Fracturing pump head body failure analysis and improvement measures

The head body of fracturing pump is one of the most easily damaged parts in fracturing equipment. Due to its own structure and working environment, it is easy to cause stress concentration, fatigue cracking and other problems. The working time of a batch of fracturing pump head body used in an oilfield is more than 300 h, and the pump head body is designed by the manufacturer for the oilfield.In this paper, the five-cylinder fracturing pump head body of this batch of cracking occurred as the research object. Firstly, the macro fracture analysis, physical and chemical properties analysis and micro fracture analysis were carried out at the cracking place. Secondly, the finite element analysis and fatigue life analysis were carried out by using ANSYS Workbench software and nCode software to explore the cause of pump head cracking failure. The results show that the crack originates from the sharp corner of the outer corner and spreads to the inner cavity. The chemical composition and mechanical properties of the pump head meet the requirements of the technical agreement. The grain size of the material does not meet the requirements of the technical agreement. The primary fracture morphology observed is indicative of fatigue streaking, while stress concentration can be identified at the location of this crack. There are two reasons for the cracking. First, the structure design is unreasonable and there is a large stress concentration at the cracking location. Second, the grain size of the material is large, resulting in a significant reduction in fatigue life, which is mainly related to the chemical composition of the material, forging process and heat treatment process, and ultimately lead to the pump head body cracking at the outside right Angle shoulder, expanding to the inner cavity, and cracking. According to the analysis results, the improvement measures of pump head body can improve the stress distribution and relieve fatigue. The results can provide a reference for the structural design and optimization of the pump head body.


Introduction
At present, people's demand for oil and natural gas and other related resources is growing day by day, and the exploitation of conventional oil and gas fields has entered the late stage, and the difficulty of exploitation has gradually increased.Therefore, the development of unconventional oil and gas fields such as shale gas, tight gas, ultra-deep oil and gas fields with greater difficulty, and low permeability oil and gas fields has become an important strategy for national energy development [1][2][3][4][5].Fracturing technology has become the main production method of oil and gas field exploitation, through fracturing can greatly improve the efficiency of oil and gas field exploitation, and has become a very important method in the oil exploitation of various countries in the world [6][7][8][9][10].The important equipment to realize fracturing technology is fracturing pump, which is composed of power end and hydraulic end [11,12].At work, the hydraulic end has been in a high-pressure state, the pump head body due to the complex structure of the inner cavity, wall thickness distribution is not uniform, easy to cause stress concentration, fatigue cracking and other problems [13][14][15][16][17][18].The penetration crack at the intersecting line of the pump head body is a typical form of the failure of the pump head body.The position of the intersecting line is shown in figure 1(c).At work, due to manufacturing, processing, structure, materials used, corrosion and other reasons, small defects start to sprout at the intersecting line.Under the action of alternating stress, they gradually expand and evolve into visible cracks until the pump head body is completely penetrated, resulting in fatigue failure, which seriously affects the development efficiency and economic benefits of oil and gas fields.High-pressure fracturing fluid can pose a serious threat to the personal safety of workers [19][20][21][22].Therefore, the pump head body is required to have high strength and service life in actual production [23,24].
The head body of an oil field fracturing pump cracked in more than 300 h on the machine, the pump head body is made of 17-4PH stainless steel, and the service time is far lower than the rated life.The technical agreement calls for a rated life of 600 h.The technical agreement refers to the technical agreement requirements for ordering by the user to the manufacturer.Among them, chemical composition, mechanical properties, metallographic structure and use time must meet the requirements of the user.The fracturing pump was a fivecylinder pump and was found to be cracking outside the pump head in the 5# pump chamber.The cracking site of the pump head body is shown in figure 1(a), the external structure of the pump head is shown in figure 1(b), from left to right is the 1 ∼ 5# pump chamber, the inner cavity structure is shown in figure 1(c), and the crack location is shown in figure 1(d).The overall dimension of the equipment is 1492.2mm, the total width is 601.3 mm, and the total height is 654 mm.In this paper, the structure of the pump head body is analyzed by macroscopic analysis, physical and chemical properties analysis, microscopic analysis and stress analysis in order to explore the reasons for its cracking and improve it, which provides a theoretical basis for structural design optimization, function and performance evaluation.

