Applications and design differences for pressure and non-pressure tanks

In many sectors of the economy, the storage of gases, liquids and semi-liquids (e.g. LPG, DME, fuels, heating oil, water, etc.) is an everyday occurrence associated, among other things, with the need to guarantee continuity and efficiency. From this point of view, it is essential to understand the differences between cylindrical steel pressure vessels and non-pressure vessels. Both types differ in their design and purpose, which has a significant impact on their operational safety, areas of application and functionality.

Pressure vessels: power under pressure

Pressure vessels are designed to ensure the safe storage of gases and liquids at high pressures different from ambient pressure. Their design must be able to withstand both the internal operating pressure and possible overloading, temperature shocks or corrosive properties of the stored substances. The use of steels with the right parameters (such as thickness, steel impact resistance, steel fatigue resistance, steel tensile strength, etc.) and the special cylindrical shape of the tank ensure even pressure distribution and increase operational safety. Pressure vessels are therefore essential in industries such as the chemical, petrochemical and energy sectors, where substances are stored under high pressure.

Non-pressure tanks: storage stability

Non-pressure tanks are tanks in which there is a slight over- or underpressure regulated by venting valves, are used for the storage of liquid substances. Their design is simpler and their wall thickness (shell) requirements are less strict than those of pressure vessels. Non-pressure cylindrical tanks are commonly used to store oils, water and various types of fuel, including diesel and heating oil. Their advantage is safety in the event of a leak – the lack of pressure means that the substance does not escape with the same force as in pressurized tanks.

Pressure tanks design and monitoring

Pressure tanks require thicker steel and reinforcement in the form of brackets to withstand internal operating pressures in excess of atmospheric pressure. The cylindrical shape of the tank is optimal as the pressure is evenly distributed over the entire surface. In addition, tanks of this type are equipped with numerous safety valves, pressure gauges and other sensors that constantly monitor their levels and immediately signal any abnormalities.

Design and monitoring of non-pressure tanks

Non-pressure tanks have a simpler design. The tank shell can be thinner as they are not affected by high internal pressure. Monitoring of non-pressure tanks is often limited to monitoring the level of the substance stored in the tank. Of course, basic safety measures such as leak detection or quality control of the stored substance are also applied.

Pressure and non-pressure tanks – one or two shells?

Steel pressure tanks do not necessarily have to be double-walled tanks. Their design can be based on a single shell. The choice between one or two shells depends on safety requirements, the type of substance to be stored, legal regulations and the specific needs of the operator in question, resulting from the type of process in which the tank is used.

Single-walled tanks are commonly used to store substances for which there are no additional environmental requirements against leakage. Examples include tanks for compressed air or pressure tanks for natural gas used in industry.

The design of bunded tanks assumes the existence of an additional ‘safety buffer’. This is the interstitial space, which is the space between the wall of the inner tank and the outer tank. The interstitial space acts as a protective barrier in the event of a leak from the inner tank. This space can be monitored to allow early detection of potential leaks. Bunded tanks are often used for the storage of environmentally hazardous substances.

In some countries, legislation may require the use of bunded tanks for the storage of a particular type of substance or for the storage of substances under particular conditions, particularly where there is a risk of environmental contamination. The final choice of tank design should be based on risk analysis, legislation and best engineering practice.

What’s inside – what substances can be stored in pressure and non-pressure tanks?

The physical and chemical properties of the substance to be stored should always be taken into account when selecting the type of container. Such parameters include vapor pressure, reactivity or chemical aggressiveness.

The following substances are most commonly stored in pressure vessels:

• Compressed gases: e.g. oxygen, nitrogen, helium, hydrogen.
• Gases liquefied by pressure: propane, butane, ammonia, chlorine, carbon dioxide, freons.
• Gases liquefied by cooling: liquid nitrogen, liquid oxygen, liquid hydrogen, liquid helium.
• Liquids with high vapor pressure: some petroleum fractions, gas condensates.
• Reactive substances: silane, phosgene, nitrogen oxides.
• Substances used for food: carbon dioxide for carbonated drinks, nitrogen for food packaging.

Non-pressure tanks are used for the storage of, among other things, the following substances:

• Non-hazardous liquids: water, waste water, saline solutions, AdBlue.
• Liquid fuels: diesel, fuel oil, petrol, bioethanol, biodiesel.
• Industrial oils: Lube oils, hydraulic oils, transformer oils.
• Chemicals: acids and bases with low chemical aggression, e.g. low concentration hydrochloric acid, sodium hydroxide.
• Food substances: vegetable oils, syrups, honey, wine, juices.
• Agricultural products: liquid fertilizers, pesticides, herbicides.
• Rainwater and treated water: Collection and storage for irrigation or industrial purposes.

The choice depends on the application

The choice between pressure and non-pressure tanks depends on the characteristics of the material to be stored and the safety requirements. Pressure and non-pressure vessels serve different purposes and are characterized by different design solutions. However, both types are an essential part of manufacturing and industrial processes where reliability and safety are paramount.