Since the change by South Africa’s Department of Labour (DoL) from the Vessel Under Pressure Regulations to the Pressure Equipment Regulations in 2009, local boiler users, among other pressure vessel users, are now required to have a risk-based inspection (RBI) scheme in place if they exceed the 36-month inspection interval, reports energy and chemicals group Sasol.

Sasol Synfuels reliability engineer Pierre Swart discussed how the new regulations impact on pressure vessel users at this year’s Southern African Asset Management Association (Saama) Conference, which took place at the Cape Town International Convention Centre, in May.

The presentation focused on the DoL’s requirements for implementing a RBI programme, as well as explaining the basic concepts of a RBI programme.

“The RBI scheme is to be certified by a certification body approved by the South African National Accreditation System. Further, users,who implement a RBI management system as an alternative to the requirements stipulated in Pressure Equipment Regulation 11 (1) (d), must now inform the DoL in this regard,” he said.

The DoL requires copies of the RBI management system, the details of the RBI specialist and their qualification, the RBI team’s competency and qualifications, the technical standard to be used, the certification body involved and a letter of recommendation from the approved inspection authority to implement RBI.

“RBI is a method using risk as a basis for prioritising and managing inspection programmes,” said Swart, adding that RBI is, therefore, concerned with the loss of containment function of pressure vessels.

“RBI rates the loss of containment, as the consequence and the occurrence of a degradation mechanism, as the probability of the risk equation. The probability of the degradation mechanism occurring drives the inspection technique and scope of the inspection, and the risk result drives the inspection interval,” he noted.

However, if the risk is unacceptable, other risk mitigation activities are required. “The RBI study is dynamic and needs to be kept current by means of reassessment over time,” states Swart.

Key areas of a RBI process, including different levels of analysis, are defined by three levels of RBI, which include qualitative, quantitative and semi-quantitative analysis.

“The qualitative level requires less resources and information than the other methodologies. The downside of this approach is that it is also less rigorous,” he says.

The quantitative methodology requires the most resources and information. “Calculations become more complex and take longer to complete, which increases cost,” explains Swart.

The semi-quantitative methodology fits in between these extremes. “This approach has the benefit of less data and resources than quantitative, but more data and resources than a qualitative approach, based on the level of quantification,” he notes.

Swart explains that data collection is dependent on the level of RBI analysis. “Typical data required include construction material data and design parameter data such as pressure, temperature and corrosion allowance. The design data is then compared against the operating data, such as operating pressures and temperatures, as well as the fluid composition and properties,” he says.

The RBI study could also take into account other parameters, such as plant population densities, local population densities, and mitigation measures, such as bunds, drains, and firefighting facilities.

Another key area is determining the probability of failure of a pressure vessel. “To determine this probability, active and credible degradation mechanisms are identified. The degradation rate and susceptibility are determined, after which the probability is determined, and with the current condition a predicted deterioration will result in failure,” says Swart.

The consequence of failure is determined by examining the loss of containment. “Factors considered may include the state of the fluid, properties of the fluid and failure mode leak size. Typical consequence categories are safety, environmental and business consequences,” he says.

Furthermore, the RBI risk is determined as the product of the probability rating and the consequence rating. “The result is typically presented on a risk matrix. One also needs to manage the risk with inspection activities. Since the active and credible degradation mechanisms are now known, as well as the susceptibility, or rate of deterioration, the inspection programme can now be defined to cater for these degradation mechanisms,” explains Swart. The inspection programme should include what to inspect, how to inspect and the extent of inspection.

However, when inspection activities alone are not sufficient to manage the degradation, other mitigation activities may be required, he states. These activities typically involve repairs, replacements, or change in operating conditions such as lowering of volumes, temperatures and pressures.

As some degradation mechanisms are time dependent, the risk profile of the equipment will change over time, specifically as the equipment nears the end of its life cycle. “For some degradation mechanisms, the rate of deterioration is also affected by the change in process conditions. Therefore, RBI is a dynamic tool and must be updated when changes occur to the equipment or process, or when new inspection results become available,” concludes Swart.