Processing plant commissioning considerations

The transition from construction to operation is the commissioning and startup. Processing plant commissioning embraces activities such as cleaning, flushing, verifications, leak tests, performance evaluation and functional tests essential for bringing a newly installed plant or facility into routine operation. The commissioning expertise is a specialized branch of engineering equivalent to the related skills of design, construction and operation. Correct commissioning is vital to the satisfactory operation of any plant or facility and it is essential that sufficient time and resources be allocated to different stages of the commissioning and startup. 

Since commissioning is the last major phase before operation, there is a risk that commissioning will take place under great time-pressure or even some delayed commissioning activities may continue after the initial operation of the plant. 

Special efforts are needed to ensure safety and reliability at commissioning. Since commissioning, startup and shutdown activities have been responsible for many accidents, failures and other issues, these activities merit special attention.

This article discusses commissioning, including pre-commissioning, core commissioning, startup and post-commissioning. Common difficulties and challenges during commissioning and startup
are described.

General notes on commissioning

Before a plant or facility is handed over for normal operation, it should be inspected, checked, cleaned, flushed, verified and tested. This process is called commissioning and involves both the contractor and operator of a facility. 

Overall commissioning includes mechanical completion, leak tests, cleaning, flushing, provisional acceptance, pre-commissioning, first startup and post-commissioning. Careful planning is necessary for commissioning and startup. Integration of different systems, particularly control systems with others, has to be seamless. This mandates a wide range of verifications, checks and tests under
realistic conditions. 

Economies of scale have led to a rapid increase in output of processing plants, which has resulted in an increase in the number of items and auxiliaries per plant. Complex automatic control schemes are being installed to reduce the likelihood of mal-operation and consequent damage to plants and facilities. Added to this, is vast and complicated condition monitoring systems, which are extensively used in modern processing plants. Commissioning of technologically advanced and computer-controlled plants and facilities usually requires a radical and advanced approach, and highly competent teams. The high performance modern plants and units leaves smaller margins for error in design, construction operation and commissioning. It makes the plant more sensitive to unforeseen phenomena or mistakes. This shows itself in many operational incidences, bringing heavy financial loss to operation companies. In many modern processing plants, there is practically no margin for error or fault in different stages of commissioning.

Pre-commissioning

The pre-commissioning stage often involves many activities such as installation; inspection of equipment, piping and machinery; checking of units and facilities against designs, such as the piping and instrumentation diagram (P&ID); flushing and cleaning; pressure testing, functional testing; simulations and more.

At this stage, the actions of all controllers and motorized valves should be checked. All valves including relief valves should be tested and verified. Major equipment should also be thoroughly inspected, monitored, and finally closed in the presence of the commissioning team. Initial measurements of critical clearances should be taken for later comparison and control.

For the inspection of equipment, it is useful to ask maintenance engineer(s) to attend the activities or even participate in the commissioning for inspection and testing of all equipment and rotating machinery including electrical or mechanical machines. After all, maintenance staff should take care of these machines and equipment during operation and the commissioning stage is the best opportunity to start this task. 

Commissioning & startup

The commissioning phase includes day-to-day involvement in making important decisions on-site. There are significant challenges and elaborations in this daily involvement as it relates to risk-taking, deviations accepted, financial control and changes in the plant. 

For each medium-sized area of a plant, a team of three to five is necessary for the core commissioning phase to be able to provide continuous cover for a period of core commissioning and startup duration. The staff engineers previously in residence in the plant, for the construction or pre-commissioning, may form part of this core team. At least half of the team, say two or three, should have good operational experience. Additionally, the team should contain specialist competence in design, maintenance and operational analysis. An ideal team consists of one commissioning area manager, one commissioning process/operational engineer, one commissioning mechanical engineer, one operation/maintenance engineer and one commissioning electrical and instrumentation and control (I&C) engineer. It is difficult to spare team members with operational experience from their normal operational team — everything should be done to make their stay as short as possible. The team should be led by a senior executive (commissioning area manager), who is able to advise on modifications and operational decisions in a situation where the line of management and authority may be much more complex than in normal settings. 

