Pressure Vessel Safety and Explosion Risks: Energy Release, Testing Methods and Safe Equipment Layout Principles

Süleyman TUCER,PhD
SELNİKEL A.Ş.

Abstract

Pressure vessels, storage tanks, and steam boilers are energy-storing equipment due to the pressure of the fluids they contain. The sudden release of this stored energy can result in pressure waves, projectile effects, structural damage, and fatal industrial accidents. The primary factor determining the severity of an explosion is not only the total amount of stored energy, but also the rapid release of that energy within a very short period of time. Since gases are compressible fluids, they store significantly more elastic energy than liquids and therefore can produce more severe consequences during sudden vessel failures. For this reason, hydrostatic testing is considered the primary method for pressure equipment testing, while testing with air or other gases presents substantially higher risks. The most common causes of testing accidents include nozzle and sleeve welding defects, insufficient weld penetration, improper weld preparation details, and unsafe connections of pressurization equipment. In facility layout design, safety distances, maintenance accessibility, fire protection, segregation of hazardous fluids, and potential axial projectile effects must be evaluated collectively. Therefore, tanks and boilers are not positioned arbitrarily; horizontal tanks are typically installed with parallel axes, vessel heads are not arranged facing one another, and sufficient separation distances are maintained between equipment.

1.Introduction

Pressure vessels are widely used in industrial applications. Air receivers, steam boilers, process vessels, separators, reactors, heat exchangers, and storage tanks all fall within this category. Steam boilers are also considered pressure vessels because they operate under pressure and store energy.

The primary hazard associated with these systems is the uncontrolled release of stored energy. When rupture, fracture, separation, or sudden opening occurs, the stored energy is released within a very short period of time. Such a release not only damages the equipment itself but may also affect surrounding personnel, structures, and adjacent equipment.

Reducing explosion energy solely to the product of pressure and volume is not sufficient. Although the pressure–volume relationship provides a quantity with energy dimensions, it does not fully explain actual explosion behavior. The real impact must be evaluated considering the work performed during fluid expansion, release duration, fluid phase, vessel geometry, weak regions, and the fragmentation behavior of the vessel.

2.Stored Energy and Explosion Effects

The energy stored within a pressure vessel originates from the compressed state of the fluid contained inside it. As long as the vessel remains intact, this energy is maintained under control. However, if the vessel suddenly ruptures or fails at a weak point, the stored energy is released instantaneously into the surrounding environment.

The severity of an explosion is not determined solely by the amount of stored energy. The critical factor is the duration of energy release. If the same amount of energy is discharged over a long period, its effects remain relatively limited. However, when the same energy is released within a very short time, extremely high power levels are generated. For this reason, sudden releases are significantly more destructive than gradual discharge events.

Damage resulting from pressure vessel explosions typically occurs through the following effects:

• Sudden overpressure
• Pressure wave generation
• Impulse effects
• High-velocity projection of vessel fragments
• Secondary mechanical effects caused by fracture and deformation
   

In many incidents, the most fatal consequence is not the pressure wave itself but the projection of fragments. Nozzles, blind plugs, covers, vessel heads, connection components, and fractured shell sections can behave like projectiles. Therefore, explosion assessment must consider not only the fluid release energy but also the mechanical fragmentation behavior of the vessel itself.

3. Gases, Liquids, Two-Phase Systems, and Nitrogen-Blanketed Systems

Steam, air, nitrogen, and other gases are compressible fluids. As a result, they can store significant amounts of elastic energy when under pressure. If a vessel suddenly opens or ruptures, the gas rapidly expands, generates a pressure wave, and can propel vessel fragments at high velocity. This is the primary reason why gas systems present substantial hazards.

Liquids, on the other hand, are significantly less compressible than gases. Under identical conditions, the liquid phase stores considerably less elastic energy than the gas phase. In a pressurized liquid system, even a small release generally causes pressure to drop more rapidly, and the classical energy release effect is much lower than in gas-filled systems. This does not mean that liquid systems are risk-free; however, they unquestionably contain less expansion energy than gas systems.

