Alignment to a machine’s running alignment position is essential for reliability and efficient machine operation. It is not enough to only consider a machine’s cold off-line running position for shaft alignment. A machines shaft alignment position will change through start up to its hot on-line running state. Certain machines will move more than others if they have extreme temperature changes and/or are connected to extensive piping. The two most common types of machines that fit into this category are boiler feed pumps and gas turbines. These machines must have their dynamic movement considered to calculate their cold alignment targets.
Dynamic movement occurs when shaft alignment circumstances change from a machine’s cold off-line state to hot on-line running conditions. Machine dynamic movement is made up of movement due to thermal growth and torque movement combined. The misalignment at the coupling represents the total change that occurs because of these two components. There is no reason to separate out the two components of dynamic change unless you plan on correcting the source of them. This will take a good amount of time and money. The best thing to do is correct for the change where it affects the machine, at the coupling. There are several different ways to determine dynamic movement.
The easiest way is not to measure it at all. Use a pre-existing alignment target. An alignment target can come from the coupling manufacturer, machine manufacturer, or an in-house specification. If there are no pre-existing alignment targets, the next easiest thing to do is to perform a theoretical calculation. This is not an exact calculation of dynamic movement at the coupling, but it is better than doing nothing. It is a theoretical calculation of how much the machine case will expand from cold to hot running conditions, paying no attention to torque movement. Multiply the machine case material constant, by the distance from the shim plane to the center of the shaft, by the difference in temperature from hot to cold. Do this for the drive end and non drive end of the machine separately. This is done because the temperature, when hot, from drive end to non drive end may vary enough to affect the angle at the coupling. The most time consuming, but most accurate way to way to generate an alignment target for your machine is to measure it. Measuring the dynamic movement of your machine is the only way to get the exact alignment target. The only other traditional method to determine dynamic movement is a hot alignment check. This is not a recommended method because it captures no torque movement and most of the thermal change has been lost by the time the hot misalignment readings have been recorded.
This best way to determine dynamic movement of a machine is to deploy a laser based real time monitoring system. This approach will ensure that all movement data is collected and the movement can be verified in several runs.
Figures one and two show a laser based monitoring system mounted to a motor – boiler feed pump machine.
Figure 1: Permalign Laser Monitor on Motor Side of Coupling
Figure 2: Permalign roof prism target mounted on boiler feed pump side of coupling
The monitor and targets are mounted to the bearing brackets, so the monitoring system will display the relative movement change across the coupling. The movement that the monitoring system records mimics the way that the shafts are moving relative to each other. Table one shows the Vertical offset of the machine over a period of three days.
Table 1: Vertical Offset and temperature of a motor to boiler pump machine.
This machine was located outside. You can see how much affect the temperature alone has on the machine throughout the day. The machine starts on 5/10/09 at 11:00 AM, even though the machine is running with a constant load the movement due to the environment does not go away. Conventional methods such as theoretical calculations or snapshot movement checks will not capture the incremental change in movement of the machine and may lead to incorrect conclusions.
This pump is supposed to have very little vertical movement because it is center mounted. The motor and the piping connected to the pump will still have an effect on the vertical alignment. Collecting this data allows you to make a decision for which conditions the machine will be aligned. This data is collected for the vertical offset, vertical angle, horizontal offset, and horizontal angle. All of the dynamic alignment change information is interpreted and converted into alignment targets. The alignment targets are used as the end state position of the machine for a cold alignment procedure.
Some laser alignment computers can accept the targets. If your alignment system does not have this capability or you are using dial indicators it is a good idea to graph out the desired final alignment position. You may have dynamic movement numbers that are generated from different sources so they will get displayed differently. That is why it is never a bad idea to graph out your alignment. It will cut down on mistakes and your total time to complete a shaft alignment. If you employ this process in all of your machines you will improve machine efficiency and increase machine up time.
