What is a pressure vessel?? In industrial refrigeration and mechanical engineering, this is purpose-built equipment for holding, transporting or combining physical and chemical processes involving media at above atmospheric pressure. Under Vietnamese safety standards (TCVN), understanding the construction and operating principles of pressure equipment is fundamental to keeping a system safe. With more than 20 years designing and installing industrial cold stores, Tan Long sets out below the inspection requirements and the operating engineering for this equipment in detail.
What is a pressure vessel?
A pressure vessel (in English: lamp shelter) is a sealed unit holding liquid or gas above atmospheric pressure. In an industrial refrigeration system, it stores and regulates refrigerant between the stages of the thermodynamic cycle, keeping the flow steady across the whole system.
Pressure vessels operate in a pressure range from 1.5 bar to over 300 bar (21.7 psi to 4,351 psi), depending on the type of fluid and specific application. Common construction materials include carbon steel (ASTM A516), stainless steel 304/316, and aluminum alloys. Equipment is certified to ASME Section VIII Div.1 or TCVN 8366:2010 in Vietnam.
Pressure vessel construction and safety principles in detail
Having established what What is a pressure vessel?is, understanding its construction in detail helps engineers and owners operate the system more safely. A standard pressure vessel at Tan Long normally comprises 6 core components, manufactured and quality-controlled to a strict standard:
- Shell: This is the main load-bearing cylindrical body, fabricated from hot-rolled steel plate 6 mm to 50 mm thick depending on the design pressure. The shell resists the hoop and longitudinal stresses as the medium inside expands with temperature, keeping the whole pressure vessel sound.
- Head: These are the end closures sealing the cylindrical shell, usually hemispherical or ellipsoidal. In industrial cold storagesystems, the 2:1 ellipsoidal head is preferred for its optimal pressure capability relative to material weight.
- Nozzles: These connection pipes are welded through the shell and properly reinforced. Nozzle sizes follow ASME B16.5 (DN25 to DN200), ensuring perfect compatibility with the pipework of refrigeration system.
- Safety relief valve: A mandatory protective device that automatically relieves pressure when it exceeds the design threshold by 10% to 25%. In Vietnam, the valve must be set to ASME PTC 25 and inspected every 12 months under occupational safety regulations.
- Pressure gauge: The instrument for monitoring the actual working state. For accuracy, the gauge range must be at least 1.5 times the maximum allowable working pressure (MAWP). For example, a vessel with an MAWP of 25 bar needs a gauge reading to at least 37.5 bar.
- Supports: These distribute the weight of the vessel and its contents to the foundations. Depending on the installation design, Tan Long uses a skirt support for vertical vessels or saddles for horizontal ones.
How a pressure vessel works in a refrigeration system
To understand how a thermodynamic system operates, we first need to know What is a pressure vessel? in the process diagram. In practice these vessels do not generate pressure themselves; they act as intermediate equipment storing, holding and regulating the refrigerant pressure throughout refrigeration compressor pushes high-pressure refrigerant vapour into the vessel, the unit separates the phases and regulates the flow, providing a steady refrigerant supply to the evaporators on the low-pressure side.
In practice this principle shows itself clearly in the role each specialised vessel plays within an industrial cold store system:
- High-pressure receiver: Stores liquid refrigerant (such as NH₃) at 10 bar to 16 bar, corresponding to condensing temperatures of 25°C to 40°C, ensuring a continuous liquid supply to the system.
- Intercooler: On two-stage compression systems, this holds the intermediate pressure at roughly 4 bar to 5 bar, optimising compressor performance.
- Liquid separator: Operating on the low-pressure side (temperatures from -40°C to -10°C), it acts as the "shield" protecting the compressor from liquid flooding, which causes serious mechanical damage.
The key to how a pressure vessel works is phase separation by difference in density. Taking NH₃ at 30°C as a concrete example, the liquid phase has a density of 595 kg/m³
Types of pressure vessel in refrigeration systems
On industrial refrigeration projects, knowing the types of What is a pressure vessel? and where each is installed is critical to optimising thermodynamic performance. Depending on the specific function, a system typically uses four types of pressure vessel:
| Vessel type | Position in the cycle | Working pressure | Main functions |
|---|---|---|---|
| High-pressure receiver | After the condenser | 10 – 20 bar (145 – 290 psi) | Stores liquid NH₃ after condensing |
| Liquid separator | Before the low-stage compressor | 1.5 – 4 bar (22 – 58 psi) | Separates wet vapour, protects the compressor |
| Intercooler | Between the two compression stages | 4–6 bar (58–87 psi) | Cools the compressed vapour, improving efficiency |
| Oil separator | After the compressor | 12 – 18 bar (174 – 261 psi) | Separates lubricating oil from the refrigerant |
High pressure tank is the largest vessel in the system, ranging from 200 litres to over 10,000 litres (53 to 2,641 gallons). Its function of holding a reserve of liquid NH₃ keeps the system stable when the heat load changes suddenly — for instance when a cold store takes in a large consignment.
