4. Non-operational consequences direct cost or repair with no possible secondary damages.
Hidden and Evident Failures. An evivent function is one whose failure will on its own eventually and inevitable become evident to the operating crew under normal circumstances. Protected device fails + protective device fails = multiple failures. A multiple failure only occurs if the protected function fails while the protective device is in a failed stated.
Failure consequences. Every failure has their set of consequences. It affects the output and production. It affects product quality or service, can shutdown a plant or part of it, others threaten safety and environment. Increase in operating cost, and have no effect at all if they occur. This combination of context, standards and effects means that every failure has a specific set of consequences associated with it, and if it is very serious, efforts should be done to prevent the failure.
Pro-active maintenance has much more to do with avoiding or reducing the consequences of failure that it has to do with preventing the failure themselves. Hence, pro-active tasks is worth doing it it deals successfully with the consequences of the failure which it is mean to prevent.
Categories of Evident Failures
1. Safety and environmental consequences. A failure mode has safety consequences it if causes a loss of function or other damage which could hurt or kill someone. A failure mode has environmental consequences it causes a loss of function or other damage which could lead to the breach of any know environmental standard or regulation. For failure modes which have safety or environmental consequences, a pro-active task is only worth doing if it reduces the probability of the failure to an acceptable level.
Here are some of the examples of the worst industrial accidents due to failures:
Chernobyl. The 1986 explosion at the Chernobyl nuclear power station was one of the worst industrial accidents in human history. The WHO estimated that the accident released 200 times more radioactivity than was released by the atomic bombs dropped on Hiroshima and Nagasaki. In the first year after the accident, 400,000 people living near the plant had to be evacuated and many have never been allowed to return to their homes.
Bhopal India. In December of 1984, 40 tons of highly poisonous methyl isocyanate gas leaked out of the pesticide factory of Union Carbide in Bhopal. More than 6,400 died (many from the effects, several years after the disaster) and 30,000-40,000 were seriously injured in the worlds worst chemical disaster. Over 500,000 men, women and children were exposed to the poison clouds and at least six thousand people died within the first week of disaster. The current death toll is well over 16,000 and rising. Pregnant women were miscarried and babies were still born. The poison affected some people's lungs and nervous system.
Amoco Cadiz Oil Sill. On March 17,2978, oil tanker spills 223,000 tons of oil in the Atlantic Ocean killing 30,000 sea birds, 230 tons of crabs, fish and a loss of jobs for the fishermen. The ship lost its steering ability during a storm and ran around rocks which cause the tank to spill and later sank.
2. Consequences can be shut down the plant. Not only will operations shut down but may cause the plant partially or as a whole to be paralyzed and shut down. Cost of plant shutdown is not acceptable.
3. Operational consequences. Primary functions of most equipment in industry is connected in some way with the need to oil refining equipment earn revenue or to support revenue earning activities. A failure has oil press operation consequences if it has a direct effect on the operational capability. Operations can be affected in four ways; they affect the total output and increase production cost thru reworks and overtimes; they affect product quality, machine no longer hold manufacturing tolerances; they affect customer service due to late delivery of products; and they increase the operating cost in the additional to the direct cost of repair.
4. Non-operational consequences. The consequences of an evident failure which has no direct effect on safety, environment or operational capability are classified as non-operational cost. Points concerning non-operational consequences will be secondary damage and stand-by or redundant components.
The objective of a maintenance program for a hidden function is to prevent or at least reduce the probability of the associated multiple failure. Protective devices works in five ways; alert the worker of abnormal conditions; shutdown the equipment in event of failure; eliminate or relieve abnormal conditions which follow a failure and cause more serious damage; take over from a function which had failed; and prevent dangerous situations from arising.
Interference Engines and Timing Belts