Vacuum Tank Risks in Process Industries
In process plants, the risk of vacuum is often less recognized than overpressure. Yet a sudden pressure drop can be just as dangerous.
From an engineering perspective, vacuum-related failures are often overlooked during design reviews because they are perceived as unlikely — a risk that falls directly within the role of the chemical engineer in industrial environments — until they occur suddenly and catastrophically.
A vacuum tank collapse may destroy equipment in seconds, interrupt production, and create severe safety hazards.
The tank walls can buckle inward in seconds, destroying the vessel and releasing its contents. Unlike corrosion, which takes time, underpressure failures are sudden and total.
Example: Atmospheric Tank and Vacuum Risk
Thin-walled storage tanks are often more vulnerable to vacuum than to overpressure. A relatively small external pressure can cause buckling long before any internal pressure limit is reached.
For some tanks, vacuum protection is clearly required.
But what about an atmospheric tank like the one shown below?
This is a typical scenario where vacuum risk is not identified as a credible deviation during early hazard analysis.
The tank is designed to store flammable solvents and is equipped with a rupture disc (PSE) set at 50 mbar(g) as its overpressure protection.
At 40 mbar(g), a breather valve acts as a process vent on the pressure side.
Would you design this atmospheric tank as a vacuum tank? After all, vacuum-resistant construction adds significant cost and complexity.
Even when a tank is not intended to operate under vacuum conditions, it is essential to carefully assess potential process deviations or abnormal situations that may lead to underpressure scenarios.
In the specific case considered, vacuum conditions may occur due to the loss of nitrogen blanketing during tank transfer or emptying operations, for example as a result of supply unavailability or malfunctions in the inerting system. Additional credible scenarios may be related to temperature variations, such as day–night temperature differences, which can cause vapor contraction inside the tank.
When the tank contains a flammable substance, as in the example discussed, the scenarios associated with a structural collapse may reach severities ranging from major to catastrophic, depending on plant design and operating conditions. In the event of failure, the sudden release of vapors or liquids may lead to secondary scenarios such as flash fire, pool fire, or vapor cloud explosion (VCE), depending on the presence of an ignition source in ATEX-classified areas.
The HAZOP analysis should therefore thoroughly assess the reliability of the inerting system, which is often common to multiple pieces of equipment within the plant and is typically equipped with several alarm signals and safety interlocks. An additional solution, frequently overlooked for atmospheric tanks, is the use of a rupture disc designed to operate under bidirectional pressure differentials. In this way, the tank can also be protected against underpressure conditions.
It is important to highlight, however, that rupture disc failure under vacuum conditions results in air ingress into the equipment. Consequently, following such an event, safe conditions must be restored through a controlled nitrogen inertization phase.
Finally, where economically and technically feasible, it is often advisable to evaluate—already during the design phase—the installation of a tank designed for full vacuum service, thereby eliminating at the source the risk of collapse associated with underpressure.
Design Considerations
A proper vacuum tank design requires evaluating all possible operating scenarios.
These include steam-out operations followed by rapid cooling, inerting with nitrogen, draining of heated liquids, or jacket operation that can induce sudden pressure drops.
Perry highlights that even small transients, such as vent line restrictions or improper sequencing of valves, can create vacuum levels high enough to exceed the allowable working stress of thin-walled storage tanks.
For this reason, engineering standards often recommend that tanks likely to experience such conditions should be specified for full vacuum service or equipped with reliable vacuum relief devices.
The additional cost of reinforcing a tank or installing protection systems is far lower than the financial and safety impact of a collapse.
Why a Vacuum Tank Collapse Creates Serious Safety Hazards
A vacuum tank collapse can create severe safety hazards due to several physical and operational factors:
- Sudden structural failure
When external pressure exceeds internal pressure, the tank can implode instantly. This may cause violent deformation, noise, and vibrations, potentially injuring nearby operators or damaging adjacent equipment. - Trigger for cascading failures
The collapse can break process lines or instrument connections, resulting in leaks of hazardous substances. In ATEX-classified areas, this may act as an initiating event for an explosion. - Loss of safety and process functions
An imploded tank can no longer be safely emptied or pressurized. Nozzles may deform, instruments may detach, and relief or isolation systems may become inoperable. - Risk to personnel safety
If the tank contains corrosive liquids or toxic gases, structural failure may expose workers to harmful substances. The sudden implosion also poses a risk of impact or crushing injuries. - Unplanned production downtime
Collapse events are immediate and unplanned, causing full production stoppage. Emergency repair, investigation, and downtime typically exceed the cost of preventive design for vacuum resistance.
Tanks and Pressure Regulations
As shown in the previous example, many storage tanks operate essentially at atmospheric pressure, with only a slight nitrogen blanketing, often below 50 mbar(g).
From a regulatory standpoint, equipment with a maximum allowable pressure not exceeding 0.5 barg falls outside the scope of the Pressure Equipment Directive (PED 2014/68/EU). Outside the European Economic Area, other pressure regulations apply (such as ASME BPVC or country-specific codes).
However, this does not eliminate the risk of collapse. Both atmospheric and low-pressure tanks are structurally weak and highly vulnerable to underpressure. If they are not designed for vacuum, even a small pressure differential can lead to buckling. The same applies to thin-walled process vessels in multipurpose plants.
Standards such as API 2000 require that these tanks are protected against both overpressure and vacuum.
In practice, loss of nitrogen supply during transfer or blocked vents can generate an underpressure strong enough to cause failure. For this reason, breather valves, vacuum breakers, or rupture discs specifically designed for vacuum protection are installed as safeguards.
Conclusion
Vacuum scenarios should always be explicitly discussed during design reviews and HAZOP studies sessions, even for equipment formally classified as atmospheric.
Operating conditions such as temperature changes, inerting, draining, or cleaning can easily generate underpressure levels capable of compromising structurally weak vessels.
Anticipating these scenarios during design and hazard reviews is the only effective way to define and implement safeguards before a failure occurs.
Ing. Ivet Miranda
Atmospheric Tank Vacuum Risk Quiz
Which situation is most likely to generate vacuum conditions in an atmospheric tank not designed for external pressure?
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FAQ
What is a vacuum tank?
A vacuum tank is a vessel specifically designed to operate under negative pressure or partial vacuum. It is commonly used in applications such as degassing, drying, solvent recovery, or chemical processing. Unlike standard atmospheric tanks, vacuum-rated tanks are structurally reinforced to resist external pressure and prevent collapse during vacuum operation.
Are all storage tanks suitable for vacuum conditions?
No. Most atmospheric storage tanks are not suitable for vacuum because they lack the structural reinforcement needed to resist external pressure. Before reusing a tank in any operation that may generate vacuum, always verify whether it was designed or upgraded for vacuum service.
How many types of industrial tanks exist?
Industrial tanks can be broadly classified as follows:
Atmospheric tanks – Designed for near-ambient pressure. Not suitable for vacuum or pressure unless specifically protected.
Vacuum-rated tanks – Reinforced to withstand external pressure when vacuum conditions may occur.
Pressure vessels – Designed for internal pressures above atmospheric.
Jacketed tanks – Equipped with heating or cooling jackets for temperature control.
Underground tanks – Used for storage below grade, often with double containment.
Transport tanks – Mounted on trucks or trailers for liquid transfer.
Each category is designed according to specific standards, depending on operating pressure, service conditions, and safety requirements.