According to many sources, maintenance cost avoidance can be built into a compressor specification.13 Maintenance cost avoidance should be confirmed during the pre-purchase machinery quality assessment (MQA). At all times, specifying, purchasing, installing and operating reliable compressors makes more sense than buying compressors based on faith, past reputation or the lowest bid.
When purchasing new compressors, the best available solutions are minimizing risks as part of the process to specify, design, build and install the machines. It is possible to design out the need for frequent or specific types of maintenance. Remember: The most desirable compressor is designed for uptime extension, low failure risk and the lowest possible lifecycle costs. On equipment that is already in service, maintenance must seek the next-best available solution. Such approach mandates that every maintenance intervention is viewed as an opportunity to upgrade. If upgrading is feasible, it should also be a cost-effective solution. An upgrading program would result in the systematic reliability improvement for previously weak links in facility-wide operating equipment.
| Fig. 1. Compressor overhaul activities. |
Photo courtesy of Julian Hanks, PE.
A prominent source attributes that 13% of all turbocompressor failures are due to errors or omissions in condition monitoring and maintenance.3 With advances made in monitoring technology and modern operating and maintenance practices, we would assume that this general number is lower with time. What are good monitoring and maintenance practices around turbocompressors? Compressor condition monitoring has these components:
- Proper response to supervisory instrumentation, such as alarms and trips
- Periodic observation and evaluation of operating parameters, such as the compressors physical condition and its performance efficiency. This would include measuring and judging the rate of deterioration of mechanical and performance conditions for input into maintenance plans. Vibration analysis and aerodynamic performance calculations are conducted. Daily visual inspections are done by operators and are structured on the principles of operator-driven reliability (ODR).2
- Evaluation of operating trends. This should include auxiliary systems, such as lubrication and seal-oil consoles, compressor online washing facilities and dry-gas seal support systems.
- Periodic testing of lubrication and seal oils. Six basic analyses and testing are: appearance, dissolved water content, flashpoint, viscosity, determining the total acid number (TAN) and additive content.3
- Periodic testing of emergency safety and shutdown devices (ESD) and other fail-to-danger components, such as exercising the compressors surge control valve loop and the trip and throttle (T&T) valve on steam-turbine-driven compressor trains
- Data logging and automated record-keeping, such as the number of unplanned trips per train per year as a basic indication of compressor reliability
- Diagnosis of problems, rating the severity of the problem and remedial action applied
- Remedial action and execution planning
- Corrective measures that should preferably be applied onstream to reduce the impact on compressor availability. Online flushing (washing) is a good example.
In general, turbocompressors have maintenance inspections, overhauls and repairs (MIO&R); these are also known as inspection and repair downtime (IRD). These terms are used interchangeably with turnarounds. MIO&R or IRD events are scheduled in periodic intervals ranging from 2 years to 10 years, depending on the service. For clean services in the hydrocarbon processing industry (HPI), maintenance intervals of 6 years to 10 years are not uncommon. The extent of MIO&R efforts ranges from simple bearing inspections to opening the compressor and replacing the rotor with a spare rotor. Used rotors are examined for rubs at labyrinth seal locations and for fissures and cracks located around the impeller eyes on radial compressors. On axial compressors, moving and stationary blades receive thorough attention. In all cases, nondestructive test (NDT) procedures are applied.
As the scheduled compressor turnaround approaches, it is best practice to carefully review the machines operating and maintenance history. If there are any defects noted at inspection, these questions should be asked:
- Are any of these defects repeat occurrences?
- If so, can they be expected at the next turnaround?
- What steps can be taken to eliminate them?
- What action should be taken at this time?
A thorough pre-turnaround review should be done to plan the work required. The review should consist of:
- An assessment of the compressors mechanical condition, solidly backed up by data
- A diligent review of the machines past history
- A well-documented performance check, with findings and recommendations accepted by operations, maintenance-technical and project-administrative personnel.
Preventive and predictive maintenance programs
Good- and poor-performing compressors must be maintained. The prevailing maintenance strategies applied are preventive maintenance (PM) or predictive maintenance (PdM). PM is time-based, whereas PdM has the goal of operating the compressor until defects start to develop and are discovered. For PdM, detection requires high-quality monitoring methods; these tools have a cost factor. Qualified personnel are needed to perform ongoing monitoring efforts with great precision. Management expects PdM to determine when a failure will occur and then plan an outage accordingly.
Certain state-of-the-art predictive routines can be used to minimize the impact of a premature failure, or to understand when a machine drifts into off-design operation. But cost savings always come back to the knowledge factor. None of the various PdM judgments can be made without experience. Real depth of experience and wisdom is needed when several seemingly minor deviations occur and converge.
