For a machine to operate efficiently and smoothly, it is essential that it be in sync with its running alignment position. Solely considering the machine's cold offline running position for the purposes of shaft alignment wouldn't suffice. The shaft alignment position of a machine changes as the machine progresses from the start up stage to the hot online running stage. Also, when subjected to extreme temperature shifts or when linked to extensive piping, certain machines depict more motion than others.

Gas turbines and boiler feed pumps are the two most commonly known machines in this particular category. The dynamic movement of these machines must be considered in order to compute their cold alignment targets.

When shaft alignment positioning changes from the cold offline state of a machine to its hot online running state, dynamic movement is said to have occurred. Thermal growth combined with torque movement cause this machine dynamic movement. A misalignment at the coupling is resulted due to the changes caused by these two components. However, you should only consider separating these components if you intend to correct their source. The process for this kind of correction involves big amounts of time and money. The more economical way to solve this problem could be to make corrections in the coupling, where it impacts the machine.

There are multiple different methods that help determine dynamic movement:

• The simplest way could be to not measure it at all. In such a case, a preexisting alignment target may be considered. This alignment target could be obtained from the machine manufacturer, the coupling manufacturer or the in house specification.

• You could perform a theoretical computation if no previous alignment targets are available. Theoretical calculation might not offer an accurate computation of dynamic movement at the machine's coupling, but it is better than having no target to rely on. Without taking into account the machine's torque movement, the theoretical method determines as to how much the machine's case will expand as it progresses from cold offline conditions to hot running ones. To calculate dynamic movement using this method, multiply the case material constant of the machine by the distance between the shim plane and the center of the shaft, and by the difference in temperature experienced when the machine runs from hot to cold conditions. Make these computations for both the drive end and the non-drive end of the machine separately. This is because the high temperature may vary significantly between the drive end and the non drive end, and this might impact the angle at the coupling.

• The most accurate method of determining an alignment target for a machine is to actually measure its dynamic movement. The measurement process is certainly tedious and time taking, but its results are worth all the hard work it demands. Also, this is the only way to find the exact alignment target of your machine. Another traditional method of determining dynamic movement is the hot alignment check. However, this method is not recommended by engineers as it does not capture the torque movement. Also, the problem with this method is that most of the thermal shift has been lost by the time you record the hot misalignment readings.

Deploying a real time laser based monitoring system is the most efficient way of estimating the dynamic movement of a machine. This approach not only ensures that all the movement related data is recorded properly but also confirms that the movement can be verified in certain number of runs.

Figures 1 and 2 demonstrate a laser-based monitoring system that has been mounted to a motor boiler feed pump machine:

The monitoring system effectively displays the relative movement shift across the coupling. The dynamic movement recorded by this monitoring system replicates the way the shafts move relative to each other.

Table 1 depicts the Vertical offset of the same machine over a period of 3 days:

The machine is located outdoors. The affect of temperature during the day on the machine is clearly visible. The machine starts on May 5th’2009 at 11:00 AM. In spite of running with constant load, the machine depicts considerable movement due to environment. In this scenario, the traditional methods such as snapshot movement checks or theoretical calculations fail to capture the incremental shift in the machine’s movement, thus resulting in wrong conclusions.

Being center mounted, the pump is supposed to have minimal vertical movement. However, the piping and the motor linked to the pump still impact the vertical alignment. Having access to this kind of data permits us to decide as to what conditions the machine needs to be aligned for. The data is recorded for horizontal offset, horizontal angle, vertical offset and vertical angle. The entire dynamic alignment change information is interpreted and then transformed into alignment targets. The alignment targets are further used as the machine’s end state position for cold alignment procedure.

Certain laser alignment computers do accept targets. However, if the alignment system you use doesn’t provide this facility, or if you use dial indicators, you might want to graph out the much desired final alignment. If you have used several different methods, you will have different dynamic movement values. So, it is a good idea to graph out the alignment position. Using this approach would reduce the time and energy invested in the shaft alignment process significantly. It will also help increase the machine up time, ultimately improving the machine’s efficiency.

ACQUIP, INC provides laser alignment services for all of equipment needs in the Oil and Gas, Petrochemical, Marine, and Power Generation Industries

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