Liquid separator protects the compressor from liquid slugging, an event that can destroy the discharge valves and pistons within seconds. The vessel is designed with a cross-section large enough that the internal vapour velocity never exceeds 0.5 m/s (1.6 ft/s), so liquid droplets settle out before the vapour reaches the compressor.
Intermediate tank appears in two-stage compression systems when the evaporating temperature falls below -28°C (-18°F). It cools the compressed vapour from the low stage before it enters the high-stage compressor, reducing the discharge temperature from 120°C to 80°C (248°F to 176°F) and improving cycle efficiency by 8% to 12% over single-stage compression.
Pressure vessel safety standards and inspection
Pressure vessels fall into the category of equipment with the strictest occupational safety requirements. In Vietnam, every vessel over 25 litres (6.6 gallons) with a working pressure above 0.7 bar (10.2 psi) must be registered and inspected.
The legal framework in Vietnam has three layers: TCVN 8366:2010 governs design and manufacture, Circular 36/2019/TT-BLDTBXH governs technical safety inspection, and QCVN 01:2008/BLDTBXH covers occupational safety for pressure equipment.
The mandatory inspection cycle under Circular 36/2019 comprises: initial inspection before commissioning, periodic inspection every 3 years, and extraordinary inspection after major repairs.
The most widely used international standards are ASME BPVC Section VIII Division 1 (USA) and PED 2014/68/EU (Europe). Vessels imported from Europe carry the CE mark; those from the USA carry the "U Stamp".
Construction Material for Pressure Vessels and Selection Criteria
The material determines service life, pressure limits and compatibility with the refrigerant. Choosing the wrong material leads to rapid corrosion, stress cracking or brittle failure at low temperature.
| Material | Standard | Minimum temperature | Popular Applications |
|---|---|---|---|
| ASTM A516 Gr.70 carbon steel | ASME | -45°C (-49°F) | NH₃, CO₂, low-pressure freon |
| SS304 stainless steel | ASTM A240 | -196°C (-321°F) | Systems requiring high hygiene (food) |
| SS316L stainless steel | ASTM A240 | -196°C (-321°F) | Corrosive environments, seawater |
| 5083 aluminium alloy | ASTM B209 | -196°C (-321°F) | LNG, deep-freeze systems |
A516 Gr.70 carbon steel is the most common choice for NH₃ systems in Vietnam, being 40% to 60% cheaper than stainless steel and fully compatible with NH₃ down to -45°C (-49°F). Note: NH₃ corrodes copper and copper alloys, so copper must never be used in an NH₃ system.
Refrigeration systems for processed food, where hygiene is mandatory, need SS304 or SS316L stainless steel to meet FDA 21 CFR and TCVN 5603:2023 food safety standards.
Pressure receivers in industrial NH₃ refrigeration systems
Ammonia (NH₃) is the most widely used industrial refrigerant in Vietnam, accounting for over 70% of all industrial refrigeration systems installed in seafood processing zones and cold stores. Pressure vessels in NH₃ systems face higher technical requirements than freon ones, because of the higher working pressure and the toxicity of NH₃.
The design pressure for an NH₃ high-pressure receiver is typically 18 bar to 24 bar (261 psi to 348 psi), 50% higher than an R404A or R507 system at the same condensing temperature. That calls for a thicker shell and welds that pass 100% radiographic examination (RT Full) under ASME.
Tan Long designs and supplies pressure vessels for NH₃ systems in volumes from 100 litres to 5,000 litres (26.4 to 1,321 gallons), with design pressures from 18 bar to 25 bar.
How to select the right pressure vessel for a refrigeration system
Choosing a pressure vessel rests on seven core technical parameters, none of which can be skipped.
| Parameter | How to determine it | Note |
|---|---|---|
| Design pressure | MAWP × 1.1 | Normally 10% above the working pressure |
| Volume | System capacity × storage factor | NH₃: minimum 20 litres/kW (5.3 gallons/kW) |
| Design temperature | Operating temperature plus a safety margin | Not below -45°C (-49°F) for A516 |
| Medium | Refrigerant used | Directly affects the material choice |
| Inspection standard | ASME, PED, TCVN | As required by the customer or the market |
| Installation orientation | Horizontal or vertical | Affects the floor area required |
| Pipe connections | DN, ASME flange, thread | Matched to the system pipework |
The most common mistake in selecting a vessel is undersizing the volume, which causes constant pressure swings, forces the regulating valves to cycle far too often and shortens system life. For a 500-tonne cold store running on NH₃, the high-pressure receiver needs a minimum volume of 1,000 litres (264 gallons) to provide an adequate hydraulic buffer.
Installing and maintaining a pressure vessel correctly
Correct installation and maintenance determine the vessel's real service life — typically 20 to 30 years when done properly, falling to 5 to 10 years if scheduled maintenance is skipped.
