Zhang Jianyun, Chen Xin, Chi Bin, Liu He, Gao Jianbin

(Shenyang Blower Group Reciprocating Machinery Co., LTD., Shenyang, Liaoning 110869)

[Abstract] : In ultra-high pressure reciprocating compressors, the plunger, as the core component for achieving gas compression, cooperates with the packing ring to achieve dynamic sealing at ultra-high pressure, which has a decisive impact on the safe and stable operation of the entire compressor. At present, some domestic application enterprises have attempted to carry out the localization of spare parts for some ultra-high pressure compressors and have achieved certain results. However, the localization substitution of plungers has not yet made a breakthrough. This article analyzes the structure, material and working characteristics of the plunger, explores the entire process of plunger localization, including raw material preparation and subsequent precision processing, and verifies the method of plunger localization through actual manufacturing.

[Key words] : Ultra-high pressure reciprocating compressor Plunger; Tungsten carbide Localization "Manufacturing

Chinese Library Classification Number: TH457 Document Code: B

Article Number: 1006-2971 (2025) 02-0058-05

1 Introduction

As the core power equipment of the high-pressure polyethylene plant, the ultra-high pressure compressor is responsible for increasing the medium pressure to the 300MPa level to meet the needs of subsequent production. At present, only two foreign enterprises in the world are capable of manufacturing this type of large ultra-high pressure compressor, which is a highly monopolized technological equipment. Some domestic application enterprises have attempted to carry out the localization of spare parts for this type of compressor and have achieved partial breakthroughs. Among them, the core and key component, the plunger, which accomplishes compression and sealing, has not yet achieved a breakthrough in domestic substitution due to its significant working importance, large failure losses, and high manufacturing difficulty.

This article discusses the localization of tungsten carbide plungers for ultra-high pressure compressors in terms of material mixing, sintering forming and subsequent precision processing.

The structure and characteristics of the 2 parts

2.1 Part Structure

The gas compression of a reciprocating compressor is achieved by the cooperation of a piston and a cylinder to form a compression chamber. As the piston moves, the volume decreases, thereby increasing the gas pressure. As the pressure keeps rising, the cylinder bore will gradually decrease, and the design and manufacture difficulty of the piston rings and support rings in the traditional piston structure will increase. In this case, by eliminating the piston rings and support rings, simplifying the piston into a plunger structure, and using the ultra-small gap between the plunger and the cylinder to achieve compression sealing, certain advantages are demonstrated.

In the development history of ultra-high pressure compressors, pistons with piston ring structures also existed in the early days. The piston is of a modular type, and the entire component is firmly assembled together with the piston rod through a nut. The structure is shown in Figure 1.

With the development of practical applications and the demands of chemical processes, it has been found that under high-pressure conditions with an exhaust pressure above 250MPa, the lifespan of the piston rings in this structure significantly decreases, making it impossible to achieve long-term stable operation. Therefore, the plunger packing type has gradually become the mainstream of ultra-high pressure compressors. The plunger packing structure simplifies the traditional piston into a smooth and simple plunger structure and, in combination with ultra-high pressure packing, achieves dynamic sealing.

2.2 Plunger Working Characteristics and Features

As mentioned earlier, the pressurization of gas is achieved by changing the volume of the compression chamber through the movement of the plunger. Therefore, the plunger is subjected to a compressive load, and as the suction and exhaust processes continue to cycle, the compressive load fluctuates to a certain extent. Therefore, the plunger needs to have sufficient compressive strength and relatively high rigidity to ensure the stability of operation.

In addition, under ultra-high pressure sealing conditions, the metal sealing ring exerts a strong contact stress on the plunger. This also requires the plunger to have sufficient surface hardness to avoid rapid wear or the breeding of surface defects that could cause major accidents.

Based on the above characteristics, sintered tungsten carbide, with its extremely high compressive strength, large elastic modulus and high surface hardness, makes it the optimal choice for plunger materials.

3. Localization of the raw material preparation process

The raw material preparation process of tungsten carbide products mainly includes three stages: mixing, pressing and sintering. Each stage is crucial to the performance of the final product.

3.1 Mixing process

The first step in mixing materials is to determine the proportion of components and the grain size grade. Through sampling and analysis of the tungsten carbide plungers that have been successfully put into service on site, it can be known that the plungers are mainly a mixture of tungsten carbide and cobalt. Cobalt exists as a binder in cemented carbide products, with a cobalt content of approximately 11% and an average grain size of about 1.0μm. The uniformity of cobalt distribution in cemented carbide is generally average.