PM encompasses periodic inspections and applying remedial steps to avoid unanticipated breakdowns and production stoppages and to prevent detrimental machine, component and control-function failures. PdM, and to some extent PM, is the rapid detection and treatment of equipment abnormalities before they cause defects or losses. This is evident from considering lube oil changes. This routine could be labeled preventive if time-based, and predictive if done only when testing shows an abnormality in the lubricant. Without strong emphasis and an implemented PM program, plant effectiveness and reliable operations are greatly diminished.
In many HPI facilities or organizations, the maintenance function does not receive proper attention. Perhaps because it was performed as a mindless routine or has, on occasion, disturbed well-running equipment, the perception is that maintenance does not add value. This may lead management to conclude that the best maintenance program is the least-cost maintenance. Armed with this false perception, traditional processing plants have underemphasized preventive, corrective and routine maintenance. Many facilities have neglected to properly develop maintenance departments, pursue proper training programs for maintenance personnel, and optimize PdM programs. Many unforeseen compressor failures and safety hazards have resulted from not understanding what this is really all about.
Correctly executed, maintenance is not an insurance policy or a security blanket. It is a requirement for success. Without effective PM, equipment is certain to fail during operation. To be effective, maintenance must be selective. Note: Selective PM results in damage avoidance, whereas effective PdM allows existing or developing damage to be detected in time to plan an orderly shutdown.
Maintenance in best-practices plants (BBPs)
Four levels of effective compressor maintenance exist. Although there is some overlap, the levels of maintenance are:1
- Reactive or breakdown maintenance. This type of maintenance includes repairing equipment after it has failed, or, in other words, run-to-failure. It is unplanned, unsafe, undesirable and expensive. If the other types of maintenance are performed, usually it is avoidable.
- Selective PM. This maintenance approach includes lubrication and proactive repair. For example, the onstream lubrication of the admission valve control linkage on certain steam turbines should be done on a regular schedule. In this instance, anything else is unacceptably risky and inappropriate.
- Corrective maintenance. This includes adjusting or calibrating equipment. Corrective maintenance improves either the quality or the performance of the equipment. The need for corrective maintenance results from PM or PdM observations.
- PdM and proactive repair. PdM predicts potential problems by sensing operation of equipment. This type of maintenance monitors operations, diagnoses undesirable trends and pinpoints potential problems. In its simplest form, an operator hearing a sound change made by the equipment can predict a potential problem. This leads to either corrective or routine maintenance. Proactive repair is a repair based on a higher level of maintenance. This higher level determines that, if the repair does not take place, a breakdown will occur.
PdM instrumentation is available for both positive displacement and dynamic compressors. It has many forms and can be used continuously or intermittently. It is available for every conceivable type of machine. PdM instrumentation schemes range from basic, manual and elementary, to totally automatic and extremely sophisticated. Recommended instrumentation depends on the compressor size and the owners sparing philosophies. For example, a facility may opt to install three 50% machines, two 100% machines or perhaps only one 100% machine in a given service. Moreover, unless the value of downtime avoidance is quantified, it will not be possible to make firm recommendations for the most advantageous level of monitoring instrumentation, shutdown strategies, etc.
There are many competent manufacturers of manual monitoring equipment. Also, manual monitoring is often used on small air compressors. Advanced PdM onstream systems are used with large process compressors to continuously monitor vibration behavior. By gathering vibration data and comparing these data with normal operating conditions, both manual and continuous systems can predict and pinpoint the cause of a potential problem. The trouble is that detecting vibration is different from eliminating vibration.
An intelligent but highly selective PM program may lead to actions that prevent bearing distress and thus prevent vibration from occurring in the first place. A selective PM program may be more cost-effective than other strategies to detect defects before failures occur. This fact establishes that sweeping management edicts that disallow all manner of PM on compressors do not harmonize with the principles of asset preservation and best practices. Traditionally, industry has focused on breakdown maintenance, and, unfortunately, many plants still do. However, to minimize downtime and equipment unavailability, maintenance programs should focus on levels 2 through 4.
Emergency repairs should be minimized
Plant systems must be maintained at their maximum performance levels. To assist in achieving this goal, maintenance should include regular inspection, cleaning, adjustment, and repair of equipment and systems. Repair events must be viewed as opportunities to upgrade. In other words, the organization must know if upgrading the failed components and subsystems is feasible and cost-justified. Conversely, performing unnecessary maintenance and repair should be avoided. Breakdowns occur because of improper equipment operation or failure to do basic preventive functions. Overhauling equipment periodically when it is not required is an expensive luxury. Upgrading where the economics are favorable is absolutely necessary to sustain profitability.
Regardless of whether or not PdM routines have determined a deficiency, repairs done on an emergency basis are three times more costly in labor and parts than repairs conducted on a preplanned schedule. More difficult to calculate, but high nevertheless, are costs that include shutting down production or time and labor lost during the event. Bad as these consequences of poorly planned maintenance are, much worse is the negative impact from frequent breakdowns on total performance, including the subtle effect on worker morale, product quality and unit costs.
