I’m seeking advice from experts regarding the alignment issue of hot oil pumps
Thread Content
As the title suggests, almost all of the pumps in our installation are mounted along the centerline. The advantage of this is that the pump body can expand evenly upward and downward from this centerline. In theory, the height of the centerline should not be affected by the temperature of the medium. However, when aligning the hot oil pumps during maintenance, we always allow a vertical deviation of 5 units either way. This is done on the assumption that the center of the pump will move upward once it is operating properly. Is this maintenance practice unnecessary? Should the force on the pump inlet and outlet due to pipeline heating also be considered?
Unless the pump data specifies a cold alignment tolerance, alignment cannot be carried out based on empirical data; if you are unsure, you can preheat the pump first and then perform hot alignment
Special instructions will be provided in the equipment installation guide. Some devices have such requirements. For example, in cases where a hydraulic coupling is used as a transmission, temperature changes occur during operation, which requires alignment of the equipment driven by it; any deviation in alignment necessitates the use of a flexible coupling. Therefore, when dealing with connection devices that experience large temperature variations, it is necessary to pay attention to the relevant instructions. And it is not possible to play cards in the conventional way; for reference only.
It can be checked again while it’s hot to adjust it to the standard.
What is the temperature of the pump’s central support? Has it been cooled?
Align the hot oil pump; if the equipment’s factory specifications provide clear guidelines, follow those values. Nowadays, oil pumps use diaphragm couplings, which compensate for even minor deviations
Your approach to maintenance is, under normal circumstances, excessive; moreover, your understanding of alignment is incorrect. When it comes to pump alignment, we need to consider why alignment is necessary. First, it’s important to understand that almost any rotating component relies on two supports (bearings) to rotate. Of these two supports, one is typically used for rotational positioning, while the other is used for axial displacement. As for how to determine which support is for positioning and which one is for displacement, that depends on the design and working principles. Focusing solely on the issue of alignment, once we understand the functions of each support, it’s clear that there will inevitably be manufacturing errors in these supports—this is what’s referred to as concentricity tolerance (the position where the bearings are installed). Once the concentricity requirements are met, it’s also important to understand that all bearings have some degree of play during installation—this is inevitable. If the pump is installed horizontally, the combined effect of the bearing play and concentricity tolerance, along with gravity, will cause the pump to deviate from its absolute center by the greatest possible amount (installing the pump vertically helps reduce this deviation). After installation, the pump will rotate around this absolute center, and the extent of this deviation determines the level of vibration. Under normal conditions, tolerances are determined during product design with these factors taken into account. Thus, these values determine the selection of bearings as well as the tolerance ranges for the manufacture of related components (such as the position of the bearing housings). As for thermal expansion, it’s important to understand that thermal expansion occurs across the entire range, and it exists between different components. However, the temperature differences resulting from heat transfer between components can be ignored, as they can be considered negligible. Under these circumstances, if both the product design and manufacturing precision meet the operational requirements, there is no need at all to increase the alignment offset additionally; in fact, if the contact angle defined by the corresponding bearing model is small, it will accelerate damage!
It is said that the experienced maintenance technicians mentioned that we used to preheat the equipment before aligning it, but this made the repair process more complicated; moreover, the data changed after each preheating, probably because it was not possible to achieve exactly the same conditions as those during preheating.
The factory documentation does not specify clear requirements for alignment.
The bracket is cooled, and its temperature after cooling is almost the same as before preheating
What you say makes sense, but thermal expansion does indeed need to be taken into account, especially when the temperature of the medium exceeds 400 degrees. The stresses at the pipe ends, as well as those on the rotor and pump casing, become quite significant in such cases. We tried aligning the pump before and after preheating, and there were noticeable differences in the data. It remains to be determined whether the design of the pump bearings took this issue into consideration. Moreover, there can be a significant difference between the operating temperature and the designed temperature; therefore, the most reliable approach is to perform alignment while the pump is in its hot state.