Commissioning is a high-risk enterprise putting personnel, facilities, the plant and materials at risk of hazard. Responsibility and authority of engineers and experts involved in the commissioning should be clear. There should be no doubt about where the responsibility lies. The correctness of important decisions and their speed of implementation profoundly affect further progress.

The overall situation and commissioning effort are often bound to interact. Sometimes it takes place in an atmosphere of commercial ease; therefore, there is no rush and activities are progressed in a normal sequence. However, this is not usually the case. For many plants and facilities, considerable time is wasted during design and initial phases of construction whereas the output of the plant is committed by an absolute deadline. In this situation, the installation and construction is rushed, supervision stretched by excessive labor build-up, and startup may take place while part of the plant is still being completed. The result is a commissioning atmosphere with a limited measure of time and financial freedom, combined with substantial technical difficulties, many deviations waiting for decisions and elements of calculated risk-taking.

Most equipment and items are first subjected to their designed operating conditions at the startup when the unit or plant is initially operated. Failures of equipment, components or facility at this time can impose heavy revenue losses on the operator, and it is therefore advisable to subject components to their designed operating conditions with great care. A correct startup sequence and procedure are needed and should be followed with utmost care and diligence. 

For any machinery or equipment, a specific set of spare parts is needed for the commissioning and startup. Generous quantities of spare parts should be provided for this phase so commissioning and startup are not delayed because of lack of spare parts or tools. Too often, the consumption of spare parts is abnormally high during startup because of rapid deterioration of inadequate equipment or items, abnormal loads on equipment because of shortcomings in design, and faulty operation due to many different reasons.

Typical difficulties & problems in commissioning

Shortcomings in basic design contribute less to commissioning difficulties than errors in detail design and construction. The reason is the fundamental
concepts have generally been the most researched, examined and evaluated by senior technical staff. Also shortcomings in basic designs are usually discovered and mitigated in the detail design. 

On the other hand, in the areas of detail design, installation and construction, surprises are not uncommon. Faulty or absent venting arrangements occur frequently. For example, if vessel vents are too close to the inlet nozzles, feed materials might be vented unintentionally. Gas locking may occur of pumps protected by non-return valves without vents on the upstream side. Errors are often made in gas-phase balancing, leading to flow and level difficulties, particularly with systems under vacuum or slightly positive pressure. 

A pump set up with insufficient suction head to meet NPSHR has been reported during commissioning of many plants. The situation may be worsened by partial suction restriction due to the plugging of a temporary strainer or similar condition. The results, in terms of cavitation, could be problematic and damaging.

Lack of detailed assessments where a flow is split often leads to unsatisfactory results. Examples are a sampling stream in parallel with the main flow and a level control maintained by recirculation as well as outflow, with a control valve on one flow only. In general, such a problem can happen in any place where the flow is split. When a control valve on a line is actuated, the flow of both lines would be affected and accurate control of these flows is not usually possible by only one valve. Sometimes a dedicated control valve is needed on each line to properly control flow of each line. 

In many operations such as different process units, reactors, separation units, recovery systems, liquid-liquid separation, liquid-solid separation, screening, size reduction machinery (mills, etc.) and gas-liquid separation, the performance is usually worse than theoretically predicted or anticipated. A wise design should assume imperfection and make contingency plans. Extensive simulations as part of pre-commissioning have proven to be worthwhile, leading on occasion to some changes and provisions prior to commissioning. But it is disappointing to learn at the startup that the equipment or system is operating far below theoretical expectations and there is no contingency plan and no safety margin/factor.  

The automatic control is another hot topic in the commissioning. Whereas realistic simulation exercises attempt to test control performance as much as possible, there have been severe limitations to do so in the absence of job fluids and normal operating conditions. Commissioning plans should therefore be viable on the assumption that some control devices will not operate when needed and alternative means of operation and control should be included. Difficulties have been experienced in nonlinear control functions and control systems with large lags. Systems involving large lags between action and detection have been the source of issues. Controls based on detection of physical variables other than temperature, pressure, flow and level — for instance, those based on electrical or thermal conductivity, radiation absorption effects, etc. — have shown difficulties and challenges. These control systems have not been commonly used and there is a wide range of problems associated with them. Complex
programming controls can also cause trouble.

