Managing ageing plant through Risk Based Inspection
06 September 2013
All plants are susceptible to ageing, and so are automatically subject to the risks of mechanical integrity failure such as loss of containment. Given this, companies should be taking action to manage their ageing plant, but at the same time all companies need more output for less input; higher production and less investment, higher reliability but lower maintenance. Risk Based Inspection (RBI) may be a solution in effectively managing ageing plant and achieving that aspiration of higher product
RBI is about inspection of the plant in a manner which is bespoke to its needs, and finding the correct and most efficient inspection regime. This can remove the need for unnecessary and costly inspections, especially those which are legally enforced under PSSR (Pressure Systems Safety Regulations).
This article gives a real-life example of RBI being used to examine the inspection requirements on a mist eliminator in a sulphonation process, which is highly toxic and corrosive. This led to significant savings in parts replacement and in down time, as well as saving logistical costs in facilitating invasive-visual-internal inspection.
Traditionally plant items have been inspected using prescriptive industry practices, for example statutory inspections of boilers and pressure vessels enforced under the PSSR. Here, the actual inspection method and interval is documented in the WSE (Written Scheme of Examination) but is based on prescriptive practices in most cases. Prescriptive practices by nature have fixed methods of inspection and inspection frequency which are based on general industrial experience. Although these prescriptive practices appear inflexible and potentially inappropriate for specialised situations, they “have on the whole provided adequate safety and reliability.” 
However, the prescriptive inspection method has inherent shortcomings;
* It does not encourage the analysis of the specific threats to the integrity of the plant in question
* It does not focus finite inspection resources on the areas of greatest concern 
This inefficient usage of resources is shown in both the inspection time and the financial outlay to prepare the logistics for inspection by the Duty Holder, the owner and user of the equipment.
This problem is further exacerbated by the fact that statutory inspections have become an over-commoditised industry. Currently, inspections of pressurised equipment are conducted via insurance companies for most large organisations. Insurance brokers acting on behalf of their clients, the plants, seek a deal with insurance companies in providing the most cost effective policy. In practice, each relevant plant item is categorised in a rough group by equipment type (i.e. reactor, air receiver, heat exchanger etc.) and each item is quoted on the number of units, a unit being a quarter of an hour of an inspector’s time. So X units for air receiver and Y units for reactor etc. and the PSSR WSE is sold as a standard commodity along with the inspection. All of this activity, which is meant to ensure plant integrity, is carried out without asking a single question about the complexity of the plant or how hazardous the materials handled are in terms of toxicity and flammability . Instead of engineers both from the plant and the inspection company speaking to one another about what the requirements of the plant really are, too often it comes down to negotiations between insurance companies and brokers.
As one would expect, this issue itself leads to many other problems:
*The WSE may not be detailed enough to highlight the specific failure mechanisms of specialised vessels. Even if that is the case when the vessel is put in service, as companies change insurance providers over time seeking to make savings, WSEs may be just passed on without being reviewed. Hence, as plants age, the latest risks to the plant may not be reflected in the old WSE. This may then lead to unnecessary or inadequate inspection methods and intervals.
*Companies end up seeing PSSR inspections as just another legal requirement. They may feel like they do not own the risk anymore and thus become disengaged from taking benefit from this professional examination; inspectors are seen as trouble makers because they stop the plant.
The preparation work for statutory inspections can also be very costly and dangerous for those handling toxic and flammable materials. It is not unusual for large plants (over 100 vessels) to be spending over £100,000 in facilitating invasive-visual-internal inspections. It is here where applying the RBI method could address these problems.
The actual methodology used to carry out RBI studies can be taken from the best practice published by the Health and Safety Executive (HSE): “CRR 363/2001; Best practice for risk based inspection as a part of plant integrity management” . The overview process for conducting an RBI is shown in Figure 1.1 above. The discussion of each stage can be found in reference .
RBI in Action
A real-life example of applying the RBI method is given but it is for demonstration purposes and so does not include the entire assessment details.
An RBI study was carried out on a large (>11 cu.m.) mist-eliminator pressure vessel in a Sulphonation process subject to statutory inspections. The study entailed a review of the process, the historical and current condition of the vessel, a mechanical integrity risk assessment and a proposal for inspection based on the RBI method.
The main reason for the RBI study was to explore the feasibility and scope in reducing internal visual inspections. This in turn would lead to other benefits:
*Decrease the risk of internal corrosion by minimising air exposure, because the reaction chemistry between the residual material inside of the vessel and atmospheric air introduced to the system when inspecting produces corrosive sulphuric acid. This serves to diminish plant ageing.
*Reduce exposure of personnel (inspectors and operators) to toxic atmospheres arising from inspections.
