1. Equipment Spacing Overview
Equipment spacing in midstream facilities serves three critical safety functions: preventing fire escalation between equipment items, providing access for maintenance and emergency response, and maintaining safe distances from ignition sources. Proper spacing is one of the most fundamental safety measures in facility design.
Fire exposure
Prevent escalation
Adequate distance prevents a fire at one piece of equipment from igniting or damaging adjacent equipment. Radiation intensity decreases with the square of distance.
Access
Maintenance & emergency
Space for crane access, equipment removal, fire truck access roads, and personnel evacuation routes.
Operations
Day-to-day function
Space for operator rounds, instrument access, valve operation, chemical injection, and routine maintenance.
Spacing Measurement Points
Distances are measured from the nearest point of each equipment item, not from centerlines:
- Vessels: From vessel shell (not nozzles or piping)
- Tanks: From tank shell
- Compressors: From compressor skid edge or package edge
- Fired heaters: From the firebox or burner opening
- Buildings: From building wall (nearest point)
- Property lines: From nearest property boundary
- Flares: From flare tip (radiant heat calculation governs)
Governing principle: Spacing distances in codes and standards represent minimums. Good engineering practice often requires larger spacing for maintenance access, future expansion, or specific hazard considerations. Always evaluate the specific hazards at your facility in addition to applying code minimum distances.
2. Applicable Codes & Standards
| Standard | Title | Scope |
|---|---|---|
| API 2510 | Design and Construction of LPG Installations | LPG storage spacing at production facilities |
| NFPA 30 | Flammable and Combustible Liquids Code | Liquid storage tank and process equipment spacing |
| NFPA 59A | Standard for LNG Production, Storage, and Handling | LNG facility spacing |
| API 2510A | Fire Protection for LPG | Fire protection requirements complementing API 2510 |
| 49 CFR 192 | Transportation of Natural Gas by Pipeline | Compressor station spacing from buildings and boundaries |
| 49 CFR 195 | Transportation of Hazardous Liquids by Pipeline | Liquid pipeline facility spacing |
| OSHA 1910.106 | Flammable Liquids | General industry flammable liquid handling |
| NFPA 58 | Liquefied Petroleum Gas Code | LPG storage at non-production facilities |
Hierarchy of Requirements
- Federal regulations (49 CFR): Mandatory; apply to interstate pipeline facilities
- State regulations: May be more restrictive than federal; always check
- NFPA codes: Adopted by reference in most state and local fire codes
- API standards: Industry recommended practices; often adopted by company policy
- Company standards: Internal spacing policies often exceed code minimums
- Insurance requirements: FM Global, Industrial Risk Insurers may require additional spacing
Compliance note: Multiple codes may apply to a single facility. The most restrictive requirement governs. Document which code applies to each spacing distance on the plot plan for future reference and regulatory audits.
3. Spacing Tables
General Equipment Spacing (Typical Industry Practice)
| From | To | Min. Distance (ft) | Basis |
|---|---|---|---|
| Process vessel | Process vessel | 10–15 | Maintenance access, fire exposure |
| Process vessel | Fired heater | 50–100 | Ignition source separation |
| Process vessel | Control building | 50–100 | Personnel safety |
| Compressor | Process vessel | 25–50 | Vibration, fire exposure |
| Compressor | Fired heater | 50–100 | Ignition source separation |
| Compressor | Property line | 100–200 | Noise, fire radiation |
| Atmospheric tank | Process equipment | 50 | NFPA 30 |
| Atmospheric tank | Property line | Varies by capacity | NFPA 30, Table 22.4.2.1 |
| Pressure vessel | Property line | 50–100 | Company/local code |
| Flare stack | Any equipment | Per radiation calc. | API 521 radiation limit |
Distances are typical industry practice and vary by company, jurisdiction, and hazard analysis. Always verify against applicable codes and company standards for your specific facility.
