of hours of work without any incidents. It is the combination of circumstances that matter, so one must view this in context. Because we cannot eliminate night shift work, especially in continuous process plants, we have to try to understand the risks, so that we can take suitable steps to minimize them.
A factor affecting sleep cycles is the way we arrange shift patterns. Lardner and Miles6 have explained why some shift patterns are superior to others from an ergonomic point of view. They propose a nine day cycle, with 2 days each in the morning, afternoon, and night shifts, with a 3 day ‘weekend’ following the night shift. The weekend may turn out to be in the middle of the week. They argue that this pattern is superior to the alternative 28 day cycle, which is quite common. The 28 day cycle consists of 7 night shifts and 7 evening shifts, followed by a 2 day weekend after each block. This sequence followed by 7 morning shifts and a 3 day weekend.
Human errors occur due to a number of reasons, and lack of knowledge and experience are not necessarily the most common. Motivation and morale are often key issues to manage. Pride in work, a sense of being wanted, and being treated fairly are all important considerations. We all want user-friendly software; similarly, workers appreciate managers who are people friendly.When this is so, we are likely to experience lower absenteeism or sickness, better participation in team effort and suggestion schemes, lower accident rates, and higher productivity.
What makes human beings distinctly different from machines is their ability to think, often in a very creative manner. Feelings and emotions change the way a person responds to identical stimuli over time, and makes it hard to predict behavior. We have provided a brief introduction to the subject in this chapter and readers can refer to Lorenzo’s excellent guide for a more detailed discussion. A check-list of potential causes of human errors is available in Appendix 4-1.
We began this chapter by defining failure in relation to the required performance standards. Failures can be critical (causing total loss of function), degraded (where the loss is partial), or incipient (where progressive deterioration has commenced, but will take some time before there is loss of function). We note the significance of the operating context, and how this explains why identical items of equipment perform differently. We saw how failures themselves provided a means of control of the process.
Our next topic was the role of maintenance in achieving the desired equipment performance. We discussed incipiency, and its use in conditionbased maintenance, using some common examples to illustrate the concepts. Thereafter, we discussed age-related failures.
Finally, we looked at human errors, perhaps the most complex issue relating to failures. We noted that there is an optimum level of stress required to keep human errors as low as possible. The work done by experts on sleep cycles shows us how they can affect the body’s natural rhythm. The experts state that some shift patterns are superior to others when planning 24-hour coverage for continuous process plants.
Feelings and emotions play a major role in affecting the way people react to situations. Therefore, managers have to focus on motivation and morale, which are key issues in minimizing human failures.
REFERENCES
1.Resnikoff, H.L. 1978. Mathematical Aspects of Reliability Centered Maintenance. Dolby Access Press.
2.Nowlan, F.S., and H.F.Heap. 1978. Reliability-Centered Maintenance.U.S.Department of Defense.Unclassified, MDA 903-75-C-0349.
3.Lorenzo, D.K. 2001. A Manager’s Guide to Reducing Human Errors: Improving Human Performance in the Process Industries.API Publication 770.
4.Swain, A.D., and H.E. Guttmann. 1983. Handbookof Human Reliability Analysis with Emphasis on Nuclear Power PlantApplications. MJREG/CR1278FSAND80200. 334.
5.Bransby, M., and J. Jenkinson. 1998. “How Alarming!” The Chemical Engineer. January 15: 16.
6.Lardner, R., and R. Miles. 1997. “Better Shift Systems.” The September 11: 28.
7.Moubray, J. 2001. Reliability-Centered Maintenance. Industrial Press, Inc. ISBN: 0831131462.
ERROR PRONE SITUATIONS
Reproduced courtesy of the American Petroleum Institute (see Reference 3 above). A check-list of work situations that could lead to human errors is listed below, based on Lorenzo.
•incomplete, inadequate, out of date, or non-existent procedures
•poor or misleading instrumentation
•lack of competence and knowledge
•conflicting priorities, especially between safety and production
•poor labeling
•inadequate feedback
•non-enforcement of policies and procedures
•excessive spurious trips, causing protective instruments to be defeated
•poor communications
•unsatisfactory plant layout
•control systems that are over-sensitive
•mental overload during emergencies
•error prone situations, typically with excessive manual operations, inadequate interlocks, or wrong use of interchangeable fittings
•improper tools and test equipment
•poor housekeeping
•excessive demand on operator vigilance
•software or control hardware faults
•poor ergonomics
LifeCycle Aspects of Risks inProcess Plants
Every process plant goes through its design, construction, commissioning, operating, and decommissioning phases. In this book, the term process plant covers any plant that uses the production or distribution process as defined in Chapter 1. It includes, for example, utility companies, paper and steel mills, and transport companies. As long as the product or service handled is physical, the principles are applicable to all of these plants. We can minimize the risks associated with each of these phases when we know the contributing causes. In this chapter, we will focus our attention on these life cycle risks, and cover the following areas:
•Quality of design and intrinsic reliability of the plant;
•Importance of simplicity in designs;
