Providing Adequate Clearances and Access for Process Equipment
Adequate clearances and access to process equipment, valves, and instruments are essential for the proper maintenance, operation, and safety of any plant. This article can be used as a guideline for the preparation of plot plans and equipment layouts for various process plants, offsite, and utilities.
General Requirements for On-shore Plant Layout
This article covers general requirements for on-shore plant layout of process units, equipment, and general facilities, utility plant, and offsite areas. It includes basic considerations for plant clearances and access, equipment elevation, paving, grading, sewers, and other related items.
Clearances and Access for Proprietary Equipment
Plant Clearances and access for the operation and maintenance of proprietary equipment shall be in accordance with the manufacturer’s or equipment vendor’s drawings/operating guidelines or manual.
Guidelines for Plant Layout Design
When designing a plant layout, certain guidelines should be taken into consideration. These include integrating process units and other facilities within a common plant area, separating them from each other where necessary, ensuring access for mobile equipment for maintenance, providing access ways for mobile handling equipment, and providing facilities such as davits and trolley beams for inaccessible areas. Additionally, the layout should account for the turning radius of mobile equipment, tanker lorries, and other vehicles, and should meet statutory and insurance requirements.
Integrating Process Units and Facilities
Process units and other facilities should be integrated within a common plant area when independent operation and shutdown for planned maintenance is not required. If separate operation and maintenance is necessary, then distance should be maintained between units and facilities with separating spaces or roads for interconnection. The distances should be based on statutory and insurance rules.
Providing Access for Mobile Maintenance Equipment
The layout should provide access for maximum use of specific mobile equipment for normal planned maintenance work. Access ways should be 6m wide with 6m overhead clearance, although this can be adjusted depending on the type of mobile equipment used. Additional facilities such as davits and trolley beams should also be provided for inaccessible areas.
Turning Radius Considerations
The layout should also account for the turning radius of mobile equipment and tanker lorries. Roads should be wide enough to accommodate the vehicles, and turning radii should be suitable for their movement. Additionally, the layout should meet statutory and insurance requirements.
Plant Clearance and Access Consideration
Proper Access is key to ensure operational efficiency and safety during plant operation. Clearance and access considerations must be taken into account to ensure that equipment, valves, and instruments requiring operational control or normal maintenance are accessible. This includes providing main operating or service levels with stairways as the primary means of access and auxiliary service platforms with vertical cage ladders as primary access.
Stairway Specifications
Stairways shall be provided as the primary means of access to main operating or service levels in structures, buildings, or furnaces. The vertical rise of any stairway shall not exceed 4.5 m in a single flight at a maximum angle of 40° and shall provide a minimum clear headroom of 2.1 m. Storage tanks shall also be provided with stairways where tank heights are more than approximately 6 m. It is preferred to provide independent staircases for lined storage tanks.
Ladder Specifications
Primary access to platforms attached to vessels, auxiliary service platforms in structures, furnaces, and storage tanks for platforms up to approximately 6 m (20 ft) high, shall be by vertical cage ladders. The vertical rise of ladders shall not exceed 5 m for a single run. Ladders that are located or extended more than 2.25 m above grade shall be provided with safety cages. Ladders that rise more than 5 m shall have an intermediate rest platform. Auxiliary exists from platforms shall be by means of ladders. Such exists shall be required when platforms are longer than 7.5 m and shall be located so that no point on an operating platform is horizontally more than 22.5 m from a primary or auxiliary exit. The dead-end length of a platform shall not be greater than 7.5 m.
Platform Access Requirements
Platforms or ladder access not less than those specified herein shall be furnished for equipment, located more than 3.6 m above grade and 2.1 m above another platform and include the following means of access: Where regular access is required, a minimum clear aisle width of 0.75 m and overhead clearance of 2.3 m shall be provided. Groups of valves at battery limits in elevated pipe racks and grade pipe sleepers shall be provided with permanent platform access. The type of valves (gate, plug, ball, etc.) shall be taken into account when locating such platforms.
Exceptions
Exceptions to the platform access requirements include flanged nozzles on vessels, block valves in pipe racks (not requiring operation except for infrequent isolation) metal temperature measuring points in piping, and orifice flanges in lower pipe racks. These shall not be provided with permanent means of access. Elevated platforms shall have sufficient space as defined herein for maintaining equipment; tall columns shall be provided with davits for removing covers, relief valves, blinds, etc. and the same shall be located such that these can be lowered safely to ground level.
