Owners and operators of tank storage need to recognise the indicators of age and deterioration and have an understanding of why this occurs and the likely rate.
Why is age important?
The age of an asset is important, not only for the expected processes of deterioration, but also because duty demands of service will frequently change progressively over many years. This is particularly the case for storage tanks, as raw materials or process duty changes are common.
The majority of storage tanks in the UK, Europe and the US are certainly over 20 years old. Many are considerably more than 30 years old. The common mechanisms by which these deteriorate from the investigations we have been involved with are as follows:
- External corrosion of the base.
- Corrosion under insulation (CUI) for insulated tanks.
- Over-stressing of base to shell weld.
- Fatigue of connections.
- Localised internal corrosion.
In order to establish why deterioration has occurred, data has been analysed from a number of sources relating to all aspects of a tank life cycle.
- Fabrication and construction
- Operating condition
- Maintenance and inspection records
It is recognised that it is not always easy to obtain this data for equipment that has been in service for many years. In many cases, partial history and comparative data (from similar equipment) can be very valuable when evaluating condition.
This information can be used to understand why problems have occurred. It is also the key to preventing a recurrence of the problem and for prioritising where maintenance and inspection resources should be directed. Unfortunately, history appears to indicate that we do not learn from these problems, as the same patterns of failure are repeated. Given an increasingly aged population of equipment in service in industry and fewer people to solve the problems, there is a real need to identify methods of preventing these problems in future.
Typically, the maintenance work undertaken in many large refinery and process plants, it is on large storage tanks. The practices are well recoded and need to be applied with diligence to monitor the condition of large tanks. The primary difficulty many operators face is in prioritising the work, when many tanks are in service at a site. Approaches to managing large tanks are described later and in more detail in the presentation.
There are many tanks on intermediate operational duties (eg catch tanks, wash tanks or settling tanks) which are given less attention during maintenance and inspection.
The first example is a contaminated process condensate tank. This is shown below in Figure 1, and suffered leakage after approximately 20 years of service.
This tank had been in service for approximately 30 years and had shown very limited signs of internal or external corrosion damage throughout its history. However, a leak occurred in service, which resulted in a plant shutdown. An emergency replacement tank was installed, as the corroded base was below thickness for satisfactory repair.
The damage was a result of corrosion under the base plate, where water had pooled as the base foundation had progressively subsided.
Investigation revealed the tank was originally designed with a 50°C operating temperature. Progressive process change had increased temperatures, until the sand bitumen support was heated to an unstable temperature of circa 80°C and began to slowly shift, under filling and empty cycles of the tank. The temperature also increased the corrosion rate, of the metal with the water which collected.
In the second example, a stainless steel hydrocarbon collection tank was found to have be suffering leakage slight in service. The plant was again shut down for repair and the cracking damage shown below, was found. This damage originated from the outside of the tank and was a result of chloride ion stress corrosion cracking (SCC). Whilst this is common on estuary locations and a well known problem in industry, this example was a little unusual.
The location of the tank was on an inland plant, in a nominally dry position in the plant. It had been in service for over 30 years, without evidence of deterioration of damage to the external insulation (fitted for heat conservation).
The problem was identified as originating from a change to the over-head air condensing units, which had water sprays added to improve efficiency. This introduced a high level of water carry over, which essentially “rained down” on equipment at lower levels. Water penetrated the insulation and chloride ions were leached from the insulation, resulting in the defects observed.
This failure again caused a plant shutdown, whilst repair was carried out.
How Ageing can be Managed
A great deal of work has been done in many plant reviews on this subject in recent years. Data is often gathered to support regulatory safety cases, establish equipment criticality or carry out Risk Based Inspection (RBI) reviews. By using selected parts of this information and relating it to the operating environment of the equipment, it is possible to assess the mechanisms of deterioration.
A key factor in this activity is to ensure the full recognition of how the design and construction may have a significant influence on the behaviour in the current operating environment. Examples are designs of insulation support leading to water retention or how fabrication / construction actions can influence residual stresses.
Change of operating conditions over years of operation also has a significant influence on equipment life. Almost all plants have progressively modified operating regimes over the years, to improve production rate, change product grades or quality. The effective management of ageing, is to assess these operational influences, in relation to the condition of the assets. Considerable benefits have been achieved, by raising awareness of operations teams to how these mechanisms of deterioration occur or making practical changes to the operating regime.
Improving Confidence in Assets
The first stage of any process is to identify key assets, where safety or production dictates there is a need to understand current condition, or to extend life. There are many methods of carrying out this step, but it must be thorough and allow rapid review of many items. Knowledge of potential problem areas and ability to identify vulnerable points, will improve the success of this activity.
Examples of potential problem areas are absence of specification and acceptance criteria for new equipment or repairs. There may be limited awareness of operations and maintenance teams responsible for process equipment and the causes of deterioration. There are many operators (and managers) who do not realise that 10 degrees of temp rise or small changes to concentrations or flow rates can double corrosion rates.
The next step is to gather data to facilitate improvement; by implementing a process to record data and look at trends, in operations and maintenance as well as from inspections. This data can be used as part of a review process, to demonstrate assets are performing and in the condition, expected.
There may also be opportunities to improve application of technology. Modern plant automation systems store data on plants operating (or maintenance) records which can be for event analysis, applied as up front problem prevention.
Our experience is that these practical methods ensure confidence in assets, with associated safety and financial benefits.