*Achieve financial savings; realised from reducing the need to facilitate an internal inspection every two years which requires logistical costs and irreversible damage to the filter causing approximately £10,000 in parts costs.
The study was carried out in several meetings over a period of a month. The team included the site Maintenance Leader, Process Safety Leader, an Operator and an expert from the inspection company. The final document was then approved by the Principal Engineer of the inspection company. Using site knowledge and expertise from the inspection company, all potential degradation mechanisms were then formally assessed. A sample of the assessment is shown in Table 1.
The historical inspection records for the vessel were reviewed; all inspection certificates and comments were studied in detail and taken into account as part of the assessment. Furthermore, other identical vessels in other production lines as well as those in other sister plants were taken into consideration. The main conclusions were:
*The only known degradation mechanism is internal corrosion.
*This corrosion is mainly concentrated at the bottom half of the vessel and at tangents to the filters on the shell of the vessel.
*The vessel appears to have a life of 10-12 years before requiring refurbishment work.
The risk assessment was carried out using a semi-quantitative approach: the 5x5 risk matrix shown in table 2:
Probability of Failure
For each of the identified degradation mechanisms, the probability of failure was assessed using the assessment tables given in CRR 363 , e.g. for internal corrosion:
Highly probable: Allowable loss is already used up
Probable remaining life: 3 - 5 years
Possible remaining life: 5 – 7 years
Unlikely remaining life: 7 – 10 years
Very unlikely remaining life:> 10 years
In this particular case, experience had shown that the vessel only requires refurbishment work once every 10-12 years. The last time the vessel was refurbished was in June 2009, so the estimated next date for refurbishment work is June 2019 - approximately 7 years from the date of assessment. Consequently, the rating is Unlikely.
The same style of assessment was carried out for the other identified degradation mechanisms. All of the results were then collated together and a qualitative assessment of the overall failure probability made. In this case the overall probability of failure was deemed to be Unlikely.
Consequence of Failure
The consequences of failures were similarly assessed using ‘guide words’ given in the practice and then rated numerically. The consequences considered different categories which included: location and personnel, impact on production, and others which took into account fluid pressures, temperatures and hazards.
These consequences were individually rated numerically and then collated to obtain an overall rating using Table 3. The numerical ratings given in Table 3 also correspond to a description which is identical to that given in the 5x5 risk matrix (Table 2), and thus the overall risk can be determined.
Very high: 16 – 19
High: 13 – 15
Moderate: 10 – 12
Low: 8 – 10
Very low: 6 – 8
Table 3: Consequence of failure rating
For example, the consequence of failure arising from location of the plant on personnel was assessed using the below criteria:
3 Heavily populated
2 Routinely accessible
1 Inaccessible without clearance
Given the vessel is located inside a building which requires authorisation for access; it was rated as 1.
Similarly, the consequence rating for all other categories (i.e. impact on production, hazards of material etc.) was considered. These ratings were added up and the total was 15 (i.e. High using Table 3).
Taking the results from the probability of failure (Unlikely) and consequence of failure (High), and using Table 2 the overall risk assessed was Low.
The overall risk rating should influence the inspection method(s) and interval, but there is currently no guidance which translates risk rating to inspection plan. The inspection plan depends on the judgement of the team and competent person carrying out the RBI.
The most important part of the inspection plan is ensuring that each degradation mechanism identified has a suitable inspection methodology and interval.
The inspection plan proposed for the mist-eliminator is shown above.
Recommendations & Conclusions
All plants are subject to ageing, and addressing this risk should be seen as an opportunity to interrogate the inspection regime of the plant. Currently, statutory inspections of pressurised equipment are prescriptive and under the influence of an over-commoditised industry. The Duty Holder’s best course of action is to question the WSE they are following and ensure it calls for a level of inspection which highlights the degradation mechanisms that are really present and is proportional to the risk. WSEs should be reviewed periodically to ensure they are still meaningful; this point itself touches at the heart of RBI and can lead to many ancillary benefits.
In this example, the RBI method was used on a mist eliminator in a Sulphonation process which led to savings of approximately £10,000 in filter replacement charges, weeks in down time, and logistical costs in facilitating invasive-visual-internal inspection.
RBI can lead to profitable operations through effective maintenance and should be part of an integrated strategy for managing the risk of ageing plants. Implementing the RBI methodology does require expertise and so care should be taken in ensuring a competent team is formed before making radical changes. Further reading can be found in CRR 363/2001.
1. CRR 363/2001, Best practice for risk based inspection as a part of plant integrity management, HSE.
2. SAFed’s Approach to Ageing Plant, May 2012, Miles Gardiner.
3. RR823, Plant Ageing Study; Phase 1 Report, 2010 HSE.