Inherent Safety: Moderate
We continue our series of posts on the topic of Safety in Design. The first seven posts were,
The principles of inherent safety vary from organization to organization, but most descriptions of the topic are structured around the following five elements.
Eliminate
Substitute
Minimize
Moderate
Simplify
In this post we consider the fourth item in the above list: Moderate.
The Moderation step looks for opportunities to operate a facility at safer operating conditions. Moderation accepts that a certain condition exists but aims to reduce its impact. It is usually achieved by reducing operating pressures and temperatures.
Safety through modification is also achieved either by changing equipment, by increasing the spacing between equipment items, or by moving people away from the site of a potential incident. Another form of moderation would be to use a lower voltage of electricity for the pump motor. Doing so makes maintenance safer and reduces the chance of a fire-creating spark from being created.
Some checklist questions to consider for use with the ‘Moderate’ term include the following:
Can the system be operated at lower temperatures and pressures?
Are pipe diameters as low as possible so as to minimize the size of a leak?
Can the facility be located so as to minimize the impact of a release?
Can the facility be located so as to minimize the transportation of materials to and from it?
Can barges be used in preference to tank cars, and tank cars in preference to tank trucks?
Equipment Modification
It may be possible to modify equipment so as to reduce its inherent risk. For example, the dead head pressure of a pump may high enough to cause a seal failure. This risk can be reduced by replacing the pump’s impeller.
Spacing
An important aspect of moderation concerns the use of space between equipment items. Blast overpressures and concentrations of toxic gas from releases go down exponentially with distance. It is better to protect equipment items from the effects of an explosion by moving them away from one another than by putting a blast wall between them. If equipment items are spaced well apart there is also less risk of a confined vapor cloud forming with its potential for a very destructive explosion.
Usually it is not realistic to move major equipment items on facilities that are already built. But increased spacing should always be considered during the design stage.
Underground Location
For onshore facilities it is generally better to run utilities such as electrical cabling and instrument lines underground. This will protect them from fires and explosions (a particularly important consideration if some of the utility systems need to be kept in operation as part of the facility’s emergency response program). The protection of utilities is also important because it is often found that, following a major accident, large equipment items and piping are soon back in service, but it can take a long time to repair the damaged utility systems, particularly instrumentation runs.
Putting utilities underground does have drawbacks. There is an ‘out of sight, out of mind’ problem — buried systems may not be inspected and checked as those above ground. They may also be more subject to corrosion than if they were above ground. A sensible compromise is to place critical pipework below grade in open trenches.
