Thread Content
In June 2000, our company commissioned and put into use a set of technical transformation equipment - the carbon dioxide upgrading system. Its function is to process the potassium carbonate solution that enters the carbon dioxide regeneration tower from the hydraulic turbine. The basic principle of this system is to use pre-flash evaporation to release hydrogen, nitrogen and carbon monoxide in the solution. The device adopts Italian technology, and the main equipment is imported from Italy. Powering the upgrading system is a horizontal single-stage single-suction centrifugal pump 1110-JB (hereinafter referred to as the upgrading pump). This pump is domestic supporting equipment, model CPK-C65-315, driven by a motor, with a rotation speed of 2950 r/min and a power of 75 kW. 1 Problems After nearly half a year of trial operation of the carbon dioxide upgrading system, we found that the upgrading pump had defects such as high flow rate and short service life of the mechanical seal (one week to half a month). In March 2001, the pump manufacturer carried out cylindrical cutting of the impeller (from Ф320mm to Ф300mm). Through this transformation, the problem of high pump flow rate was better solved, but the problem of short service life of the mechanical seal did not improve. Long-term leakage of the mechanical seal will not only cause the loss of potassium carbonate solution, but also cause pollution to the environment around the pump. What is more serious is that once the leaked potassium carbonate solution enters the bearing chamber, the temperature drops and crystallizes, it will directly cause damage to the bearing. 2 Cause search and treatment What causes the service life of the mechanical seal of the upgrading pump to be too short? Let’s first study the sealing structure of the pump. Since the pump factory did not provide equipment drawings, the understanding of the pump structure and operating characteristics was achieved through multiple inspections of the pump. The sealing structure of this pump is basically the same as other single-suction centrifugal pumps in design. The seal also adopts a single-end built-in mechanical seal. The mechanical seal and the shaft are also connected through a sleeve and a key. The elastic compensation force of the seal is provided by a spring leaf installed in the moving ring seat. The sealing between the moving ring and the shaft sleeve, and between the static ring and the sealing gland are all achieved by "O"-shaped rubber rings. ； The seal between the sleeve and the shaft is achieved by a PTFE gasket installed on the end face of the sleeve. The sealing liquid is supplied in a self-sealing form, that is, the working medium is introduced from the pump outlet to the sealing chamber as the sealing liquid. We all know that there are many reasons for mechanical seal failure: Wear of dynamic and static rings, damage to elastic compensation parts, aging and failure of "O" rings and other rubber parts in various parts, sealing fluid problems, etc. Therefore, we also analyze the problems of the quality improvement pump one by one. Let’s start with sealant. We all know that the sealing fluid of mechanical seals is very important. Its main function is: Provide the end face specific pressure required for dynamic and static seals, lubricate the friction surface of the seal, take away the heat generated by friction, etc. Since it acts directly on the mechanical seal, it has strict requirements on its temperature, pressure, corrosiveness and cleanliness. For the upgrading pump, the original design adopts a self-sealing form. Since the working medium is potassium carbonate solution, the medium will crystallize when it is lower than 70°C, and the presence of solid particles will cause wear of the sealing surface. The temperature of the sealing fluid is usually controlled between 20 and 50°C. This is because the temperature of the sealing fluid is too high, which not only causes the cooling effect * * It will also increase the aging of rubber seals, so potassium carbonate solution should not be used as a sealing fluid. For this reason, we changed the sealing liquid supply method of the pump and chose the same sealing water source as the lean liquid pump. After the transformation, the operating condition of the mechanical seal of the upgrading pump has been improved, and the service life has been increased (about 1 month). However, compared with other pumps (usually 3 months to half a year, and can run continuously for 1.5 years in good condition), the service life is still too short. To this end, we carried out further work, and this time we chose to start with the analysis of the structure of the mechanical seal and the auxiliary seals. (1) Adjust the spring compression of the mechanical seal. Since the mechanical seal of this pump uses a spring leaf as an elastic compensation element, the compression amount is very small (the total amount is about 6mm), which determines that the adjustable range is very small. During the maintenance, we also found that although the seal leaked, the relatively soft static ring was not seriously worn. The seal compression was adjusted between 2 and 4 mm, but the adjustment did not extend the time for the seal to leak again. (2) Problems with various auxiliary seals. Auxiliary seals include rubber parts made of various materials, which play a vital role in sealing. In order to better play its role, the main indicators we usually need to control are the material and size of the rubber parts. The material of rubber parts depends on the composition of the working medium. The working fluid of the upgrading pump is potassium carbonate solution. The medium is alkaline salt solution. The main applicable rubber materials are:: Fluorine rubber, nitrile rubber, ethylene propylene rubber, etc. By comparing the cost performance of several rubbers, we finally selected nitrile rubber. The size of rubber parts is of course also a very important factor, but general rubber parts factories can guarantee it, so I won’t go into details here. (3) Other issues. In order to further find out the root cause of the seal leakage of the upgrade pump, we purchased a stroboscope. Through the stroboscope, we used the principle of persistence of vision to observe the relatively "static" characteristics of the rotating equipment components. We can clearly see the occurrence of leakage. Through observation, we found that the leakage occurred along the gap between the sleeve and the shaft, rather than from the dynamic and static sealing surfaces. This is consistent with the small wear on the sealing surface and the failure of the "O"-shaped rubber ring we saw during maintenance. Through analysis, we believe that the leakage comes from the damage to the PTFE gasket installed on the end face of the shaft sleeve δ=1mm. During the maintenance, we did find the fact that the gasket was seriously damaged. For this reason, we thickened the PTFE gasket to δ=2mm. After it was put into use, we found that the situation did not improve, but worsened. After another inspection, we found that the PTFE gasket was damaged and the sealing end faces of the impeller and shaft sleeve were damaged to varying degrees. After in-depth analysis of the reasons for this phenomenon, we draw the following conclusions:: After turning the outer circle of the impeller, no dynamic balancing test of the impeller was performed, and the impeller was unbalanced. During such operation, the impeller will inevitably swing. There are PTFE gaskets on both sides of the impeller. The continuous swing of the impeller will produce a repeated extrusion impact force on the PTFE gasket. This force will intensify as the PTFE gasket continues to become thinner, leading to gasket seal failure and leakage. Since the PTFE gasket is soft, the thicker the gasket, the more serious the impeller will be. To fundamentally solve this problem, on the one hand, the dynamically balanced impeller should be replaced, and the material of the sealing gasket should also be changed. We chose a copper gasket with δ=0.5mm. After installation and commissioning, the equipment ran well, fundamentally solving the problem of seal leakage of the upgrade pump. 3 Experience Summary By solving the case of seal leakage of the carbon dioxide upgrading pump of the ammonia synthesis unit, we deeply realized that the modification of the equipment must be fully demonstrated and carried out in strict accordance with the specifications. As for the reasons for equipment failure, we should also open up ideas, start from many aspects, and strive to see the essence through the phenomenon. Only in this way can the key points not be missed, the problem can be completely solved, and detours can be avoided.