Effectiveness of selective PM
When used properly, selective PM can produce considerable savings. Sweeping, broad-brush maintenance, including the routine dismantling and re-assembling of compressors, is wasteful. It has been estimated that one out of every three dollars spent on broad-brush, time-based PM is wasted. A major overhaul facility reported that 60% of the hydraulic pumps sent in for rebuild had nothing wrong with them. This is a prime example of the disadvantage of performing maintenance to a schedule as opposed to the individual machines condition and needs.
However, when a selective PM program is developed and managed correctly, it is the most effective maintenance plan. The proof of success can be monitored and demonstrated in several ways, including:
- Improved plant availability
- Higher equipment reliability
- Better system performance or reduced operating and maintenance costs
- Improved safety.
A plant staffs immediate maintenance concern is to respond to equipment and system functional failures as quickly and safely as possible. Every maintenance event must be viewed as an opportunity to upgrade to avoid repeat failures. Good maintenance refers to relatively frequently scheduled work. Systematic upgrading will extend allowable intervals between shutdowns.
Know your existing program
The starting point for a successful long-term selective maintenance program is to obtain feedback regarding effectiveness of the existing program from personnel directly involved in maintenance-related tasks. Such information can provide answers to several key questions, and these answers will differ from machine to machine and plant to plant. Your in-plant data and existing repair records will provide most of the answers to seven questions listed in Table 1. A competent and field-wise consulting engineer will provide the rest.
Changes should not be considered in areas where existing procedures are working well, unless some compelling new information indicates a need for a change. In short, focus on known problem areas. To keep focus, continuity of information and proper activities relative to maintenance programs, some facilities assign responsibility for well-delineated plant systems to a knowledgeable staff person. All maintenance-related information, including design and operational activities relating to such a system, are funneled through this expert. The maintenance expert is responsible for refining maintenance procedures and reshaping the PM program into a selective PM system.
Problems associated with machine uptime and quality output will affect several functional areas. Many people, from plant managers to engineers and operators, make decisions and take actions that directly or indirectly affect machine performance. Production, engineering, purchasing and maintenance personnel, as well as outside vendors, use their own internal systems, processes, policies, procedures and practices to manage sections for the business enterprise. These organizational systems are interactive and dependently interact; yet, at times, these systems constrain efficiency and effectiveness for the company as a whole. Some constraints are appropriate; others can have disastrous consequences on equipment reliability.
Program objectives must be clearly defined. Table 2 lists the elements and objectives of an effective maintenance program. Following these general guidelines of Table 2 for centrifugal compressors will provide positive results.
What we have learned?
The main lesson is that one deviation alone might not be enough to bring on a compressor failure, but, when several more deviations combine, the failure risk increases exponentially. So, while it may be possible to avoid more serious failures by implementing an automatic compressor unloading scheme, or by adding bells and whistles that annunciate excessive temperatures and vibrations and seal deficiencies, there will never be any substitute for the human brain supplying both logical root-cause failure analysis and upfront failure prevention processes.
Achieving these up-front processes before calamities and finger-pointing occur requires both training and accepting accountability. The operator, supervisor or manager accepting a deviation from established practice should be motivated or compelled to understand the potential ramifications of bypassing or not following established guidance as documented in writing. As documentation requirements are enforced, fewer deviations are tolerated. Accept the initial incremental cost outlay needed to do things right. The apparent expenditure of time and money will ultimately bring rich rewards in safety, reliability and increased profitability.
If youre stuck with an existing compressor, view every maintenance event as an opportunity to ask if upgrading is feasible. If it can be shown to be cost-justified, then DO IT. Its your professional duty to the stakeholders. Stakeholders are not just investors; more importantly, they are employees, families and the community. HP
1 Bloch, H. P. and J. J. Hoefner, Reciprocating Compressors: Operation and Maintenance, Gulf Publishing Company, 1998.
2 American Petroleum Institute, API 617, 9th Ed.
3 Bloch, H. P. and F. K. Geitner, Compressors: How to Achieve High Reliability and Availability, McGraw-Hill, New York, 2012.
4 Allianz Insurance Handbook of Loss Prevention, Munich, Germany, 1984.
5 Geitner, F. K., Pipeline and Gas Technology, December 2005.
6 Bloch, H. P. and F. K. Geitner, Maximizing Machinery Uptime, Elsevier-Butterworth-Heinemann, Stoneham, Massachusetts, 2006.
Fred K. Geitner is a registered consulting engineer in Brights Grove, Ontario, Canada, engaged in process machinery consulting. He retired from Imperial Oil with 20 years of service that included a position as engineering associate with Esso Chemicals Canada, Inc. He teaches reliability improvement courses worldwide. Mr. Geitner graduated from the Technical University of Berlin, Germany, and has had extensive equipment selection and upgrading experience.