Overpacks, Salvage Packaging & Pressure Differential Standards

Overpacks, Salvage Packaging & Pressure Differential Standards

Beyond standard individual packages, air cargo operations frequently utilize specialized packaging configurations to streamline handling, protect cargo, or manage damaged packages. Understanding the legal definitions, marking rules, and pressure standards for overpacks, salvage packagings, and pressure differential compliance is critical for dangerous goods certification.

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1. Overpacks: Definition, Rules & Construction

An overpack is defined under IATA DGR Section 5.0.1.5 as an enclosure used by a single shipper to contain one or more dangerous goods packages to form a single handling unit for convenience of handling and stowage. Examples include a shrink-wrapped wooden pallet, a plastic stretch-wrapped crate, or a large protective outer box.

+-------------------------------------------------------------------------+
|                        OVERPACK CONFIGURATION                           |
|                                                                         |
|  +-------------------------------------------------------------------+  |
|  |                       OVERPACK ENCLOSURE                          |  |
|  |  Markings: "OVERPACK" (12 mm min), PSN, UN ####, Labels, Arrows |  |
|  |                                                                   |  |
|  |  +-----------------------+     +-----------------------+          |  |
|  |  | Compliant Package 1   |     | Compliant Package 2   |          |  |
|  |  | UN 1263 Paint (4G)    |     | UN 1263 Paint (4G)    |          |  |
|  |  +-----------------------+     +-----------------------+          |  |
|  +-------------------------------------------------------------------+  |
+-------------------------------------------------------------------------+

Fundamental Overpack Rules

1. Independent Compliance: An overpack is not an outer packaging of a combination package. Every individual package placed inside an overpack must independently meet all DGR packaging, UN specification testing, marking, labeling, and net quantity rules.
2. Segregation Compliance: Incompatible dangerous goods that require segregation under DGR Section 9.3.2 (Table 9.3.A)—such as Class 3 Flammable Liquids and Division 5.1 Oxidizers—must never be placed inside the same overpack.
3. Single Shipper Requirement: An overpack is by definition assembled by a single shipper. Packages from different shippers must not be combined into one overpack.

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2. Overpack Marking and Labeling Requirements

Unless all marks and labels representative of every dangerous goods package inside the overpack are clearly visible from the exterior, the shipper must reproduce all required package markings and labels on the outside of the overpack.

Exterior Overpack Markings Checklist

• The Word "OVERPACK": Printed in lettering at least 12 mm (0.5 inches) high.
• Proper Shipping Name (PSN) & UN Number: For every dangerous good contained within (e.g., UN 1263, Paint).
• Shipper & Consignee Information: Full name and physical address.
• Hazard Warning Labels: Primary and subsidiary hazard labels for each package contained.
• Handling Labels: Special handling labels such as Cargo Aircraft Only (CAO) or Environmentally Hazardous Substance labels.
• Package Orientation Arrows ("THIS SIDE UP"): Placed on two opposite vertical sides of the overpack whenever liquid dangerous goods packages are enclosed.

Shipper's Declaration Documentation

When packages are tendered in an overpack, the Shipper's Declaration for Dangerous Goods (DGD) must explicitly include the notation "Overpack Used" adjacent to the package details, indicating to ground acceptance staff that packaging limits apply to individual inner packages within the overpack.

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3. Salvage Packaging ('T' Marking & Operational Use)

When a dangerous goods package is found to be damaged, defective, leaking, or non-compliant during handling or transit, it cannot be loaded onto an aircraft in that condition. It must be safely enclosed within a salvage packaging.

UN Code Identification for Salvage Packaging

Salvage packagings are specially tested UN specification containers designated by the capital letter 'T' inserted immediately after the packaging type code:

$\text{UN Mark Example: } \mathbf{1A2T \text{ / Y300 / S / 26 / USA / +AA9876}}$

• 1A2T: Removable-head steel drum specifically tested and certified as a salvage packaging.
• 1H2T: Removable-head plastic drum tested as a salvage packaging.

Salvage Packaging Codes:
Removable-head Steel Drum:   1A2T
Removable-head Plastic Drum: 1H2T

Operational Salvage Transport Procedures

• Absorbent Placement: Sufficient inert absorbent material must be placed inside the salvage drum to surrounding the damaged inner package to absorb all free liquid.
• Exterior Markings: The salvage drum must be marked with the Proper Shipping Name and UN number of the damaged substance, plus the word "SALVAGE" in bold lettering.
• Documentation: The Shipper's Declaration must state "Salvage Package" in the quantity and type of packing section.

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4. The Aviation Internal Pressure Differential Standard (95 kPa)

During flight, commercial aircraft cargo holds undergo cabin altitude pressure changes. As external ambient pressure drops, internal pressure inside sealed liquid containers pushes outward against container walls.

Aircraft Ascent Profile & Internal Pressure Differential:
Ground Level (101 kPa)  --> Cruise Altitude (75 kPa Ambient Pressure)
                             |====================================|
                             | Sealed Liquid Container            |
                             | Internal Pressure: 170 kPa         |
                             | Differential: 170 - 75 = 95 kPa    |
                             |====================================|

The Mandatory 95 kPa Rule

All inner packagings (or single packagings) intended to contain liquid dangerous goods for air transport must be capable of withstanding, without leaking, an internal pressure differential of:

$\text{Required Internal Pressure Differential} \ge 95 \text{ kPa (13.8 psi / 0.95 bar)}$

Or a pressure derived from the total vapour pressure of the liquid at 55°C, whichever is greater. This requirement is independent of the packing group and applies to every liquid offered by air.

High Vapor Pressure & Low Pressure Exceptions

• Packing Group I Liquids: The UN design-type hydraulic test for a PG I single packaging is at least 250 kPa, so such packagings comfortably exceed the 95 kPa air requirement. Do not confuse that design-type test pressure with the 95 kPa differential.
• No lower alternative exists: 95 kPa is a floor, not a target. There is no reduced differential for short sectors, cargo aircraft, low-altitude routings, or non-viscous liquids — any figure below 95 kPa is a misreading.
• Temperature Integrity Range: Pressure differential integrity must be maintained across ambient transport temperatures ranging from -40°C to +55°C (-40°F to +131°F).