At the same time, by comparing the performance of the finished products with similar ratios and grain grades of cemented carbide enterprises and the sampling analysis results, the main performance is basically consistent. For instance, the compressive strength is ≥2800MPa, the hardness is ≥HV1300, and the elastic modulus is approximately 540GPa. At the same time, considering the working conditions of the plunger, the above indicators also meet the requirements for the strength, rigidity and wear resistance of the plunger.

By comparing with the mainstream hard alloys in the market, the composition ratios, grain size grades and hardness of some common grades at present are shown in Table 1.

From this, it can be known that the composition ratio of the plunger in the ultra-high pressure compressor belongs to the common proportion in hard alloys, the grain size grade is at the intermediate level, and the mixing degree is average. Conventional drum mixing can meet the requirements. It can be seen from this that there are no difficulties in the domestic production of the mixing process of the plunger, and it is easy to achieve.

3.2 Pressing Process

The most common pressing processes for cemented carbide are extrusion and cold pressing. Among them, the extrusion process is relatively simple. The mixed powder and molding agent are extruded by extrusion equipment and can be directly extruded into rod-shaped billets. However, due to the fact that the maximum weight of raw materials in a single barrel of the production equipment is 200kg, it is impossible to extrude the blank of the plunger product at one time. In addition, the powder produced by the extrusion method contains water-containing materials. If it is too hard and too soft, it is not conducive to the transposing of large parts such as plungers. Therefore, the extrusion method is not suitable for the production of plungers.

The cold pressing method is to first overcome the reduction of voids between powders under the action of external pressure. As the pressure further increases, the powder particles will undergo severe deformation and fracture. At the same time, under the action of friction, the powder particles will undergo a certain degree of atomic diffusion, promoting the further strengthening of the bond between the powders. After the pressing process is completed, a blank material with certain strength and density can be obtained. Common cold pressing methods include unidirectional pressing, bidirectional pressing, and floating pressing, etc. To achieve better compactness of the plunger blank, cold isostatic pressing is chosen for forming. Due to the simple overall structure of the plunger, especially its common cylindrical shape during the pressing stage, it is most suitable to use an isostatic press to achieve the powder pressing of the plunger.

Cold isostatic pressing can be further divided into dry bag isostatic pressing and wet bag isostatic pressing. The wet bag method is mainly used for large-sized plates or cylinders with a height not exceeding 200mm. Generally, after pressure forming, they are placed in bags, vacuumed, and then placed in CIP to improve product density. Dry bag isostatic pressing products can be used to press large-sized rods, containing only wax and no water, which is convenient for semi-burning on carbon plates or direct sintering. It can be seen from this that the dry bag static isobaric technology is more suitable for pressing the powder of the plunger.

After the pressing is completed, the plunger blank needs to undergo simple processing to reduce the amount of post-sintering processing. This process is relatively simple, and the key is to ensure that the density of the blank is not damaged during the processing.

Through the above analysis of the pressing process, it is found that the overall flow is conventional. However, in the actual implementation of localization, some local bottlenecks have also been discovered. Mainly because the diameter of the plunger is close to 150mm and its length is close to 1500mm

This specification and size belong to the super-large size products among hard alloys, which puts forward higher requirements for the production and manufacturing equipment. Among them, the capacity of the dry bag type static isobaric equipment is the bottleneck in this process. Fortunately, the existing mature models of this equipment cover the demand for plunger pressing. After purchasing additional equipment, the difficulty of localization has been solved.

3.3 Sintering Process

After pressing, the plunger billet has certain mechanical properties and compactness, but this is only a simple mechanical interlocking state of powder particles, and the performance is far from the target required value. Therefore, sintering of the billet is necessary. Under certain temperature and pressure conditions, the powder particles are made to bond with each other and further shrink and densify. The intergranular bonding state of the green material is transformed into an intergranular polymer, achieving the internal material migration of the green material. Eventually, the pores in the plunger are basically eliminated, the microstructure is uniform and compact, and the strength and density are significantly improved.

The common sintering methods at present include atmospheric pressure sintering, hot-press sintering, microwave sintering, and spark plasma sintering, etc. Atmospheric pressure sintering is not suitable for the sintering of tungsten carbide cemented carbide due to its abnormal grain growth and unstable quality. The industrialization of microwave sintering technology is still not mature. Discharge plasma sintering is also difficult to apply due to the size limitations of the products. Therefore, the most common approach in the current hard alloy industry is the combination of vacuum and hot-pressing sintering. Vacuum hot-pressing sintering can reduce the pollution of harmful substances in the atmosphere (such as water, oxygen, nitrogen and other impurities), thereby avoiding adverse reactions such as decarburization, carburization, reduction, oxidation and nitriding. At the same time, high-pressure gas is used as the pressure medium to act on the plunger preform, subjecting it to balanced pressure in all directions during the pressurization process. Through the combined effect of high temperature and high pressure, the material densification is achieved.

