Paving and Drainage | Safety Considerations for Plant Layout Levels
Paving Hydrocarbon Spill Areas
The type and extent of paving shall be determined by area usage as follows:
a. Areas not subject to hydrocarbon spills, but used for vehicle access for maintenance, are usually asphalt concrete of sufficient strength to bear the anticipated loads.
b. Areas subject to hydrocarbon spills, but not accessible to vehicular traffic, should be reinforced concrete at least 100 mm thick.
c. Air-entrained concrete shall be provided for low-temperature [below -46 °C] hydrocarbon exposure.
d. All areas subject to hydrocarbon spills and vehicular traffic (especially areas in front of shell-and-tube exchangers) shall be reinforced concrete at least 150 mm thick and of sufficient strength to bear the anticipated loads. The concrete in front of exchangers shall be at least as wide as the bundle length plus 1.5 m.
e. Selected, unpaved areas within the battery limits should be treated with a soil sterilant and covered with crushed stone 100 mm thick. However, avoid small islands or peninsulas of gravel in a paved area.
f. Areas under pipeways within battery limits, which are considered subject to hydrocarbon spills and light crane traffic, shall be paved with concrete.
Road and Accessways
17.2.1 Primary Road
Width of primary road shall be 6.75 meters with 1.25 meters hard shoulders, except at main entries. Main entries shall have two 3.5 meter lanes divided by a 3.5 meter median and 1.25 meter hard shoulders.
17.2.2 Secondary Road
Width of secondary road shall be 6.75 meters with 1.0 meter hard shoulders.
17.2.3 Tertiary (Fire Fighting or Remote Access) Road Width shall be 6.75 meters with 1.0 meter hard shoulders.
17.3 Polyethylene and Polypropylene Units
In outdoor paved areas where polyethylene pellets may collect, drainage shall be provided in open grating
covered trenches rather than in buried pipe. Flows shall be directed towards collection sumps so that the
pellets may be skimmed off and recovered prior to discharge into the main contaminated water systems.
17.4 Drainage and Sewer Systems
Segregation of Sewer Systems
a. Two sewer system shall be provided as follows:
(i) An oily water sewer system with necessary fire seals
(ii) A clean water system for storm water or firewater drainage
Capacity
The capacity of the sewer systems shall be sufficient to avoid liquid backup under various types of loading, such as rainfall, firewater and process drainage. Process unit surface drainage shall be sized on the basis of process drainage plus rainfall, or process drainage plus design firewater capacity, whichever is greater.
Process Area Drainage Design
Area grading shall be provided for drainage away from vessels toward catch basins, to prevent accumulation of flammable liquid or vapor adjacent to or beneath vessels.
Pump Rows Drainage
Drainage from pump bases and glands shall enter an equipment drain bell that shall be provided with drip-type bases to facilitate goo housekeeping and drainage. The maximum number of pump drains connected without seals to the same sewer branch shall be four.
Blowdown Drains
The process drainage system shall be provided for liquid hydrocarbon drainage, including blowdown drains from high pressure vessels and blowdowns from level gauges.
Open Catch Basins
Open catch basis with sealed outlets shall not be located closer than 9 m (30 ft) to fired heaters or similar constant ignition sources. Dry box catch basins shall be located adjacent to fired heaters and separately drained through sealed outlets into the open catch basins.
Sewer Design
See Atmospheric Venting Standard for sewer design considerations.
Administrative and Shop Area Drainage
The sewer shall be designed so that flow through the system shall be from administrative and shop areas toward the process and tank field areas.
Open Drainage Ditch Design
General – Drainage ditches may be used to collect storm water and firewater outside process unit battery limits subject to the following limitations.
a. They shall not be used as part of the process sewer system.
b. They shall not be located under pipeline sleepers.
c. Drainage ditches shall be separated from pipe racks and sleepers by a horizontal minimum distance of 5 m and from process equipment by 10 m.
d. Pipeways crossing drainage trenches shall be protected by a concrete slab or culvert, or the piping shall be provided with fireproofing insulation. Where pipes cross ditches, pipe flanges and other mechanical-type joints shall be avoided.
e. Location of the drainage ditch relative to equipment and piping must be carefully considered so as to prevent fire exposure in the event of a flammable-liquid spill and fire in the ditch.
Ditch Roadway Crossings and Fire Stops
Ditch crossings for roadways shall consist of a roadway culvert at least 9 m (30 ft) in length, arranged with a fire stop on one end.
Fire stops between areas shall be of the inverted weir, downward elbow, or other design providing a water seal at least 152 mm (6 in) deep. The seals shall be designed to facilitate cleaning and removal of silt and debris.