- Detail

Bearing design of centrifugal compressor

1 introduction

the requirements of centrifugal compressor for sliding bearings are: safe and reliable, stable operation, good seismic resistance, long service life. At present, the multi block thrust bearing in domestic and foreign activities is to participate in the Olympic Games in Rio de Janeiro, Brazil as the representative of the U.S. team. The thrust bearing is mainly divided into two categories, one is Mitchell bearing, and the other is Kingsbury bearing. Michaelis bearing has strong adaptability to variable load, but the load is not evenly distributed on each pad, which leads to uneven wear of thrust bearing pads. Kingsbury bearing overcomes the above shortcomings, and its advantages are that the load distribution is uniform, the adjustment is flexible, and it can compensate for the misalignment and deflection of the rotor

JS Kingfisher bearing is a series of general centrifugal compressor bearing components formed through structural improvement and design based on the products of Kingsbury company in the United Statesthe structure is formulated according to the original Kingsbury bearing series, and the standard series specification adopts the structure of Kingsbury thrust bearing used in 200000 t/a synthetic gas compressor

according to practical experience, the Kingsoft bearing series is divided into 7 common specifications, namely 5, 6, 7, 8, 9, 10.5 and 12, a total of 7 specifications (the number represents the outer diameter of the thrust surface of the pad), which is preferred and recommended in the compressor design of our factory

2 structural features

Kim's thrust bearing is a laminated self balancing thrust bearing, which is composed of several thrust blocks. The thrust block is padded with an upper level block, a lower level block and a base ring, which is equivalent to three layers of parts stacked on the base ring. The thrust block and the level block are in contact through a spherical fulcrum. Its working principle: when the load of each thrust block is different, it will cause the imbalance of the bearing, and the thrust block will deflect due to uneven stress. At this time, the load on each thrust block can be automatically adjusted through the upper and lower overlapping leveling blocks until the load on each thrust block is the same, and the bearing is re balanced. That is, when the rotating shaft has a large deflection and fulcrum angle, the positions of each pad can be balanced to produce uniform oil film pressure

typical structure of King's thrust bearing. Its main body is composed of tiles, level blocks swinging up and down, bearing plates and oil control rings. The lubricating oil enters the bearing cavity from the notch at the bottom of the bearing plate, and is brought into the pressure bearing surface of the thrust pad through the gap between the pads and the rotation of the thrust plate. The oil is brought to the outer circumference by centrifugal force, and then flows to the outside of the bearing through the oil drain hole on the upper part of the oil control ring

3 design specification of main parts

3.1 dimension standard

Kingfisher thrust bearings adopt inch system. In order to facilitate manufacturing, except that the internal and external diameters of pads, thickness, the matching dimensions of bearing plates and positioning keys are still converted into metric system according to inch system, and two decimal places are taken as the nominal dimension, the rest dimensions shall adopt metric rounding value as far as possible

3.2 tile design standard

the arc radius of the tile support pad is related to the relative position between the support point and the tile width after the tile swings. Therefore, it should be the function value corresponding to the tile width. This series uses the same ratio of Mitsubishi of Japan to determine the half diameter r of other specifications, and calculates the contact stress. The calculation formula is: = 0.388 (PE 2/r 2) 1/3, in which the contact stress, kg/cm2

p axial force of rotor, kg

e modulus of elasticity, kg/cm2

r average radius of bearing surface, and the calculation results of cm

are shown in Table 1. Table 1 specifications 125 150 175 200 225 265 300 19569 18993 19627 18374 18839 19334 18697

among them, specification 200 is the Mitsubishi structure of Japan, and the other stresses are close, but they are slightly higher than the allowable stress specified for the transmission system (4000 ~ 18000kg/cm2)

each set of thrust bearings is equipped with two groups of main and auxiliary bearings, which have the same structure and can be interchanged as a whole without being limited by the rotation direction of the compressor. However, if it is necessary to measure the pad temperature, each group of bearings can be equipped with pads with thermocouples, which have rotation direction requirements

