(osama mohammed elmardi suleiman khayal) #1

The interactions between the design and application of instrumentation and safety are many and diverse. The correct utilization of instrumentation for monitoring and control reduces risk. An obvious example is a fire detection and control system, but even a simple cistern control which prevents a water tank from overflowing affects

overall safety. Any instrumentation which contributes to maintaining the designed status of an installation can arguably affect safety. However, instrumentation can increase the danger in an installation, usually by being incorrectly designed or used. The principal direct risks from electrical instrumentation are electrocution and the possibility of causing a fire or explosion by interaction between the electricity and flammable materials, which range from various insulating materials used on cables to the more sensitive oxygen-enriched hydrogen atmosphere of a badly ventilated battery charging room. Some aspects of the safety of lasers and the risks from radiation are dealt with elsewhere in this reference book, Part 3, Chapters 21, 22, and 24. Toxic materials should also be considered (see Substances Hazardous to Health in the References). These risks pale into insignificance when compared with the full range of possibilities of misapplying instrumentation to a process plant, but nevertheless, in an overall safety analysis all risks must be minimized.

It is important to recognize that nowhere is absolute safety achievable, and that the aim is to achieve a socially acceptable level of safety. Quite what level has to be achieved is not well defined; it is perhaps sufficient to say that people are even more reluctant to be killed at work than elsewhere, and hence the level of safety must be higher than is generally accepted. For example, the risk level accepted by a young man riding a motorcycle for pleasure would not be acceptable to a process operator in a petrochemical plant. There are similar problems in determining how much financial expenditure is justified in achieving safety.

As well as the moral responsibilities implicit in not wishing to harm fellow mortals there are, in the majority of countries, strong legal sanctions, both civil and criminal, which can be used to encourage all designers to be careful. In the United Kingdom, the Health and Safety at Work Act 1974, together with the Electricity Regulations, provides a framework for prosecuting anyone who carelessly puts at risk any human being, including himself. (In the United States, the same functions derive from the Occupational Safety and Health Administration, part of the federal government, with similar agencies in each state and some municipal authorities.) The Act places responsibilities on manufacturers, users, and individuals in some considerable detail, and the requirements are applied in almost all circumstances which can conceivably be regarded as work. For example, manufacturers are required to sell only equipment which is safe for its intended use, test it to check that it is safe, provide adequate installation instructions and be aware of the "state of the art." The Act was derived from the Robins Report, which is a very readable, well-argued discussion document which sets a reasonable background to the whole subject of industrial safety. The Act lays great stress on the need to recognize, record, and evaluate levels of danger and the methods of reducing the risk to an acceptable level, and consequently, there is a

need for adequate documentation on the safety aspects of any installation. In the majority of installations the enforcing organization is the Factory Inspectorate, who have awesome powers to enter, inspect, and issue various levels of injunction to prevent hazards. Fortunately, the majority of factory inspectors recognize that they do

not have quite the infinite wisdom required to do their job, and proceed by a series of negotiated compromises to achieve a reasonable level of safety without having to resort to extreme measures. It is important to realize that the legal requirement in most installations is to take "adequate precautions." However, in the real world the use of certified equipment applied to the relevant British Standard Code of Practice is readily understood, easy to document, and defensible, and is consequently the solution most frequently adopted. In the United States, the National Electrical Code, promulgated by the National Fire Prevention Association, is the controlling set of specifications for electrical safety.

In addition, the reader is referred to ANSI/ISA standards as follows:

ANSI/ISA84.01-1966 "Application of Safety Instrumented Systems to the Process Industries".

ANSI/ISA91.01-1995 "Identification of Emergency Shutdown Systems & Controls That Are Critical to Maintain Safety in the Process Industries".

ANSI/ISA RP12.6-1995 “Recommended Practice for Hazardous (Classified) Locations…”.Safety.pdf (487.7% u)