The solenoid valve manufacturer provides an FMEDA report that lists “useful life = 12 years.” Useful life is a reliability engineering term that indicates a point in time when the failure rate of a device will begin increasing rather rapidly. Why do we care about this time interval? That means that any safety function equipment operated past that point in time will not achieve the designed risk reduction factor. The IEC 61511 standard states that our maintenance procedures must require that all equipment be replaced before the end of its useful life rating. OK, there is a good reason for this.
Then we ask the maintenance supervisor to sign the purchase requisition for the new device. First response is a question: “Do you know we had to reduce the budget this year?” Then another: “Please explain again why we need to purchase a new solenoid valve to replace one that hasn’t even failed yet? It may last for several more years.” It may not be easy to explain the loss of risk reduction and it is never easy to explain risk cost. But without that replacement, a higher risk cost is the result. Often the risk cost is much higher than the price of the new instrument so we better learn how to explain risk cost. There are times when I feel like asking: “Do you want this place to blow up?” But we all hope no accident ever happens and using a product beyond useful life only increases the chances of an accident, that usage does not guarantee an accident. Perhaps just remember, you must replace all products before the end of useful life in order to maintain a risk management program. One may challenge the useful life number by gathering good field failure records to prove a longer time interval but always replace the equipment on time.