3.3 determination of oil feeder

this series adopts the inter pad oil injection structure, and its specification is determined according to the following method:

according to the Nuovo Pignone e.c.c standard, the oil consumption of various specifications has clear data, and has a certain proportion relationship. Based on the 8 bearing of Mitsubishi Corporation of Japan, the small hole flow formula is

q= D 4/128 L △ P, in which Q mass flow, m3/s

d small hole diameter, m

dynamic viscosity of oil, Pa.s

L length of oil feeder, m

△ P differential pressure, PA

under the assumption that, l, △ P and constant, the flow is proportional to D4, so as to calculate the total injection hole area (relative to the actual area of 8), and then determine the number of holes that may be distributed on the length of oil feeder and the diameter of small holes

3.4 determination of thrust disc thickness

the thickness of this series of thrust discs is less than the specified value of typical Kingfisher bearings, which may be due to the fact that the structure uses an oil control ring to collect oil, and the oil needs to flow out between the oil control ring and the groove of the thrust disc, which requires a certain space, while the new structure cancels the oil control ring, so the thickness can be appropriately reduced. See Table 2 for the comparison of the thickness of the new and old thrust discs

Table 2 specifications 6810.512 the original standard thrust thickness 25.4 34.925 44.45 50.8 the thickness determined in this standard 24304045

3.5 nbsp; Determination of the axial clearance between the thrust disc and the pad

about the axial clearance between the thrust disc and the pad, Mitsubishi designed it to be about 1.36 times the average value of the typical structure, which may be because the lubricating oil of the typical structure can flow out through the gap between the pads in order to cool the bearings. The new structure has an oil feeder between the pads, which limits the flow of oil, so the clearance is enlarged. According to the gap flow theory, the flow is proportional to the third power of the gap, If the gap is increased to 1.5 times, the flow can be increased to 2.5 times. Therefore, this standard is given by 1.36 times the average gap value of typical structures (see Table 3). Table 3 bearing specifications 6810.512 the original standard clearance is 0.25 ~ 0.35 0.28 ~ 0.38 0.33 ~ 0.43 0.38 ~ 0.45 the standard clearance is 0.36 ~ 0.46 0.4 ~ 0.5 0.47 ~ 0.57 0.63 ~ 0.73

3.6 bearing capacity

axial load is divided into two parts, which includes the axial thrust of the compressor and the axial speculation produced by the coupling. Automatic calculation of maximum air force, upper and lower yield strength, hysteresis ring method, gradual approximation method, non proportional extension strength Tensile strength, compressive strength, elongation strength at any point, load extension at any point, modulus of elasticity, elongation, maximum value, minimum value, average value, bending modulus and other forces in the stripping interval. The calculation formula is:

f=f1+f2, where F1 compressor axial thrust, kn

f2 coupling axial thrust, kn

compressor axial load calculation formula: f=0.25 9550 pr/nrd, where f external thrust, Kn

pr rated power, kw

nr rated speed, r/min

d shaft hole diameter of coupling, mm

axial load calculation shall be in accordance with the provisions of 4.6.3.3 of hg/t226292 technical conditions of centrifugal compressors for oil refining and chemical industry. Nuovo Pignone e, C and C standards list the bearing capacity charts of various specifications of bearings, which can be used as design parameters for this series of bearings. The load borne by the thrust bearing should not exceed 50% of the rated bearing capacity of this chart

4 conclusion

(1) the oil supply mode of Kim's series bearings can be measured: the full range is not divided into grades to fully lubricate and cool the pads. At the same time, this structure also reduces the thickness of the whole bearing

(2) the oil volume of the upper and lower half of the original structure bearing is uneven, and the pads of this series further improve the automotive power performance. The lubricating oil supply of the block is sufficient, so the oil control ring is cancelled, and the thickness of the thrust disc is reduced at the same time

(3) the lower swing bearing pad of typical structure is in direct contact with the bearing plate from the arc surface at the bottom, and the contact stress is too high, which is easy to crush the bearing plate. A support block is added to the new structure. Although the structure is complex, it increases the reliability of use

in a word, the improved structure is better than the original structure and can better meet the working requirements. The serialization of bearings will provide convenience for the design of centrifugal compressors, shorten the design cycle and ensure the design quality

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