Sudden major leaks from piping or equipment after pressure testing have been reported in some occasions; but such leaks are relatively uncommon if the piping and facilities are properly fabricated, inspected and tested. However, other types of disfunctional pressurization and leaks have been common in the commissioning. There have been dangerous incidents where fluid from a high-pressure circuit leaked into an ancillary low-pressure circuit through a faulty item such as defective equipment, tube or valve. This resulted in release of a large quantity of pressurized material and even high-pressure flammable material. Careful attention should be given to all systems including low-pressure auxiliary and ancillary systems and their potential interfaces and exposures to other process or high-pressure circuits. For instance, there was a case that high pressure gas in a compressor pressurized the lubrication oil system because of a faulty internal part inside the machinery which caused extensive damages.  

Vacuum leaks are a common problem during commissioning of vacuum systems, particularly air leaks into a vacuum system. There have been many reported cases of air leaks into barometric legs, the drain connection and piping connected to the condenser drain as a tail leg, vulnerable to air leak. Less obvious is the leakage of air through safety valves provided for relief of pressure operation in the same equipment.

The need for relocation of machinery an equipment can also slow things down during commissioning. For instance, there were positive-displacement pumps that were located high in a structure that
demonstrated high vibrations. Many of them were relocated on the ground floor. As such, if a relocation needs new base, piping, etc., it can cause delays
and difficulties. 

Many problems and errors can arise with many different types of mechanical handling equipment and machines, even apparently simple ones. Cranes, hoists, belt conveyors, overhead cranes, powder sifters, filling machines and material handling machinery manufactured by many different vendors — even well-known suppliers — have created many problems and difficulties. For instance, in some cases, overhead cranes cannot provide the coverage required; the laydown area does not exist or it is not sufficient for a normal operation.

Wrong or imperfect application of materials of construction have been widely reported as serious problems in commissioning. Two examples are wrong initial material selection and, secondly, errors in manufacturing, fabrication and welding. Welding of the less common metals or dissimilar materials have nearly always been associated with problems and controversies; for example, a careless case of a contractor who simply welded galvanized steel to carbon steel, which is unacceptable. 

Tank and vessel linings have been a known source of problems and poor performance during commissioning. In theory, lining is a good and cost-effective solution. However, in practice, the internal coverage of lining, the gaps between lining plates and attachment of lining to the inside of equipment, vessel, etc., have caused problems. Unfortunately, in many cases, defects and problems became apparent at the
commissioning stage.

Startup & post-commissioning

The first few days and weeks of operation is a critical and challenging period that needs great attention. The commissioning team continues their work and activities during this time. The commissioning period should not be terminated until broad satisfactory operation is achieved, though the senior members and experts of the commissioning team might be gradually withdrawn after the first weeks of operation. The fact is plants and facilities often face some sort of challenges and difficulties at the initial period of operation. The rate of problem solving and development is bound to slow down once the
commissioning team is disbanded.

The aim during the initial period of operation is to ease operation, remove bottlenecks, reduce costs and improve quality. The provision of additional items and equipment will also be discussed and decided in this stage by a committee that consists of different parties such as commissioning team, operation team, maintenance team, etc. Special attention should be paid to post-commissioning deterioration of mechanical conditions affecting the operation of the plant. Factors and parameters such as fouling of heat-transfer surface, reduction in pumping rates, vacuum deterioration, etc., if reported, should be properly investigated by the commissioning team that just commissioned the plant.

One special field of cost reduction involves some mitigation measures such as improvements or even replacement of high-maintenance pieces of
equipment. Financial situations would be properly evaluated case by case and a key factor in such studies is the interruption of the operation (production) and associated financial losses.

Proper transfer of "as-commissioned" documents, knowledge, procedures and experiences to operation and maintenance teams is important to final success. Great care should be taken not only to transmit operational routines, but also control and maintenance procedures. It is helpful to train all client’s staff in addition to the operational team. When conveying maintenance experiences and procedures, be mindful of differences in maintenance concepts between different teams and companies.