Air Compressor Vibration Analysis

Vibration measurement is the most direct method for judging compressor operating condition. Get a machine's vibration data in hand, compare against standard limits, condition good or bad is immediately clear. Follow up with spectrum analysis, fault type can be pinpointed eight or nine times out of ten.

Diagnostics

Vibration Measurement

Vibration Measurement Locations

Measuring vibration requires all three directions: horizontal, vertical, axial. Measuring only one direction easily misses problems. Misalignment faults show up prominently in axial direction. Imbalance is more pronounced in radial. Three-direction data together for judgment is accurate.

On airend, focus on drive end bearing housing. Horizontal direction sensor mounted on side of bearing housing, axis perpendicular to rotor. Vertical direction on top. Axial on end face, sensor axis parallel to rotor. Non-drive end some manufacturers also measure, some figure drive end can represent overall condition and skip it.

Motor has more measurement points. Drive end and non-drive end both measured. Each position three directions, six points total. Motor non-drive end has the fan. Vibration at this position sometimes relates to fan dynamic balance. Looking only at drive end data will miss this type of problem.

Vibration Limit Standards

ISO 10816 is most commonly used vibration evaluation standard. Uses vibration velocity RMS value to grade, unit in/s RMS.

For 20 to 100 hp power range units, vibration velocity below 0.07 in/s is excellent. New machines with good bearing condition and high alignment precision can reach this level. 0.07 to 0.18 is good. Most normally operating units are in this range. 0.18 to 0.28 is acceptable range. Can continue running, gotta watch the trend. Over 0.28 is unacceptable. Keep running and bearing life will be shortened.

Large power units have relaxed limits. 100 to 400 hp units, below 0.11 counts as excellent. 0.11 to 0.28 good. 0.28 to 0.44 acceptable. Over 0.44 unacceptable. Rotor weight is big, same residual imbalance produces larger vibration response. That's why limits are relaxed. Some sites use small unit standards to judge large units. Everything tests as exceeding limits. Actually wrong standard was used.

Vibration acceleration metric mainly looks at high frequency, used to judge bearing condition. Velocity metric covers mid-frequency range, reflects overall machine vibration level. Use both metrics together. Velocity normal but acceleration elevated, usually bearing starting to have problems.

Vibration Frequency and Fault Correlation

Getting vibration data, just looking at amplitude isn't enough. Gotta do spectrum analysis. Time domain waveform becomes frequency domain plot. Each frequency component's amplitude is clear at a glance. Different faults look different on spectrum.

1x frequency vibration, most common cause is imbalance. Rotor mass distribution uneven, heavy side during rotation produces centrifugal force, wobbles once per revolution, frequency equals rotation frequency. 1x vibration increases with speed squared. Speed doubles, vibration amplitude quadruples. Shaft bow is also mainly 1x. Bent shaft is like built-in eccentricity, rotating effect is similar to imbalance. Distinguishing these two faults requires looking at phase. Imbalance phase is stable. Shaft bow phase changes with speed.

2x frequency vibration, high possibility pointing to alignment problems. Coupling angular misalignment, two shafts have an angle. Rotor wobbles up and down twice per revolution, produces 2x. Parallel misalignment is two shaft centerlines parallel but not coincident, also 2x characteristic. Bearing housing looseness also shows as 2x. Foot suspended or bolt loose, unit taps twice per revolution. Belt drive units also consider pulley eccentricity. Running drags whole drive end into periodic offset.

Bearing characteristic frequencies are high frequency components. Rolling bearings have four characteristic frequencies: outer race pass frequency, inner race pass frequency, ball spin frequency, cage frequency. Calculation formulas relate to bearing geometry parameters. Number of rolling elements, contact angle, pitch diameter, plug these in and can calculate. Spectrum showing these characteristic frequencies or their harmonics means corresponding component has damage. Outer race fault easiest to identify, frequency stays fixed. Inner race fault spectrum has running speed sidebands, because inner race rotates with rotor, damage point periodically enters and exits load zone.

Vibration Abnormality Handling Process

After measuring vibration, compare data against limit table, what to do is clear.

Within good range, no special action needed. Continue monitoring per original plan. Record data after each measurement. Over time plot trend chart. See if vibration is slowly climbing or staying stable. Machine with stable vibration at one level, bearing and rotor condition should be fine.

Entering acceptable range, need to pay attention. Monitoring interval shortened. Originally once a month changed to every two weeks or weekly. At this point need spectrum analysis. Just looking at total vibration value doesn't tell where problem is. Pull up spectrum, see if it's 1x high or bearing frequency high, have some idea. Maintenance plan can start being scheduled. Handle hidden issues during next planned shutdown.

Unacceptable range, must act immediately. First analyze cause. Spectrum data, phase data, temperature data, synthesize for judgment. Cause clear and not developing fast, can limit load operation and hold out until nearest shutdown window. Vibration still rising or spectrum shows bearing already has damage, shutdown for repair can't wait. After repair must re-measure. Vibration back to good range before resuming normal operation. Some sites don't measure after repair and just start up. Result is problem wasn't fully solved. Runs two days, stops again. Hassle.

Common Causes of Sudden Vibration Increase

Vibration suddenly jumps high during operation, few directions to focus investigation.

Coupling

Alignment Check

Bearing

Housing Inspection

Fastener looseness ranks first. Anchor bolts loose, whole machine stiffness drops, vibration energy that was being absorbed now all shows up. Coupling bolts loose is more direct. Drive system constraints changed, vibration characteristics change with it. When checking, tighten all visible bolts, especially those inside coupling guard.

Coupling rubber element aging is also very common. Rubber pads or spider in flexible couplings, over time they harden, crack, cushioning performance drops. Some directly break off a piece, each revolution the gap location is one impact. Open guard cover and look, rubber element damage is visible.

Bearing having problems, vibration will rise, and temperature is often abnormal too. Use stethoscope or listening rod on bearing housing to listen. Normal bearing is uniform swishing sound. Damaged bearing has periodic clicking or sharp whistling. IR gun measures bearing housing temp. 20-30°F higher than usual should raise alertness.

Belt drive units also check belt condition. Belt worn thin, edges fraying, tension loose, all make vibration rise. Multiple belt drive, one belt breaks, load distribution on others changes, vibration characteristics immediately different.

Inlet valve sticking causes airflow fluctuation, reflected in vibration as low frequency components rising. Inlet valve should open but can't, should close but won't seal, compressor intake surging, vibration bouncing up and down.

Recommended Vibration Monitoring Intervals

Monitoring interval depends on what stage equipment is at.

Trending

Data Collection

Schedule

Monitoring Intervals

New machine first month after commissioning, measure frequently. Weekly. Break-in period has higher probability of problems. Installation alignment has deviation, bearing has early defect, assembly stress not released, these issues most easily surface in first month. Break-in period data is also baseline for later comparison. Measuring detailed at this stage has benefits.

After stable operation, once a month is enough. Equipment condition stable, vibration level changes slowly, measuring too often won't find anything new. Enter each month's data, trend chart can show long-term changes. Over one or two year span, whether vibration is slowly climbing.

Old machines running over eight years, monitoring should be more frequent. Every two weeks. Mechanical wear accumulated to certain level, bearing clearance larger, rotor fit looser, seal leaking more, various small issues start popping up. More frequent monitoring can catch early signs, handle before fault develops.

Machines already showing vibration abnormality, measure every day. Keep measuring until vibration returns to normal. Purpose of intensive monitoring is tracking fault development speed. Vibration stable at certain level versus rising every day, two situations have completely different handling strategies. Stable can hold out and wait for shutdown window. Rising every day, schedule repair quickly.