissioning.
Cleanliness of a machine's lubrication system is extremely important. So, during commissioning, flush the entire system using the same oil specified for lubrication. Any dirt or debris should be collected at the lubricant filter (or additional strainers) during flushing. Allow sufficient time (usually several days) for proper flushing.
The cleaning capacity of the lubricant is better at relatively high oil velocities and temperature and low viscosity. The transportation capacity of the lubricant is best when the oil is relatively thick at relatively low temperature and high viscosity. For proper lubrication system flushing, vary lubricant temperature within specified low and high limits several times.
MECHANICAL AND GAS SEALS
During commissioning and startup of a rotating machine, mechanical seal damage occurs particularly frequently. Modern mechanical seals usually come as cartridge units (pre-mounted) and most often don't require adjustment during machine assembly, installation or commissioning. Increased imperfections of concentricity and run-out in the shaft can lead to high vibrations and significantly decrease the service life of the seal. Use stainless steel pipes with sufficient cross-section for the entire seal network to ensure adequate supply of required clean seal fluid at any operating condition.
Carefully control gas seal leakage during startup and initial operation. The warning limit for leakage often is five times the expected (normal) leakage value (per sealing stage).
During operation, a dry gas seal needs a positive differential pressure (usually a minimum of 1–3 bar) to provide for sufficient seal cooling, and to be able to set suitable switch points for monitoring. Commissioning or starting up with too low a differential pressure may lead to seal damage from overheating, hang-up, wear or contacting sliding faces. Operational safety dictates limiting of the pressure change rate (most often to the range of 10–30 bar/minute).
Start supplying bearing oil only when the bearing sealing system is provided with separation gas.
Operation of a dry gas seal requires minimum values for machine speed. If the machine doesn't rotate, you must provide a sufficient differential pressure to ensure the sliding faces lift off statically.
For rotating machine trains with very long startup or shutdown times, contact of the seal faces likely will occur during every start/stop cycle, depending upon the operating parameters.
Foundation and mounting systems under rotating equipment often will deteriorate over time. When that happens, the dynamic forces or deflections in the system can exceed permitted levels. This can necessitate very costly foundation (or mounting) system repair and re-grouting if the plant can accept the downtime. Alternative approaches (such as shimming) that don't require machine downtime to repair (or renew) mounting systems have proven successful and recently become very popular.
The tight manufacturing clearances and complex geometry of many rotating machines make refurbishing difficult in the case of major damage or catastrophic component failure such as bearing destruction. Sometimes, the rotor may end up digging and melting into the casing. If bent, the rotor should be replaced. If not bent, refurbishing may be possible, but this usually is a difficult process. Spraying often can restore damaged sealing edges. Casing repair via machining is both hard and risky — welding repairs are even more so.
AMIN ALMASI is lead rotating equipment engineer at WorleyParsons Services Pty Ltd, Brisbane, Australia. E-mail him at Amin.Almasi@yahoo.com