Designing out the risk

Box 7.9. Example of designing out the risk on a coastal engineering project... 

The hardest part of engineering risk assessment has turned out to be the prediction of the modes of failure. Serious accidents at nuclear installations, such as those at Three Mile Island or at Chernobyl, have been caused by modes of failure that had not been analysed at all. For example, the report of the Presidents Commission on the Accident at Three Mile Island (Presidents Commission, 1979, p9) highlighted that the concentration of the assessment process on more obvious large break scenarios meant that the eventual mode of failure, which was a result of a chain of a number of more minor events, was not even considered. Despite the use of significant resources in the design process, the risk assessment had been unable to characterize the complex system adequately, a system that was totally human-made and defined. In particular, the risk assessment process had not been able to identify modes of failure caused by humans involved in the operations of the reactor behaving in unexpected ways. 

If buying a more complex or custom-built machine the buyer should discuss their requirements with potential suppliers. For a custom-built piece of equipment, there is the opportunity to work with the supplier to design out the causes of now on agreeing the health and safety risks. 

Designers, such as architects, must ensure that their designs minimise the risks to persons carrying out the construction or cleaning work, and to persons who may be affected by the work. 

It has already been pointed out that a liquid even when subjected to simple atmospheric distillation may become superheated and then bump violently in consequence this danger is greatly increased during distillation under reduced pressure and therefore a specially designed flask, known as a Claisen flask, is used to decrease the risk of superheating. In Fig. i2(a) a Claisen flask D is shown, fitted up as part of one of the simplest types of vacuum-distillation apparatus. ... 

Designing vessels for full vacuum eliminates the risk of vessel collapse due to vacuum. Many storage and transport vessels have been imploded by pumping material out with the vents closed. 

This calculation assumes, of course, that corrosion is uniform. Finally, implicit in the design will be boundary conditions on the way the plant can be run, outside of which the risk of corrosion is high. These should be clearly set out in the operating manual for the plant. 

Errors in design and changes in operation will occur which increase the risk of corrosion. Corrosion-monitoring systems give advance warning and enable remedial measures to be worked out and adopted. 

After obtaining the prototypes, tests must be made to determine the utility. Generally these include a short time destructive test to determine the strength and to check out the basic design. Another test that is done is to use the product in the projected environment with stress levels increased in a rational manner to make for an accelerated life test. Other tests may include consumer acceptance tests to determine what instructions in proper use are required, tests for potential safety hazards, electrical tests, self-extinguishing tests, and any others that the product requires. In the case of high risk products, the test program is continued even after the product enters service. 

There are several attractive features of dense phase conveying which warrant its use in some circumstances. In general, pipe wear is less and separation of the product is easier. KONRAD(79) has carried out a comprehensive review of the current state of the art. He points out that the technique is not yet well established and that there are considerable design difficulties, not least a satisfactory procedure for predicting pressure drop. The risk... 

Table 13.16 sets out the major waste disposal methods, and potential hazards from toxic waste deposition are indicated in Table 13.17. The range of precautions required at land tips depends upon the risk, e.g. the nature and degree of contamination and the work to be undertaken. It will, however, encompass personal protective equipment a high standard of personal hygiene enclosure, possibly pressurization, and regular cleaning of vehicle cabs vehicle washing facilities site security, and control of designated dirty areas. Air monitoring and medical surveillance may be required.

Another mechanism designed to distribute the financial burden of pharmaceuticals is the deductible ( >), the sum of money that patients are required to pay out of their own pocket before the effective insurance coverage is activated. In other words, patients must in all cases pay the first D monetaiy units. Above that figure, the insurer begins to assume the financial burden of the drug. In addition, maximum or minimum amounts are sometimes fixed to limit the risk of one of the parties. 

The inherent safety is the pursuit of designing hazards out of a process, as opposed to using engineering or procedural controls to mitigate risk. Therefore inherent safety strives to avoid and remove hazards rather than to control them by added-on systems. The inherent safety is best considered in the initial stages of design, when the choice of process route and concept is made. 

By contrast, the nature of certain accident scenarios could prove to be quite sensitive to some design parameters. It should not be ruled out during the risk assessment phase, especially during detailed design, that discoveries during consequence analysis could lead to the revision of the design basis of the facility or some equipment or components. 

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