The purpose of shaft alignment between the driver and driven element in a turbo-machinery train is so that they are co-linear when in normal operation. It is desirable to have them in a common plane both vertically and horizontally. Design elements are incorporated to make this easier. In most designs, one component is designed to be the “movable” and one component is designed to be the “stationary”. The moveable component in a motor/compressor train will almost always be the motor. In order to accommodate the moves necessary, the motor will be designed to have some nominal amount of adjustable shim under each support foot. We require a 1/8” nominal shim to allow for adjustment up or down.
The alignment is accomplished using laser equipment capable of meeting the required tolerance. In our facility, we would normally expect the alignment to be within 0.002” of the target in both the vertical and horizontal planes. The targets are based on the expected movements of the machines in the running condition. For a typical API centrifugal compressor, the OEM would provide the alignment targets. Even though the compressor is supported by feet at the elevation of the shaft centerline, some amount of growth is unavoidable. Yesterday, we completed the alignment of a 4000 HP, 9000 rpm Coker wet gas compressor. The train consists of a motor, gear increaser and compressor. The targets called for the motor to be set 0.010” higher than the gearbox and 0.006” to the East. This allows for the fact that the gearbox will grow much more in the vertical direction than the motor. The horizontal offset allows for the spread of the gears at full speed and load.
The targets for compressor alignment required that the compressor be set 0.015” higher at the inboard support feet and 0.006” higher at the outboard feet. The compressor was to be set 0.004” to the West. The difference in vertical offset on the two ends of the compressor allowed for the different temperatures primarily driven by the heat of compression. The outboard end was the discharge end which runs much hotter than the inlet end.
For machines with no gearbox, the alignment targets usually only require a vertical offset. Both the motor and compressor would be designed to grow straight up, with very little horizontal movement.
Even with an alignment tolerance of 0.002”, a flexible coupling is needed. It would be impractical to achieve and maintain an alignment precise enough to allow for a rigid flange coupling. Without the flexible coupling, the shaft would fail from high cycle fatigue in bending. Coupling manufacturers will brag about the capability of their flexible coupling to accommodate misalignment. But, no matter what the supposed capability of the coupling, a precise alignment will reduce the loads on the bearings, seals, gears and other components. A precise alignment will result in lower vibration levels and increased reliability.
Johnny Pellin
