current compressor condition monitoring and lubrication programs seems to be active and effective
There is proper management of all safety and reliability programs for these compressors
Conclusion
If we assume each valve failure leads to a two-day compressor outage, then the current consequence level is $50,000 per day times 2 days per outage, equals $100,000 per event, which we will consider a medium consequence level. Because there is a medium consequence level for a process outage and that compressors are failing at a rate of twice a year, we can conclude the current risk level is high (see cell with dashed oval in Table 1.4). It appears that the best way to reduce the risk level is to reduce the frequency of valve failures. Reducing the valve failure frequency from several times a year to once every 1 to 10 years moves the risk level from high to low.
Table 1.4 Risk matrix for machinery reliability assessment example.
Note: Installing an additional compressor to provide spare capacity is another way to reduce the overall risk level. However, a fifth compressor does not appear to be the most cost-effective solution to the current reliability problems. I will leave to the reader to analyze the pros and cons of installing a fifth compressor versus improving the reliability of the individual compressors.
In this example, the major recommendation is to review the present compressor valve designs to better understand why they are unreliable. First, the site needs to identify the root cause of the valve failures. Perhaps the failures are due to materials of construction, some type of gas contamination, or even the basic valve design. Every effort must be made to improve the service lives of these valves. Improving the reliability of these valves should be top priority for the site until their reliability of these four compressors are improved to acceptable levels.
Closing Remarks
I hope readers now have some understanding of how machinery professionals approach reliability reviews. We have discussed the three facets of machinery reliability assessments: A review of machinery criticality, equipment history, and safeguards. These three aspects of a machine are required to obtain a snapshot of the current state of reliability. After determining the criticality of a machine, we must evaluate the cause-and-effect relationships between adverse machine conditions and the various failure modes. We need to ask ourselves: Are these cause-and-effect relationships understood and repeatable? If they are, then we can begin to design proper safeguards to avoid future failures as a way of improving availability. For example, if we know that a low pump flow causes high vibration due to internal recirculation, which in turn causes a mechanical seal failure, then we may need to consider an automatic flow spillback valve to preclude low flow conditions. Keep in mind that every safeguard must be cost justified, i.e., the cost of each safeguard must return value in the form of reduced risk associated with the improvement.
Reliable operation requires: 1) proper physical safeguards, such as surge control, temperatures and vibration monitors, load monitors, speed sensors, etc., to prevent and detect unwanted operating conditions, 2) well-written operating procedures, and 3) judicious design improvements. We must be ever vigilant if we wish to keep our rotating machinery out of harm’s way and operating reliably by avoiding undesirable operating conditions at all stages of a machine’s lifetime. Before writing compressor operating and monitoring procedures, you need to clearly understand your machinery’s operating limits. If in doubt, talk to the original equipment manufacturer about any concerns you may have. Always try to be conservative when setting operating limits. If you faithfully protect your process machinery with effective safeguards, you will be rewarded with many years of safe and reliable service.
2
Useful Analysis Tools for Tracking Machinery Reliability
By Robert X. Perez
Figure 2.1 Machinery reliability metrics are essential to ensure that a site’s reliability efforts are effective.
Reliable machinery is essential to realizing efficient and profitable plant operations. Most sites achieve world-class reliability through steady and continuous improvements in design, repair, and operating procedures. If we want to continue to improve the mechanical reliability of our machinery, we need to adopt and then maintain metrics and reporting tools that will help track and analyze how your equipment is doing with respect to reliability (Figure 2.1). Carefully selected machinery reliability metrics and reports can:
1 Assist in identifying problem areas or applications.
2 Assist in identifying significant changes in reliability performance.
3 Document the impact of design improvements.
4 Document the impact of reliability programs.
5 Provide visual trend of machinery failures.
6 Allow us to compare the reliability performance of different sites and benchmark your overall performance with industry data.
7 Assist users in making economic evaluations of design modifications.
8 Assist in evaluating maintenance department productivity.
9 Assist in maintenance budgeting and planning.
When we talk about machinery reliability tracking, we typically break down the discussion into spared machinery and unspared, or critical machinery categories. Spared machines are less critical machine installations that have installed backup units (see Figure 2.2) to ensure the process is not interrupted in the event of one machine failure. On the other hand, critical machines are unspared machines that are essential to the operation of the process. If critical machines fail or shut down for an extended period of time, the entire process has to shut down, resulting in significant economic losses.
Figure 2.2 Main and spare pump arrangement is used to ensure reliability pumping service. If one pump fails, there is a standby pump ready for service.
Because these machinery categories expose the plant to different types of risks, their reliability performance is measured and reported differently:
1 1. Spared machines usually make up 90 to 95% of the plant population, which means they tend to consume most of the day-to-day maintenance resources. Therefore, the primary aim of tracking the reliability performance of spared machinery is to control maintenance resources, i.e., costs.