LPG Storage Spacing (API 2510)
| From LPG Storage | To | Min. Distance (ft) |
|---|---|---|
| LPG tank (< 30,000 gal) | Property line / public way | 50 |
| LPG tank (30,000–70,000 gal) | Property line / public way | 75 |
| LPG tank (> 70,000 gal) | Property line / public way | 100 |
| LPG tank | Control building | 50 |
| LPG tank | Fired equipment | 50 |
| LPG tank | Other LPG tank | 5 ft or 1/4 sum of diameters |
| LPG loading rack | Property line | 25 |
Tank Spacing (NFPA 30)
| Tank Type | To Adjacent Tank | To Property Line |
|---|---|---|
| Floating roof (< 150 ft dia.) | 1/6 sum of diameters | 1/2 diameter (min 5 ft) |
| Fixed roof (< 150 ft dia.) | 1/6 sum of diameters | 1/2 diameter (min 5 ft) |
| Fixed roof (crude, > 150 ft dia.) | 1/4 sum of diameters | Varies |
| Pressure vessel (horizontal) | 5 ft or 1/4 sum of diameters | 50 ft |
Spacing tables are minimums: These tables provide code minimum distances. Good practice adds spacing for crane swing radius, pipe rack clearance, future expansion pads, and specific maintenance access requirements identified during HAZOP or constructability reviews.
4. Compressor Station Layout
Compressor stations have unique spacing requirements due to the presence of high-pressure gas, rotating equipment, noise, and vibration. Layout must accommodate both safety distances and operational access.
Compressor Station Spacing (49 CFR 192)
| From | To | Requirement |
|---|---|---|
| Compressor building | Nearest building (off-site) | Per DOT Class location (Table 192.111) |
| Compressor building | Property line | Minimum 200 ft recommended |
| Gas piping | Buildings not part of station | Per Class location requirements |
| Compressor unit | Compressor unit | 10–20 ft (maintenance access) |
| Compressor building | Control building | 50 ft minimum |
Typical Compressor Station Layout Zones
- Compressor zone: Compressor packages, suction and discharge scrubbers, coolers. Central location for piping efficiency.
- Piping and valve zone: Suction and discharge headers, mainline valves, bypass piping. Adjacent to compressors.
- Utilities zone: Fuel gas system, instrument air, lube oil storage, electrical switchgear. Separated from process by distance or fire wall.
- Control zone: Control building, MCC building, telecommunications. Located upwind and at maximum practical distance from process.
- Flare zone: Flare stack or enclosed ground flare. Located downwind per radiation calculations.
- Access zone: Truck access roads (minimum 20 ft wide), crane pad areas, laydown areas for maintenance.
Access Road Requirements
| Road Type | Minimum Width (ft) | Purpose |
|---|---|---|
| Main access road | 20–24 | Two-way traffic, emergency vehicle access |
| Perimeter road | 16–20 | Emergency access around facility |
| Equipment access aisle | 10–15 | Maintenance vehicle and forklift access |
| Pedestrian walkway | 4–6 | Operator access routes |
Layout tip: Orient the compressor station with the control building upwind (prevailing wind direction) of the process area. Position the flare downwind. Ensure that at least two emergency exit routes exist from any point in the facility.
5. Fire Protection Considerations
Thermal Radiation Limits
Equipment spacing for fire exposure is based on limiting thermal radiation intensity at adjacent equipment and personnel locations:
| Radiation Level (BTU/hr·ft²) | Effect | Application |
|---|---|---|
| 500 | Equipment failure after prolonged exposure | Minimum distance for unprotected steel vessels |
| 1,500 | Piloted ignition of wood | Minimum distance to combustible structures |
| 2,000 | Pain threshold (prolonged exposure) | Maximum at fenceline / property line |
| 3,000 | Blistering in 20 seconds | Maximum at areas of limited occupancy |
| 5,000 | Structural steel weakening | Maximum at any normally occupied area |
Fire Wall and Barrier Requirements
When minimum spacing cannot be achieved due to plot constraints, fire-rated barriers may be used as alternatives:
- Fire walls: 2-hour fire-rated walls between equipment items. Must extend 2 ft beyond the protected equipment on each side.
- Blast walls: Reinforced concrete or steel blast walls for explosion resistance. Typically 12–18 inches thick concrete.
- Fire-proofing: Cementitious or intumescent fireproofing on structural steel, vessel skirts, and pipe supports within fire exposure zones.
- Deluge systems: Water spray systems can substitute for some spacing requirements. Provides cooling during fire exposure.