Valve Positioning Rules for Manual Operation
Valves that require manual operation during plant operations or in an emergency must be placed in a specific manner. This includes valves with chain operators or extension stems. For horizontally installed valves that are 6 inches (150 mm) or smaller, the bottom of the handwheel should not be more than 2.25 m above the grade or platform. For valves 8 inches (200 mm) or larger, the maximum height to the centerline of the handwheel should be 1.95 m above the grade or platform. The preferred height to the centerline of all valves should be 1.5 m above grade or platform. Additionally, horizontally installed wrench-operated plug, ball, or butterfly valves should be positioned such that the wrench movement arc is no higher than 2.3 m above the grade or platform.
Equipment Clearance and Access Guidelines
Layout and Safety Design Rules for Furnaces and Fired Heaters
Layout Requirements
Several furnaces in the same or different services and part of the same process unit or facility, together with associated close-coupled equipment, may be located in a single area and shall be segregated within that area only as required for operational and maintenance requirements. To avoid a hazard, a furnace and close coupled equipment or a furnace area shall be located not less than 15m and preferably upwind (prevailing) from other equipment containing flammable fluids, except as follows: The distance shall be measured from the outside of the nearest of furnace walls to the nearest point of the equipment considered.
Minimum Distance Requirements
Not less than 15 m from air coolers, containing flammable fluids. Gas or liquid reforming furnaces, as used in ammonia and similar plants, may be located at a minimum distance from associated process equipment consistent with operating design and maintenance requirements. Reforming and desulphurizing furnaces may be located at a minimum distance from their reactors and feed/effluent exchangers, consistent with operating and maintenance requirements. Not less than 15m from switch rooms, un-pressurized control houses, and the compressor or pump house containing equipment in hydrocarbon service. These distance requirements shall generally be governed by statutory / TAC regulations. Furnaces shall be located at a maximum practical distance from process equipment containing liquefied petroleum gas or similar materials in accordance with applicable Tariff Advisory Committee (TAC) rules with a minimum distance of 15 m.
Exceptions
Integral-type fired heaters such as start-up heaters shall be considered exceptions. Fired furnaces of the following types that are used only for planned intermittent and start-up service may be located at a minimum practical distance from the equipment which they serve and not less than 6 m from other equipment, including air coolers containing flammable fluids. Furnaces having welded coils and no header boxes. Furnaces with header boxes facing away from other equipment. Vertical cylindrical furnaces.
Platforms Requirements
Areas for tube pulling shall be as shown on the plot plan and may extend over roads that are peripheral to the unit and not required for access to other plot areas. Furnaces shall be provided with platforms for operation and access as follows: Platforms for maintenance of soot blowers and dampers. Platforms for burner operation when inaccessible from grade or in accordance with client requirements. Platforms for access / observation doors except that when the doors are located less than 3.6 m above grade or another platform, access shall be by ladder only. Platforms for header boxes containing removable plug fittings. Platforms for decoking / swing elbow connections.
Plant Clearance and Access Considerations for Heat Exchangers, Air Coolers & Cooling Towers
Heat Exchangers Clearance and Access Considerations
Shell and tube heat exchangers should generally not be stacked more than two units high and the bundle weight should not exceed ten tonnes. Permanent steel or reinforced concrete structures with monorail beams should be provided for supporting bundles during pulling and lowering to grade when specified mobile equipment is not suitable, shells are stacked more than two units high, or the shell centreline is more than 3.6 m above grade. Platforms should only be supplied when required for operation and the layout should be arranged to facilitate maintenance.
Air Coolers Clearance and Access Considerations
Bundle removal of elevated air coolers may be done over adjacent equipment where suitable cranes are available, otherwise, crane accessways should be provided. Access should be provided for the maintenance of fan drives and headers as appropriate. Air-cooled exchangers containing flammable fluids should NOT be installed directly above control rooms, MCC rooms, transformers, and other major switchgear. Water sprays should be provided over equipment in certain services located directly beneath air-cooled air exchangers containing flammable fluids.