In practice, tanks are rarely filled completely with liquid. Liquids heat up and expand, cool down and contract, while liquid levels continuously change during filling and discharge operations. Therefore, tanks are typically designed with a free volume or vapor space. In actual process systems, the following configurations are commonly encountered:

• Tanks containing liquid in the lower section and gas or vapor in the upper section
• Partially filled tanks with a free gas volume above the liquid level
• Nitrogen-blanketed storage tanks
• Process vessels operating under vapor-liquid equilibrium conditions
• Separators
• Flash tanks
• Pressurized product storage tanks
• Horizontal or vertical tanks with variable gas volumes depending on filling level
   

Nitrogen-blanketed systems deserve special consideration. In these systems, nitrogen occupies the upper portion of the tank. Nitrogen is commonly used to prevent product oxidation, avoid moisture ingress, maintain an inert atmosphere, and enhance process safety.

However, nitrogen itself is also a compressible gas. Therefore, even if a nitrogen-blanketed tank primarily contains liquid, the gas phase still stores energy and may create serious hazards in the event of sudden rupture.

The conclusion is clear: a tank being predominantly filled with liquid does not automatically ensure safety. If gas, vapor, or a nitrogen blanket is present inside the tank, that volume directly influences the explosion behavior of the system.

4. Safety Relief Valves and Rupture Discs

Tanks and pressure vessels are equipped with protective devices designed to prevent excessive pressure accumulation. Among these devices, safety relief valves are the most commonly used. In certain applications, rupture discs are also employed.

Safety relief valves open when the system pressure exceeds a predetermined set value and discharge fluid to prevent the equipment from reaching dangerous overpressure conditions. Rupture discs, on the other hand, are protective devices designed to burst at a specified pressure, providing instantaneous pressure relief. They are particularly used in applications where rapid pressure rise is expected and where leak-tightness is critical.

The presence of these protective devices is important; however, they are not sufficient on their own. Even in systems equipped with safety relief valves or rupture discs, serious accidents may still occur due to welding defects, localized weaknesses, connection failures, improper testing procedures, or incorrect installation practices. Protective devices cannot replace proper design, qualified manufacturing practices, and safe operational procedures.

5. Pressure Testing and Test Safety

Pressure equipment testing is generally performed using water. The reason is straightforward: water has very low compressibility and therefore stores very little energy during testing. If a failure occurs during a hydrostatic test, the amount of energy released remains limited.

Testing with air or other gases carries significantly higher risks. Since gases are compressible, they can store considerable amounts of energy. In the event of an explosion, this energy is released suddenly. As a result, metal components may be projected at high velocity, effectively transforming pressure equipment into a fragmentation hazard. Such incidents can result in severe injuries and multiple fatalities.

The fundamental rule is clear: Hydrostatic testing is the primary and preferred method for pressure equipment testing. Pneumatic testing should only be performed when absolutely necessary, under additional safety precautions, and at the lowest practical pressure level.

Even during hydrostatic testing, gas pockets must not remain trapped within the system. In a system that appears to be completely filled with water, residual air pockets may still exist, causing the system to store more energy than expected. Therefore, complete air venting prior to testing is essential.

6. Major Causes of Accidents During Pressure Testing

The most common causes of fatal accidents during pressure vessel testing occur not in the vessel shell itself, but rather at connection regions. The most critical areas include nozzles, sleeves, blind connections, plug locations, and temporary testing apparatus.

The primary causes of accidents include:

• Defects in nozzle welds
• Manufacturing defects in sleeve connections
• Insufficient weld penetration
• Improper weld preparation details
• Welding practices that do not comply with design specifications
• Lack of fusion and inadequate weld cross-sections
• Weak temporary test connections
• Unsafe or improperly secured pressurization equipment connections
   

These defects may result in sudden failures during testing. Nozzles and sleeves can separate from the vessel and become high-velocity projectiles, leading to severe or fatal accidents. Likewise, if pressurization equipment connections are not properly secured, sudden separation may occur and connection components may be projected at high speed.

The fundamental reality is clear: one of the most dangerous events during pressure testing is the projection of improperly manufactured or inadequately secured components under pressure.

Therefore, testing safety cannot be ensured solely by controlling the test pressure. Welding quality, connection details, the structural integrity of temporary equipment, installation accuracy, and overall test area safety must all be evaluated collectively.

