Facility layout study

Even before considering safety distances between different units or between a unit and a sensitive environment (residential, protected or industrial area), rules of good practice should be applied to the installation of equipment or industrial units.
Namely:

A savoir :

• Locate ignition sources and fuel sources, taking into account the direction of prevailing winds, to minimize the risk of ignition in the event of a fuel leakage.

• Properly arrange the units whose equipment generates hazardous areas and those whose equipment does not generate hazardous areas.

• Judiciously arrange fuel sources, taking into account the direction of prevailing winds to minimize the consequences of a potential fire.
• Limit areas of congestion and unnecessary obstacles to facilitate natural ventilation.
• Keep air inlets from non-hazardous equipment or buildings away from hazardous areas as far as possible.
• Adapt the design of buildings, especially if they are located in front of industrial units at risk.
• Keep explosion risk units as far away from technical and administrative buildings.
• Limit the release of valves to the atmosphere and favor relatively high discharge points to facilitate the dispersion of a gas cloud, for example.

• Install new equipment preferably in an open environment with natural ventilation, to minimize the risk of explosion.

• Position an industrial furnace or any equipment with air inlets (air compressor for example) appropriately with respect to the prevailing winds.

In addition to the above, other complementary measures may be considered, although not associated with installation rules, but which improve the safety of the facilities, such as:

• Install drip pans around rotating equipment if there is a risk of flammable or toxic liquid spilling.

• Provide a depressurization system connected to a flare network.

• Provide escape routes and at least one muster area.
• Provide access to equipment for maintenance.

• Provide a good drainage system to keep a sensitive or dangerous unit away from hazard.

  etc.

Although the general rules of good practice for the installation of industrial equipment and units have been applied, they do not ensure that the distances between the units or equipment and the areas neighboring the site are sufficient.

Indeed, moving a building away from a hazardous area does not guarantee that the distances are sufficient.

There are 2 main methods to define safety distances, namely:

• The method based on pre-established matrices – Qualitative method
• The method based on simulations of accidental events (gas leak, fire) – Quantitative method

The first method to consider is the qualitative method because it is very simple and fast. Indeed, depending on the type of unit or equipment, it is possible to determine the minimum distance required between each unit. There are many supports from internal standards of large groups, but also guides from insurers for example.

Most often the information is listed as a matrix as the example shown below:

Although this method is the simplest and fastest, it nevertheless has the disadvantage that the distances given are often very restrictive. It can, however, make it possible to define a preliminary layout. 

For an optimized layout, it is more appropriate to go with the second method.

The quantitative method has the advantage of giving more precise and more suitable distances to the units and equipment of the project. On the other hand, it is more complex to implement and requires more time since it is based on simulations of accidental events using 2D modeling software.

In order to apply this method, it is essential to define all the criteria to be taken into account to establish the simulations. The most important criterion is the scenarios to be considered. This exercise, which can be subject to a lot of discussions, must be carried out rigorously on tangible data. Thus, it is not realistic to consider pipe breaks to estimate safety distances.

Therefore, the scenarios will preferably be determined initially on the basis of expert judgment (jet fire, pool fire, dispersion of a flammable and/or toxic gas cloud, etc.) but also on statistics of accidents/incidents or failure. For example, depending on the diameter of a pipe, it is possible to identify the most likely leak size that could occur. There are many databases for defining leak frequencies. Below are examples of leak frequencies according to different references:

It should be noted that in the context of studies on flares or on vents/safety valves to the atmosphere, the scenarios will not be based on the size of the leak, but rather on flow rates and operating conditions. 

In order to determine the safety distances, the target values for each type of scenario should also be considered. These correspond to a certain threshold of domino effect (e.g. thermal radiation threshold of a jet fire, threshold concentration of flammable gas dispersion).

Once all the scenarios have been identified, the simulations stage can be launched in order to determine all the safety distances to be taken into account for the project. 

These safety distances will thus facilitate the installation of industrial equipment and units. This type of study provides a solid basis for justifying to the authorities that your new installation has been designed with strict safety distances in mind.