Impoundment and Drainage
- All liquid-containing equipment must have adequate drainage to prevent burning liquid from flowing to adjacent equipment
- Impoundment areas (dikes, curbs) around tanks and vessels contain spills
- Drainage should direct away from equipment, roads, and property lines
- Impoundment volume: minimum 100% of largest tank + rainfall allowance
Fire protection hierarchy: The preferred approach to fire safety is (1) adequate spacing first, then (2) fire-rated barriers if spacing is insufficient, then (3) active fire protection systems (deluge, foam) as supplementary measures. Active systems require maintenance and can fail; passive measures (distance and barriers) are inherently more reliable.
6. Plot Plan Development
Plot Plan Development Process
- Step 1: Develop equipment list with sizes, weights, and utility requirements
- Step 2: Identify applicable spacing codes and company standards
- Step 3: Create spacing matrix (equipment-to-equipment minimum distances)
- Step 4: Determine prevailing wind direction, topography, and drainage
- Step 5: Place equipment starting with the largest items and highest-hazard items
- Step 6: Route access roads and establish emergency exit paths
- Step 7: Verify all spacing requirements are met with a compliance check
- Step 8: Review with operations, maintenance, and safety personnel
Equipment Placement Priority
| Priority | Equipment | Rationale |
|---|---|---|
| 1 | Control building / MCC | Personnel safety; upwind location; access for operations |
| 2 | Flare / thermal oxidizer | Fixed by radiation calculation; downwind |
| 3 | Compressors | Central for piping; vibration isolation; noise |
| 4 | Separators / scrubbers | Adjacent to compressors for piping efficiency |
| 5 | Storage tanks | Impoundment area; fire spacing from process |
| 6 | Coolers (ACHEs) | Adequate air circulation; avoid recirculation |
| 7 | Fired equipment | Separated from hydrocarbon sources |
| 8 | Utilities | Near point of use; away from process hazards |
Future expansion: Always allocate plot space for future expansion when designing a new facility. Reserve space for additional compressor units, treating equipment, or pipeline connections. It is far less expensive to reserve space during initial design than to retrofit spacing later.
7. Best Practices
Layout Review Checklist
- All code-minimum spacing requirements verified and documented
- Spacing matrix completed and attached to plot plan
- Two emergency exit routes available from every point in the facility
- Crane access paths clear for equipment removal (all major items)
- Prevailing wind direction considered for control building and flare placement
- Drainage slopes away from equipment and buildings
- Fire truck access road continuous around the facility perimeter
- Air cooler recirculation prevented (adequate clearance between units and from buildings)
- Noise levels verified at property line and nearest receptor
- Future expansion areas identified and reserved
Common Layout Mistakes
| Mistake | Consequence | Prevention |
|---|---|---|
| Insufficient crane access | Cannot remove equipment without disassembly | Plan crane swing radius for all major equipment |
| Blocked emergency exits | Personnel trapped during emergency | Model evacuation routes; verify with walk-through |
| Ignition source too close to HC release | Immediate ignition of release | Follow spacing tables; locate fired equipment upwind |
| No expansion space | Costly retrofit or new facility needed | Reserve 30–50% additional plot area |
| Drainage toward equipment | Burning liquid flows under equipment | Grade away from all equipment |
| Air cooler recirculation | Reduced cooling capacity in summer | Minimum 1.5x fan diameter clearance between ACHEs |
Constructability Considerations
- Module delivery: If equipment is delivered as pre-assembled modules (skids), verify that access roads and gates accommodate module dimensions and weights.
- Soil conditions: Foundation design affects spacing. Poor soils may require larger foundations that reduce available clearance between equipment.
- Underground utilities: Map all underground piping, electrical conduit, and cable trays before finalizing the plot plan. Underground conflicts are expensive to resolve after construction begins.
- Above-ground piping: Pipe rack routing affects equipment spacing. Plan pipe rack locations early in the layout process.
Review process: Conduct a formal layout review (also called a plot plan review or constructability review) with participation from process engineering, mechanical engineering, operations, maintenance, safety, and construction. This multi-discipline review catches spacing and access issues that single-discipline reviews miss.
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