Modification of A301 mechanical seal in the hottes

2022-08-09
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Modification of A301 mechanical seal in polypropylene plant

the sealing device is an important part of the machine and equipment, and the sealing performance has a great impact on the working efficiency and reliability of the machine. Zhongyuan ethylene polypropylene plant is the production technology of montail loop method introduced. A301 is a power separator located at the top of propylene flash tank. Its function is to remove the polypropylene powder entrained in propylene gas after flash. It is one of the key equipment in the polypropylene process. Once the equipment fails, the whole plant will shut down. Zhongyuan A301 is produced by zucchetti company in Italy. It was put into use in 1996. Its operation has been poor in the first six months. It is often shut down for maintenance due to mechanical seal leakage, which seriously threatens the smooth operation of the device. Through systematic analysis and accounting of the equipment, we have found out the main causes of the fault and solved the problem. Now it is introduced as follows

1 introduction to sealing system

a301 seal adopts double face mechanical seal, and white oil is used as sealing fluid. The model of mechanical seal is 909b (balanced double face mechanical seal, and the auxiliary seal ring is tetrafluoro wedge ring). The operating parameters and sealing system are shown in Figure 1

figure 1a301 sealing system schematic diagram

relevant operating parameters:

a301 rotating speed 1480r/min

flash tank operating pressure 1.8MPa (g), operating temperature 80 ℃

mechanical seal cavity pressure 2.1MPa, but it does not need too high strength and hardness (g), the required temperature is 30 ℃

seal liquid cooling water inlet pressure 0.45MPa (g), Temperature 32 ℃

cooling water flow 0.5m3/h

2 existing problems and cause analysis

during the nine inspections before and after 1996, it was found that the moving ring at the atmospheric end of the mechanical seal was seriously worn, so the main reason for the failure of the mechanical seal was that the moving ring at the atmospheric end was worn too fast, and the average service life was 8 days. Although the straightness of the shaft was checked, the bearing was replaced, and the device of the mechanical seal shaft sleeve was adjusted many times, the compression of the upper and lower mechanical seals was changed, and the sealing system was adjusted, the problem was still not solved, which seriously affected the long-term operation of the device

causes the machine seal end face to wear quickly. 2. There are many reasons for the frequent application of the jaws of the hydraulic universal testing machine, mainly including: ① foreign impurities invade between the seal end faces; ② Poor lubrication of sealing face; ③ Axial opening and closing movement is performed between the end faces of the dynamic and static rings of the mechanical seal; ④ The manufacturing quality of moving ring is poor; ⑤ Insufficient amount of sealing fluid or poor cooling effect, resulting in excessive temperature of the seal; ⑥ The compression of the mechanical seal spring at the atmospheric end is too large; ⑦ The specific pressure of the end face is too large. According to the situation of many starts and uses, combined with the design, installation and other aspects of the mechanical seal, we believe that the possibility of the first four reasons is relatively small, so we analyze the last three reasons

(1) the sealing liquid circulation is insufficient or the cooling effect is not good enough to deposit the transparent silica gel in the droplets, causing the sealing temperature to be too high. The cooling water inlet pressure required by the process design is 0.45MPa (g), but in fact, because the installation height of A301 is 32m, the inlet pressure of its seal liquid cooling is only about 0.15Mpa (g), which indicates that the actual cooling water circulation cannot reach the design 0.5m3/h, which may cause the seal liquid temperature to be too high, the friction heat dissipation of the seal end face is poor, and the end face wear is too fast. In view of this situation, they checked the circulating temperature of the sealing fluid: the inlet temperature of the sealing fluid is 45 ℃, the outlet temperature is 49 ℃, and the temperature of the sealing cavity wall is 72 ℃ (the operating temperature of the flash tank is 80 ℃, which is caused by heat conduction). The actual measured temperature is not high for the selected sealing ring material (the moving ring is graphite and the static ring is silicon carbide), and the factor of too high temperature of the seal can be eliminated. (2) The mechanical seal spring at the atmospheric end is compressed too much. From the structure of A301, the mechanical seal shaft sleeve should be located in the middle of the sealing cavity, but the spring compression of the atmospheric end mechanical seal and the medium end mechanical seal can be changed by slightly adjusting the relative position of the mechanical seal shaft sleeve and the sealing cavity. Therefore, during the maintenance, we tried to check and adjust the position of the shaft sleeve many times to reduce the compression of the atmospheric spring to varying degrees, but the mechanical seal at the atmospheric end was still worn quickly, the longest running time was 12 days, and the mechanical seal at the medium end was worn greatly after adjustment. Therefore, this reason is not the root cause

