Storage Tanks & Suitable Gas Detection
Storage tanks are a huge part of refineries in the oil and gas industry, providing safe containment for a wide range of liquids and gases, from crude oil and refined fuels to chemicals and liquefied gases. The design and structure of these tanks vary depending on the type of product stored, the storage conditions required, and safety considerations such as pressure, volatility, and temperature.
In this blog, we’ll take a closer look at four of the most common types of storage tanks used across the industry: Fixed Roof Tanks, Floating Roof Tanks, LNG (Liquefied Natural Gas) Tanks, and Spherical Tanks. Each of these designs serves a distinct purpose and comes with unique operational advantages and safety challenges.
Fixed Roof Tanks
Fixed roof tanks are among the most cost-effective options for liquid storage and are primarily used for storing low-pressure liquids. Emissions from these tanks typically result from changes in temperature, pressure, and liquid levels during filling and emptying operations.
When the tank is being filled, vapours are displaced and vented directly into the atmosphere. Conversely, as the tank is emptied, fresh air enters to replace the lost volume. Because these tanks offer limited control over fugitive emissions, refineries often need to estimate the total amount of vapor released annually based on tank activity and throughput.
To help minimise vapour losses, fixed roof tanks are equipped with pressure-vacuum vents. These vents allow fresh air to enter during cooler conditions to balance internal pressure, reducing unnecessary vapour discharge to the atmosphere.
Floating Roof Tank
A floating roof tank is designed so that the roof rests directly on the liquid surface, rising and falling with changes in the liquid level. The roof is guided along the tank wall, typically using rollers, and a seal between the roof and the tank wall prevents vapours from escaping into the space above the liquid. Because the roof remains in constant contact with the liquid, there is no vapour space above it. This eliminates the need for venting during filling or for adding inert gas during emptying.
Some floating roof tanks are open to the atmosphere, allowing the position of the roof to be visible from above, while others are fitted with a fixed outer roof for additional protection from weather and contamination.
Lightning is a significant cause of tank fires, whether from direct strikes or induced electrical charges.
Because the area above an internal floating roof can contain hazardous vapours, it is considered a confined space. Any entry onto the internal floating roof must comply with confined space entry procedures.
LNG (Liquefied Natural Gas) Tanks
Liquefied Natural Gas (LNG) is natural gas that has been cooled to approximately –162°C (–260°F), transforming it into a liquid state to make storage and transportation more efficient. LNG storage tanks are specifically engineered to maintain this extremely low temperature, preventing the gas from reverting to its gaseous form.
These tanks are critical components in the LNG value chain, ensuring the safe storage, handling, and distribution of cryogenic liquids. Depending on their design and intended use, LNG storage tanks come in several configurations:
Single Containment Tanks:
The most basic design, consisting of a single shell that holds the LNG. While cost-effective, these tanks provide minimal secondary protection in the event of leaks or structural failures.Double Containment Tanks:
Featuring an inner tank for LNG storage and an outer shell for secondary containment, this design enhances safety by reducing the risk of environmental contamination in case of a spill.Full Containment Tanks:
Considered the safest option, full containment tanks include both inner and outer tanks, with the outer wall typically constructed of reinforced concrete to withstand external impacts. These are commonly used in large-scale LNG terminals.Underground Tanks:
Installed below ground level, these tanks are often lined with specialised materials to prevent leakage. The extreme cold of the LNG can freeze the surrounding soil, naturally forming an impermeable barrier that enhances containment.Membrane Tanks:
Built with a thin metallic membrane, usually stainless steel or other specialized alloys these tanks are lightweight and space-efficient. They are widely used in LNG carriers for maritime transport due to their high capacity and optimised design.
Spherical Tanks
Spherical tanks are among the strongest and most durable types of storage vessels, outperforming common designs such as fixed roof, open-top, and floating roof tanks. The spherical shape provides an even distribution of stress across both the internal and external surfaces, effectively eliminating weak points.
Conventional storage tanks are typically constructed from multiple metal plates welded or bolted together, either in the workshop or on-site. While effective for many applications, welds and seams can become vulnerable under high-pressure conditions. In contrast, a spherical tank is constructed using fewer individual components and therefore has fewer welded joints, significantly enhancing its overall strength and reliability.
