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Reducing plant downtime

The benefits of including a non-intrusive approach within a risk based strategy for the inspection of pressure vessels.

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Article Summary
Minimising plant downtime is a business imperative throughout the process industries. The inspection of pressure vessels, to maintain safe operation and fulfil statutory requirements, is a major driver of the frequency and duration of planned production outages.

The internal inspection of pressure vessels has traditionally involved putting an inspector inside the vessel to carry out a visual examination. This requires the equipment to be shut down and a considerable amount of work is involved in preparing a vessel for inspection and returning it to service afterwards.

For non-intrusive inspection (NII), the pressure vessel is inspected from the outside which avoids the requirement for entry into confined spaces to perform the examination. In many instances, the inspection can be undertaken with the equipment online. This approach is increasingly used in the process industries and has delivered significant financial benefits without compromising process safety.

The external examination may be performed using non-destructive testing (NDT). Increasing computing power and the development of new techniques means that the capability of NDT is increasing year on year.

NDT techniques
A range of NDT techniques can be used to inspect the equipment (see Table 1 and Figure 1). The table clearly demonstrates the importance of identifying degradation mechanisms and the resulting defects before selecting the inspection technique. Indeed, for an effective examination, more than one technique may need to be employed. It should be noted that, on its own, visual examination is not capable of detecting the full range of defects.   

The effectiveness of the inspection technique is also highly dependent on the competence of the operator. It should not be a surprise that the probability of detection becomes lower as the flaw size decreases.

For the examination of vessels where the consequence of failure is high, it is recommended that more than one technique is utilised. Confidence in the probability of detection can be increased by producing test pieces with defined defects for qualification of the technique and to identify the better performing NDT technicians. The competence of NDT technicians relies on a combination of suitable training, accreditation and experience. They should be qualified to an appropriate standard, for example:
• British Institute of Non-Destructive Testing (BINDT) – PCN (Personnel Certification in Non-Destructive Testing)
• American Society for Nondestructive Testing (ASNT) – Personnel Qualification and Certification in Non-destructive Testing.

The organisation providing the NDT technicians should be an accredited provider of inspection services.
There are limitations in current NDT technology which mean that certain vessel features and vessel types are difficult to examine. For example, packed columns and shell and tube heat exchangers may not be suited to an NII approach.

Vessels with internal linings may need to have a periodic internal visual examination to inspect and where necessary repair or replace the lining. The materials of construction specified for the vessel shell and the lining are important considerations, especially where a lining with a limited life has been applied. There is usually more opportunity to move to NII where the shell has an internal metallic overlay than for vessels coated with some types of non-metallic lining.

There can be process related requirements for periodically entering a vessel, for example to remove the build-up of deposits or fouling that affects the process performance of the vessel. In these circumstances, it is worthwhile taking the opportunity to undertake an opportunistic internal visual examination of the vessel. 

Application of NII
Within the UK there is no legal requirement to carry out an internal examination of a pressure vessel. The examination must, however, be effective at detecting the predicted damage mechanisms. It must also provide sufficient information to enable the vessel’s integrity to be assessed and to determine the safe interval until the next examination.

A traditional intrusive inspection (sometimes supplemented by NDT for an intermediate inspection) can do this subject to appropriate controls. For a vessel to be examined without an entry, the NDT technique (or combination of techniques) selected needs to be at least as effective as carrying out the intrusive inspection.

There are three situations where non-intrusive examinations are technically justifiable. The first is where the combination of the materials of construction of the vessel and the chemical and physical properties of the process fluid means that there are no credible internal degradation mechanisms. The second covers situations where there are credible internal degradation mechanisms, for which an NII strategy is determined to be capable of adequately detecting and monitoring the degradation. The third situation is one in which the consequences of failure of the vessel are deemed to be low and there are significant difficulties or integrity threats associated with taking the vessel off-line to perform an internal entry.

It is necessary to identify the deterioration mechanisms where they may occur and the expected rate of progression. A risk based inspection (RBI) study is a highly effective means of doing this. Once the deterioration mechanisms have been identified, a technical assessment can then be undertaken to determine whether it is possible to detect all these mechanisms externally using a combination of proven NDT techniques.

NII should therefore be considered as an integral part of the RBI process. It is one of the tools to manage the integrity of the pressure vessel throughout the remainder of its operating life. NII focused on detecting the predicted defects can be at least as good if not better than an unfocused, intrusive visual examination.

Guidance relating to the application of NII can be found in DNV-RP-G103. This document is the output of a joint industry project covering NDT best practice in the oil and gas industry. It has been adopted as the industry standard for NII. The screening procedure and decision guidance chart are widely used to determine whether a vessel is suitable for NII. However, many companies then prefer to use an RBI process to define the appropriate NDT techniques.

There is, nevertheless, more scope for the further application of NII, especially within the oil and gas industry. In some companies there is a resistance to change from established inspection practices together with a lack of knowledge about the capability of the available NDT techniques.

The use of NII offers significant safety, environment and financial benefits. For example, a survey undertaken in 2017 by the Oil & Gas Technology Centre (OGTC) and ABB found that NII technologies could deliver increased production and lower maintenance costs worth up to £242 million per year to the UK oil and gas industry alone.
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