TURBO TIPS: TURBOMACHINERY PACKAGES

PRACTICAL GUIDELINES FOR THE COMMISSIONING OF GAS TURBINES AND POWER PLANTS

In the commissioning of turbomachinery packages, sufficient time should be given for planning and preparation — the keys to success.

Commissioning is the most critical and busy phase of a project. Design flaws, hidden manufacturing issues and errors in construction or installation appear during the commissioning stage. Any necessary modifications or corrections, therefore, should be rapidly implemented simultaneously to ongoing commissioning activities.

A realistic plan for commissioning is an important factor. But it must be supported by the correct mix of engineers. Mechanical, turbomachinery, piping, electrical, instrumentation and other engineering members should be part of the commissioning team for turbomachinery packages. Each team member should be empowered to fully implement their responsibilities in a timely manner.

In particular, sufficient time should be allowed for the realignment of turbomachinery. This aspect of commissioning planning is often neglected. It is not unusual to have last minute corrections and small modifications to turbomachinery packages or piping changes. These and other activities can alter the load on nozzles as well as having other impacts on turbomachinery alignment. Thus realignment has to be carried out.

Re-commissioning is another oft-forgotten area of planning. Even if the turbomachinery package has been commissioned, it should be re-commissioned once it is hooked up to other systems as an integrated unit. Only at that stage can the package be tested in conditions that resemble real-world full load. The necessary time and resources should be provided for re-commissioning.

Yet another underestimated area is the time required for oil flushing. Based on practical experience, it generally requires two or three times more time than commonly used estimates. For more complex lubrication oil systems, a complete oil flushing can even take up to a week.

When the turbomachinery package is handed over from construction to commissioning, it should be properly inspected for cleanliness. It is rarely as clean as it needs to be. Therefore, a correct time frame should be set for cleaning checks, internal inspections and boroscoping.

Temporary equipment and facilities, too, are usually required for commissioning. In some modern plants, electrical power is generated within the plant. At the start of the commissioning process, there is usually not enough electrical load to adequately perform power generation unit testing and deliver enough electrical load to support the commissioning of other facilities, packages and systems simultaneously.

Details, such as the number of independent generators or the variable load capability (minimum load capability) of onsite power generation equipment, are important to know in advance. In some cases, temporary power generators are required at the beginning of commissioning; they are needed until the site power requirements reach the level of a permanent generator working at minimum load.

“Early planning and proper preparation are the keys to commissioning success.”

For many plants, air, nitrogen, oxygen, water systems and packages, or other utilities may be commissioned late because of a delay. If this is the case, these utilities should be provided via temporary packages to facilitate the commissioning of core areas. These temporary units consist of expensive rental equipment; and special care should be taken to ensure their presence at the correct time.

The identification of packages that comprise commissioning is an essential aspect of preparation. Each commissioning package should be an operable entity that the operation team can take over and actually run.

In other words, a commissioning package is a system that can be finally tested, and it should be big enough for operations to take it over and run it. For each package, therefore, a dedicated drawing should be prepared. There should be no confusion concerning the scope of the commissioning package.

Pre-commissioning activities should commence in a timely manner. This should not be too early, since the involvement of the commissioning team during construction can cause inefficiency. All engineering and installation changes, therefore, should be implemented before the start of pre-commissioning activities to avoid rework. As a result, pre-commissioning checks cannot be started early, since changes and modifications are commonplace.

On the other hand, the commissioning team should be actively involved in final completion activities, tests, cleaning & flushing and end-stage inspections. As an indication, some of the commissioning team should become involved when about 80% of the construction and installation job has been finished. Full presence of the commissioning team should start in earnest at around the 90% mark.

The commissioning “punch list” should be monitored closely. A well-prepared, properly detailed punch list is a good indication of quality commissioning. It is a team activity. Where possible, the instrument engineers, electrical team and other engineers should accompany mechanical and machinery engineers when preparing the punch list.

Usually one or two members check items in a package (for example, machinery components, lubrication in a machinery, gasket presence, bolt tightening, piping supports, instruments, valve installation, access and labeling). Others record overall status and note any defects in the punch list as well as relevant package documents.

A missing gasket in a machinery package piping system, for example, is a common item detected during the creation of a punch list. If left undetected, this could surface as a serious leak during a leak test as part of actual commissioning activities. The compilation of a punch list is a good way to avoid such delays. It should be drawn up when 85%-90% of construction and installation activities are complete.

The package leak test, in fact, is one of the first commissioning activities to be done after punch list compilation. Common leak points are turbomachinery seals, pipe or equipment joints and gaskets, and valve stem, packing and bonnet-gaskets.

Leak test pressure can be set at around 1.1 times normal operating pressure. Hold points are often set at 25% and 65% (in addition to 100%) of leak test pressure. This enables early inspection of the package to check for a major leakage before full pressure is reached.