Design & Simulation / Reliability & Maintenance

Head Off Hassles During Startup

The right preparation and team are essential

By Amin Almasi, rotating equipment consultant

Commissioning/startup is the most critical and busiest phase of a project. Design flaws, hidden manufacturing problems, and construction and installation errors will become apparent during commissioning/startup and usually will mandate immediate modifications and corrections. Addressing such issues adds to the high stress already posed by the tight schedule and the economic penalties of any delay.

Three factors are key to a successful and less-stressful commissioning/startup:

1. developing a correct and realistic plan;
2. assembling an appropriate team, i.e., one that includes a suitable mix of mechanical, machinery, piping, electrical, instrument and other engineers for the particular process and equipment; and
3. properly empowering all team members to implement their responsibilities.

I generally don’t recommend delegating commissioning/startup to the engineering/construction contractor responsible for building the unit. Commissioning and startup involve very special types of work that require very specific skills. This expertise normally resides within operating companies, specialized commissioning/startup agencies and, often, equipment vendors. Keep the construction/installation team and the commissioning/startup team separate to the maximum extent possible.

Commissioning Concerns

Many commissioning/startup efforts suffer from avoidable errors such as:
• mis-sized control valves, actuators and equipment;
• incorrectly configured instrumentation and control systems;
• undersized hydraulic return lines;
• improperly laid out piping;
• misaligned equipment;
• insufficient amount of heat tracing;
• inadequate coverage by the fire water system; and
• lack of essential spare parts.

Correcting these mistakes during commissioning/startup takes time; getting a necessary part can cause considerable delay. Moreover, the needed modifications can have consequences such as changes in machinery alignment, requiring a realignment. Commissioning planning often neglects the time demanded for such realignment work.

Recommissioning activity is another item frequently forgotten in commissioning planning. If machinery (or a package) has been commissioned, it should be recommissioned when hooked up with all other systems in the integrated unit. Only then can the equipment be tested in a situation that resembles the actual working full-load condition. So, it’s essential to provide adequate time and resource allowances for the recommissioning activities for different packages.

The time needed for the oil flushing of equipment nearly always is under-estimated. The actual oil flushing usually should continue for two or three times longer than the time given by commonly used estimates. For special/complex lubrication-oil systems, a complete/high-quality oil flushing could take 3–12 days (depending upon the specifics of the machinery and the oil system).

In addition, too little time usually is allocated for cleaning checks, internal inspections and borescoping. The machinery package handed over by construction to commissioning should be properly inspected for cleanliness; usually equipment isn’t as clean as required.

Take full advantage of equipment installed for construction and maintenance activities (Figure 1). Commissioning often can benefit from temporary facilities, too. For example, for a plant that will be generating its own power, at the start of the commissioning process there’s usually not enough electrical load to adequately test the power generation unit. Temporary generators can supply power until equipment with sufficient electrical load to meet the minimum load requirements of the permanent generator is operating.

Because the main power generator or the main power transmission systems usually are on the critical path of a project, such temporary power-generation facilities could offer good flexibility in the commissioning.

At many plants, air, nitrogen, oxygen and water systems/packages or other utilities may be commissioned late because of a problem, delay or issue. Temporary sources for the affected utilities can facilitate the commissioning of the core areas. These temporary units are expensive rental equipment; special care is required to ensure correct timing.

More Tips

Identifying the commissioning (or takeover) packages is an important step in preparing for commissioning. Each package should consist of a unit or system suitable for integrated final testing and big enough for the operations team to take over and actually run. Prepare a dedicated drawing for each commissioning package. There should be no confusion about the commissioning package scope.

It’s essential to start precommissioning activities (and then commissioning efforts) at the right time. Getting a commissioning team involved too early often can cause inefficiencies and, sometimes, even problems. Complete all engineering and installation changes before the start of precommissioning activities to avoid rework and mandatory rechecking. The commissioning team should be involved in the final completion activities — the tests, the cleaning/flushing jobs and the end-stage inspections. As a rough guide, the commissioning team should start their involvement when construction/installation activities are around 80% finished, and should be out in full force when construction/installation is about 90% completed.

