If the operator is not physically present when the event takes place, is it an evident or hidden failure? For example, a pump seal may leak in a normally unattended unit. There will normally be some evidence of the leak, such as a pool of process liquid on the pump bed. Merely because the operator was not present and did not see it does not change the event from an evident to a hidden failure. If the operator had been present, the leak would have been obvious, and a second event is not necessary. The question is not whether a witness was present, but whether the consequence occurred at the same time as the failure. To identify a hidden failure a second event must take place, and unless this condition occurs, it is an evident failure. Thus the time the operator sees the failure is not an issue.
To revert to the earlier question of the brake lights, you know that at the time you inspected the vehicle it was road-worthy, and the lights were working. If you ask a friend to stand behind the automobile while you press the brake pedal, you will soon know the answer. This is an example of a test on an item subject to hidden failures.
4.1.5 Incipient failures
If the deterioration process is gradual, and takes place over a period of time, there is a point where we can just notice the start of deterioration. Incipiency is the point at which the onset of failure becomes detectable. As the deterioration progresses, there is a point when the performance is no longer acceptable. This is the point of functional failure. The incipiency interval is the time from onset of incipiency to functional failure. When the failures are evident and exhibit incipiency, it is possible to predict the timing of functional failures.
The operating context describes the physical environment in which the equipment operates, demands made on it and the details of how it is used. The way in which we operate equipment has a bearing on how it performs, and affects its rate of deterioration. How close to the duty point does it operate? What is the external environment in which it operates? Does the internal environment affect its performance? What is the loading roughness? Does it have an installed spare unit that can come on stream if it fails? If the net positive suction head (NPSH) available to a pump is just acceptable, is the suction piping alignment such that the spare pump has as much NPSH as the duty pump? The answers to these types of questions will help define the operating context.
To illustrate this concept, let us take the example of an automobile or bus, and examine how we use it. For the purpose of this discussion, consider the following two contrasting requirements:
•We use it for long distance travel, mainly on freeways (highways, autobahns, or motor-ways);
•We use it for city travel only.
In the first case, the vehicle operates in a steady state, generally at cruising speed for much of its operating life. So the vehicle is operating predominantly at constant loads, well below duty point and with a smooth loading. In the second case, there will be frequent starts and stops, and driving speeds will be changing most of the time. The load on the engine will be variable due to the rapid changes in speed. The fluctuating power requirement from the engine means there will be more wear on the main elements of the power transmission, such as the clutch and gearbox. One should expect that brakes, tires, and indicator lights will need more frequent replacement.
We now add the driver profile, and the situation becomes more complex, for example,
•The driver has many years of experience, and has a ‘clean’ license, or
•The driver received the (first) license three weeks ago, and has already had one accident.
Turning to driving styles, we know that some drivers like to accelerate rapidly and use brakes frequently. Some are fond of taking corners at high speeds. Others prefer to cruise at a steady pace most of the time, use brakes infrequently, and take corners on all four wheels! Assume that you are buying a used car, and have the following options. One car belongs to a person who drives at a steady 40 mph, accelerates gently, and uses brakes sparingly. The other car, identical in make, model, vintage, and miles on the clock, belongs to a person who comes in screeching round the corner and slams on the brakes. If the price of the two cars is about the same, which one do you choose? It is an easy call, and you will decide quite quickly. The example highlights the significance of loading roughness, which contributes greatly to wear and tear.
External factors are next on our list of variables. These include dust or sand in the air, road surface, and weather conditions. One can see that the differences in performance as a result of these factors can be quite important.
Each of the changes in operating context will affect different sub-systems or components differently. For example, demanding driving habits will result in accelerated wear and tear on brakes, clutches, and tires, whereas dusty conditions will clog up air and lubrication filters more frequently. In an industrial context, the situation is quite similar. People wonder why identical pieces of equipment in the same process service perform differently. They believe that a pump is a pump is a pump! When we examine the differences in operating context, the reasons for performance variations become evident. As in the case of the vehicle, the operating context is one of the most significant contributors to performance.
4.3 THE FEEDBACK CONTROL MODEL
Let us examine how the driver of a vehicle controls it. The driver’s eyes measure the position and attitude of the vehicle. These measurements are with respect to the edge of the road, other vehicles on the road, as well as any pedestrians who may be trying to cross the road. The change in position and attitude is being measured all the time. This information reaches the driver’s brain, which compares these measurements with acceptance standards. The brain calculates the rate of change in position and attitude, and checks them against the norms. The driver’s knowledge of the traffic regulations and past experience determines these acceptance criteria. The brain computes deviations from the norms, generating error signals. These signals initiate control actions, which are similar to those in section 1.4. The driver’s brain instructs the hands to move the steering wheel, or the foot to press the brake or accelerator pedals, so that the car remains in control.
Figure 4.1 Input signal, amplifier, output signal, and feedback loop.
Other control systems follow a similar process, whether the unit in question is a battle tank gun control or a chemical-process control system. Figure 4.1 shows a model illustrating the control mechanism.
Would it not be wonderful to have life without failure? The fewer the failures, the higher the reliability we can enjoy. A good designer tries hard to make the product or service as reliable as possible, within given economic and technical constraints.
A marble rolling along a smooth glass surface may roll on for a long time. However, controlling its movement can be difficult. Similarly, an astronaut doing a space-walk faces a handicap. In the absence of friction or gravity, it is very difficult to navigate, because the only way to do so is to use reaction forces, applying the principle of conservation of momentum. Thus a lack of resistance or opposition may make the process energy-efficient,but control is more difficult. One could extend this approach to explain why democracies are superior to dictatorships, or why market forces are better than price controls. Seen in this context, failures can be useful, as they help identify deviations from expected performance and, hence, the scope for improvement.
Failures