Shao Xiaoguang                                                Xue Jun

Wenzhou Ruide Valve Co., Ltd.                         Jiangsu Huachang Chemical Co., Ltd.


Abstract: By improving the first-stage piston assembly of Shenyang 6M50 compressor, the operating safety of the piston assembly is improved, maintenance costs are saved, the stable operation of the compressor is ensured, and good economic benefits are achieved.

1. Preface

Jiangsu Huachang Chemical Co., Ltd. completed its overall relocation at the end of 2006. Its ammonia synthesis units were put into operation at the end of June 2005 and the end of November 2006 respectively. The core operating equipment includes nine 6M50 nitrogen and hydrogen compressors, with an annual output of 40,000 tons of ammonia per unit. . Five of them use 6M50 nitrogen and hydrogen compressors produced by Shenyang Gas Compressor Co., Ltd.

2. There is a problem

The machine was put into production from trial run. After many years of operation, some problems were exposed in the first-stage piston assembly. Through cooperation with Wenzhou Ruide Valve Co., Ltd., the local structure of the piston assembly, the support ring and the piston ring were improved. After more than a year of operation, the previous problems were eliminated, the operation was safe and stable, and the transformation was relatively successful. . Since the original first-stage piston assembly has been in operation, the following problems have occurred:

2.1 Piston body fragmentation During normal operation, the piston body suddenly fragmented.

2.2 Piston rod fracture This fracture occurred on the back side of the step where the piston rod contacts the piston body, and the fracture is very neat. After careful inspection of the cross section, no hidden scars including cracks were found.

2.3 Abnormal wear of the piston ring. The structure of the piston ring is as follows: a basic ring is made of cast iron, and a anti-friction layer of Babbitt alloy is inlaid and cast on its outer periphery. After partial detachment, the sealing performance will become worse and the air volume of the compressor will be insufficient.

2.4 The piston body support ring lacks function. The piston assembly support ring is only 120°, and the cylinder inner wall mirror has a small supporting effect. During the operation of the compressor, the first-stage cylinder block of some machines vibrates too much.

3. Cause analysis Based on the analysis of the problems that occurred during the operation of the first-level piston assembly of the machine, combined with the actual production analysis, the main reasons are as follows:

3.1 The main reason for the problem of piston body fragmentation is that the compressor uses a steel annular air valve. After the air valve has been running for a period of time, the valve plate breaks and falls into the cylinder. The piston body is inlaid with a large number of broken valve plates. , the strength of the piston body is greatly reduced, and after running for a period of time, the piston body will break. In addition, after the primary piston assembly has been used for a long time, the aluminum piston body core undergoes plastic deformation under alternating loads. The preload force between the piston rod and the piston body decreases, creating a gap in the piston body core, and the piston body is subject to additional impact loads. And rupture occurs.

3.2 The problem of piston rod fracture. From the analysis of the fracture surface of the piston rod, the piston component has been subjected to lateral forces during high-speed operation. According to other conditions when the piston rod breaks: the support ring used to support the piston body is greatly worn and protrudes from the support ring groove. Since the gap between the piston body and the cylinder is small, additional bending moments are generated after being partially blocked by the support ring, causing the piston rod to break at the above-mentioned location. In addition, the 120° support ring only relies on the positioning blocks at both ends to define its circumferential position, and the positioning blocks are fixed with two M6 screws. After the compressor has been running for a period of time, the positioning blocks often fall off, causing the support ring to After wear, it is easier to come out of the support ring groove.

3.3 The main reasons for abnormal wear of piston rings are:

3.3.1 There is a lot of dirt in the primary air intake. The dirt is mainly sulfur, tar and moisture entrained in the semi-water gas, which affects the lubrication effect of the first-stage cylinder of the compressor, causing piston rings to appear early in use. Unusual wear and tear.

3.3.2 The oil injection pipe gasket of the first-level cylinder is made of aluminum, and the circulating cooling water of the cylinder corrodes it, causing leakage. Due to the special operation of the first-level cylinder, the first-level exhaust pressure is different from the circulating cooling water of the cylinder. In addition, when the piston moves to inhale, the pressure in the cylinder is much lower than the circulating cooling water pressure, so a large amount of circulating cooling water enters the cylinder, exacerbating the damage to the cylinder lubrication condition. As a result, the babbitt anti-friction layer around the first-level piston ring rapidly wears and the seal fails, seriously affecting the first-level air pumping capacity of the compressor.

3.4 The problem of lack of function of the piston body support ring has existed for a long time. The support ring groove on the original design of the piston body was shallow, only 5mm, and the thickness of the support ring was only 10mm. Therefore, after the piston assembly is newly installed, the ring The groove has a limited role in restricting the support ring. Once the support ring suffers from poor lubrication and abnormally rapid wear, it can easily come out of the ring groove and get stuck between the piston body and the cylinder wall, thus generating additional bending moments. Eventually, As a result, the piston rod broke due to the dual effects of tensile force and additional bending moment. Furthermore, since the support ring groove is shallow, after a period of use, its right angles are easily worn into rounded corners, and even the right-angled steps are ground flat, which further increases the possibility of the support ring coming out of the ring groove.