Macro-analysis of cracks
In order to facilitate the analysis, the pump head body was cut using a sawing machine and wire cutting.Figure 2(a) shows the outer wall cracking sample and figure 2(b) shows the inner cavity cracking sample.It can be observed from figure2 that the crack length of the outer wall is about 148 mm, and the crack length of the inner cavity is about 35 mm.When the crack cutting area is opened, the macroscopic morphology and propagation trend of the crack fracture are observed and analyzed, and it can be seen that the crack originates from the sharp corner of the outer corner and spreads to the inner cavity.

Chemical composition analysis
Using a sawing machine and wire cutting method, the sample was sampled at the inner 2in of the body surface of the pump head, and the size was 10 × 10 × 5 mm sample block.The main chemical components were analyzed by ARL-3460 direct reading spectrometer, and the contents of C, Si, Mn, P, S, Ni, Cr, Cu and Nb were measured.The measurement results are shown in table 1, and the main chemical composition of the pump head body meets the requirements of the technical agreement.

Mechanical performance test
Tensile test, impact test and Brinell hardness test were carried out on the mechanical properties of the external wall and the inner cavity of the pump head body.Transverse and longitudinal rod-like tensile specimens were used for tensile tests, respectively.The diameter within the standard distance was 6.25 mm and the test temperature was room temperature.The tensile test equipment is WDS100 electronic universal material testing machine.The test results are shown in table 2. The tensile strength, yield strength, elongation and section shrinkage of the pump head body all meet the requirements of the technical agreement.
The longitudinal and transverse Charpy V-type notched impact samples were used in the impact test, the size of which was 10 × 10 × 55 mm.The temperature of the transverse and longitudinal samples was normal temperature.The equipment was pendulum type high and low temperature automatic impact testing machine, and the impact sample making equipment was GB/T229 hydraulic metal Charpy impact sample notched broking machine.The three groups of data were measured respectively and the average value was taken.The test results are shown in table 3. The impact power of the pump head body at normal temperature meets the requirements of the technical agreement.
Brinell hardness test was carried out on 50 × 50 mm sample blocks in outer wall and inner cavity of pump head body respectively.DHB-3000 electronic Brinell hardness tester was used for hardness test.5 groups of data were measured respectively, and the test results are shown in table 4. HBW10/3000 means that the cemented carbide ball with a diameter of 10 mm is maintained for 10s-15s under the test force of 29420 N (3000 kgf).The measured hardness values are in the range of 300 ∼ 350, and the hardness meets the requirements of the technical agreement.

Metallographic analysis
Longitudinal samples were taken from the outer wall and inner cavity of the pump head respectively for observation.The microstructure was observed with a metallographic microscope (VMM5000R/RT) at an adjusted pitch of 100 μm.It can be observed that the outer wall and the inner cavity are tempered martensite, and the tissues contain δ ferrite.The metallographic structure of the outer wall is shown in figure 3(a), and that of the inner cavity is shown in figure 3(b).Tempered martensite appearance is black needle shape, with high strength, hardness, wear resistance and toughness.δ ferrite is a crystal structure of iron formed at low temperatures, in the form of plates or bands, with high strength, hardness, corrosion resistance and magnetic properties.The grain size of the outer wall and the inner cavity is measured.The average grain size of the outer wall is 3.0, and the grain size difference is 5.5 in the same field of view.The grain morphology is shown in figure 3(c).The average grain size of the inner cavity is 3.0, and the grain size difference is 5 in the same field of view.The grain morphology is shown in figure 3(d).The technical agreement requires that the average grain size is not less than 5 grades, and the difference between the grain size and the same field of view is not more than 2 grades.It can be concluded that the grain size of the pump head body is large, and the anti-fatigue crack initiation ability of the material will increase with the decrease of the grain size.

