|Mechanical Engineering Design - Year 2 - DSGN 215 and DSGN 221
1. Failure - General Introduction
Although this module deals primarily with preventing straightforward mechanical engineering failures, students should note that when safety critical
components and systems are being designed, particularly for applications in aerospace and the petrochemical industries, a much
more detailed assessment of possible failures is needed and carried out. This will often include a Failure Mode and Effects Analysis
(FMEA) which would be carried out on every component that could possibly have an effect on the functioning of the plant or system.
As an example, for a new petrochemical plant this would probably involve the assessment of every valve. The assessment is further
complicated by the fact that a valve that controls the passage of fluid will have several possible modes of failure, and the possible
outcomes of each of these modes of failure must be investigated. For example such a valve could fail by:
- seizing fully open
- seizing fully shut
- seizing in an intermediate position
- starting to leak
- if fitted with a position sensor, this could fail giving incorrect indication of position, possibly leading to alarms
and actions that made matters worse
The effects of each of these failures must be considered. One consequence of this is that unless a component is essential for
the function of the plant or system it is omitted from the design as this not only reduces risk of problems, but it also saves the cost
of purchasing, inspecting, installing and maintaining the component!
However systems will often contain many such components and control strategies / computer software (and emergency plans) must
include capabilities for handling all modes of failure and their effects on the system operation.
In the case of modern aircraft this involves multiple sensors, backup computer systems, software, power supplies and actuators.
Detailed operating procedures and risk assessments will be produced for aircraft, petrochemical and nuclear power plants. In the latter
two cases, risks to the environment will normally have to be considered.
Dr David J Grieve, 8th June 2009.