Impact: LowEnvironmental: High Fire Risk: Low Safety Impact: Low
The controlling category is Environmental. Therefore, these pumps receive an overall criticality rating of High. It is recommended that they have permanent monitoring instrumentation and shutdowns installed (see Table 3.2 and follow the black path).
Example #2
There is a single multistage centrifugal pump that boosts kerosene flow from the crude oil processing unit to storage. If the pump is unavailable for service, the crude oil unit will have to reduce production by about 50%.
The highest risk is the potential of a greater than 24-hour loss of production if the pump shuts down unexpectedly. For this example, the pump receives the following criticality ratings:
Maintenance Impact: Low
Production Impact: Medium Environmental: Low Fire Risk: Low Safety Impact: Low
Because the pump is in a remote location, the recommendation is to install permanent monitoring instrumentation and shutdowns, and initiate weekly or daily operator inspections (see Table 3.2 and follow the grey line path).
Simplified Method of Economic Justification
In the business world, a detailed economic analysis is often required to determine if a machine is critical enough to warrant permanent monitoring. This type of analysis requires 1) an income stream, which contains the economic consequence of an undetected failure times the probability of the event occurring for each year of the analysis, and 2) the estimated cost of the monitoring system. Many electronic spreadsheets are available to assist you in performing this type of economic analysis. Every organization has its own hurdle for justification. However, in our experience, any monitoring project that has a payout of 2 years or less will probably be approved.
Here is a simplified method that can be used to justify condition monitoring hardware projects. First you need the following information:
•Annualized economic risk of the “do nothing” case
•Annualized economic risk of the improvement case
•Payback period required by management
•Probability of detecting the machine fault in its early stages
Table 3.2 Machinery Monitoring Recommendations Based on Criticality
Note: Machine monitoring typically includes vibration, bearing temperatures, RPMs, driver load, and process pressure and flow measurements, which can either be taken and recorded periodically or be monitored continuously, depending on the risks involved.
Economic risk is a product of two quantities: consequence and frequency (or probability). For justification, consequence is usually presented in monetary units. If a pump fails catastrophically, a potential consequence is $50,000. If this event occurs at a frequency of once every ten years, this represents an annual economic risk of $5,000/yr, but if it occurs every two years, the annual risk is $25,000/yr.
Assume the annualized economic risk for a given machine can be lowered from $30,000 to $5,000—with a certainty of 80 percent—by installing condition monitoring hardware. This scenario represents a project income of ($30,000 – $5,000)/yr ×.80 = $20,000/yr. If management requires a two-year simple payback, then we can justify $20,000 × 2 ($40,000) for a condition monitoring project. If we go on to assume the installed cost is twice the cost of the basic hardware, we know we can only justify the purchase of $20,000 in hardware.
Let’s explore a simplified justification formula. To use this formula, we need to know a) the estimated probable maximum loss (PML) per failure event without monitoring hardware and b) the estimated probable maximum loss (PMLCM) per failure with the proposed monitoring hardware. (Probable maximum loss events are the worst losses that can be expected, not the worst losses that can be imagined.) Starting with these assumptions, we propose the maximum amount of condition monitoring equipment justified (CM) is determined by the following formula:
where:
1.CM is the maximum amount of condition monitoring equipment justified.
2.PML is the probable maximum loss predicted without condition monitoring. (Include probable losses due to fire and environmental release fines, etc.) The equipment OEM is one good source for this information.
3.PMLCM is the probable maximum loss predicted with condition monitoring. Again, the equipment OEM is one good source for this information.
4.D is the probability of detecting the machine fault in the primary state.
5.tPB is the payback period required by management
6.TPML is the expected time between PML events. Use five years for unproven, isolated, or severe service machines; ten years for typical industrial duty machines; and 20 years for proven, light-duty service machines.
7.Sum these terms for all components in the machine train to determine the total amount CM equipment justified.
Consider a spare pump example involving two 500-hp spare pumps. No production loss is associated with pump failures. If we consider only secondary damage due to an undetected primary failure, we decide (PML – PMLCM) is $75,000. Now assume this installation is an isolated one. Therefore, select a TPML of five years, a D of 75 percent, and a management-required payback of two years. This analysis will lead us to the conclusion that an investment in CM hardware of up to [($75,000 × 0.75 × 2) / 10] = $11,250 is justified.
Additional Analysis Notes
•In cases where multiple machines can be monitored with a common monitoring system, first calculate the CM value for each machine in the area, then sum the values to determine the total amount of condition monitoring equipment justified.
•If a permanent system cannot be justified, consider a walk-around monitoring approach.
•More condition monitoring hardware may be justified if a more detailed risk assessment is performed.
Once the maximum amount of condition monitoring equipment justified is determined, work with the condition monitoring hardware provider to determine how to best spend this money. The types of condition monitoring hardware will depend on the answers to the following questions:
1.What types of machines are going to be monitored? Compressors, pumps, gearbox, etc.?
2.What are most predominant machine failure modes? Bearings, seals, etc.?
3.What bearing types do you have? Rolling element bearings (REB) or hydrodynamic?
4.What is the transmissibility ratio at the bearings, i.e., the ratio of casing vibration to shaft vibration?
5.What
