What is Alignment?
When speaking of a pumping system, proper alignment of two different types is important: the alignment of the pump shaft and the drive shaft, and the alignment of the pump flanges with the connecting piping.
The alignment of the pump shaft to the drive shaft of the motor, gear, or engine driving the pump is called shaft alignment. The alignment of the pump flanges to the connecting piping is called flange alignment.
Importance of Shaft Alignment
The shafts of the pump and drive unit must be closely aligned. Failure to achieve proper alignment, a condition referred to as misalignment, will result in increased pump vibration, decreased bearing life, and has the potential to cause mechanical seal leakage and issues with the coupling.
Importance of Flange Alignment
It is critical that the piping be carefully aligned to the pump flanges, and that the piping not be forced into place when the pipe flanges are bolted to the pump flanges. Poor flange alignment will place a tremendous amount of force on the casing – a condition referred to as flange loading – and may result in shaft misalignment as the casing shifts, increased vibration, bearing failures, mechanical seal failures, and cracks in the pump casing.
Types of Misalignment
When checking alignment – shaft alignment in partiular – there are two types of misalignment that may be encountered: parallel and angular.
Parallel misalignment may be horizontal, vertical, or a combination of both parallel and horizontal. Parallel horizontal misalignment occurs when two shafts are moved away from each other horizontally. Parallel vertical misalignment occurs when one of two shafts is higher than the other.
Angular misalignment occurs when two shafts are at an angle to each other and are not parallel. Just as with parallel misalignment, angular misalignment can be vertical when one shaft is at a downward angle toward the other, or horizontal where the shafts are in the same horizontal plane but at an angle to each other.
Misalignment often includes a combination of both angular and parallel misalignment.
Frequently Asked Questions
How do you check shaft alignment?
Shaft alignment can be checked in a rudimentary fashion using a straight edge placed on the pump and drive unit coupling hubs. This method is sometimes called rough alignment, and may be performed prior to certain construction activities to confirm that the pumping unit installation is adequate to continue work.
For most centrifugal pumps, once installation is complete, much more precise alignment is necessary. There are two common methods for checking shaft alignment: dial indicator alignment or laser alignment.
Dial-indicator alignment involves the use of two dial indicators: one mounted on the pump shaft and one mounted on the drive shaft. The shafts are slowly rotated and measurements are taken to determine the degree of misalignment. The position of the drive unit is adjusted until shaft alignment is achieved within the tolerances set by the pump manufacturer or equipment owner.
If you would like to learn more about shaft alignment, this article from John Crane provides a good overview of dial indicator and laser alignment methods.
How do you check flange alignment?
Flange alignment does not have to be as perfect as shaft alignment. When checking flange alignment the goal is to ensure that the piping is not exerting force on the flange bolts, and through them, the pump casing.
In order to verify that the piping is not loading the pump flanges, while the piping and pump are empty and out of service loosen all of the flange bolts at the same time and verify that they slide in and out of the bolt holes easily. If the piping is pressing the bolts against the bolt holes in the pump flange, then the bolts will not slide easily and the piping is not adequately aligned to the pump flange.