TML Flow Table Test: Transportable Moisture Limit Determination for Bulk Cargo Safety

The TML flow table test is an internationally recognized laboratory method used to determine the Transportable Moisture Limit of fine-grained solid bulk cargoes that may liquefy during sea transportation. Recognized under Appendix 2 of the International Maritime Solid Bulk Cargoes (IMSBC) Code, the test establishes the maximum moisture content at which cargoes such as iron ore fines, nickel ore, and mineral concentrates can be safely loaded and shipped. This article explains the test principle, apparatus, procedure, reporting requirements, and practical applications of the TML flow table test in the bulk shipping industry.

Technical Overview

The Transportable Moisture Limit (TML) flow table test is a standardized laboratory procedure used to determine the maximum moisture content at which a fine-grained solid bulk cargo can be loaded, transported, and discharged without the risk of liquefaction. The method is formally recognized in Appendix 2 of the International Maritime Solid Bulk Cargoes (IMSBC) Code, which became mandatory under the SOLAS Convention, and it is one of the principal test procedures available to shippers and competent authorities for evaluating cargoes classified as Group A, meaning cargoes that may liquefy if the moisture content exceeds the transportable moisture limit.

Liquefaction is the central safety concern addressed by this test. When a saturated or nearly saturated granular cargo is exposed to cyclic stresses during a voyage, such as engine vibration, rolling, and pitching, the particles tend to compact, pore water pressure rises, and the effective stress between grains collapses. As a result, the cargo loses shear strength and begins to behave like a viscous slurry that can shift rapidly inside a cargo hold, leading to loss of stability and, in severe cases, capsize. A series of bulk carrier losses involving nickel ore and iron ore fines has demonstrated the potentially fatal consequences of shipping Group A cargoes above their TML.

In the flow table test, a series of specimens prepared at progressively higher moisture contents are compacted in a conical mold and subjected to repeated standardized jolts on a flow table. The moisture content at which the specimen first exhibits visible flow or plastic deformation is recorded as the Flow Moisture Point (FMP). The Transportable Moisture Limit is then calculated as 90 percent of the FMP, providing a deliberate safety margin for commercial transport. The flow table method is best suited to fine-grained materials with a maximum particle size of approximately 1 mm, and it is widely applied by accredited laboratories, mineral producers, port terminals, and marine surveyors worldwide.

Testing Samples

The flow table method is designed for fine, cohesive, granular materials that retain interstitial water and are therefore susceptible to liquefaction. Typical sample types submitted for TML flow table testing include the following.

Mineral Concentrates

Concentrates of copper, lead, zinc, nickel, and iron are produced by fine grinding and flotation followed by dewatering. Their very fine particle size distribution and high moisture retention make them classic Group A cargoes, and they are among the most frequently tested materials for TML certification.

Iron Ore Fines

Sinter feed, direct-shipping fines, and other fine iron ore products are routinely tested because stockpiles are often exposed to rainfall before loading. Moisture can migrate and accumulate within the cargo mass during storage and transport, making reliable TML and moisture content data essential for safe shipment.

Nickel Ore and Lateritic Ores

Nickel ore and other lateritic ores contain significant clay fractions that bind large quantities of moisture. These cargoes have been implicated in multiple fatal liquefaction casualties and are subject to particularly rigorous TML testing and moisture monitoring regimes.

Coal Fines and Coal Slurry

Fine coal products, including coal slurry and coal sludge, may present both liquefaction and chemical hazards, placing them in combined Group A and B categories. TML flow table testing supports the moisture-related part of their cargo classification and documentation.

Other Fine-Grained Bulk Materials

Fluorspar fines, pyrites concentrates, manganese fines, silica sand, screening fines, and similar fine mineral products may also be evaluated using the flow table method, provided the particle size is within the applicable range and the material shows the cohesive behavior required for meaningful results.

Testing Items

A complete TML flow table testing program typically includes the following determination and verification items, which together support the cargo declaration required by the IMSBC Code.

• Flow Moisture Point (FMP) determination, identifying the moisture content at which the test specimen first exhibits flow behavior under standardized jolting.
• Transportable Moisture Limit (TML) calculation, taken as 90 percent of the determined Flow Moisture Point.
• Cargo moisture content (MC) determination by oven drying, allowing direct comparison of the actual cargo condition against the certified TML.
• Particle size distribution verification to confirm that the material is fine enough, generally with a maximum grain size of about 1 mm, for the flow table method to be applicable.
• Sample homogenization and representative sub-sampling checks to ensure that the tested portions accurately reflect the cargo lot to be shipped.
• Evaluation of the acceptance criterion, confirming whether the declared moisture content of the cargo remains safely below the established TML.

Testing Methods

Test Principle

The flow table test applies controlled dynamic loading to a compacted, moistened specimen in order to reproduce, in a simplified and repeatable manner, the cyclic stresses that act on a cargo during a sea voyage. As moisture content increases toward saturation, the specimen progressively loses its ability to maintain its compacted shape. The transition point at which the material begins to deform plastically and flow under jolting identifies the Flow Moisture Point, from which the TML is derived.

Apparatus and Equipment

The test requires a dedicated set of laboratory equipment constructed and calibrated in accordance with the reference procedures of the IMSBC Code.

