TBM vs THT: Choosing the Right Odorant for Pipeline Conditioning and Long Term Stability

Selecting the correct odorant for a natural gas system is more than a purchasing decision. Odorant chemistry directly affects pipeline conditioning behavior, odor stability, and long term operating cost. Two sulfur based odorants dominate North American pipeline systems: tertiary butyl mercaptan and tetrahydrothiophene.

Both compounds meet regulatory odorization requirements, yet they behave very differently inside pipelines, particularly during commissioning and conditioning. Decades of industry research and field data show that odorant selection plays a major role in odor fade, surface reactions, and overall system performance.

Understanding the differences between TBM and THT allows utilities to make informed decisions that improve safety, reduce odorant consumption, and shorten stabilization time during startup.

Why Odorant Chemistry Matters in Pipeline Systems

Natural gas odorants are sulfur containing organic compounds designed to be detected at extremely low concentrations. While their purpose is simple, their behavior inside pipelines is governed by chemical structure, reactivity, and interaction with internal surfaces.

Research has shown that odorant loss is driven by adsorption, absorption, and chemical reaction with pipe materials. The extent to which each mechanism occurs depends heavily on the odorant selected.

TBM and THT differ significantly in molecular structure, and those differences explain why they perform differently during pipeline conditioning and steady state operation.

Overview of Tertiary Butyl Mercaptan

Tertiary butyl mercaptan is the most commonly used odorant in natural gas systems across North America.

TBM has an extremely low odor detection threshold, strong odor impact, and good soil penetration. These properties make it highly effective for public safety applications.

However, TBM is also more chemically reactive than some other odorants, particularly when exposed to iron oxide surfaces found in steel pipelines.

How TBM Behaves in Steel Pipelines

Industry research demonstrated that TBM reacts readily with active iron oxides present on the internal surface of steel pipelines. During initial exposure, TBM concentration can decrease rapidly until reactive sites are neutralized.

This behavior makes TBM highly effective for pipeline conditioning, since it actively quenches reactive surfaces. Once conditioning is complete, TBM odor levels stabilize and normal injection rates become effective.

Utilities performing pipeline pickling and conditioning often rely on TBM for its predictable surface interaction characteristics.

Overview of Tetrahydrothiophene

Tetrahydrothiophene is a cyclic sulfur compound with higher chemical stability than mercaptans.

THT has a higher odor detection threshold and lower soil penetration than TBM, but it is significantly more resistant to oxidation and surface reaction.

These properties make THT attractive in applications where odor stability is prioritized over aggressive surface interaction.

How THT Behaves in Steel Pipelines

Research confirmed that THT reacts more slowly with iron oxide surfaces than TBM. As a result, THT experiences lower odorant loss during initial exposure to steel pipelines.

This reduced reactivity means that THT requires less total odorant to achieve stabilization, but it also means that surface conditioning occurs more slowly.

Utilities that prioritize odor stability over conditioning speed sometimes select THT or TBM THT blends as part of their pipeline odorization planning strategy.

Conditioning Speed Versus Long Term Stability

One of the most important differences between TBM and THT lies in how quickly they neutralize reactive pipeline surfaces.

TBM reacts rapidly, resulting in faster conditioning but higher initial odorant consumption. THT reacts more slowly, resulting in lower initial loss but extended conditioning periods.

This tradeoff is critical during commissioning of new steel pipelines. Selecting the wrong odorant for the application can extend startup timelines or increase odorant usage unnecessarily.

Utilities often balance these factors by choosing odorant blends that combine the advantages of both compounds.

Odorant Blends and Their Role in Conditioning

Industry research examined blends of mercaptans and sulfides to evaluate their stability and odor impact.

Blends containing TBM provide strong odor impact and effective surface quenching, while the inclusion of THT or sulfides improves stability and lowers freezing points.

Blended odorants are commonly used in systems that experience wide temperature swings or require flexibility across multiple operating conditions.

During pipeline commissioning and startup, blends can offer a practical compromise between conditioning efficiency and odor stability.

Temperature and Pressure Effects on TBM and THT

Both odorants are influenced by operating temperature and pressure, but TBM is more sensitive to changes in these variables.

Higher temperatures increase TBM reaction rates with iron oxide surfaces, accelerating odorant loss during conditioning. THT shows a similar trend but at a slower rate.

Pressure increases adsorption for both odorants, particularly during low flow or stagnant conditions. Understanding these effects helps utilities anticipate odorant demand during seasonal transitions.

Incorporating temperature and pressure considerations into odorization best practices improves predictability and reduces reactive adjustments.

Soil Penetration and Leak Detection Considerations

TBM provides superior soil penetration compared to THT. This characteristic improves leak detection in buried pipelines and is one reason TBM remains widely used in distribution systems.

THT, while stable, may produce weaker odor perception above ground in some soil conditions. Utilities must weigh this factor when selecting odorants for specific service areas.

In many cases, odorant selection is influenced by both pipeline behavior and public safety performance.

Cost Implications of Odorant Selection

Odorant cost is not determined solely by purchase price. Consumption rate, conditioning efficiency, and long term stability all influence total cost of ownership.

While THT may require lower initial quantities, extended conditioning periods can increase labor and operational complexity. TBM may require higher initial volumes but allows faster stabilization and transition to steady state injection.

Utilities that align odorant selection with pipeline conditioning services often achieve lower overall odorization costs.

Best Practices for Odorant Selection

Research and field experience support several best practices when selecting between TBM and THT.

Match odorant chemistry to pipeline material and condition.Consider conditioning speed versus long term stability requirements.Account for temperature, pressure, and seasonal operation.Use blends strategically when flexibility is needed.Coordinate odorant selection with commissioning and conditioning plans.

Odorant selection should be treated as an engineering decision rather than a commodity purchase.

Applying Odorant Selection in the Field

At Burgess Pipeline Services, odorant selection is integrated into the broader pipeline conditioning and commissioning process.

Projects are evaluated based on material type, internal surface condition, operating environment, and safety objectives. Odorant chemistry is selected to support predictable conditioning and stable long term odorization.

This approach supports consistent outcomes across new construction, pipeline rehabilitation, and system expansion projects.

TBM and THT are both effective odorants. Choosing the right one at the right time is what ensures safe, reliable, and cost effective pipeline operation.