Compiling the commissioning punch-list is a critical step in any project. A well-prepared, properly detailed punch-list is a good indication of high-quality commissioning. Developing this punch-list should be a team activity. Where possible, the instrument engineers, electrical group and other specialists should accompany the mechanical/machinery engineers for the punch-listing. Usually one or two members check the items in the facility (for example, machinery components, equipment lubrication, presence of a gasket, bolt tightening, piping supports, instruments, valve installation, access, labeling, etc.) and other members record the status and defects in the punch-list (and other project documents). Missing gaskets are a common punch-list item; if not addressed, they could lead to a serious leak during the leak test during actual commissioning activities, and could result in a long delay. You should compile a primary punch-list at around 85%–90% completion of the construction/installation job.

The facility/package leak test (not to be confused with the construction pressure test) is one of the first commissioning activities after completion of the punch-listing. The test indicates all potential leak paths within a complete facility/package. Common leak points include machinery seals, pipe or equipment joints/gaskets, valve stems/packings/bonnet gaskets, and the like (Figure 2). As a rough guide, use a test pressure around 1.1 times the normal operating pressure. Consider hold points of 25% and 65% (in addition to 100%) of the leak test pressure, which allow for an inspection to check for a major leakage.

Quite sensibly, operating companies generally resist using prototype equipment or new designs. However, being a pioneer sometimes is unavoidable. The commissioning schedule should account for this and include suitable allowances.

A realistic performance test (such as ASME-PTC-10 type-1 for turbo-compressors) at the manufacturer’s shop can facilitate the startup. Indeed, it could save 2–3 months during the commissioning/startup. Moreover, any necessary modifications or repairs probably will cost one-third of what they would at the plant site.

Startup Blunders

Mandating uniform bolt tensioning during construction and installation isn’t enough. Many pieces of equipment require retensioning of bolts, especially those at piping connections, at the startup. Another lapse is forgetting to remove isolation valves/devices; this can cause serious damage.

Lubricant-related bearing issues plague many startups. So, to ensure lubrication is adequate, check:

• lubricant properties (e.g., viscosity);
• the quality of the lubricant and the lubrication system; and
• operating conditions (e.g., pressure, temperature and flow) of the lubrication system.

Insufficient lubrication can damage a bearing’s surface, which very quickly can cause bearing failure. Deficient lubricant flow leads to inadequate heat removal from the bearing, which could result in discoloration (a black color on bearing components) and reduction in mechanical strength. Other signs of inadequate lubrication and its associated effects include shiny areas, frosty appurtenances and glassy surfaces.

Large and critical rotating machines always get lots of attention. However, small- and medium-size equipment often doesn’t receive sufficient scrutiny. Such machines usually use ball and roller bearings. The major reasons for the early failure of such bearings are: mechanical damage (for example, due to the misalignment), poor lubrication and material defects. Ceramic-type rolling element bearings (e.g., ball bearings with special stainless-steel races and silicon nitride balls) can offer better reliability but such advanced bearings remain relatively rare in process equipment.

Machinery load and machine speed usually determine the installation tolerances of the bearings (also the looseness or the tightness). Installation greatly affects performance and reliability. Too tight a bearing/shaft fit can cause, for example, high loads on the rolling elements. Distorted housing bores can radially pinch an outer ring.

Some rolling-element bearings only can tolerate very limited amounts of misalignment. For instance, a deep-groove ball bearing can generate complex and destructive patterns of load zones as the result of misalignment. Cylindrical roller bearings, tapered roller bearings and angular-contact ball bearings can’t endure misalignment. Alignment checking is critical during commissioning/startup.

Shaft seats and housing bores may not be properly sized (either oversized or undersized). They also may be tapered or deformed. These defects could result in a premature bearing failure. The bearing ring could contort under loads and cracks could appear along the bearing raceways.

Foreign matter (for example, from unclean lubrication oil, improper flushing at precommissioning or insufficient sealing for the bearing) can result in high rates of wear and considerably reduced bearing life. Impact damage during delivery, installation or startup can cause a small defect that could quickly develop into a large one.


 

AMIN ALMASI is a rotating equipment consultant based in Sydney, Australia. E-mail him at amin.almasi@ymail.com.