Cooling Towers Clearance and Access Considerations
Equipment adjacent to cooling towers should be located at a sufficient distance downwind as determined by each season’s prevailing winds from cooling towers, to minimize detrimental effects during fog-creating seasons. The location of cooling towers with respect to roads should be considered carefully as the fog created under certain conditions of temperature and humidity presents a serious driving hazard. If necessary, roads should be provided on both sides of critically located cooling towers; public highways should be located at min. 45 m from cooling towers.
Reactors, Towers, and Vessels Clearance and Access Considerations
Monorail beams should be provided for charging reactors only when the use of a crane is not feasible.
Design Rules for Structure Clearance and Access
Ladders: Rules for Side Exit and Step-Through
Ladders are generally arranged for the side exit of a structure. Step-through ladders may be used for runs from grade up to a height of 6 m or for elevated runs of 3 m. If ladders are not provided for access between platforms, intermediate steps should be provided where the elevation difference is greater than 0.35 m.
Handrailing: Requirements for Structure Clearance and Access
Handrailing must enclose all stairways and platform areas where the clearance between the equipment and the edges of flooring is greater than 0.3 m. Toe plates should be provided around floor openings for permanent equipment where the clearance to the edge of the flooring is greater than 50 mm but less than 0.3 m. Handrails must also be provided at the periphery of tank roofs adjacent to access stairways and dip hatches.
Sheds for Pumps, Compressors and Other Equipment
Where compressor sheds (or utility sheds) are provided, they should be open-sided steel frame structures, sheeted from roof eaves level down to a maximum height of 2.45 m above the compressor house floor level, with ventilation by means of natural draught. Pump or equipment sheds should also be steel framed with sheeting as described above.
Compressor House Facilities
Compressor houses should be provided with suitable facilities for handling the heaviest machine component during normal maintenance. This includes a laydown area and suitable road access adjacent to the compressor house, as well as a design of the ground floor to carry the internal parts of the machines, but not the top halves of casings, during maintenance.
Removal of Equipment
The design of structures containing equipment should not provide for the removal of equipment, not considered a normal maintenance requirement, except to provide access to mobile equipment, clearance within the structure, or for the removal of sections of the structure where specified. Facilities for breakdown maintenance of vital equipment, such as standby generators and power units, should be provided.
Layout rules for Pumps and Compressors
Pump Isolation Valves
To ensure the efficient removal of pump internals without dismantling the piping or valves, a flanged spool piece or strainer should be provided on the pump nozzle. Pump isolation valves should be located as close to the pump as possible and accessible from ground level. The hand wheels of the valves should be oriented to provide clearance over the pumps for maintenance and operation.
Alignment of Pumps
When installing groups of pumps in process and utility plant areas, the drive ends should line up. In offsite areas, the pump ends of the plinths of the pumps should line up. Large pumps can be orientated parallel to a pipe rack or passage.
Installation of Alignment Bellows
To avoid transferring excessive loads to the pump nozzles, alignment bellows should not be installed directly on the pump nozzles. A flanged spool piece of adequate length should be installed and anchored between the pump nozzles and the bellows.
Clearance and Spacing Rules for Buildings
Safety Requirements
Central control rooms, MCC rooms, and substations shall serve several process units, utility plants, and offsite facility areas as far as practicable. Isolation of utility supply shall be provided as necessary. However, when impracticable or per the client’s operating philosophy, separate control/MCC rooms may be required.
Locating Control Rooms and Switch Rooms
Control rooms and switch rooms shall be located either in a safe distance from the nearest normal source of flammable hazard (as per statutory or TAC rules) or of pressurized design with the top of the air intake stack located in a safe area. In such cases, the construction may have to be blast-proof.
Rules for Height of Atmospheric Discharge
The top of stacks and continuously operating vents that release hazardous vapors must be at least 3m above any platform or flat building roof, within a 21m radius. If the platform or building roof is larger than 21m, the height of the stack or vent should be reduced by the same distance. However, the height should be determined in accordance with the State Pollution Control Board regulations and HAZOP/hazard studies.
Height Requirements for Relief Valves and Intermittent Vents
The top of the outlet piping from relief valves and intermittent vents emitting hazardous vapors must be at least 3m above any platform or flat building roof within a 15m radius. If the platform or building roof is larger than 15m, the height of the piping should be reduced by the same distance.
Non-Flammable Vapors
The discharge of steam, air, or similar non-flammable vapors from relief valves and continuously operating vents should be located to avoid any potential risk to personnel. Operator-controlled minor vents may be released to the atmosphere close to the equipment vented, provided they meet the restrictions mentioned above.
