Figure 1.7 A set of safeguards are designed to prevent major events from occurring or to mitigate their effects.
Table 1.3 Here is a risk matrix for a hypothetical process machine. In this table, risk is defined as the frequency of an event occurring times the consequence level of the event (Risk = λ x C). Therefore, the cells in the upper-left corner of the matrix represent the highest levels of risk, and the cells in the lower-right corner represent the lowest levels of risk. Reliability professionals should strive to lower risk levels to acceptable levels using procedures, machine improvements or other safeguards.
It is helpful to drill down to the failure mode level to understand the true nature of your risk. For example, if you have a mechanical seal that is failing more than one a year, causing a costly unit outage (high risk level), then you quickly realize you must improve the reliability of the seal. Other design efforts to reduce risk will likely be ineffective in significantly reducing the failure rate.
Layers of Machinery Protection
A set of safeguards should work together to provide multiple layers of protection (Figure 1.7) designed to eliminate failures and mitigate the effect of machinery failures. Here is a list of generic safeguards that can be used as a starting point for a review. As you go down the list, the consequences increase in severity. For example, items 1 through 4 represent normal operation, items 5 and 6 go into action if an upset has occurred and some type of automatic action is required for correction, and finally, items 7 through 9 are required when a product release has occurred.
You must decide if this list makes sense in your situation.
1 Detailed operating procedures are recommended in all cases. There should be specific operating procedure for start-ups, shutdowns, and normal operations.
2 A set of operating targets for flow, pressure, and temperature limits help keep your equipment away from unsafe operating conditions
3 Reliable process control systemsFlow, pressure, and speed control
4 An alarm and operator Intervention planPeriodic walk-through inspectionsMachine alarms (temp, vibration, speed, etc.) to alert the operating of an unsafe condition
5 Safety Instrumented systems (SIS) for high consequence events, such as:Overspeed trip systemsTemp and vibration monitoring system set to tripAutomatic surge control
6 Physical ProtectionRelief valves for overpressure eventsElectrical breakersMechanical overspeed trips
7 At this point of our hypothetical scenario, there has been a full loss of containment. The potential released volume and flammability of the liquid or gas released will dictate the severity of the event. Physical containments can be used for mitigation, such as:Isolation valves (with fusible links) & battery limit valvesDikesDitchesBarriers and walls
8 If the severity of a release is deemed great enough then a plant emergency response plan may be required.
9 If the severity of a release is deemed great enough then a community emergency response may be required.
When it comes to safeguards, the following adage fittingly applies: Plan for the worst but hope for the best.
Machinery Reliability Assessment Example
Background
Four 1200 rpm, 1500 hp, reciprocating, natural gas compressors (see Figure 1.8) are installed in parallel service. Each compressor is driven directly with a synchronous, electric motor. The gas processing facility needs all four compressors running to achieve design processing rates. If one of the compressors is down, the processing plant must cut back its throughput until it can be repaired and restarted. A process slowdown loses the plant about $50,000 per day when one compressor is unavailable.
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:
There are adequate operating procedures in place
Controls and safety systems are appropriate for the installation
Field instrumentation are appropriate for these compressors. Note: there are thermocouples installed on all the valve covers to detect valve failures.
The
