Loading Rack Fundamentals — Truck / Rail / Marine

Loading Rack — Engineering Fundamentals

Truck / rail / marine bay design, top vs bottom loading, vapor recovery, NSPS XX trigger logic.

Truck load

180–200 bbl

15–30 min at 250–500 gpm per bay.

Vapor capture

≥ 95% (NSPS XX)

Required for gasoline loading with TVP > 1.5 psia.

Preferred fill

Bottom loading

Submerged fill, no splash, closed-loop vapor return.

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.

Frequently Asked Questions

What is the difference between top and bottom loading?

Top loading inserts an arm through the dome hatch and can cause splash, foam, and static. Bottom loading uses a dry-break coupling at the undercarriage for submerged fill, eliminating splash and simplifying vapor recovery. Bottom loading is required for gasoline and volatile liquids under most state codes.

When does NSPS Subpart XX apply to a loading rack?

40 CFR 60 Subpart XX applies to bulk gasoline terminals loading gasoline with TVP > 1.5 psia, requiring ≥ 95% capture of organic vapors. Many state implementations extend the trigger to other volatile liquids such as high-RVP crude, condensate, and NGL.

How is the number of loading bays determined?

Loads per bay per day = (operating hours × 60) / load time. Bay count = ⌈annual throughput / (loads/day × load size × operating days)⌉. Add one standby bay for racks larger than four bays to cover maintenance and queueing.