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Machine Train Alignment
- 13. Case Study
Notes de l'éditeur
- Welcome to our presentation on machine train alignment. Machine trains can offer some of the most extreme challenges to even the most experienced shaft alignment professionals. In order to overcome the obstacles presented by a machine train, we first need to understand the objective and how to achieve it in the most efficient manner. The following presentation will provide an insight on how to attack the machine train alignment…
- Before we can approach the alignment, we first need to understand the definition of a machine train. Simply put, a machine train is three or more machines that require alignment to each other. There are many combinations of machine trains. Some common configurations include a pump-gearbox-motor or multiple-stage turbines connected to a generator. There can also be larger and more complex sets.
- Machine trains, like all directly coupled rotating equipment, must be aligned. If alignment can be improved, machinery failure rates decrease. Equipment failures are a major maintenance expense and have numerous incidental or associated costs. Proper shaft alignment will increase the life of machine components such as bearings, seals and couplings, thereby prolonging the life of the machinery. As a result, costly unscheduled downtime is reduced. If machinery failures cost money, and improving the alignment reduces the number of failures, then clearly there is money to be saved through good alignment.
- The objective of any machine train alignment is to align the equipment within the specified tolerances at operating conditions. One goal during the alignment process is to minimize movement and still reach the objective. To achieve this goal it is necessary to graphically display the “as found” condition, whether this is with dial indicators and graph paper or with a laser alignment tool that graphically displays alignment conditions on screen. Once the whole picture is in view, the smallest possible move can be calculated, thereby reducing the chances of becoming bolt bound or base bound during a move.
- Once the whole picture is in view, the smallest possible move or “optimal move” can be calculated. By doing so, the risk of becoming bolt-bound or base-bound is reduced. One of the most common problems that will test an alignment technician’s ability as well as patience while doing a machine train alignment is becoming bolt-bound. When the stationary machine in a train is angled on it’s base, the other machines in the train need to be aligned to this angle. The amount of movement needed in the train may be impossible to achieve. When moving the last machine in the train, the movements required to achieve tolerances may not be met because the move required is greater than the space remaining between the bolts and the bolt holes. Before a decision is made to physically alter the base, feet or the bolts, one needs to explore the optimal move options which may save a considerable amount of time and labor. This can be done by several means as shown.
- In the example of a Pump-Gearbox-Motor machine train, a quick glance would assign the gearbox as stationary and the Pump and Motor as moveable. What if the alignment cannot be completed with this configuration? Alignment options must be explored. In this example, the static foot function available on the Rotalign Ultra shaft alignment system is used. This feature permits viewing alignment conditions throughout the whole machine train on one screen. Further more, the static foot function offers great versatility, allowing the user to view different individual pairs of static feet, not only entire static machines, thereby permitting a solution with the smallest possible move to be found for the bolt-bound machine in order to achieve alignment tolerances. In fact, with the Ultra, the entire concept of "stationary" machine becomes obsolete: you can find a truly optimized solution for all the machines in the train, moving all of the feet if necessary. Once the best solution is found, the moves are made utilizing the “live move” function on the Rotalign Ultra. The machine train is then re-checked to verify the alignment.
- Next we are going to examine a case study of a machine train set at a power generation facility that was aligned using the Rotalign Ultra laser shaft alignment system. The machine train consisted of a gas turbine, boiler feed pump, gearbox and booster pump. This machine train possessed many alignment obstacles, which had to be conquered. The turbine base had sunk over time due to environmental issues. The booster pump was subjected to considerable machine frame distortion, or soft foot, due to excessive pipe stress. Prior to the outage, the gearbox was overhauled and the bearings were replaced.
- The standard compact chain brackets, used to mount the laser and receiver, were not sufficient to take readings at each of the three couplings.. Each coupling required additional accessories for mounting. Coupling 1, between the turbine and the main feed pump, utilized the standard chain brackets, using the provided 600mm long chains to accommodate the large shaft diameter. Coupling 2, between the main feed pump and the gearbox required narrow brackets, due to extremely limited axial clearance. Coupling 3, between the gearbox and booster pump, used magnetic brackets with offset support posts because of obstructions to rotation, and limited axial clearance. The coupling bolts protruded from the coupling hub requiring a different option other than straight support posts. The hardware obstacles are resolved with different accessories supplied by the vendor of the laser alignment kit.
- At couplings one (1) and three (3), a turning gear is necessary to rotate the shafts which does not provide a smooth and consistent rotation which is desirable when using the sweep measurement mode. To compensate for this the multipoint measurement mode is utilized. Measuring in this mode permits rotation of the shafts until a desired location is reached, where the shaft is relaxed into its natural position where a reading is then taken. Multipoint mode achieves alignment readings without the forces from continuous rotation influencing alignment results. If excessive vibration from nearby equipment were present, it is possible to take more samples during each multipoint reading to mitigate erroneous information. This is called increasing the averages. This alignment took place during an outage with no other equipment in operation; therefore the averages were not increased because there was no vibration from surrounding running machines. Coupling two (2) provided a challenge for most laser shaft alignment tools. The spool piece was removed during the alignment process. For uncoupled shafts, the pass mode measurement mode was used to overcome this challenge. Rotating both coupling halves simultaneously is usually a concern. When measuring with pass mode the laser is simply rotated past the receiver at least five times over at least 70 degrees of rotation to achieve alignment readings. With misalignment values at all couplings recorded, we can now take a look at an overall picture of our “as found” alignment condition.
- Now that we have a graphically detailed overall picture of our alignment condition to scale; we can utilize the static foot function on our alignment system. The static foot or optimization function allows us to view different combinations of stationary feet until we determine which combination results in the most minimal moves to bring the machines into alignment, perhaps even one where no feet are stationary at all!
- The “as found” alignment conditions shows us that three machines need to be lowered to achieve the vertical alignment. By changing the stationary feet with the static foot function, the optimal move is found and a base-bound situation is quickly and easily avoided. Movement of machinery commenced at the main feed pump, continuing with the gearbox and then the booster pump. The live move mode is instrumental, informing the user when tolerances are met. Live move monitors each piece of equipment while it is moved. By monitoring the move at each pair of feet, guesswork is eliminated. This assists in moving the machinery into its proper location in a straightforward manner, with the chances of over shooting the correction being reduced .
- The alignment is considered complete when all equipment is within tolerance, in this case manufacturer-specific tolerances. The manufacturer also provided coupling target values. Targets are offset and angularity values that deliberately misalign each coupling when cold to compensate for thermal growth or load-related displacements that occur after a machine reaches its normal running condition. Once the targets are entered into the alignment computer, all values are automatically compensated for, eliminating the need for cumbersome calculations on graph paper.
- After each piece of equipment was moved into tolerance, a new set of readings was taken at each coupling. Each coupling is within manufacturer’s tolerances, while incorporating targets. The appearance of the smile symbol ensures the speed related tolerances are met at each individual coupling.
- The time taken to complete this alignment task was reduced from four days to two days. This reduction was made possible through the use of a tool designed for machine train alignment, encompassing all features necessary to solve the challenges that were encountered. There are a myriad of machine train configurations for every possible application. Machine trains are often the lifeblood of a plant or mill. Their alignment is crucial due to the cost of down time, cost of the equipment and their vitality. Using a versatile alignment tool will reduce frustration, save time and help accomplish the original goal of aligning a machine train within specified tolerances.
- We hope you have enjoyed our presentation. Thank you very much for your time in viewing it. If you have any questions or would like further information, please feel free to contact us. Good bye!