Scanning electron microanalysis of fracture
The fracture morphology of the crack source region and expansion region was observed by SEM.The scanning electron microscope equipment is the Sigma300 type field emission scanning electron microscope produced by ZEISS.The fracture morphology in the source region is shown in figure 4(a), and that in the extended region is shown in figure 4(b).The morphology is fatigue grain, and the pattern is short and curved like a river, with more tearing edges.Energy spectrum analysis was performed on the fracture surface, and the results of the fracture surface in the source region and the extended region were shown in figures 5(a)-(b).The elements at the fracture are mainly matrix elements, which meet the requirements of the technical agreement.The content of elements is shown in table 5.By analyzing the fracture at the crack, it can be concluded that the failure mode of the crack is fatigue crack.

Analysis of results
Based on the macroscopic fracture analysis, physical and chemical properties analysis and microscopic fracture analysis of the failed pump head body, it is concluded that the chemical composition, tensile strength, yield strength, elongation, room temperature impact power and brinell hardness of the pump head body meet the requirements of the technical agreement, the grain size does not meet the requirements of the technical agreement, and the fracture morphology is fatigue grain.It can be concluded that the cracking failure occurs due to fatigue and the grain size does not meet the requirements.

Pump head body model construction
The pump head body is a 5-cylinder structure, the structure is relatively complex, so this paper first uses Unigraphics NX 3D design software for 3D solid modeling, the ratio between the model and the actual equipment is 1:1.Then the model was directly imported into ANSYS Workbench through the menu generated by associating Unigraphics NX software with ANSYS Workbench software.In order to study the cracking causes of the pump head body and the improvement measures, the stress analysis of the pump head body model is carried out.The pump head body is made of 17-4PH stainless steel with an elastic modulus of 204 GPa, Poisson ratio of 0.291, density of 7790 kg m −3 and yield strength of 975 MPa.

Unit type selection and mesh
The structure of the pump head body is divided into tetrahedral structures, and the unit type is Solid187, which has the advantages of easy grid generation, strong adaptability to the model, convenient numerical calculation, etc.The basic size of the global analysis model unit is 5 mm, the number of nodes is 3506631, and the number of units is 2504893.The mesh model is shown in figure 6.

Applying constraints and loads
Constraint application: By analyzing the actual working condition of the pump head body, it is necessary to apply a fixed support at the fixing bolt hole.There is no displacement of the pump head body in the YOX plane in the Y direction at the fixed bolt, and the displacement constraint is applied here.Load application: The maximum working pressure output by the pump head body during operation is 103.5 MPa, which is the pressure required by the user when performing fracturing work.There is a regulating valve at the outlet of the pump to adjust the opening of the valve to achieve the adjustment of the pump outlet pressure.The pressure is obtained by connecting the pressure gauge at the output.The pump head body has two working conditions at work, namely suction condition and discharge condition.In the suction condition, the suction valve is opened, the discharge valve is closed, the pressure of the suction chamber is almost 0, and the pressure of the discharge chamber reaches the maximum pressure.The maximum working conditions were numerically simulated, and 103.5 MPa pressure was applied to each chamber of the pump head body for analysis.Constraints and load application are shown in figure 7.  between the cylinder cavity and the plunger cavity of the pump head body, the position of the fracturing fluid discharge channel and the position of the external edge.The pump head is made of 17-4PH stainless steel with a yield strength of 975 MPa and the maximum stress is located in the fracturing fluid discharge channel.The maximum stress is 565.43 MPa, which is less than the yield strength.There is also a large stress concentration at the intersecting line of the inner cavity and the outer edge, which is consistent with the cracking of the pump head body in the actual fracturing site.It can be concluded that the reason for the cracking of the pump head   body is fatigue and unreasonable structural design.The angle of cracking position is 93.8°, the radius of rounded curvature tends to zero, the stress concentration coefficient tends to, and the stress concentration is large.For the inner cavity intersection line, although also belongs to the stress concentration site, but the radius of curvature of the rounded corners is larger than the outer edge corners, and its stress concentration factor is smaller than the outer edge corners.Mesh convergence analysis was performed before solving, as shown in figure 9.The stress values are shown in table 6.

