Loading Rack — Engineering Fundamentals
Truck / rail / marine bay design, top vs bottom loading, vapor recovery, NSPS XX trigger logic.
1. The three modes
| Mode | Load size | Load time | Typical rate |
|---|---|---|---|
| Truck | 180–200 bbl (8,400 gal) | 15–30 min | 250–500 gpm |
| Rail (DOT-117 / TC-117) | 700–800 bbl | 60–120 min | 250–500 gpm |
| Marine (barge / ATB) | 10,000–80,000 bbl | 4–12 hr | 2,000–10,000 gpm |
2. Top vs bottom loading
Top loading: arm inserted through dome hatch. Allows splash + foam + free fall, generating static. Free-fall ban (≤ 6 in tip clearance to bottom) for volatile liquids. Legacy in older racks.
Bottom loading: dry-break coupling at undercarriage; submerged fill so liquid enters below surface — eliminates splash, simplifies vapor recovery. Required for gasoline / volatile liquids per most state codes. API RP 1004 standardizes the coupler interface.
3. Vapor recovery + NSPS XX
40 CFR 60 Subpart XX applies to bulk gasoline terminals loading gasoline with TVP > 1.5 psia: ≥ 95% capture of organic vapors required. Typical control: VRU adsorption (carbon bed) or vapor combustor (flare). NSPS XX trigger extends to other volatile liquids (crude with high RVP, condensate, NGL) in many state implementations.
Vapor return: balanced loading captures displaced vapor in the tanker via a closed line back to the loading-rack vapor manifold. Bottom load + balanced vapor = closed-loop loading.
4. Bay-count math
Loads per bay per day = (operating hours × 60) / load time. Bay count = ⌈annual throughput / (loads/day × load size × operating days)⌉. Add 1 standby bay for > 4-bay racks for maintenance / queueing.
5. Safety / static
- Bonding / grounding: clip + cable to vehicle chassis before loading; interlocked with valve.
- Spill containment: drip pan with auto-shutoff valve.
- Emergency shutoff: push-button at each bay + dispatcher remote.
- Operator training: API RP 1007 details loading procedures and emergency response.
6. References
- NFPA 30 §28 — Loading & Unloading.
- 40 CFR 60 Subpart XX — NSPS bulk gasoline terminals.
- API RP 1004 — Bottom Loading / Vapor Recovery for MC Cargo Tank Motor Vehicles.
- API RP 1007 — Loading & Unloading of MC-306/406 Cargo Tanks.
- API MPMS Ch. 17 — Marine Measurement.