Things about the foundation for pumps (excerpt from Pump and Valve Home)

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Pumps have two main foundation options: concrete foundations and pile foundations. These dynamic study results show that for certain large pumps, pile foundations are sometimes the only option that can provide sufficient frequency separation to avoid resonance. The main natural frequencies of a concrete foundation include: 1. Transverse, 2. Longitudinal, 3. Oscillatory. All of these natural frequencies should be far away from the operating frequency of the pump, in order to avoid resonance and meet the vibration limits for the pump’s foundation. Pump sets sometimes generate both low-frequency excitation and high-frequency excitation simultaneously. In foundation design, the low-frequency range (below 3000 rpm) is more critical than the high-frequency range (above 4000 rpm). 　　Foundation for pumps The concrete foundation option should be the first choice for all pumps. To prevent resonance, various methods should be attempted to create sufficient frequency separation between the natural frequency of the foundation and the excitation frequency of the pump. Increasing the floor area can raise the natural frequency of the foundation, while increasing the weight of the foundation can lower it. 　　The natural frequency of a concrete foundation on a relatively rigid base may be in the range of 30–50 Hz (1800–3000 rpm), which is usually close to the operating frequency of some common pumps, motors, or other drives. Generally, for such foundations, it is impossible to increase the longitudinal natural frequency without also increasing the lateral natural frequency. However, pile foundations are easier to control than concrete foundations, as it is possible to adjust the natural frequency of the foundation individually. By changing the number and diameter of the ground piles, the inherent longitudinal and swaying frequencies can be increased, while keeping the inherent lateral frequency unchanged. 　　Pile foundations can fine-tune the lateral natural frequency while over-tuning the longitudinal and sway natural frequencies (which may be a requirement for certain special pumps). 　　For key large-scale power pumps (over 1 MW), the following standards for dynamics and foundation design are recommended: 1. For the center of the foundation, the vibration limit is usually 0.3–1 mm/s. 　　2. At the foundation surface (or at the location of the foundation bolts), the commonly adopted vibration limit is 1-2 mm/s. 　　3. For the bearing housing, the vibration limit may be specified as 2-4 mm/s. 　　The limitations mentioned above apply only to situations where resonance (or fatigue failure) does not occur. In other words, a margin of 15-20% should be reserved between all excitation frequencies and natural frequencies. At construction sites, the soil shear modulus obtained from soil tests can vary significantly. 　　If the soil is relatively hard (or soft), it may increase (or decrease) the natural frequency, thereby potentially leading to resonance. The best approach is to combine the two results to create two composite soil state profiles, showing respectively the softest and the hardest soil states. Sensitivity analysis helps determine the final foundation design for the pump. The design of the ground piles (pile diameter, number of piles, and others) should be optimized. 　　Precautions regarding the pump foundation The pump foundation should be large enough, constructed properly, and maintained in good condition. One of the most important principles is that the weight of concrete is approximately three times that of a dynamic pump (centrifugal or axial flow pump). During the pump’s service life (which can be over 30 years), regardless of the operating mode, the foundation/installation system must meet the following requirements: 1. Support the pump and related equipment. 　　2. Effectively transmit the vibration and the dynamic pressure it generates through the foundation (and soil), thereby reducing or eliminating the harmful effects of such vibrations. 　　The main problem in designing and implementing the foundation and installation system of a pump is \"vibration\". Vibration energy enters and exits the foundation in the form of waves, thereby being absorbed by the soil. However, if the overall structure of the pump and foundation breaks, is damaged, or separates, it will hinder the transmission of vibration waves. 　　The foundation should be a single structure; the pads, concrete foundation, cement slurry, and pump should all form one unified whole. If the pump must be installed on a skid-mounted unit, it is best to use one of the following two approaches: ■ Use anchor bolts at the top of the skid base. 　　■Fill the gaps in the skid base with appropriate cement slurry. 　　Special attention should be paid to the dimensions of the foundation and its center of gravity. The dynamic pressure of the pump and the height difference between the center of gravity of the pump-foundation system should be minimized as much as possible. Very roughly speaking, for small pump sets (3 MW), the length of the anchor bolts should be greater than 0.5 meters. 　　Foundation bolt sleeves or caps are always recommended for use. Wavy anchor bolt sleeves are the best choice. The bolt should be kept away from the epoxy mortar above half of its length. To this end, the upper half of the bolt needs to be wrapped with tape or covered with pipe insulation foam (or something similar). Bolts and grooves should be as far away from the edge of the foundation as possible (minimum 250 millimeters)