Fatigue life analysis of pump head body
In this paper, ANSYS nCode DesignLife software will be used to analyze the overall failure number (fatigue life) and failure location of the pump head body under actual working conditions, and the calculation results of finite element analysis of the pump head body will be analyzed in combination with the software.ANSYS nCode DesignLife software is a finite element calculation and analysis software for fatigue life prediction.Firstly, the material selection, structure model, mesh division and calculation results of finite element analysis are imported into the fatigue calculation module.Secondly, the material property parameters were set, and finally the solution was performed to view the life cloud map of the pump head.

Material property parameter setting
According to the working nature of pump head, SN Time Step module in nCode software is selected to carry out fatigue analysis.The material of the pump head body is 17-4PH stainless steel, which has been defined during the finite element analysis.When fatigue analysis is carried out, it is necessary to map and edit the material according to the strength limit and tensile strength of the material, and the S-N curve of the material at the background can be accurately obtained and fitted.Material 17-4PH of the pump head body is created in the Material Map of nCode.The performance parameters of the material are input and the S-N curve is obtained after taking the tensile strength into account.The material parameter Settings are shown in figure 10.

Select fatigue algorithm
In the stress cycle, the stress range and the average stress of the load spectrum have important effects on the life of the material, and the stress correction should be carried out before the calculation and analysis.The commonly used mean stress correction methods include Morrow correction, Goodman correction and Gerber correction, among which Morrow correction is applicable to low-cycle fatigue, and Goodman and Gerber correction is applicable to high-cycle fatigue.Therefore, for the pump head body, Goodman correction was used to correct the average stress of the load spectrum in fatigue calculation, as shown in figure 11.

Fatigue result analysis
As shown in figure 12, the fatigue analysis results show that the failure times of the 1# cylinder and the 5# cylinder of the pump head body are larger than those of the other three cylinders, and the locations where each  cylinder is prone to fatigue failure are all at the intersecting line of the suction cavity.In the 1# cylinder and 5# cylinder, the most prone to fatigue failure is near the position C at the intersecting line, and affected by the position of the corner of the external edge, it can be concluded that the most prone to fatigue failure is basically   and it is calculated that the pump head body will be damaged in about 299.3 h, which is similar to the actual cracking time, and the failure location is consistent with the cracking location.

Improvement measures
Through the failure analysis and stress analysis of the failed pump head body, it can be concluded that the main reason for the cracking of the batch of pump head body is that there is a large defect in the structural design, which leads to the failure of the pump head body in normal operation.The grain size of this batch of manufacturing materials is large, which should be paid attention to during manufacturing.In this paper, the structure of the pump head body is improved.

Structural improvement
The numerical simulation results of the original pump head structure show that there is stress concentration at the cracking position.In order to improve the stress concentration and prolong the service life, the structure of the pump head is improved.Therefore, two corresponding improvement measures are proposed.The first improvement method is to appropriately increase the fillet radius at the maximum stress, that is, the location of the fracturing fluid discharge channel, and reduce the maximum stress value.The first improvement method is to increase the radius of the rounded corner at the intersection line of the inner cavity and adopt a chamfered and flush structure at the external edge.In order to avoid stress concentration at the side position, the side structure adopts a whole flush structure to reduce the risk of cracking of the pump head body.The original structure is shown in figure 13 5.2.Finite element analysis at maximum stress ANSYS Workbench software was used to conduct numerical simulation of the improved model, and the maximum stress data was obtained, as shown in figure 14.It can be concluded that the maximum stress value can be reduced by properly increasing the radius of the fillet at the position of fracturing fluid discharge channel.When the radius of the fillet increases to R20, the stress will no longer decrease.Considering the location and sealing of the fracturing fluid discharge channel of the pump head body, the fillet radius should not be too large.

