Why do industrial storage tanks fail sooner than expected?

Industrial storage tanks play an important role in many production processes. They are used for the storage of, amongst other things, chemicals, process water, foodstuffs, fuels and raw materials. This is generally based on the assumption of a long and predictable service life.

In practice, however, this service life does not always prove to be so predictable.

Some tanks require increasing attention over time. Inspections are more frequently revealing anomalies, minor repairs are recurring, and a tank’s condition can deteriorate more rapidly than would be expected based on its age.

This raises an important question: why do some industrial storage tanks develop problems sooner than expected?

The answer can rarely be attributed to a single cause.

A storage tank may look perfectly fine on the outside for years, whilst the material is slowly deteriorating.

This makes material degradation difficult to detect. Problems do not usually arise overnight, but develop over a longer period under the influence of the conditions in which a tank is used.

Consider, for example, moisture, temperature, the stored medium, contaminants and periods of inactivity.

Individually, these factors do not necessarily lead directly to problems. It is precisely their combination that can cause deterioration to progress more rapidly than expected.

One of the best-known causes of material degradation in industrial storage tanks is corrosion.

Corrosion occurs when a material reacts with its environment. The rate at which this process takes place varies greatly from one situation to another.

Moisture, oxygen, temperature, chemicals and contaminants can all influence the extent to which a tank wall is affected.

It is particularly important to note that corrosion does not always occur evenly across the surface. A tank wall may still be virtually its original thickness in many places, whilst significant corrosion may already be present at a single specific location.

As a result, the actual condition of a tank may differ from what might be expected at first glance.

An example of localised corrosion is pitting corrosion.

Pitting corrosion results in relatively small areas where the material is corroded to an ever-greater depth. The surrounding material may be barely affected.

It is precisely this localised nature that makes pitting corrosion so insidious.

On the surface, the damage sometimes appears limited, whilst the remaining wall thickness may have been significantly reduced in one specific location. As a result, a relatively small corroded area can ultimately have a major impact on the integrity of a tank.

Conditions that may be associated with pitting corrosion include:

• water collecting at the bottom of a tank;
• sediment and other deposits;
• areas of stagnant water;
• prolonged contact with corrosive substances;
• temperature fluctuations and condensation.

These are conditions that occur regularly in many industrial processes and therefore easily become part of normal day-to-day operations.

When a storage tank shows signs of deterioration, attention often turns first to the product being stored.

That makes sense. The chemical properties of a medium can have a significant impact on the material.

However, the medium is only one aspect of the conditions to which a tank is exposed throughout its service life.

Factors such as fluctuating temperatures, condensation, moisture exposure, cleaning cycles and periods of inactivity can also affect the condition of the material.

Furthermore, conditions are not the same throughout a tank.

Around weld seams, connections, flanges, the tank bottom and other specific areas, conditions may differ from those on large, open surfaces. As a result, corrosion can develop more severely in certain places.

An additional complication is that material degradation often occurs slowly.

Minor localised deterioration does not have any immediately noticeable consequences for the production process. The tank continues to function and nothing out of the ordinary is visible on the outside.

Meanwhile, the material may continue to deteriorate.

It is only when the deterioration reaches a certain threshold that the consequences become more apparent. For example, this may be because anomalies are detected, the wall thickness has decreased significantly in certain areas, or a leak occurs.

As a result, a problem that has developed over many years may ultimately be perceived as an unexpected fault.

Even tanks used for similar applications can exhibit very different patterns of degradation.

This is because service life is not determined by any single factor.

It is a matter of the interaction between, amongst other things:

• the stored medium;
• moisture and condensation;
• temperature;
• contaminants and sediment;
• downtime;
• cleaning cycles;
• local design details;
• mechanical and chemical stresses;
• properties of the material used.

A change in any one of these conditions may also affect other factors.

Consequently, it is not necessarily the case that two tanks of the same age will develop in the same way from a technical perspective.

When an industrial storage tank starts to show problems earlier than expected, it can sometimes appear to be a sudden technical fault.

In reality, however, this may have been preceded by a much longer process.

Corrosion, localised pitting, moisture, sediment, temperature fluctuations and other operating conditions can affect the material over a period of years without this being immediately apparent.

Therefore, the age of a storage tank alone does not tell the whole story about its actual condition.

The more interesting question is what conditions cause material degradation to occur and why that degradation develops much more rapidly in some places than in others.

A specific form of corrosion illustrates this particularly well. In this process, micro-organisms can play a role in localised corrosion of metals: MIC (Microbiologically Influenced Corrosion).

We will explore this in more detail in the following article.