Trend analysis - interpreting the results

Oct. 11, 2006
The question is, how do you get more production out of the equipment you have? The company's budget is tight. You just can't get approval for the new or back-up equipment needed to ensure production schedules are met.

The question is, how do you get more production out of the equipment you have? The company's budget is tight. You just can't get approval for the new or back-up equipment needed to ensure production schedules are met.

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Increasingly, companies use trend analysis to maximize equipment performance and minimize unscheduled down time. Oil analysis and vibration analysis are examples of two services meeting this need today. While there are differences between the two in data acquisition and data application, there are also some important similarities. For the purposes of this article, the examples illustrate trending of oil analysis results on a rotary screw compressor. However, the general concept presented works equally well for vibration analysis.

Isn't there an easier way? After all, promises of new high tech equipment that result in quicker analysis or even eliminate the need for trend analysis permeate the marketplace. Some of these methods attempt to replace trend analysis by taking a single sample and comparing it to a generic data base specially compiled by the equipment manufacturer. These databases might consist of the manufacturers specifications and characteristics for individual components.

Before selecting a quicker, cheaper, and perhaps less effective method, consider this. The power of trend analysis is the ability to observe how equipment operates and how it changes over time. The trend data you gather does not apply to any piece of equipment--it is on your piece of equipment, sitting in your facility, running in your environment. The data you acquire is unique in many ways and cannot be considered generic. Vibrations, heat, contaminants, and noise from adjacent equipment influence analysis results. If not taken into account these factors could lead to unnecessary repairs or increased maintenance intervals that bleed precious operating budgets dry. It is the ability to treat each piece of equipment uniquely that convinces many companies to optimize their maintenance program by incorporating a quality trend analysis program.

The cost

The cost to analyze an oil sample ranges from 20 to 40 dollars. A visit from your local vibration specialist typically ranges from 300 to 1,000 dollars. Two to four analyses a year are usually sufficient for typical industrial environments.

While analysis bills can add up over the year, analysis that prevents just one unscheduled repair justifies its cost easily. Then, improved operating efficiency and elongated equipment life becomes icing on the cake.

Taking the sample

While much has been published on taking samples, the importance of consistent samplings is worth emphasizing. Determine how to be properly take the samples. Usually the analysis facility or the equipment manufacturer can provide a recommended sampling protocol.

Write a brief, easy to read procedure for taking samples. Sampling consistency is the single most important step in trend analysis. By reducing the sampling procedure to writing you ensure that future samples will be taken in a manner consistent with the previous samples.

Take the first few samplings and establish a baseline data for comparison to future data. Without a baseline, it is nearly impossible to know if future analysis results indicate a problem.

File the data properly for future reference. Make sure your filing system allows you to find this data for years to come. Without the equipment's history, trend analysis is rendered virtually useless.

Make sure that future samples are taken using the same procedure.

Reviewing the results

Once you receive the analysis results, it is not enough to just look at the values. Even with proper sampling, the results from a single sampling can be misleading. For example, an oil analysis report may reveal high levels of iron, lead, and other metals indicative of wear. However, the metals may be from the equipment's environment and not wear. It is also possible that the sample was contaminated before the lab analyzed it. Then, the lab could have made an error. There are many possibilities. This ambiguity is the weakness of scheduling maintenance or repair work based on a single analysis. Often, it may be reasonable to re-sample and rule out possible errors.

The ability of trend analysis to build a history dampens one's reaction to a single analysis result. If an analysis report indicates a jump compared to previous samples, a certain amount of healthy skepticism prevents unnecessary repairs. In short, comparing the results to historical data improves the effectiveness of analysis dramatically. Trend analysis makes it easier to predict future samples and to react properly to unusual analysis results.

When reviewing analysis results consider the following. Compare each analysis result to the previous results as well as to the baseline analysis. The better labs provide the equipment's previous analysis results on the current report for easy comparison. While this can be useful, some caution is advisable. Check against your own data as well. The analyzing facility may not be aware of changes, repairs, or maintenance you have performed. Obviously, consider such changes when evaluating the analysis results.

Note any trends that form. Trend analysis is best described as an observation of changes. Often, the actual levels are inconclusive unless compared to previous analyses. For example, multiple analyses on a compressor's lubricant shows that acids (reported as total acid number) are forming in the oil. Low levels of various metals, dirt, and acids are considered normal.

If noted trends do not suggest the equipment is in imminent danger, select the next sampling date. For compressors operating around the clock, a minimum of four samples per year is fairly routine. However, if sample levels are of concern (but do not require immediate action), additional samples at more frequent intervals make sense.

In our earlier example, the acid level in the oil appeared to be increasing. However, the level was still considered to be acceptable. Since the levels may continue to build to damaging levels, it is wise, in this case, to take another sample a little sooner then originally planned.

Taking action

How do you know when to take action? Experience is the best teacher. While many experts may be able to help, no one will know how your equipment operates in your facility better then you. As a start, talk to the laboratory, a specialist, or the equipment manufacturer. Find which areas are of concern. Eventually, you will know which data is significant and when action is required. You will become the expert on your equipment.

If trends suggest the equipment is in imminent danger, schedule the equipment's repair. Continuing with our example, subsequent analyses revealed that the acid level continued to build. Note that there are now elevated levels of copper, zinc, and iron, as well as further increase in the total acid number. Since there are brass and iron components in the compressor, this may suggest the early stages of acid damage to the compressor's internal components. Apparently, we should not allow the acid level to build to this level in the future. To eliminate the acid right now, we changed the compressor's oil. A brief inspection suggested additional component replacement was not necessary. We filed a note recommending that the total acid number not be allowed to increase above 3. This should eliminate the possibility of future acid damage to internal parts.

As you make repairs, give careful attention to possible causes. In our example, we determined that the components in the compressor or the compressor's lubricant are not creating acids. Therefore, acids must be coming from the intake air. We noted this observation on the analysis report.

If maintaining acceptable analysis levels requires frequent repair or maintenance, review possible causes noted during previous repairs. While defective parts are always a possibility, repeated failures or unusually frequent maintenance are often preventable. The effectiveness of a modification can then be monitored during subsequent samplings.

In our example, further action was necessary to extend the life of the lubricant and, perhaps, the compressor itself. In an attempt to eliminate acid from the compressor's intake, we installed a duct that allowed the compressor to bring in fresh outside air. Subsequent oil analyses revealed that the acid levels changed only slightly for the duration of the lubricant's life.

The cost of not properly maintaining equipment gets more expensive every year. Dwindling natural resources and increasing demand drives costs higher while company budgets become leaner. In the future, only the most efficient companies, capable of meeting customer's demands on time, can hope to survive. Trend analysis provides a low-cost tool to look into your equipment's future. Remember...

  • Take samples the same way every time.
  • Compare the results to previous analyses.
  • Take corrective actions to minimize repair frequencies and eliminate unscheduled down time.

These are the keys to effective trend analysis and are the way of the future.

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