Less invasive inspection...how can it be justified?

TWI Bulletin, July - August 2010

Get it right, and the savings can be huge. Get it wrong, and the cost is unimaginable...

The consequences of a serious pressure vessel failure don't bear thinking about. Even worse, a catastrophe caused by an operator trying to penny-pinch on inspection is the stuff of nightmares and law courts. Experts within TWI's structural integrity team Philippa Moore and John Wintle, have made a study of the issues surrounding the requirements for the internal examination of high hazard process plant. Getting it wrong by adopting an inappropriate inspection regime irresponsibly is of course unforgivable, but there's increasing evidence to suggest that present inspection regimes for internal examination can in some circumstances be changed safely by applying knowledge, experience and modern non invasive NDE technology.

For process vessels containing non-corrosive, clean and dry products under stable and benign conditions, there may not be any degradation mechanisms affecting the vessel materials for a long time. Taking account of the low chance of failure from internal defects, it may be possible to justify a longer interval between shutdowns for internal examination. Even so, it is important that this judgement is made carefully and correctly, according to a defined process, and that adequate safeguards are in place so appropriate action may be taken if the expected conditions of operation or process fluid change.

This is the topic of recent work by TWI commissioned by the UK Health and Safety Executive. HSE Research Report 725 outlines a six-step approach for assessing and justifying internal examination requirements for high hazard process plant. As Moore and Wintle reveal, the work has been developed partly through consultation with leading UK petrochemical companies and inspection bodies that are proactive in applying risk-based inspection. It builds on earlier TWI work on best practice for risk based inspection as a part of plant integrity management which was published as HSE Research Report 363 in 2001.

Experience from consultation - the voice of industry

To find out how leading companies operating high hazard sites in the UK approached the issue of internal examination of process vessels, TWI conducted a series of visits during 2008. The companies visited were those that were known to have a progressive approach to risk based inspection and operations of equipment used in the manufacture and processing of potentially hazardous products and chemicals.

As a direct result of the visits it was found that in the UK there are particular applications of pressure equipment where extended intervals are specified between internal examinations, subject to a periodic review. Such practices are accepted for vessels storing pure liquified gases at cryogenic temperatures but their application to vessels containing other products at higher pressures is relatively new. Regular and on-going review of the decision not to open and enter a vessel for inspection may mean that the interval between such examinations can be extended for a considerable period. In practice, intervals beyond 12 years are uncommon.

The industrial visits allowed TWI to establish the approach that was being used to implement risk based inspection in each case. These discussions helped establish a six step process for determining the requirements for internal examination. The approach begins with the RBI team assessing the conditions to be free from active degradation during service.

Absence of degradation

Pieces of equipment for which a less invasive examination regime might be considered would primarily be those described as 'clean non-corrosive service'. This means equipment that is not expected to suffer from active internal degradation mechanisms over time in normal service or from transient events within the design basis. It is equipment where the product is continuously sealed so that water or other impurities cannot reside in the system, where stresses are below code fatigue thresholds, and with high quality and documented welding.

Where any active degradation (such as wall thinning, stress corrosion or fatigue cracking) might occur from the inside, regular internal examination is the main method of ensuring on-going structural integrity. These kinds of equipment would be outside the scope for justification for less invasive examination intervals.

Evidence of the absence of active degradation will be supported by the RBI assessment, in conjunction with previous inspection records over the duration of the equipment's lifetime. This evidence can be difficult to obtain for third party inspectors when the contracts may be changed every turnaround. Further evidence may come from other equipment operating under an equivalent service, but the weight placed on this evidence must not be too high.

The kinds of chemicals under consideration are mainly the range of organic chemical products, which in their pure forms tend to be non-corrosive. It is important to assess the risk of contamination or ingress of water into such systems, for example from leaks, openings or other parts of the system, which can lead to degradation.

Many products that are considered corrosive are stored under conditions where they do not corrode, namely very pure chemicals stored within vessels made of suitable containment materials. In these 'non-corrosive systems', where there are no expected degradation mechanisms, the same approach can also be applied. However, the threat of corrosion from even minor contamination with water or other impurities may be significantly higher for these corrosive products, such as in the case of concentrated acids. These types of products therefore need very careful consideration when using RBI to set inspection strategies.

What if...?

If there are no identified corrosion mechanisms and nothing, other than pure product within the operational pressure and temperature ranges, is ever contained in the vessel, then high confidence can be taken that no environmental degradation will occur and internal examination will not find anything. However, for a robust justification to be made, we must ask what happens if something changes or goes wrong?

The most likely contamination of a clean product is from water. This can be from the production process, leaks in steam heating or water cooling systems, moisture ingress from air as the product is removed from storage, from steam cleaning during outage, or possibly as a result of degradation of the product. For this reason atmospheric storage tanks would normally require continued regular internal examination where these may be exposed to moisture ingress. The risk from atmospheric water entering the system is that it will contain oxygen and/or salt, which can result in rusting, corrosion, cracking or pitting.

It is also possible that new understanding of degradation mechanisms can come to light for products normally assumed to be non-corrosive. An example exists in the US experience of ethanol stress corrosion cracking of steel. Keeping up to date with world knowledge along with regular review of the RBI assumptions may avoid ignorance of potential risks.