4. Measures taken To address the above-mentioned problems of the first-stage piston assembly, the following measures have been taken:

4.1 In view of the problem that the steel annular valve plate is easy to break and is embedded in the piston body, the PEEK valve plate produced by Wenzhou Ruide Valve Co., Ltd. is used to replace the original steel annular valve plate air valve. On the one hand, it greatly extends the service life of the valve; on the other hand, even if a broken valve piece falls into the cylinder and is hit by the piston body, the valve piece will only be shattered and not embedded into the piston body, eliminating the need for valves. The film weakens the strength of the piston body. In addition, for the newly made piston body, the core diameter and the thickness of the internal stiffening plate are appropriately increased, which also improves the strength of the first-level piston body to a certain extent.

4.2 Increase the depth of the piston body support ring groove and correspondingly increase the thickness of the support ring so that the matching size between the two reaches 10mm. At the same time, increase the support ring from 120° to 360°, leaving only the thermal expansion gap. In this way, even after the support ring is worn, it is difficult to come out of the support ring groove. At the same time, the cylinder mirror also supports and guides the reciprocating motion of the piston body, and also reduces the vibration of the cylinder body to a certain extent.

4.3 The piston rings whose cast iron surface is inlaid with Babbitt alloy have also been replaced with materials. The original idea was to replace it with polyether ether ketone (PEEK) resin material. Although this material has been successfully used on the fifth and sixth and seventh stages of this type of compressor, this material is relatively expensive after all. For larger piston rings and supports, environment, the renovation costs will increase significantly. Later, after discussion, it was decided to use polyethersulfone resin (PES) material to make piston rings and support rings. The performance of this material is better than that of currently commonly used polytetrafluoroethylene, and its manufacturing cost is much lower than that of polyetheretherketone (PEEK) resin material.

Polyethersulfone resin (PES) is a thermoplastic polymer material with excellent comprehensive properties developed by the British ICI Company in 1972. It is one of the few special engineering plastics currently in use. It has excellent heat resistance, physical and mechanical properties, insulation properties, etc., especially its outstanding advantages such as continuous use at high temperatures and stable performance in environments with rapid temperature changes. It has been widely used in many fields. The main properties of polyethersulfone resin (PES) are as follows:

a. Heat resistance: The heat distortion temperature is 200~220℃, and the continuous use temperature is 180~200℃.

b. Hydrolysis resistance: It can withstand hot water or steam at 150~160℃, and is not corroded by acids and alkali at high temperatures.

c. Creep resistance: Its creep resistance is the best among thermoplastic resins in the temperature range below 180°C. In particular, glass fiber reinforced PES resin is better than some thermosetting resins.

d. Dimensional stability: Its linear expansion coefficient is small, and its temperature reliability is also small. In particular, the linear expansion coefficient of 30% glass fiber reinforced PES resin is only 2.3×10-5 ℃-1, and it can still maintain a value similar to that of aluminum until 200℃.

e. Impact resistance: It has the same impact resistance as polycarbonate.

f. Chemical resistance: PES is resistant to gasoline, engine oil, lubricating oil and other oils, and its solvent cracking resistance is the best among amorphous resins.

PES resin has the above-mentioned good physical and chemical properties. Combined with the relevant pressure and temperature conditions of the actual operation of the first-level cylinder, the PES resin meets the conditions for making the first-level support ring and piston ring.

4.4 In addition, strict operation management of the compressor, improving the purification degree of the first-level inlet semi-water gas, and strengthening regular inspection and maintenance of the compressor wearing parts are also important measures to ensure the safe and stable operation of the compressor.

5. Effect of use: Wenzhou Ruide Valve Co., Ltd. designed the PES piston ring and PES support ring by itself according to the relevant technical parameters and requirements for the operation of our compressor. The improved first-level piston body assembly was replaced on July 28, 2010. So far, after monitoring its actual use, the gas discharge pressure and temperature of this stage are in line with the designed process indicators, and the use effect is good.

6. Economic benefit analysis

6.1 Maintenance costs: Every time an accident occurs, it will not only cause damage to the piston assembly, but also the replacement of the piston rod and connecting rod bolts. The total cost is more than 180,000 yuan. Coupled with maintenance labor costs, etc., each accident will cause huge economic losses.

6.2 Indirect costs affected by maintenance: Each replacement of the first-level piston assembly takes at least about 10 hours. According to the production capacity of a single unit of this machine, which is 40ktNH3/a, that is: 5tNH3/h, each maintenance will affect more than 50 tons of synthetic ammonia. . This does not include the electricity and gas consumption caused by additional driving and parking. In addition, the invisible damage to other parts of the compressor caused by the accident cannot be estimated.

7. Summary

Through the transformation of the first-stage piston assembly of the Shenyang 6M50 compressor, the operating safety of the first-stage piston assembly of the compressor has been improved, a large amount of maintenance costs have been saved, the impact of sudden shutdowns and maintenance on synthetic ammonia production has been reduced, and compression has been ensured. The machine operates stably and achieves good economic benefits.