Before installation, the floor must be able to carry the static load: a 1,000-litre vessel full of liquid NH₃ weighs roughly 1,600 kg (3,527 lb), before counting the vessel's own weight of 300 kg to 500 kg (661 lb to 1,102 lb). A minimum clearance of 600 mm (23.6 inches) is needed all round for inspection and maintenance.
Scheduled maintenance covers four main items. A monthly visual inspection catches surface rust, leaks at the joints and the condition of the insulation. The safety valve is checked every 12 months by lifting the valve lever to confirm it is not stuck. Shell thickness is measured by ultrasonic testing (UT) every 3 years at potential corrosion points such as the bottom of the vessel and where it contacts the supports. A hydrostatic test is repeated whenever the shell thickness is found to have dropped more than 15% below the original design.
Tan Long supplies pressure vessels for industrial refrigeration systems
Tan Long designs and builds industrial refrigeration systems with more than 20 years of experience in the Vietnamese market. The pressure vessels Tan Long supplies are fabricated from A516 Gr.70 steel or SS304/316L, with 100% radiographic weld examination, hydrostatic testing to 1.5 times the design pressure before leaving the works, and a complete technical file for inspection registration in Vietnam.
Alongside pressure vessels, Tan Long supplies complete refrigeration equipment: industrial compressors, evaporative condenser, industrial refrigeration and cold stores. Contact the Tan Long engineering team for advice on choosing the right pressure vessel for your refrigeration system.
A pressure vessel is an indispensable intermediate component in an industrial refrigeration cycle, performing three jobs at once: storing refrigerant, separating the liquid and vapour phases, and regulating flow throughout the system.
Investing in a pressure vessel built to standard from the outset saves operating cost and reduces the risk of production stoppages over the long term — particularly important for food and seafood processing plants with continuous operation and high safety requirements.
If you need a pressure vessel to a specification not available off the shelf, see the process for manufactures pressure equipment to order.
Frequently asked questions about pressure vessels
How does a pressure vessel differ from an ordinary vessel?
A pressure vessel is designed and certified to withstand internal pressure above atmospheric, typically from 0.7 bar to several hundred bar (10 psi to several thousand psi). An ordinary vessel has no pressure certification requirement. A pressure vessel must be registered with the authorities before being put into service.
How often does a pressure vessel need inspecting?
Under Circular 36/2019/TT-BLDTBXH, a pressure vessel is inspected every 3 years. An extraordinary inspection is also required after an incident, after a major repair, or when the vessel is returned to service after more than 12 months out of use. The safety valve is checked separately every 12 months.
Are pressure vessels in NH₃ refrigeration systems dangerous?
NH₃ is a toxic refrigerant, immediately dangerous to life at 300 ppm (IDLH). However, an NH₃ pressure vessel designed, fabricated and inspected to ASME or TCVN 8366:2010 is safe under normal operating conditions. The risk comes mainly from maintenance outside the proper cycle or from incorrect installation.
What is the difference between a high-pressure receiver and a liquid separator?
A high-pressure receiver stores liquid refrigerant after the condenser, operating on the high-pressure side of the cycle at 10 to 20 bar (145 to 290 psi). A liquid separator operates on the low-pressure side at 1.5 to 4 bar (22 to 58 psi), its main function being to separate wet vapour and protect the compressor from liquid flooding.
How is the volume of a pressure vessel calculated?
Pressure vessel volume is calculated from the system's cooling capacity, the refrigerant and the required storage time. A practical formula for NH₃: volume (litres) = cooling capacity (kW) × 20. For example, a 50 kW system needs a receiver of at least 1,000 litres (264 gallons). This figure is adjusted to the actual range of heat load variation at the plant.
Frequently asked questions about pressure vessels
What is a pressure vessel?
A pressure vessel is a sealed unit for holding or processing a medium above atmospheric pressure. In a refrigeration system, the group comprises the high-pressure receiver, the liquid separator, the oil separator, the intercooler and the condenser vessel.
Is inspection of a pressure vessel mandatory?
Yes. Pressure vessels fall within the category of equipment with strict occupational safety requirements, and must undergo technical safety inspection before being put into service and at intervals during operation.
What role does a pressure vessel play in a refrigeration system?
The receiver holds a reserve of liquid refrigerant for the system; the liquid separator prevents liquid refrigerant returning to the compressor; the oil separator recovers lubricating oil; the intercooler cools the compressed vapour between stages on multi-stage systems.
What does manufacturing a pressure vessel require?
The material and thickness must be calculated against the design pressure, the welds must be made by certified welders and subjected to non-destructive examination, and the vessel must come with a design file, a manufacturing file and pressure test records as required.
Does Tan Long manufacture pressure vessels?
Yes. Tan Long manufactures pressure vessels for industrial refrigeration systems, complete with technical documentation and inspection under the applicable regulations.
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With over 18 years of practical experience in directing, consulting, designing, and constructing heavy industrial refrigeration systems, ASRS automated warehouses, and IQF freezing systems across more than 29 major national projects.