Justifying internal inspection intervals

An overview of the six step approach that can lead to sound decisions on internal examination intervals is summarised in Figure 1 (at end of text). A Written Scheme of Examination where the requirement for internal examination is reviewed periodically may be justified, provided the technical basis for the decision is sound, and adequate safeguards such as non-invasive inspection are used. It is important that a robust management policy is in place to review the justification in rapid response to any change to expected conditions.

A company culture that is proactive to reporting changes, and that has good communications between the operations and integrity management teams, is seen as being essential to sustain the justification. This must be supported by a comprehensive information management system to manage the operational and inspection history of the equipment.

The justification for a less invasive examination regime is based upon a risk based inspection assessment that demonstrates the absence of active degradation mechanisms and that also includes considerations of the consequences of failure. The demonstration should be based on past inspection history and technical understanding. The RBI assessment must outline clear limits over the conditions of operation where the assumption of absence of degradation is still valid, and when it is not. The justification should be conditional on being reviewed periodically as part of the review of the Written Scheme of Examination, and in the event of unexpected conditions occurring or new knowledge being available, by there being a robust management of change policy in place.

For some high hazard situations, where the scale and consequences of failure are very high, moving to a completely non-invasive inspection scheme where internal examination is practicable is unlikely ever to be tenable consistent with reducing risks ALARP (as low as reasonably practicable). In these cases there are grounds for continuing to carry out some internal examination for defence in depth, even though no active degradation mechanisms are identified.

Non-invasive inspection

Given that it is justified to undertake internal examination less infrequently, there are actions which can also be taken to compensate for the inherent side benefits of regular internal inspection, and to safeguard against potential drawbacks. One of the main safeguards is non-invasive inspection.

Information about the thickness and the condition of the internal surfaces of vessels can under some circumstances be obtained without entering the vessel using non-invasive techniques. These techniques include NDT methods applied from the outside of the vessel, such as radiography, ultrasonic testing, shearography and thermography. Non-invasive techniques also include those where endoscopes or laser probes can be used to examine the inside of a vessel 'visually' when inserted from a remote location through a hand-hole for instrument access, or long range guided wave ultrasonic techniques.

Non-invasive (also called non-intrusive) inspection is not always cost saving. The methods can be slower and more expensive to apply than internal visual examination, and require more highly trained operatives. However, it can offer advantages where human access is difficult and emptying the vessel disruptive.

When applying non intrusive inspection it is important to know where to look. Welds, and in particular weld roots, are usually the most critical locations to inspect. But when the weld caps have been ground flush, the roots can be hard to locate from the outside. External examination using ultrasonics can detect wall thickness loss or hydrogen induced cracking, but should not be relied upon to find internal surface breaking stress corrosion cracking or pitting.

Appraising the Risk Based Inspection approach

When RBI is being used to set inspection strategy for pressure vessels with high consequences of failure, it is good practice for companies to undertake an independent peer review and audit of the management of their RBI assessment process. This might be an internal review within a large company by those with suitable expertise but not involved directly in the RBI or plant in question. An alternative is for an external review, possibly by an independent competent body or consultancy organisation. There is an audit tool available in HSE Research Report 725 that can help provide a structure for such audits.

The intention of the peer review would be to examine the stages of the processes that the RBI team has gone through, and the factors that were considered in reaching the inspection decisions. It is not intended that the peer review effectively does the whole job a second time. The experts comprising the RBI team need to update their knowledge of on-going developments in their respective fields continually. A review of how the members of the team have been selected, how their competencies have been demonstrated and maintained, and what information has been gathered for the RBI assessment is an essential part of the whole process.

Conclusions

When an RBI assessment can foresee no possibility of internal degradation, then a Written Scheme of Examination that specifies an extended interval between internal examinations, subject to internal review, may be justified for high hazard process plant. This applies only if the technical basis for the decision is robust and sufficient non-invasive inspection is achievable or other adequate safeguards are used. In such cases it is important to review the justification periodically, as well as in response to any changes or excursions from expected operating conditions or to process fluids, and to take opportunities for examination when they arise for other reasons.

It should be noted that there may be some very high hazard inventories where a completely non-invasive inspection approach is unlikely ever to be tenable. High potential failure consequences demand the need for defence in depth consistent with reducing risks ALARP. Full details of the approach outlined in this article were published last year in HSE Research Report 725.

• Include people with the all the required knowledge, experience and competencies to carry out Risk Based Inspection (RBI)

• Analysis of all possible damage mechanisms
• Combination of process fluids and containment material is stable and inert
• Good quality (documented) fabrication
• Operating/inspection history of vessel

• Set written scheme of examination (WSE) with review date
• Set operating/process limits for policy validity within a Risk Based Management (RBM) framework
• Reduce the risk as low as reasonably practical (ALARP)

• Non-invasive inspection and maintenance
• In-service monitoring of process, pressure and temperature etc
• Periodic reviews of written scheme and inspection policy
• Opportunistic examinations policy

• Good knowledge of RBI process settings
• Reactive to equipment modification or changes in product
• Live process ie fast response to upset or abnormal conditions

• Peer review and audit of management and operating procedures
• Maintaining awareness from world experience
• Sustaining the competencies of the RBI team

 Fig.1. Overview of the process to justify and support extended internal examination intervals