The words ‘siting’ and ‘layout’ are often used interchangeably, but, strictly speaking, they have different meanings. Siting is concerned with the location of a facility. For example, if a company is planning on building a new chemical plant its management may consider the relative merits of sites in Texas, Mexico or China. Layout, on the other hand, is to do with the locations of equipment, piping and buildings at the selected site and how they connect with one another.
Layout of Operating Units
The layout of operating units, equipment, buildings and roads is critically important for safe and efficient operation of the facility. A good layout is the most effective means of preventing an incident from escalating. It also provides good access for emergency response services.
It is necessary to pay close attention to this topic in the early stages of a project because, once the major items have been installed, it is difficult and expensive to relocate them. This means that there needs to be close coordination between all the engineering disciplines as well as construction, operations, maintenance and HSE (Health, Safety and Environmental). A 3-D model of the facility (and/or isometric drawings) should be created as early as possible during the design. This gives everyone involved an opportunity to ‘walk through’ the facility as if they were operating and maintaining it.
The layout of equipment at a facility is developed in stages. The first stage is to decide where the major operating units are located. Then the location of equipment within those units is determined. For example, in on oil refinery the relative locations of major units such as the Cat Cracker, Alkylation and Boiler House will be considered first. Following this the location of individual equipment items such as distillation columns, heat exchangers and fired heaters can be made. Requirements for vehicle and personnel traffic, security, emergency evacuation, firefighting and access for maintenance, operation and workovers also need to be considered along with the proper positioning of emergency shutdown and depressuring valves, ventilation inlets and outlets, cranes, engine air intakes and exhausts, vents, fired heaters, control rooms, offices, living quarters and maintenance equipment.
Although most layout decisions are made during the initial design phase, the topic also needs to be considered when making facility modifications, particularly when new equipment or piping is being added.
One of the most important layout decisions is to identify those activities and functions that can be moved away from the hazardous area altogether. For example, one refinery purchased a disused school building that was located about 1 km from the facility. They then moved most of their administrative functions into that building. Doing so created some inconvenience and expense. But it did mean that, were there to be an explosion or fire the people in that building would be safe.
Once safety and environmental issues have been addressed equipment spacing should be organized so as to minimize the expense of the intermediate piping and valving, particularly when expensive alloys are being used.
In general, keeping items well away from one another improves safety for the following reasons:
- A fire is less likely to spread.
- There is less likelihood of someone injuring themselves during routine operations or maintenance.
- There is more access space for the emergency response team and their equipment.
The following general comments and guidance apply to the layout of equipment within process facilities once the major operating units have been located. (Due to the lack of space on a platform or drilling rig, the layout of offshore facilities may not be able to follow this guidance.)
- Process flow generally dictates the overall layout of equipment within an operating unit.
- High hazard and low hazard areas should be separated from one another. High hazard areas are those that contain materials that meet one or more of the following criteria:
- Flammable liquids at a pressure of 35 bar(g) or greater.
- Combustible liquids at a temperature above their flash point.
- LPG (liquefied petroleum gas), butane, hydrogen, ethylene, acetylene at any pressure.
- Highly reactive or toxic compounds such as hydrogen fluoride or anhydrous ammonia.
- Spacing is required between the facility’s equipment and the property line so that members of the public or workers at another facility are not impacted by an explosion or release of toxic materials. Greater spacing should be considered if adjoining property contains process plants, large storage tanks, or other potential hazards.
- Spacing between major process units should 25 meters or more. Spacing greater than 60 meters, except for specialized operations such as emergency flares, provides little decrease in risk and may actually increase the overall hazard because piping runs will increase in length.
- The likelihood of future expansion of any major process unit should be considered in the layout decisions. The selected spacing between units should not be viewed as a location for future expansions.
- The facility or plant site should be level. If this is not possible sloped topography can be used to achieve additional design safety. For example, large storage facilities should be located at a lower elevation than the process areas. Then, if there is a large leak from a tank the liquids or heavy vapors will not flow downhill into the process.
- It should be possible to build a unit without lifting heavy items over operating equipment and piping.
- Issues to do with traffic and roads should be considered. These include:
- Suitable barricades between process equipment and adjacent roadways
- Vehicular traffic restricted where pedestrians are present
- Overhead pipe racks protected from crane impacts
- Adequate access for emergency vehicles
- Blockage of access roads by trains, congestion or construction equipment
- Sharp curves in access roads
- The drainage system should remove spilled liquids to a safe location with minimal exposure to piping and equipment. The potential for vapor accumulation in the drainage system should also be considered.
- Good access should be provided for normal operations, emergency response and escape and evacuation. In particular, there should be direct, unobstructed access ways that are continuous from one end of the unit to the other and that are connected to roads surrounding the unit.
- Fire hoses must be able to reach any area that could be affected by a fire or chemical release.
- The prevailing wind should be considered when locating ignition sources such as fired heaters and flares.
- Continuous ignition sources should be located away from probable points of release of flammable materials. In particular, pumps and compressors, the most likely source of accidental leaks, should be located at least 15 meters from sources of ignition such as fired heaters and internal combustion engines.
- Air intakes for equipment and building ventilation should be located away from potential releases of combustible or toxic vapors.
- When operating conditions necessitate locating adjacent pieces of equipment closer than desirable from a fire protection standpoint, such as is often found on offshore platforms, additional fire protection measures should be considered.
Access to equipment and other layout issues affecting personnel can be optimized through the use of 3D models.
The sketch below shows how equipment and safety equipment (in this case a safety shower) can be laid out.
The sketch illustrates the following layout decisions to do with the safety shower.
- It is upwind of the pump seals (the most likely source of a leak);
- It is in a non-restricted area (outside the yellow line); and
- It has a flashing light that can be seen by personnel outside the area of a likely leak.
Regulations and Standards
There are many regulations, codes and standards to do with equipment spacing and layout. Examples are:
- API Recommended Practice 752 — Management of Hazards Associated with Location of Process Plant Buildings;
- The National Electrical Code 70; and
- API Recommended Practice 14J — Design and Hazards Analysis for Offshore Production Facilities.
In addition, many companies have their own standards and guidance. The use of standards can be supplemented by vapor dispersion analysis and other types of modeling.