Reasons for Failure and Improve Design of Rotating Axial Seal in Titanium Dioxide Processing Pump Dong Zongyu et al Reasons for failure of original axial seal in titanium dioxide processing pump were analyzed. On the experimental investigation basis, suitable materials of a pair of friction components were selected, two improved design constuctions were used for industrial test, the results indicates that operating life of improved rotating axial seal is 4 ~ 6 times longer than original one. Operating life will be much longer. Keywords: titanium dioxide processing pump, seal, reasons for failure, improve design 1 Introduction Titanium Dioxide Plant in Panzhihua Iron and Steel Company uses concentrated sulfuric acid decomposition ilmenite powder production line of titanium dioxide in its production process has a series of titanium liquid pump, due to titanium liquid has a strong corrosive, and contains solid particles [1] , causing the pump Overcurrent components and sealing components are subject to corrosion and wear double role and easily damaged or abnormal failure, frequent replacement of sealing elements, and serious environmental pollution. In recent years, the factory titanium liquid pump shaft seal average life expectancy is only about 50h. Obviously, this is an urgent issue to be solved in terms of reducing production costs or reducing environmental pollution. 2 existing shaft seal problems The factory processes titanium pumps are used FSP corrosion-resistant pumps, the original configuration of the shaft seal structure shown in Figure 1. The shaft seal for the single-end exterior, exterior mechanical seal. Silicon nitride (Si3N4) static ring with a gland mounted on the pump body flange, filled with PTFE ring in an intrusive way embedded in the pure Teflon bellows groove end face of the groove. The clamping ring is fixed on the impeller shaft through the inner surface of the spring seat by the clamping ring so as to rotate with the shaft and simultaneously compress the spring to load the sealing end surface. Above the back of the static gland, a radial groove is communicated with the external water pipe for flushing and cooling the outer periphery of the sealing surface. Figure 1 FSP pump original seal structure Although this mechanical seal has the advantages of compact structure and corrosion resistance, the intermittent operation of the titanium liquid pump, due to frequent parking and start-up, decreases the temperature of the titanium liquid after stopping and increases the viscosity and precipitates fine crystals of titanium sulfate, titanium oxide and the like Particles, often appear the following problems caused by the rapid failure of the seal. (1) Severe wear and deformation of sealing surface When starting again after stopping, crystal particles and other solid impurities precipitated in the titanium liquid will cause abrasive wear on the sealing surface, while the original sealing end surface adopts silicon nitride / filled polytetrafluoroethylene Of the material group, both in the titanium liquid although have good corrosion resistance, but its poor friction and wear characteristics, especially filled with PTFE, the friction reduction is good, but not wear-resistant, when the seal Between hard particles, hard particles easily embedded in the surface of the material, the relative sliding in the process of cutting the surface of the material, resulting in smooth and straight sealing surface multi-groove-like wear marks. In addition, PTFE under the action of external force easily produce "cold flow" and deformation [2], especially the outflow structure, in order to compensate for the sealing ring face wear after the sealing force required for the fit, often in the beginning Mechanical seal installed, intentionally increase the spring pressure, resulting in the end pressure is too large, resulting in a serious deformation and excessive wear PTFE seal ring, which is leading to premature sealing failure important reason. (2) The compensation performance of the corrugated pipe is lost. The corrugated pipe structure of Teflon used in the original sealing has a rectangular wave shape and a large number of waves with small wave pitch. Although the machining is convenient and the axial compensation performance is good, Medium containing solid crystalline particles, once the particles sink into the bellows groove is difficult to discharge, will seriously affect the bellows axial elastic compensation performance and lead to seal failure. (3) Auxiliary sealing function to reduce the external flow spring mechanical seal structure, when the use of PTFE or rubber bellows to do moving ring auxiliary seal, usually by the application of pressure or interference with the bellows tail close to the shaft Shoulder or circumferential sleeve tighten the sleeve in order to prevent the fluid axial leakage purposes. The moving ring or ring installed ripple and the minimum inner diameter of the head and the sleeve should be a certain gap between the end face of the moving ring to ensure that the axial float after the wear, while pressure sealed fluid through this gap into Moving ring sealing surface back sealing surface can play a self-tightening effect. However, the original titanium liquid pump mechanical seal structure did not consider these issues, but also with the impeller made of plastic sleeve and cylindrical bellows inner circumference of the size and shape accuracy are poor, neither for axial compensation, but also can not The fluid into the back of the sealing surface, thus greatly reducing or even completely lost the bellows auxiliary seal function. (4) poor flushing effect Original seal design flushing method is to run through the static gland mounted on the gland cover and the back of the radial groove rushed to the stationary ring and moving ring contact surface, in order to achieve flushing and The purpose of cooling the end. However, in this structure, the sealing medium actually enters the sealing surface through the clearance between the inner periphery of the stationary ring and the sleeve. Therefore, flushing only flushes the medium that has leaked through the end surface to the ground, preventing the leakage and preventing leakage. Not conducive to the improvement of operating environment. 