Correctly Commission Rotating Equipment

Appropriate attention to a number of elements can preempt problems.

By Amin Almasi, WorleyParsons Services Pty. Ltd.

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Proper installation, pre-commissioning, commissioning, and startup can play crucial roles in the performance, reliability and safety of process machinery. Unfortunately, many plants lack the necessary expertise or fail to demand sufficient precision in such activities. This often results in expensive damage to machines or production interruption.

So, let's look at several key elements — machine alignment, field balancing, electrical issues, lubrication systems, and mechanical and gas seals — that frequently don't get the attention they deserve during installation and commissioning.

MACHINE ALIGNMENT
Good alignment can greatly extend bearing and seal life, lower vibration, improve reliability, and foster better overall performance. Improper alignment can cause excessive vibration, premature wear and early failure. Adequate clearance for each machine casing (for example, driver, gear unit and driven equipment) is essential to permit proper alignment. Some experts recommend a shaft interface fit (alignment tolerance) of around 0.0005 times the shaft diameter (approximately 0.01–0.02 mm for typical 25–50-mm shaft diameters). Some textbooks suggest an alignment tolerance of around 0.01 mm overall, regardless of shaft diameter. Special rotating machine trains may need tighter alignment tolerances. On the other hand, machinery trains with flexible couplings (such as those that transmit torque through elastomeric materials) sometimes can tolerate looser interface fits. Coupling spacer length also is important because parallel misalignment accommodation is directly proportional to this length.

Alignment tolerances given by coupling manufacturers, while right for the coupling itself, may not be tight enough for coupled rotating machines. The real criterion for alignment is the machinery's running vibration. If excessive, particularly at twice running speed (or axially), further alignment improvement is required. Analysis of failed components such as bearings, couplings and seals also can indicate the need for improved alignment.

Commonly used alignment methods fall into three broad categories:
1. reverse-indicator;
2. face-and-rim; and
3. face-face-distance.

Reverse-indicator alignment is the preferred method for modern rotating machines. Axial movement of the shafts in sleeve bearings doesn't affect its accuracy. Both shafts should turn together (generally, both shafts should be rotatable and coupled), so coupling eccentricity or surface irregularities don't reduce accuracy of the alignment readings. Geometrical accuracy usually is better than with other methods. Moreover, alignment is very convenient and generally doesn't require disconnecting the coupling. The reverse-indicator method readily can handle complex alignment situations where thermal growth or multi-casing trains are involved. Usually single-axis leveling suffices for machines using rolling element bearings; two-axis leveling could suffice for machines employing sleeve bearings. The method does have some limitations. If the coupling diameter exceeds the available axial measurement span, geometric accuracy may be poorer than that of other methods such as face-and-rim. Nowadays, the general trend is toward high-torsional-stiffness couplings (metallic-flexible spacer-type ones) — so, the reverse-indicator method nearly always should be selected.

The face-and-rim method, a traditional alignment procedure, was popular decades ago. It suits large and heavy rotating machines whose shafts can't be turned. (Of course, some run-out error may occur due to shaft or coupling eccentricity.) It may offer better geometric accuracy than the reverse-indicator method for couplings with short spans (small span-to-diameter ratio). Generally, face-and-rim is better and easier to apply on short coupling spans (or small noncritical machines). If used on a machine with sleeve bearings, it can give significant axial float error; a special procedure usually is required. As a rule, machines with rolling element bearings require two-axis leveling and those with sleeve bearings three-axis leveling. (Reverse-indicator needs leveling in one fewer axis for each.) For long spans, the face-and-rim method requires spacer removal to permit face mounting.

The face-face-distance alignment method only makes sense for long spans such as trains that use a long transmission shaft instead of a coupling. It doesn't need an elaborate long-span bracket or other special considerations. The geometric accuracy of this method normally is lower than other methods.

Today, laser optic alignment has become very common. Devices usually use a semiconductor emitting a laser beam in the infrared range (wavelength around 800 mm) along with a beam-finder incorporating an infrared detector. Physical contact with the rotating machine isn't required. Modern laser optic alignment methods generally provide 1-micron accuracy. The data automatically obtained from the sensor enable instantaneous generation of necessary horizontal and vertical alignment adjustments.

Many machinists correct alignment by trial and error. A conscientious person may spend two days aligning a machine this way. However, knowing how to calculate the corrections or using an advanced laser alignment module could cut this time to two hours or less.

A machine's thermal growth (or contraction) may or may not be significant for alignment purposes. It's the relative movement of one machine versus other(s) that matters. Movements due to pipe loads, fluid forces and torque reactions usually have important effects. Vibration can indicate whether thermal movements and other operational effects are causing a misalignment problem during startup or operation. If so, consider thermal-operational-growth correction in the machinery alignment during commissioning. One of the best methods is to take mechanical measurements of the machine as it's operating after it's on its foundation and has final piping. Another useful approach is to make machine and piping adjustments while the machine is running, using vibration as the primary reference.

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