Up Time Magazine - February/March 2009 - (Page 35) Spares Inventory Good Good In-Service Population Good Rogue Spares Inventory Good Rogue In-Service Population Good Good Good Good Good Rogue Good Good has ensured that they are sorted out to the most disadvantageous place in the asset management process – the spare inventory. In Figure 7, they have comprised 75% of the spare inventory. There are documented cases where the entire spare inventory had been replaced by rogue components. Rogue Component Effect As mentioned earlier, there are a number of facets of the organization that are affected by the development of rogue components. To illustrate the impact on Maintenance Effectiveness, the following scenario describes a real-life event: Figure 2 When one of the in-service components develops a rogue failure, it is removed and sent to the repair facility. Since the failure is not addressed by the standard bench test or overhaul procedure, it is not duplicated and resolved. It checks normally, scoring a “No Fault Found” (NFF) and returns to the spare inventory as depicted by Figure 3. Spares Inventory Good Rogue Good Good Good Good Figure 5 Through the “natural selection” process, only 50% of the spare inventory is comprised of truly serviceable components. Again, as long as the in-service population continues to operate normally, the new rogue component will simply join the first in the spare inventory. However, if the rogue failure is not recognized and resolved, Figure 6 demonstrates how the spare inventory can be severely compromised when another in-service component develops that unique failure. Spares Inventory Good Rogue Rogue Rogue “Real Life” Case in Point: There is a system that allows air to be vented to the atmosphere, comprised of an electromechanical control unit, sensing units A through C, a control feedback sensor and the vent valve. A system malfunction occurred that caused the vent valve to intermittently lock up in mid-position during high operational demands. The maintenance technicians could not duplicate the fault, so they replaced the control unit as it was the most likely part that could cause this problem. The problem repeated. Since the control unit did not resolve the problem, the vent valve was replaced, which required considerable system down time and maintenance resources. Now when the system operated during high demand periods, the valve intermittently modulated open and closed, when it should remain in a fixed position. Again, the problem could not be duplicated and since this new issue surfaced immediately after the installation of the valve, it was replaced again in the assumption that it was defective from stock. The system was down again for a considerable amount of time during this second replacement. The modulation problem repeated. Next, the control feedback sensor was replaced and again the problem repeated. It was suspected that there could be an intermittent fault in the interconnecting wiring, which might have been caused when the valve was replaced. Several maintenance technicians spent many hours checking the wiring for faults, finding no problems. In-Service Population Good In-Service Population Good Figure 3 As long as the in-service population operates normally, the rogue component will remain in the spare inventory. This rogue failure can begin to proliferate when the next in-service component ‘goes rogue’, as illustrated by Figure 4. Spares Inventory Good Rogue Rogue Good Good Good Good Good Figure 6 Like the other rogue components, when this new one is removed from service and sent to the repair facility, it scores NFF and returns to the spare inventory as shown in Figure 7. Spares Inventory Rogue Rogue Good Rogue Good Good In-Service Population Good In-Service Population Good Figure 4 This new rogue component is also removed from service and sent to the repair facility, and, like the first rogue component, it is also NFF and returned to the spare inventory, as shown in Figure 5. www.uptimemagazine.com Figure 7 Though the rogue components make up a very small part of the total component population, the “natural selection” process 35 http://www.uptimemagazine.com
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