Practical Safety and Reliability Assessment

K.C. Hignett

• Publisher: Routledge
• Year: 2002
• ISBN:
  9781135816841
• Edition: 1
• Topic: Architecture
• Subtopic: Méthodes et matériel d'architecture

An integral part of any engineering or manufacturing process is a continuous process of assessing its safety and reliability. This work provides a guide to the practical application of safety and reliability principles wherever risk is a consideration. The theory and mathematics are kept to a minimum, whilst a practical working model of the technology is presented for everyone involved in general engineering disciplines. It reduces the high cost of using professional consultant practitioners, introduces an advanced methodology of common mode failure analysis and modelling, with potential savings on system capital costs, and provides an illustration of working principles by graded tutorial projects.

1
Terminologies in process safety engineering

1.1 PLANT SYSTEMS

1.1.1 Hazard states

Hazard states can be categorized as:

• dangerous process hazard states manifested by events which may lead to fatalities and/or injuries and/or plant system damage;
• safe process hazard states manifested by events which result solely in economic penalties due to loss of production consequent to enforced plant shutdown and when conditions implied by 'dangerous' are absent.

It should be recognized that those events which produce dangerous hazard states most often encompass monetary losses arising from loss of production, replacement cost of damaged plant and possible reparations to personnel.

1.1.2 Plant system demands

When considering safety, a demand is defined as a process system abnormality which, if allowed to proceed, would be expected to result in a dangerous process hazard state. Demands are normally assumed to be random or exponentially distributed with time.

1.1.3 Hazard and risk

The presence of any process abnormality which represents a dangerous or potentially dangerous state is known as a hazard. In high hazard scenarios, by way of example, it may refer to a detonation or perhaps a release of toxic materials. In lesser terms it may refer to, say an inadequate mixing of chemical constituents or build-up of dangerous byproducts which may lead to the high hazard. Potential hazard states, particularly in chemical processing, are most often identified by carrying out hazard and operational studies known as HAZOPs.

Risk normally refers to the statistical annual frequency of the particular hazard state. For example, once per year would be very high risk, whereas once in 10 000 years would be considered a relatively low risk.

To illustrate the subtle difference between hazard and risk, consider a sharp-pointed pencil in the context of a hazard whereby it is capable of causing serious injury to a personˊs eye. When lying on a table, it poses an insignificant risk. However, if it should be taken up and pointed closely towards the person's eye then the risk of injury will be significantly greater.

1.1.4 Consequence

The presence of a process hazard state does not represent the ensuing consequences which may result from the said hazard when it becomes established. For example, taking a detonation as a hazard state, the range of possible outcomes could typically depend on such factors as proximity to personnel, containment etc. A single hazard state most often results in multiple outcomes which range from the insignificant to major accident scenarios. They are normally evaluated by the use of event-tree logics, characterized by a single input hazard event which propagates through multiple paths to outputs representing the range of possible consequences.

1.2 SAFETY SYSTEMS

A safety or safeguards system may be defined as a system of logic which is designed to await and recognize the onset of a specific process demand with the objective of preventing a dangerous process hazard. The commonly accepted design philosophy of a safety system is that it should be independent of the process system to which it affords the required safeguard. For purposes of clarification, independence implies that safety functions should not be derived from those measurement and control systems which provide...