The internal pressure within any storage tank is closely linked to the temperature of the stored liquid, which can fluctuate due to ambient environmental conditions. This relationship is similar to how water expands and contracts within a sealed container when heated or frozen. The spherical shape of the tank provides exceptional resistance to these pressure variations while minimizing the external surface area exposed to the atmosphere. This reduced surface area limits heat transfer from the surrounding environment, helping to maintain stable internal conditions and improving overall safety and efficiency.
Suitable Gas Detectors
BW Clip to detect H2S
Hydrogen sulphide (Hâ‚‚S) is commonly found in various areas of tank storage facilities. Many crude oil tanks naturally contain Hâ‚‚S, which can evolve into the vapour space during storage, creating a toxic atmosphere above the liquid. Produced water and slop oil tanks also pose a risk, as water separated from crude oil during production often contains dissolved Hâ‚‚S. In refinery and petrochemical storage, tanks holding residual fuels, asphalt, sour condensates, or certain chemical intermediates can release Hâ‚‚S vapours. Fixed roof tanks and their vents may allow vapours to accumulate inside or escape through pressure-vacuum valves. Additionally, pipeline and manifold areas can be sources of Hâ‚‚S exposure when leaks occur or during venting operations, posing hazards to personnel and the surrounding environment.
The BW Clip Series is one of the most user-friendly, reliable, and cost-effective solutions for ensuring safety, compliance, and productivity. For Hâ‚‚S, a two-year version is available, which can extend its operational life even further when used with a hibernation case for periods of inactivity of a week or more, effectively prolonging service life for up to a year.
For more information, click here: BW Clip 3 Year Single Gas Detector | Rockall Safety
BW Ultra for H2S, CO, LEL, O2 and VOCs
Storage tanks can contain a variety of hazardous gases that pose both health and safety risks. Hydrogen sulphide (Hâ‚‚S), found in sour crude oils and some refined products, can accumulate in tank vapour spaces and is highly toxic and flammable, requiring careful detection. Carbon monoxide (CO), though less common, may form near burners, heaters, or hot surfaces due to incomplete combustion or reactions with flammable gases. Volatile organic compounds (VOCs), including benzene, toluene, xylene, and other hydrocarbons from crude oil, gasoline, and diesel, are highly flammable and a major concern for emissions and fire hazards. Combustible gases, measured via their Lower Explosive Limit (LEL), such as methane and other hydrocarbons, can cause fires or explosions if concentrations exceed safe levels. Oxygen (Oâ‚‚), present from air ingress or tank operations, must be monitored closely because it supports combustion and can exacerbate fire risks.
The Honeywell BWâ„¢ Ultra is a five-gas detector engineered for safe sampling and monitoring of confined spaces, both before and during entry. It features advanced sensor technology, clear gas-readout visibility, enhanced comfort, and reliable connectivity, even in the harshest environments. The device simultaneously monitors the four gases typically required for confined-space safety, while a fifth customisable sensor allows you to track an additional gas of your choice, providing an extra layer of protection for every confined-space entry.
For more information, click here: BW Ultra 5 Multi-Gas Detector | Rockall Safety
UltraRAE 3000+ for VOCs & Benzene
Volatile organic compounds (VOCs), including benzene, are commonly present in tank storage containing crude oil, gasoline, diesel, and other petroleum products. These compounds readily evaporate from the liquid phase into the tank’s vapour space, creating flammable and potentially toxic atmospheres. Benzene, a specific VOC found in many hydrocarbons, is particularly hazardous due to its toxicity and carcinogenic properties.
The UltraRAE 3000 is a leading Compound-Specific Monitor, offering exceptional versatility and precision. Its Photoionisation Detector (PID) provides an extended detection range of 0.01 to 10,000 ppm in VOC mode and 10 ppb to 200 ppm in benzene-specific mode, making it ideal for a variety of applications from pre-entry screening during refinery and plant maintenance to hazardous material response, marine spill incidents, and downstream refinery monitoring.
For more information, click here: UltraRAE 3000 Rae Systems Benzene Detector
