Inspection and quality control

‘Inspection’ and ‘quality control’ are two overlapping activities. In a general engineering context, ‘inspection’ includes both quality and quantity examination and verification, and indeed the exercise of due dili-gence in examining anything of importance, which in the case of a project may include the site conditions or the wording of a contract. ‘Quality control’ includes observation and record of product quality, analysis and observation of the production process (including all aspects of engineer-ing and management), surveillance of the actions taken to maintain quality within acceptable limits, and rejection of non-conforming work.

The stages of quality control prior to commissioning can be divided into three: engineering, procured items, and fabrication and construc-tion work. Engineering quality control has been addressed in previous chapters. In the following, our focus is on procured items and shop fabrication, and we will in general use the term ‘inspection’ loosely to embrace both activities of inspection and quality control, i.e. the observation and the feedback loop.

In the pursuit of providing a quality product (that is, a product having any specified quality level), inspection is arguably just as impor-tant as design and production. There is no gain in designing features which do not materialize in practice. Product inspection, in all its forms of quality, quantity, and general conformance checking, has long been recognized as an integral part of any productive activity or commercial transaction. One of the fundamental laws of purchase of goods that already exist, caveat emptor, ‘let the buyer beware’, stresses the presumed negligence and lack of recourse of the buyer who fails adequately to inspect.⁴ The tradition in engineering is generally to assume that errors and unacceptable work will inevitably follow poor quality control, both in design and construction. All important work must be checked, and corrected if necessary.

In the present limited context, our aim is to establish what inspection, and how much inspection, is required when procuring items for a process plant. Evidently this is very similar to the question of how much engineering work to do; too little inspection is likely to result in unacceptable quality, while increasing the inspection expenditure will ultimately become uneconomic as the costs exceed the benefits.

Theoretically, the optimum is reached when the marginal increase of inspection costs equals the marginal decrease in direct field costs and other savings arising out of the additional inspection. ‘Other savings’

may flow from reduced site erection problems, reduced commissioning time, increased plant availability, reduced maintenance costs, and so on. Clearly, the optimum amount of inspection for an item is influenced by the quality of the system by which it is produced, and by the consequences of a flawed product being accepted.

Some possible consequences of poor product quality are unaccept-able, and the corresponding items should be identified by review of criticality if they have not already been identified by the process technology package or by statutory regulations. Clearly these items must be inspected to certain standards, and the engineers should ensure that the items are known and the standards are defined. Pressure vessels are an example.

A process plant is finally inspected when it is handed over as a complete entity, usually prior to commissioning. However, this is not a practical nor an economic time to concentrate inspection activities;

4 This is not universally applicable; not for instance when there are other overriding laws, or when the purchaser has ‘qualified-out’ his obligation to inspect, or where there is a warranty from the seller.

impractical because most of the critical features become inaccessible during construction, and uneconomic because of the cost of delays and of dismantling to carry out rectification. Activities and components have to be inspected before their incorporation into the plant, so that ideally the final inspection should be a verification that all of the planned, preceding inspections have been completed and deficiencies remedied.

Inspection can be classified according to who is responsible for the activity; there are usually many levels to this chain. Typically, there may be the inspectors of the project management and engineering team, the quality department of the procured item suppliers and contractors (and, separately, their factory production workers, using inspection as a production control tool), and a chain of sub-component and raw material suppliers and their quality departments.

There may also be superimposed inspection from the plant owner or third-party bodies, sometimes acting within statutory or licensing agency authority. We will disregard the latter as there is usually little choice to be exercised in how their input is applied, and because they relieve the project team of no responsibility. There is however an additional burden in managing the interface with such agencies and of ensuring that documentation and work practices follow their requirements, which should not be omitted when drawing up inspection budgets.

For most project inspection activities, the interests of economy and efficiency are best served by utilizing the internal quality control systems of the suppliers/contractors, and monitoring the control system rather than the end product. Thus standard conditions of purchase should specify the quality management systems required of suppliers/contrac-tors, and the facilities for surveillance of the systems. End-product inspection, if carried out by the project team inspectors, should essentially be regarded as a tool for monitoring the effectiveness of the quality control system, and if failures are detected both the system and the end product have to be upgraded. For instance, it is common practice, when specifying sample radiometric inspection of welding work, to require that if detected flaws exceed a certain percentage of samples inspected, the percentage of welds to be inspected, both retroactively and in the future, increases to perhaps 100 per cent. This may continue until such time that the pass rate improves sufficiently, and is all at the contractor’s expense.

The practice of purchasing according to specification, which the supplier is responsible for meeting, allows the buyer to escape the

responsibilities of caveat emptor.⁵ The buyer’s inspector must be careful to assume no responsibility by the act of inspection, exactly according to the reservations of the buyer’s engineer in the case of design approval.

There is a simple saying, ‘You cannot inspect quality into a product’, which is in fact a half-truth that causes some misconceptions. The intent of the saying is that the quality of a product is primarily a result of the production process. There are several ways that inspection augments quality or, conversely, that inadequate inspection leads to quality deterioration.

• At the most fundamental level, any form of quality control is impossible without inspection. Control of any form depends on a closed loop of inspection and correction.

• As in quantum physics, observing a process changes the process. The performer who is watched jacks up his act, and if he gets into a situation where he cannot cope, any part of his workload which is not subject to inspection tends to suffer first.

• In the case of 100 per cent-inspected product, once all the rejects have been separated the increase in quality rating of the remainder is obvious. Even the rejects can be increased in quality by assigning a new or restricted duty to their usage, and for this revised application the quality may be regarded as adequate.

• In conjunction with adequate record-keeping, inspection facilitates the identification and rectification of problems which may occur over the lifetime service of a component or group of components.

This is especially the case where such problems may arise out of design or specification inadequacy – the existence of the inspection and test documentation has ongoing potential value, rated as essential where the consequences of failure are severe.