• A flow table comprising a rigid tabletop mounted on a mechanism that raises and drops the table through a fixed, calibrated height, delivering standardized jolting energy to the specimen.
• A truncated cone-shaped mold together with a strike-off device for forming a compacted specimen of consistent geometry on the table surface.
• A hand tamper for compacting the material inside the mold in a uniform and reproducible manner.
• A precision balance suitable for weighing the specimen and moisture-determination portions to the required accuracy.
• A ventilated drying oven capable of maintaining approximately 105 degrees Celsius, or a lower temperature where the material is heat sensitive, for moisture content determination to constant mass.
• Mixing bowls, spatulas, sealable containers, moisture cans, and a supply of distilled or potable water for specimen preparation.

Sample Preparation

A representative sample of the cargo is obtained using recognized sampling practice and reduced by splitting or quartering to obtain test portions of suitable mass. The material is oven-dried to constant mass so that the initial moisture state is known precisely, and it is allowed to cool in a sealed container to prevent moisture uptake. Water is then added in measured increments, and each batch is mixed thoroughly and allowed to condition so that the moisture is distributed evenly before testing. Testing begins at a moisture content safely below the expected flow point and proceeds toward the critical condition in small, controlled steps.

Test Procedure

The core of the flow table test consists of a repeated cycle of specimen preparation, jolting, observation, and moisture determination, performed as follows.

• Position the truncated cone mold at the center of the flow table and fill it with the prepared material in successive uniform layers, compacting each layer with the standard tamper.
• Strike off the surface to obtain a level, compacted specimen of defined geometry, then carefully lift the mold vertically to leave the freestanding specimen on the table.
• Jolt the flow table a standard series of 25 drops through the calibrated fall height, subjecting the specimen to repeatable dynamic loading.
• Immediately examine the specimen for evidence of flow, including a marked increase in the base diameter, plastic deformation of the cone, visible wet sheen, or migration of water to the specimen surface.
• Take a representative portion of the tested material and determine its actual moisture content by oven drying to constant mass.
• Repeat the full procedure at successively higher moisture contents in small increments until flow is clearly observed, then bracket the Flow Moisture Point between the last non-flowing and first flowing moisture contents and confirm the result with additional replicate tests.

Calculation and Reporting

The Transportable Moisture Limit is calculated as 90 percent of the determined Flow Moisture Point, expressed as moisture content by mass of the wet or dry material as specified by the applicable reporting convention. Plotting the measured specimen deformation against moisture content assists the laboratory in identifying the transition to flow behavior and improves the reliability of the FMP determination. The final test report should identify the test method reference, the sample designation and origin, the sampling and test dates, the particle size observations, the determined FMP and TML, the measured moisture content where applicable, and the details and signature of the testing laboratory responsible for the certification.

Applications

TML flow table testing serves a wide range of commercial, regulatory, and safety functions across the dry bulk supply chain.

• Shipper certification and compliance with the IMSBC Code, providing the TML information that must accompany cargo declarations for Group A materials.
• Mine, port, and terminal quality control before shiploading, including the management of stockpiles exposed to rainfall and the verification of dewatering performance.
• Pre-shipment surveys and protection and indemnity club surveys, where independent verification of TML and moisture content protects carriers and cargo interests.
• Due diligence by charterers, traders, and receivers who require reliable moisture safety data before committing to shipment contracts.
• Verification testing for port state control officers and competent authorities investigating suspected non-compliant cargo.
• Cargo handling research, liquefaction risk studies, and crew training programs that build awareness of Group A cargo hazards.
• Technical investigation of marine casualties, where retrospective TML and moisture data help establish the causes of cargo failure and loss of stability.

FAQ

What does TML mean and why is it important?

TML stands for Transportable Moisture Limit, which is the maximum moisture content at which a bulk cargo is considered safe to transport without liquefying. It is important because a cargo loaded above its TML can liquefy under the motion of a ship, shift inside the hold, and cause the vessel to lose stability or capsize.

Why is the TML set at 90 percent of the Flow Moisture Point?

The 10 percent reduction applied to the experimentally determined Flow Moisture Point provides a deliberate safety margin. This margin accounts for variations in cargo moisture throughout a parcel, moisture migration during the voyage, sampling limitations, and the inherent scatter of laboratory results.

Which materials are suitable for the flow table test?

The flow table method is intended for fine-grained materials with a maximum particle size of approximately 1 mm. For coarser cargoes, alternative IMSBC-recognized procedures, such as the Proctor and Fagerberg methods or the penetration test, are generally more appropriate, and the choice of method should always be justified by the material characteristics.

Is the onboard can test an acceptable substitute for the flow table test?

No. The can test, in which a sample is struck against a hard surface to observe whether free moisture or a wet surface appears, is only an indicative supplementary check that can be performed on board. It cannot replace laboratory determination of the TML and does not satisfy the certification requirements of the IMSBC Code.

What happens if the cargo moisture content exceeds the TML?

The cargo should not be loaded, and the master is entitled to refuse it, unless the specific special provisions of the IMSBC Code apply. The cargo will typically require drainage, drying, or other approved treatment, followed by re-sampling and re-testing to demonstrate compliance before shipment.

How current must the test results be?

As a general rule, moisture content sampling should take place as close as practicable to loading, commonly within seven days before loading operations, and retesting is expected whenever conditions change, such as after heavy rainfall. TML results should be refreshed whenever the characteristics of the material change, and many administrations and industry bodies treat certifications as current for approximately six months under stable conditions.

Who should perform the TML flow table test?

Testing should be carried out by competent laboratories with appropriate accreditation or acceptance by the relevant competent authority, using trained personnel and calibrated equipment. The laboratory report and certificate must clearly identify the responsible organization, since shippers rely on these documents when making the statutory cargo declarations to carriers.