Failure Analysis of Centrifugal Pump Shaft and Impeller

My company two sets of oil refinery atmospheric vacuum tank centrifugal pump for the cantilever centrifugal pump, the design of the pump shaft material requirements for the 35CrMo, quenched and tempered, hardness HB = 269 ~ 302. Impeller material is G25 steel, static balance tolerance is 8g. Pump shaft end has a M24 × 1.5 thread, for the fixed impeller. Pump head 150 m, volume flow 200 m3 / h, speed 2 950 r / min, wax density 0.919 kg / m3. From December 1995 onwards, the pump occurred more than a shaft accident, fracture parts are in the shaft end of the thread undercuts, broken shaft time interval smaller and smaller, the shortest only about a month. At the same time many holes appear on the impeller surface, affecting the normal operation of the device.

1 analysis and calculation
1.1 Macro Analysis of the Shaft (1) Fracture Macroscopic observation of the fracture site of the failed shaft shows that the surface of the macroscopic fracture can be obviously divided into three zones: the fatigue crack zone, the fatigue crack growth zone and the final fracture zone. A careful observation of the edge of the shaft shows several fatigue crack steps, indicating that there are multiple sources of fatigue cracks in these fractures. These sources of fatigue cracks reflect the stress concentration at the axial relief. The fatigue crack growth area accounts for most of the total area of ​​the fracture surface, and the final fracture area is small, indicating that the tensile stress suffered by this shoulder is small. Because here the transition fillet radius R is very small, will have a greater stress concentration. Cyclic loading in the role of the stress concentration of the largest part of the thread undercut groove, the shaft fatigue fracture occurred. The cyclic load is the axial impact generated by unbalance caused by impeller weight loss. Impeller has not been replaced for a long time, causing more dynamic unbalance, the pump shaft breaking time is getting shorter.
(2) Metallographic analysis In the vicinity of the fracture sample analysis, found that the axis of the microstructure mainly tempered troostite, the organization can still see there are many lath martensite morphology, shown in Figure 1. It shows that when the shaft is quenched and tempered, the high temperature tempering temperature or time is not in place and the tempered sorbite structure is not obtained, resulting in the increase of the notch sensitivity of the shaft and accelerating the initiation and propagation of the fatigue crack.

Figure 1 axis microstructure 350 ×

(3) Hardness test Sampling hardness test, Rockwell hardness HRC values ​​were 31,32,31,34, these values ​​higher than the HB value specified in the drawings, further that the organization of non-tempered sorbite.
(4) Scanning electron microscopy (SEM) analysis After scanning, the microscopic morphology of the shaft fracture of the pump shaft was observed with a scanning electron microscope, and the fatigue streaks were found in the fatigue crack zone and the fatigue crack growth zone, as shown in FIGS. 2 and 3 Failure is caused by fatigue failure, and the microscopic morphology of fatigue crack propagation is a cleavage pattern.

Fig.2 Fatigue stripes near the source region of the fatigue crack

1.2 impeller (1) macroscopic observation and analysis of the pump inlet impeller surface there are many honeycomb-shaped perforation, the hub surface obvious signs of friction. Impeller front and rear surface erosion pits, foundry is also more loose. From the overall assembly point of view, the impeller weight loss perforation site and the axial end of the corresponding source of fatigue crack.
(2) Analysis of Chemical Composition The content of each component is as follows: w (C) = 0.162%, w (Si) = 0.218%, w (Mn) = 0.119%, w (P) = 0.015%, w 0.030%, from the chemical composition, carbon and manganese content is low.

Figure 3 Fatigue cracks in the fatigue crack growth zone

(3) Metallographic analysis of the organizational characteristics of massive and acicular ferrite + pearlite, is a typical Wei-Wei body tissue, shown in Figure 4. Coarse grains, the average grain size of 3. The pearlite content is lower, which is related to the low carbon content, and intergranular inclusions exist. This shows that the poor quality of the impeller casting, there is a large number of shrinkage, and the microstructure is not uniform, segregation is more serious, resulting in uneven metal surface conditions, the presence of non-metallic inclusions in the metal to form a gap, resulting in a physical Unevenness and incompleteness. Due to the higher temperature wax oil, acid value is higher, and contains sulfur and other elements, making inclusions around the point of origin of pitting. Pitting from the impeller surface can be inferred that the local site of perforation is the place where inclusions are enriched, and the low manganese content may be caused by the accumulation of this Mn-rich sulphide. At the same time, the segregation in the microstructure causes the grain boundary to weaken, so that the combination of pitting and intergranular corrosion accelerates the local corrosion.

Figure 4 impeller microstructure 100 ×

(4) The net positive suction pump head calculation As the pump allows the NPSH Δh unknown, here by the inhalation ratio of the number of revolutions to solve:

Where, n is the speed, r / min; qV is the volume flow, m3 / min; S is the pump suction than the number of revolutions. Centrifugal pump for ordinary design, regardless of the number of revolutions, can take 1 200. Thus we can get Δh = 7.4 m.
Effective net positive suction head Hg can be calculated using the following formula:

Wherein, Hg is a net positive suction head, m; p1 is the absolute pressure at the pump inlet, Pa; v1 is the average flow velocity at the inlet of the pump, m / s; pv is the vaporization pressure of the oil at the delivery temperature, Pa ; g is the acceleration of gravity, m / s2; ρ is the density of oil, kg / m3; Hf is the pipeline head loss, m.
In the calculation process, taking Hf = 6 m, the saturated vapor pressure of the wax solution under this condition is 169 kN / m2, and the calculated Hg = 11.9 m. From the above calculation shows that under normal operating conditions, cavitation can not occur.

2 Improvement Measures â‘  Increasing the fillet radii R, it is recommended that R = 1.5 ~ 2 mm, in order to improve the degree of stress concentration in this part. â‘¡ In strict accordance with the requirements of the design implementation of heat treatment process to ensure that the shaft to obtain a good overall mechanical properties. â‘¢ impeller installation, the nut preload should be appropriate, not too big. â‘£ impeller surface treatment to improve corrosion resistance.

3 Conclusion The oil and gas centrifugal pump pump shaft and impeller modified according to the above measures, in May 1997 after the installation of the device overhaul and put into use, has been running for more than a year, running well, there is no shaft accident, that the transformation is successful.

About the author: Yang Huosheng (1962-), male, Hubei Tianmen City, engineer, master's degree, is engaged in static equipment research, failure analysis, safety assessment and pressure vessel inspection and other aspects of work.
Research Institute of Zhenhai Refining & Chemical Co., Ltd., Ningbo 315207, Zhejiang, China

[1] Gao Mingquan. Cause and prevention of cavitation of circulating pump impeller [J]. Petrochemical Equipment Technology, 1986,7 (1): 42-43.

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