Stress analysis at edge
The stress analysis of the improved model was carried out respectively, and the value of the improved stress at the edge was obtained, as shown in table 7. The fillet radius at the position of fracturing fluid discharge channel remains unchanged, and the fillet radius R8 around the inner cavity increases to R10.The results show that the stress concentration can be improved by the three improvement measures of the external edge.The overall equivalent stress and the stress at the intersecting line of the inner cavity are reduced by 5%-8%.The stress at the edge is reduced by about 50%.Stress concentration is improved, reducing the risk of cracking at edges.Among the three edge improvement measures, the edge rounded process is more complicated.The edge chamfered technology is convenient for processing and has the lowest maximum stress value.The flush structure at the edge has the greatest stress and increased weight.Considering the processability, economy and processability, the improvement method of edge chamfered is the best.

Fatigue life analysis of the improved model
The fatigue life analysis of the three improved structures was carried out respectively, and the working conditions were actual working conditions.The Settings of material property parameters and load spectrum in the analysis were the same as those of the original structure.The calculated results were compared with those of the original structure.According to figure 15 and table 8, the service life of the pump head body can be improved after the structure is improved, in which the service life of the external edge with flush structure is improved the most, and the service life of the fillet structure is improved the least.

Conclusion and recommendations
Through the stress analysis of the pump head body by using ANSYS Workbench software, it is found that there is stress concentration at the cracking position of the pump head body.The fracture morphology under scanning electron microscope is fatigue streak.It can be concluded that the fatigue cracking of the pump head body is caused by structural design defects.Three measures are put forward to improve the position structure of edge edge, which can improve the stress concentration phenomenon, increase service life, relieve fatigue and reduce the risk of cracking, among which the chamfered measure at the outer edge is the best.

Figure 2 .
Figure 2. Macro-photomorphology of the fracture of the pump head body.

Figure 3 .
Figure 3. Pump head body microstructure and grain morphology.
Workbench software to analyze the stress of the pump head body, the stress distribution of the pump head body is obtained.The stress cloud diagram of the inner cavity is shown in figure 8(a), and the stress cloud diagram of the edge is shown in figure 8(b).The stress concentration occurs at the intersecting line

Figure 4 .
Figure 4. Electron microscope morphology of pump head body fracture.

Figure 5 .
Figure 5. Energy spectrum analysis of fracture surface of pump head body.

Figure 11 .
Figure 11.The average stress is corrected.

Figure 12 .Figure 13 .
Figure 12.Overall life cloud of pump head body.
(a).Edge rounding is shown in figure13(b).Edge chamfered is shown in figure13(c).The edges are flush as shown in figure13(d).

Figure 14 .
Figure 14.Maximum stress value after increasing the rounded corner.
Figure 15(a) shows the rounded life cloud of the outer edge, figure 15(b) shows the beveled life cloud of the outer edge, and figure 15(c) shows the flush life cloud of the outer edge.The fatigue life (crack formation life) of the improved pump head is shown in table 8.

Figure 15 .
Figure 15.Improved exterior edge life cloud image.

Table 2 .
Pump head body tensile performance test results.

Table 3 .
Pump head body impact performance test results (J).
Average value at room temperature 20 J, Single value 18 J

Table 5 .
Results of energy spectrum analysis of fracture surface.

Table 6 .
Table of pump head body stress values.

Table 7 .
Stress values for original structure and modified.

Table 8 .
Life of the improved structure.