Guideline for Flare Location
When determining the location of a flare, it is important to consider the requirements of the client, the recommendations of the vendor, and the limitations of API – RP.521’s ‘Guide for Pressure Relief and Depressurising Systems’. It is generally preferable that the flare is positioned upwind of any process units or tankages, as this will reduce the risk of any potential vapor leaks being ignited.
Safety Zone around Flare Stack
To ensure safety, a safety zone should be created around the flare stack, in which the radiation level at grade is acceptable. This zone should be isolated by a fence.
Equipment Handling and Maintenance Design Rules
Design and installation of maintenance facilities shall be based on the parts of equipment that require frequent or routine service and which are inaccessible to the available plant handling facilities. Such facilities should not be designed to handle heavy parts which are normally unaffected like bedplates, rotating equipment, bodies of compressors, machinery frames, etc.
Considerations for Major Machines
Special considerations must be taken when dealing with major machines such as multi-case compressor trains where heavy lifts are likely and laydown areas are required. Clear passages should be provided for workers to safely handle the parts. Monorail beams, overhead traveling cranes and hoist trestles should be designed to lift, transport or lower parts to designated maintenance areas or to grade. From there, they should be removed with skids or hand trucks to other more suitable areas for maintenance.
Requirements for Paving, Surfacing, and Grading
Paving Requirements
The areas which are to be paved are walkways to buildings, areas below oil-fired or combustible liquid-containing furnaces, around groups of two or more pumps located outdoors, around process equipment, around rail and road loading installations, additive and metering facilities, areas below pressurized spheres and bullets. Curbs may be used to retain spilled materials, and the minimum slope of the paving should be 1 in 125, with a maximum fall of 150 mm.
Surfacing Requirements
Roads and access ways for maintenance vehicles should be as shown on the plot plan. Unpaved areas within the battery limits of utility plant and process unit areas, administration and parking areas, should be graded and surfaced with a minimum of 50 mm of gravel, crushed stone, or other suitable material. Offsite areas such as tank farms inside bund walls, pipe racks, and areas alongside roads may not normally be surfaced except where required for maintenance or as specified in statutory regulations.
Grading Requirements
The finished grade elevation of different plant areas or within the battery limits of an area may be varied and established to permit adequate drainage. Roads should be ramped over piping at the intersection with grade-level pipe sleepers, if possible. If not, suitable culverts should be provided for the passage of pipes.
Design Considerations for Pipe Trenches and Pits
Pipe Trench Considerations
When designing pipe trenches, concrete or brick side walls should be used, with an open top covered with grating or plate and bottom surfaced with crushed stone. Paving with acid-resistant tiles should also be considered in corrosive services, with a sloped surface for drainage and a drainage sump at the lowest point. Firestops should be included where trenches cross-unit battery limits, or to separate areas such as totally enclosed buildings, furnaces, or grouped equipment. Additionally, trenches near fired heaters or any sources of ignition should be backfilled with sand and be provided with sealed covers. The minimum width of pipe trenches should be 450 mm, with a minimum clearance of 100 mm between the outside of the pipe, flange or insulation, and walls, and 50 mm to the high point of trenches bottoms.
Covering Pipe Trenches
When covering pipe trenches, those in paved areas should be covered with plate or grating set flush with the top of the paving, and in unpaved areas should be covered with a steel plate set 50 mm above the high point of adjacent grade. Trenches inside buildings should be covered with MS/CI grating set flush with the floor, and trenches subject to vehicle traffic should be provided with removable reinforced concrete covers set flush with the surface.
Pit Considerations
When considering pits, they should have concrete/brick walls and their floors should be surfaced with concrete. Pits should also have a sump for the collection of drainage, with the floor sloping towards the sump. Access to pits should be by ladders or ladder rungs cast into the walls, with stairways added for pits containing equipment such as vessels, pumps, and exchangers. Auxiliary exit ladders should also be included if any point on the pit floor is more than 7.5 m from a main or auxiliary exit. Open pits should be provided with handrailing and 150 mm high curbs above the grade level, while closed pits should be provided with suitable removable covers set flush with paving in paved areas or 50 mm above grade in unpaved areas. Sealed covers with a vent, piped to a safe location, should be provided for pits located near fired furnaces or similar sources of ignition.