3 sealed improved design Through the inspection of the leakage of the shaft seal during the operation and the detection of the failure of the seal element, the above analysis conclusion is confirmed. To this end, without changing the structure of the original main pump under the premise of this article focuses on the selection of friction materials, strengthen the cooling measures flushing, improve the function of compensation agencies and other aspects of the seal reform. 3.1 friction material selection Through the static corrosion test of titanium carbide (SiC), high silicon cast iron, tungsten carbide (WC-Co), dip glass graphite and other materials in titanium liquid (see Figure 2) and graphite / SiC, graphite / WC-Co , Graphite / Al2O3, polytetrafluoroethylene / SiC, PTFE / WC-Co, PTFE / Al2O3, WC-Co / SiC, WC-Co / WC- (See Figure 3) shows that the corrosion resistance of silicon carbide in titanium is best compared to several other materials; the corrosion resistance of various materials in clear media The coefficient of friction is small, indicating that several material groups have good friction compatibility [3], and from several materials, respectively, and graphite group wear value with PV curve (see Figure 4) [ 4], SiC is the lowest. So from the overall characteristics of graphite / SiC group is more reasonable, if the production site operating conditions can not be fully guaranteed good washing conditions to prevent the solid particles into the sealing surface, the choice of SiC / SiC friction pair material group is more appropriate. Figure 2 several kinds of material corrosion results Figure 3 Several material pairs of the friction coefficient Figure 4 PV of several materials - wear curve 3.2 static ring structure and flushing cooling system design For external flow mechanical seal, when the medium is sealed with solid particles, if the pressure is slightly higher than the sealing medium pressure flushing coolant from the external piping system introduced into the back of the stationary ring, both with the solid particle medium from the plugging effect , At the same time lubrication of the seal ring surface cooling effect, which is to improve the mechanical seal life is very effective. Due to the pump structure can not change the restrictions, it is necessary to achieve the above purpose of flushing cooling, but also to meet the static ring material corrosion resistance and wear resistance of titanium liquid needs to be difficult to use silicon carbide ceramic ring made of static, but The stationary ring is designed as a combination of a titanium ring seat and a SiC ring. Flushing water from the outer circumference of the static tube take over the radial hole into the back and inner ring of the ring, effectively blocking and lubricating the end face of the purpose (see Figure 5). Figure 5 improved seal structure 3.3 moving ring and auxiliary sealing and loading mechanism design According to the site conditions, designed two programs, shown in Figure 5. Figure 5 (a) of the program I is the silicon carbide ring installed in the compression ring, constitute a combined ring; corrugated bellows section designed to trapezoidal; loaded with a small spring structure. Figure 5 (b) of the program â…¡ friction material, static ring combination structure and flushing cooling method and the same program â… , the corrugated pipe is a trapezoidal cross-section, except that the silicon carbide ring directly embedded in the polytetrafluoroethylene The bellows end ring groove, extending axially from the ring groove 3 ~ 4mm. 3.4 End pressure ratio calculation It is known that the maximum outlet pressure of titanium liquid pump is 0.35MPa, the maximum working pressure of sealed chamber is P0.25MPa, the working speed of pump is n = 2900r / min, the working temperature is T≤60 ℃, the diameter of sleeve is d = 50mm. Pc is calculated by the following formula: Pc = Ps + (K-λ) P In the formula, the spring pressure Ps, small spring Ps = 0.518MPa; large spring Ps = 0.369MPa; load coefficient K, small spring K = 0.55; large spring K = 0.27; liquid film pressure coefficient λ ≈ 0.5 , The calculation results: small spring structure Pc = 0.531MPa, slightly larger; large spring structure Pc = 0.31MPa, more appropriate. 4 field operation test Will improve the design of small springs and large spring structure of the two mechanical seals were installed in the titanium pump and titanium thin titanium pump on the running test, due to field test conditions, large spring temporarily without internal flushing, static ring material Still using the overall silicon nitride, moving ring material impregnated furan resin graphite. Tests show that the improved seal in the thick titanium pump and titanium pump thinner than the original seal life increased by 4 to 6 times. However, due to the low pressure of flushing water on site, it failed to give full play to the effectiveness of internal flushing, greatly limiting the further improvement of service life. 5 Conclusion (1) mechanical seal friction material used Si3N4 / filled PTFE, not suitable for titanium liquid medium conditions. Change to Si3N4 / graphite, the life expectancy increased by 4 to 6 times, but the graphite wear is still high, to SiC / SiC, life expectancy will have a greater increase. If you give full play to the role of flushing, SiC / graphite group can also get a longer life expectancy. (2) According to the structure size and working parameters of the titanium liquid pump, the elastic element of the mechanical seal is designed as a large spring structure or a small spring structure. However, due to the fact that the scene environment conditions and the pump accuracy are low, the large spring structure More practical. (3) After the shape of the corrugated pipe is changed from rectangle to trapezoid, not only the state of force is good but also the solid particles are not easily deposited, which is beneficial to give full play to its elastic compensation function. (4) For the mechanical seal with solid particle medium, the design of flushing structure is the key technology to improve the sealing life.
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(Medium: water, the average sliding speed 3.46m / s)
Based on the analysis and experimental research on the failure causes of the original titanium seal of the titanium liquid pump, the suitable friction material was screened out and two improved design schemes of the structure were put forward. After the test results of the production operation proved, Improved seal than the original shaft seal life increased by 4 to 6 times, under the conditions to ensure that the flushing, the life can be further improved.