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Figure 1.8 Reciprocating gas compressor cylinders.

      There are several types of potential consequences associated with these four compressors. First, there is a significant economic penalty related to a process outage. The actual loss is equal to the length of the outrage times the length of the outage. A two-day outage will lose $100,000, while a 30-day outage represents a loss of $1,500,000. Second, there is a safety risk related to a major product release and subsequence fire scenario. Based on the existence of detailed emergency operating procedures and automatic isolation valving, the safety consequence level can be seen to be at medium. The third type of potential consequence is related to machinery damage. A catastrophic compressor failure could result in major damage and repairs costs. For example, a main bearing failure could potentially result in a crankshaft failure and even frame damage. If an undetected primary failure goes undetected, repair costs can escalate rapidly. In our example, since each compressor is rated at 1500 hp, we will rate the potential consequences of a major failure to be high.

      After assessing all the potential consequences, we have rated the process consequence at a medium to high level, the potential safety consequences at a medium level, and the potential consequences of machine damage at a high level.

History

      Our next step is to review the history of these compressors. During your review of the failure history, we discover that on average each compressor has experienced two valve replacements every year and therefore each one is down twice a year. A review of the valve failure data indicates that failures are time dependent, which suggests that valve monitoring may make sense as a way to mitigate the impact of a failure by allowing the process to better plan outages. There are no other significant compressor failure modes of interest. Furthermore, the synchronous electric motor drivers have demonstrated they can easily run between unit turnarounds without any need for maintenance. It is clear by reviewing the historical data that valve failures control the overall reliability of these machines. Interviews with operators indicate that there seems to be an acceptance that valve failures are the norm and that they have learned to live with the valve problems.

Safeguards

      A review of existing safeguards indicates:

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.