7. Equipment Layout Principles for Tanks, Boilers, and Pressure Equipment

Maintaining safety distances between tanks, boilers, and other pressure equipment is essential. These separation distances are not only intended to reduce the effects of potential explosions, projectile hazards, pressure waves, fires, or domino effects; they are also necessary to ensure safe operation, periodic inspections, maintenance activities, repair work, equipment removal and installation procedures, and emergency response operations.

Placing equipment too close together creates two direct consequences. First, failures or accidents occurring in one piece of equipment may affect neighboring systems. Second, maintenance and repair activities become significantly more difficult. Insufficient spacing can restrict personnel access, complicate lifting and handling operations, limit access to valves and nozzles, hinder inspections and testing procedures, and delay emergency response actions.

Equipment layout planning should not be based solely on process flow and space utilization. The following factors must be evaluated together:

• Maintenance accessibility
• Safe working areas
• Lifting and handling capabilities
• Valve and nozzle accessibility
• Ease of testing and inspection
• Firefighting accessibility
• Emergency escape routes
• Potential projectile directions
• Pressure wave effects
• Domino risk
   

Storage systems containing flammable fluids should not be located adjacent to systems containing oxidizing or combustion-supporting fluids within the same area. The presence of such materials together increases both fire and explosion risks. Therefore, stricter separation requirements are applied, and where necessary, physical barriers and firewalls are implemented.

Tanks may be installed in either horizontal or vertical configurations. This distinction affects liquid-gas distribution inside the vessel, nozzle arrangements, head behavior, and hazard directions.

In vertical tanks, gas or vapor spaces accumulate in the upper portion of the vessel. Safety valves, upper nozzles, and roof connections are typically located in this region, making the upper section particularly critical.

In horizontal tanks, the liquid and gas phases distribute differently depending on the vessel geometry. Head sections, side nozzles, and support effects become more pronounced. Additionally, axial projectile effects require particular attention in horizontal installations.

For cylindrical pressure vessels and tanks, the most critical projectile direction is frequently along the longitudinal axis of the vessel. Therefore, when designing horizontal tank farms, tanks are generally installed parallel to one another and vessel heads are not positioned directly facing each other.

The main principle is straightforward: Horizontal tanks should be arranged with parallel axes, and vessel heads should not face one another directly.

This arrangement reduces the likelihood that fragments released during vessel failure will travel directly toward adjacent equipment. Safe installation is achieved through the combined application of parallel-axis arrangements and adequate separation distances.

8. Conclusion

Pressure vessels, storage tanks, and steam boilers are energy-storing systems. The sudden release of stored energy may generate pressure waves, projectile effects, and severe structural damage. The primary factor determining explosion severity is the rapid release of energy within a very short period of time. Because gases are compressible, they present greater hazards than liquids. For this reason, pressure equipment testing is generally performed using water, while testing with gases carries significantly higher risks. The major causes of testing accidents include nozzle and sleeve welding defects, insufficient penetration, improper weld preparation details, and unsafe pressurization equipment connections.

Real process systems are rarely single-phase systems. Tanks frequently contain liquids together with gas, vapor, or nitrogen blanketing systems. Therefore, risk assessments must always account for two-phase behavior and gas volume.

Equipment layout safety is also an integral component of engineering design. Safety distances between tanks, boilers, and pressure equipment are necessary not only for explosion protection but also for maintenance, repairs, inspections, operational access, and emergency response activities. Storage systems containing flammable and oxidizing fluids should be separated. Horizontal tanks should be arranged with parallel axes and should not be installed head-to-head. This arrangement minimizes the risk of axial projectile effects directly impacting neighboring equipment.

Ultimately, pressure equipment safety is achieved through proper engineering design, appropriate equipment layout, correct testing methods, qualified welding practices, secure connections, and effective pressure relief devices. Failure to follow these principles may result in severe injuries and fatal accidents.

References

• ASME Boiler and Pressure Vessel Code (BPVC), Section VIII, Division 1 — Rules for Construction of Pressure Vessels
• ASME Boiler and Pressure Vessel Code (BPVC), Section I — Rules for Construction of Power Boilers
• ASME Boiler and Pressure Vessel Code (BPVC), Section IX — Welding, Brazing, and Fusing Qualifications
• API Standard 510 — Pressure Vessel Inspection Code: In-service Inspection, Rating, Repair, and Alteration
• Health and Safety Executive (HSE) — Plant Layout
• NFPA 30 — Flammable and Combustible Liquids Code