(3) the specific pressure of the end face is too large. According to the wear theory of contact end face seal, under the condition of reasonable mechanical seal design, the wear amount of end face is in direct proportion to the specific pressure of end face. If the specific pressure of end face is too large and the cooling effect is not good, the friction heat between seals will not dissipate, which will lead to intermittent vaporization of liquid film between seal faces, increase the thermal stress on the seal face, and intensify the wear of end face

measure the relevant structural dimensions of the original mechanical seal as follows (both ends are the same):

outer diameter of the end face of the moving ring D2 = 82.5mm

inner diameter of the end face of the moving ring D1 = 70mm

balance diameter d0 = 73mm

number of springs 1

number of effective coils 12.5

spring size d=6.5mm

known seal cavity pressure P cavity =2.1mpa (g)

total compression after spring installation x = 16.7mm

the spring is made of carbon steel, Take the elastic modulus g=75000n/m2

sealing fluid as oil, and take the liquid film back pressure coefficient according to the recommended value in the design manual λ= Calculate the specific pressure of the end face below 0.33 ~ 0.5

moving ring end face area s=15cm2

spring stiffness K1 = gd4/8nd3 = 0.182kgf/mm

single spring working load f spring = K1X = 3kgf

total spring force F total = f spring × 12 = 36kgf

spring specific pressure P spring = ftotal/s = 2.4kgf/cm2

load coefficient k = (d22-d02)/(d22-d21) = 0.775

atmospheric end medium action pressure difference P large = P cavity-p outer = 21kgf/cm2

medium end medium action pressure difference P medium = P cavity-p medium = 3kgf/cm2

atmospheric end mechanical seal end face specific pressure P large ratio = P end +p medium (k- λ)= 8.175 ~ 11.7 45kgf/cm2

specific pressure of mechanical seal end face of medium end P medium ratio = P elastic +p medium (k- λ)= 3.225 ~ 3.735 kgf/cm2

through the calculation of the specific pressure of the end face, it can be found that the specific pressure of the end face of the atmospheric mechanical seal is far greater than the normal range of the specific pressure of the end face of 3 ~ 5kgf/cm2, while the specific pressure of the end face of the sealing medium is within the range of normal values. This shows that the manufacturer focused on the medium end seal when selecting the mechanical seal, ignoring the influence of the sealing cavity pressure of high-performance resin on the atmospheric end seal. Therefore, the excessive specific pressure on the end face of the mechanical seal at the atmospheric end should be the main reason for the serious wear of the A301 mechanical seal at the atmospheric end

3 transformation measures

in view of the problem of severe wear caused by excessive pressure on the end face of the A301 mechanical seal at the atmospheric end, there can be a variety of transformation schemes, such as using more wear-resistant materials for the dynamic and static rings of the mechanical seal, shortening the axial size of the mechanical seal ring seat at the atmospheric end, shortening the axial size of the intermediate pump ring, changing the size of the spring, changing the end face size of the dynamic ring and reducing the load coefficient, etc. However, each scheme also has its disadvantages. For example, the use of more wear-resistant materials for the dynamic and static rings does not reduce the friction heat; Shortening the axial size of the sealing ring seat of the atmospheric end machine may cause inconvenience to the installation

the above different transformation schemes have their own advantages and disadvantages. Therefore, we studied and analyzed together with Sichuan Sealing Technology Research Institute, and decided to adopt the scheme of only changing the size of the ring end face (the same at both ends) to reduce the load coefficient, which will be manufactured and processed by Sichuan sealing technology research institute. This transformation scheme has the following advantages:

(1) the moving ring is a vulnerable part, and it is the most economical and easy to change the structure

(2) keep the size of the moving rings at both ends the same to reduce installation errors. After reducing the load factor, the end face specific pressure of the atmospheric end seal can be close to the normal value

(3) reducing the end face size of the moving ring is more conducive to the concentric rotation between the moving ring and the shaft, and is beneficial to the transmission of friction

after many trial calculations and comparisons, combined with the structural dimensions of mechanical seal shaft sleeve, mechanical seal seat and other components, the final end face size of the moving ring is determined as: outer diameter 79.5cm, inner diameter 68.3cm. Now it is calculated as follows:

moving ring end face area s=12.8cm2

spring specific pressure P elasticity = f total/s = 2.8kgf/cm2

load coefficient k = (d22-d02)/(d22-d21) = 0.599

atmospheric end mechanical seal end face specific pressure P ratio = P elasticity + P medium (k- λ)= 4.85 ~ 8.4kgf/cm2

specific pressure of mechanical seal end face of medium end P medium ratio = P elastic + P medium (k- λ)= 3.09 ~ -3.6kg f/cm2

through calculation, it can be seen that after the transformation, the lower limit of the end face specific pressure of the atmospheric end of the mechanical seal enters the normal value range, while the change of the end face specific pressure of the medium end is very small, which is also within the normal value range. (end)

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