At present, the capacity of domestic vacuum hot-pressing sintering equipment has reached 1700kg per furnace, with the maximum size being 2.4m, fully meeting the requirements of plunger sintering.

3.4 Semi-burning process

As mentioned earlier, after tungsten carbide is sintered, its hardness is ≥HV1300. It is very difficult to process it under this hardness condition. There are also inclined surfaces for positioning and center holes required by the processing technology on the plunger. If all these processing procedures are set after the complete sintering of the cylindrical plunger blank is completed, the overall processing volume will be large, the processing time will be long, and the economy and efficiency will be poor.

Therefore, before complete sintering, the plunger billet is pre-sintered to achieve a certain degree of density and mechanical strength, but the hardness is controlled below HRC40, and the dimensional change compared to the final sintered material is controlled within 20% to 25%. At this point, the semi-burned plunger is mechanically processed to measure the dimensions of the inclined plane, center hole, reduced diameter and other positions to a certain size. After that, vacuum hot-pressing sintering will continue to be carried out to meet the final performance requirements. Due to the setting of the semi-burning process, the control of the carbon content in the plunger blank during the semi-burning process, as well as the control of the carbon content and cobalt magnetism, are also the core control points of this process.

4. Localization of precision manufacturing

4.1 Processing Position and requirements of tungsten carbide plungers

As previously introduced, the structure of the plunger is relatively simple. However, since it is sintered from tungsten carbide as a whole, it has extremely high hardness and is thus very difficult to process. At the same time, when operating under ultra-high pressure and the load it generates, the plunger must always maintain good centering performance; otherwise, it will have a serious impact on the stability of the unit. In addition, to ensure the service life of the sealing ring and avoid excessive frictional heat generated during operation, the surface roughness of the plunger needs to reach a relatively high precision grade. Therefore, the processing and manufacturing of plungers must overcome high hardness while ensuring ultra-high precision manufacturing quality.

The processing of the plunger must first ensure a 15° inner and outer fit bevel accuracy, which is crucial for the centering performance of the plunger. In addition, the R20 transition fillet and R2 transition fillet have complex processing shapes. However, it can effectively alleviate the stress concentration problem of the plunger. The outer circle runout, roundness and coaxiality are required to reach 0.01mm level, the runout between the end face and the inclined surface is 0.01mm level, and the roughness of the friction surface is required to be Ra0.01 level. These are all the core concerns in the processing and manufacturing of plungers.

4.2 Discussion on the Processing Procedure

First, process tests were conducted by purchasing specimens made of integral tungsten carbide material. According to the lathe test results of the specimen, the turning tool did not move during processing, and the tool kept vibrating during the processing, resulting in unstable processing. This determined that the processing equipment could only be a grinding machine, and diamond grinding wheels were used for grinding processing.

For the processing requirements of the cylindrical structure and end face of the plunger, an end face cylindrical grinding machine is usually used for processing. The end face cylindrical grinding wheel is used to process the end face and outer circle of the plunger separately. The extremely high roughness requirement of the outer circle needs to be achieved by the grinding machine being equipped with a polishing wheel. For the mating inclined surfaces, the common grinding methods are as follows:

(1) The worktable of the grinding machine is arranged at the corresponding Angle, and grinding is completed by the longitudinal feed of the common cylindrical grinding wheel. However, due to the 15° Angle of the plunger's mating inclined plane, which is too large, the maximum swing Angle of the general grinding machine's worktable is only about 5°, and the longer the worktable, the smaller the swing Angle. Moreover, the swing Angle of the workbench cannot be made very precise, which is not conducive to ensuring the processing quality of the inclined surface. Therefore, this method is not applicable to the processing of plunger inclined surfaces.

(2) Use a formed grinding wheel, directly shape the grinding wheel into the corresponding form, and complete the grinding by longitudinal feed. However, the formed diamond grinding wheel needs to be dressed with a set of formed silicon carbide dressing wheels. If the Angle is changed later, a new set of formed dressing wheels with the corresponding Angle needs to be ordered, which is likely to cause structural rigidity.

(3) A common cylindrical grinding wheel is used. The grinding wheel spindle rotates at a certain Angle, and the grinding is achieved by using the interpolation program for oblique feed. This method only requires that the grinding wheel spindle can rotate, and there are no special requirements for the grinding wheel.

