Technical Feasibility of Condition Monitoring

There are hundreds or maybe thousands of equipment in a plant, started from wiring, breaker, ducting, motor, pump etc. If we want to start Condition monitoring the question is what kind of equipment that i can put on my Condition Base Monitoring program.

These criteria below may give you some direction. The condition monitoring are technically feasible if;

1. Potential failure condition can be detected clearly. This could be increased vibration, temperature, noise, etc.
2. P-F interval is reasonably consistent. P-F interval is time between Potential failure (point where we know that something is failing) and Functional failure (point where an equipment has failed or unable to fulfill it’s function)
3. it is practical to have monitoring frequency at interval less than the P-F interval
4. the net P-F interval is long enough to be some use ( long enough to take action to prevent it from reaching functional failure)

Overhung fan balancing

The characteristic vibration of overhung fan is shown in figure 1.

Figure 1. Overhung Fan

Bearing B vibration is related with plane C and bearing A vibration is related with plane D.

Step for onsite balancing for overhung fan are as below:

1. Connect accelerometer in bearing A and perform single plane balancing on plane D or put your accelerometer on bearing B and perform single plane balancing on plane C. See vibration correlation between bearing and plane in figure 1.

2. If the vibration level on bearing A (if you choose bearing A on the first step) are acceptable, move your accelerometer to bearing B and measure vibration.

3. If the vibration on bearing B is not acceptable, then put weight on plane D that equal with trial weight in plane C but 180^o apart.

4. Perform static balancing on plane D till vibration is acceptable. Sometime it’s may needed to put equal trial weight on plane C on 180^o opposite when you put trial weight on plane D.

Water contamination on lubricant

Double suction centrifugal pump running at 1000 rpm with oil bath lubrication reported has noisy outboard bearing. Vibration data taken and shows an increase noise floor (figure 1).
It was also found a leak on gland packing that cause water spray out toward bearing casing. Increasing noise floor was suspected caused by improper lubrication, this was caused by water get into the bearing.
Recommendation was made to drain oil and inspect if the water has enter the bearing.

Figure 1. Vibration spectrum after water contamination

The suspicion that water has entered the bearing was true. They found some amount of water on the drained oil.
After flush bearing housing and change the oil, the pump was put back in service and noise floor was decreased. Vibration spectrum after and before oil replacement are shown in figure 2 .

Figure 2. Vibration spectrum after oil replacement

Unbalance

Unbalance is a condition where shaft geometric centerline and mass centerline do not coincide or the center of mass does not lie on the axis of rotation.

Vibration spectrum will be dominated at 1x running speed and the time waveform will sinusoidal for pure unbalance.

There are three types of unbalance condition :

1. Static unbalance : In this type of unbalance we have a heavy spot at a single point in the rotor, it will shows up even when the rotor is not running or if you put it on the frictionless bearings the rotor will turn so the heavy spot is at the lowest position.

The vibration signal at each end of the machine in the same direction will be in phase. There will be 90 ^o + 30 ^o phase different between horizontal and vertical direction.

Figure 1. Static unbalance

2. Couple Unbalance : A rotor with couple imbalance seems well balanced in static condition (the rotor not turn when placed on frictionless bearings). But it going to produce centrifugal force on the bearings when the rotor is rotating.

Vibration signal at each end of the machine taken at the same direction will be 180 ^o out of phase. There will be 90 ^o + 30 ^o phase different between horizontal and vertical direction.

figure 2. Couple unbalance

3. Dynamic Unbalance : is a combination of static and couple unbalance. Spectrum at 1x running speed will dominated overall vibration. The highest vibration usually at horizontal direction where the machine can move more freely. Vibration signal at each end of the machine taken at the same direction will be 0 ^o to 180 ^o. There will be 90 ^o + 40 ^o phase different between horizontal and vertical direction.

figure 3. Dynamic unbalance

Misalignment

Misalignment is a condition where the centerlines of coupled shafts do not coincide. If the misaligned shaft centerlines are parallel but not coincident, then the misalignment is said to be parallel misalignment. If the misaligned shafts meet at a point but are not parallel, then the misalignment is called angular misalignment. Almost all misalignment conditions of machines seen in practice are a combination of these two basic types.

Type of misalignment :

1. Parallel Misalignment : condition where the misaligned shaft centerlines are parallel but not coincident. This type of misalignment produce shear forces and bending moment on the coupled end of each shaft. Vibration are dominated at 1x and 2x running speed at radial direction and most often spectrum at 2x lf are higher. Axial vibration at 1x and 2x running speed will be low.

Figure 1. Parallel misalignment

2. Angular Misalignment : condition where the misaligned shaft meet at a point but not parallel to each other. This misalignment produces a bending moment on each shaft. And generate high vibration at 1x and some vibration at 2x and 3x in the axial direction at both bearings. The vibration will be 180 degree out of phase across the coupling in the axial direction, and in phase in the radial direction.

Figure 2. Angular misalignment

Softfoot

Soft foot is a condition where you have uneven level of feet. This cause distortion on the machine frame as you tight or loosen the bolt. Effect of soft soft foot could be damaging for an equipment or at least decrease it’s operating life. Softfoot or casing distortion will cause several problems like :

1. Internal misalignment : Casing distortion will cause misalignment between the internal bearings. As the machine frame distorted due to softfoot, the bearing move relative to each other and when this relative movement exceeds bearing tolerance it going to deflect the shaft . Every time the shaft turn there will be back-and-forth bending moment, after a period of time, the shaft may develop fatigue crack and fail.

2. Distorted Bearings : As the machine distorted, the shaft bearings are also distorted. The effect of this distortion, the bearing will have two load zones 180 ^o apart. Softfoot is one of the major causes of outer race distortion in properly installed bearing.

Softfoot problem on a motor can be identified by the appearance of spectrum on 2x lf. This is caused by uneven air gap between rotor and stator due to frame distortion.

Vibration Criteria for Lobe Blower

1. Units of measurement : Rotary lobe blower vibrations are measured in inches/sec. Measurements of spike energy is not recommended for judging blower condition because the rotary lobe blower has inherent impacting bearing loads.

2. Measurement location : vibration should be measured at bearing locations on the housing

These are guideline for assessing rotary lobe blowers rigidly mounted on the stiff foundations.

Unfiltered Vibration ( in/sec peak)
>0.62 thru 1.0 ( satisfactory)
>1.0 ( review required)

If the blower is operating at "review required" levels then the installation must be fully evaluated to determine the source or cause of vibration and the cause shall be corrected.

In general, blower vibration levels should be monitored on a regular basis and the vibration trend observed for progressive or sudden change in level.