For the transition fillet of R20, only longitudinal feed grinding with a formed grinding wheel can be used. For the excessive convex fillet of R2, if a formed grinding wheel is used, it needs to be dressed into a concave fillet of R2 for grinding. Due to the small Angle of R2, it is difficult to process with a formed grinding wheel. The R2 can be divided into 3 to 5 transition bevels for processing by rotating the grinding wheel, and then manually ground to ensure there are no sharp corners and avoid stress concentration.

To sum up, in order to meet the processing requirements of the plunger, at least three grinding wheels are needed in total (one end face outer cylindrical grinding wheel, one forming grinding wheel with R20 on one side, and one ordinary outer cylindrical grinding wheel) and one polishing wheel. And considering the processing cost and efficiency, to avoid frequent replacement of grinding wheels during processing, the grinding wheel frame of the end face and outer cylindrical grinding machine needs to be rotatable and capable of being equipped with multiple grinding wheels simultaneously.

Through the technical exchanges with domestic and foreign grinding machine manufacturers in the early stage, a domestic manufacturer's end face and outer circle compound grinding machine has come into our view with its high precision and significant price advantage. And after more than a year of technical preparation, equipment adjustment and grinding wheel purchase, the plunger specimens were test-processed. The processing procedures are as follows:

4.2.1 Preface One

As shown in Figures 2 and 3, the grinding of the small end is carried out first in the first sequence. The equipment needs to be equipped with T1 forming grinding wheel, T2 end face outer circle grinding wheel, and T3 outer circle grinding wheel.

The T1 forming grinding wheel is trimmed with an R20 fillet on one side and a 15° bevel on the other side, which is used for grinding the outer circle of the middle transition part, the R20 fillet and the 15° conical surface. The T2 end face cylindrical grinding wheel is used to grind the end face of the small head and

Outer circle The T3 cylindrical grinding wheel is used for grinding 15° outer conical surfaces. In addition, the transition fillet R2 between each surface is cut 3 to 5 times by rotating each grinding wheel at a certain Angle to ensure there are no sharp corners, and then manually ground evenly. All surfaces are ground to

Ra0.2.

4.2.2 Preface Two

As shown in Figures 4 and 5, the workpiece is turned over for clamping, and the grinding wheel equipped on the equipment remains unchanged. The T2 end face and outer circle grinding wheel is used to grind the end face and outer circle of the large end. The T3 cylindrical grinding wheel is used for grinding 15° outer conical surfaces. Similarly, the transition fillet R2 between each surface is cut 3 to 5 times by rotating each grinding wheel at a certain Angle to ensure there are no sharp corners, and then manually ground evenly. All surfaces are ground to Ra0.2.

4.2.3 Preface Three

This sequence requires the replacement of the grinding wheel. As shown in Figure 6, replace the T2 'fine diamond grinding wheel and the T3' polishing wheel. Use a T2 'fine diamond grinding wheel to grind the outer circle of the large end to Ra0.05 and grind it to the finished product size. Then use the T3 'polishing wheel to polish the outer circle to Ra0.01.

4.3 Development of Specialized Equipment

Through the above experiments, customizing domestic specialized developed compound grinding machines and continuously improving technical details can precisely ensure the processing quality of the plunger. The special grinding machine adopts a Siemens numerical control system to separately control the feed, rotation of the grinding wheel frame and the movement of the worktable. Maximum grinding diameter: 500mm, maximum grinding length: 1500mm, maximum workpiece weight: 500kg. The X-axis positioning accuracy of the machine tool is 0.004mm, the X-axis repeat positioning accuracy is 0.002mm, the Z-axis positioning accuracy is 0.008mm, and the Z-axis repeat positioning accuracy is 0.005mm. Moreover, according to the above processing procedure, grinding the plunger requires one forming grinding wheel, one end face cylindrical grinding wheel, one coarse diamond cylindrical grinding wheel, one fine diamond cylindrical grinding wheel and one polishing wheel. The formed diamond is dressed by diamond roller interpolation, and the outer diamond grinding wheel is dressed by silicon carbide grinding wheel.

The grinding wheel frame of the compound grinding machine can be equipped with three grinding wheels. Through the rotation of the grinding wheel frame, the three grinding wheels are respectively used. During the processing, only one grinding wheel needs to be replaced to meet the requirements of plunger processing, thus avoiding multiple changes of grinding wheels during the processing and affecting the processing efficiency. The rotation Angle of the grinding wheel turret of the compound grinding machine is 225°, and the rotation resolution can reach 0.0001mm. It can precisely control the rotation Angle of the grinding wheel frame, thereby ensuring the accuracy of the mating inclined surface.

5 Conclusion

The breakthrough in the localization of ultra-high pressure compressor plungers is an important part of the localization of the entire compressor. It will also provide more economical and convenient spare parts resources for in-service units and offer a new option for the economic, efficient and controllable operation of the owners.