Abstract: By analyzing the causes of the frequent failure of mechanical seal of lye circulating pump in ethylene plant, the sealing structure and friction pair material are improved, and the good sealing effect and economic benefit are obtained. Ethylene plant compression system concentrated lye circulation pump is the bottom tower circulating pump caustic tower, it will be cracked gas caustic lye transported to the top of the tower, so that lye recycling. The pump in the actual operation, frequent leaks, mechanical seals in the installation and operation of a week or so began to leak, 1 to 2 months the amount of leakage increased, can not continue to use. Seriously affected the device's environmental standards and long-term stable operation. 1 basic situation: The pump is a single suction cantilever horizontal centrifugal pump, the main technical parameters in the following table: Media components containing gelatinous, particulate impurities, 20% NaOH liquid Medium temperature 45 ° C Medium viscosity 1070CP flow 30m3 / h Head 18m Inhalation pressure 1.078MPa Discharge pressure 1.272MPa Shaft power 2.7KW Motor revolutions 2950r / m Krane mechanical seal will be disintegrated, the damage is as follows: (1) moving ring and ring seat, the gap between the sleeve is blocked (2) moving ring and static ring ring wear marks (3) sealed cavity coke-like polymer, particles and other impurities and crystals (4) serious spring rust, jams by impurities and crystalline objects, lack of flexibility 2 Failure Analysis: The main cause of seal failure generally caused by the end liquid film failure, incompatible materials and media, and manufacturing and installation problems. End pressure is an important mechanical seal performance parameters, the end surface pressure within the scope of the design (built-in mechanical seal is generally 0.2 MPa ~ 0.4 MPa), in order to ensure the mechanical seal face with a good fit, so that the sealing surface between the formation of effective solution membrane. Fb = Ft (spring force) + Fp (medium acting pressure) - Fm (liquid membrane acting pressure), the end face specific pressure is calculated as: Pb = Pt + Pp-Pm Pb = Pt + (K-λ) P Where: d2 = 50mm d1 = 32mm d2 = 38mm Spring pressure Pt = Ft (spring force) / [π / 4 (d22-d02)] = 0.3MPa (factory design value) Load factor K = (d22-d02) / (d22-d12) = 0.715 Film pressure coefficient λ = 0.742 Medium pressure P = 0.45 MPa It can be calculated that the specific pressure Pb of the original 8B1-P type mechanical seal end face is 0.28 MPa, which is in accordance with the design specifications and can meet the production requirements under normal working conditions. This is in line with the actual situation, in the early use of mechanical seals do not leak, then the cause of its failure is what? This must be from the mechanical seal of the working environment for a comprehensive analysis. Since the pump is at the bottom of the caustic washing tower, the corrosion products of pipes and equipment as well as impurities and heavy polymer in the pyrolysis gas are deposited on the bottom of the tower. At the same time, NaOH in the tower reacts with H2S and CO2 components to form Na2S and Na2CO3 Will produce crystals. Pump inlet filter was 40 mesh, often blocked in actual production, the pump time, we filter to 20 mesh, the basic solution to the problem of blockage of the entrance, which also resulted in the lye into the pump contains a lot of impurities And crystals, which are inevitably deposited in the sealed chamber due to their own flushing mode. So that the moving ring and moving ring seat, the gap between the sleeve is blocked, resulting in moving ring in the axial movement is not flexible or even stuck, the spring is eroded Kaibu, lack of flexibility, can not provide the original design of 0.28 MPa end pressure, Resulting in the sealing surface can not be properly affixed and the liquid film balance is destroyed, then the seal will be slightly leakage, with the crystal and impurity particles into the friction side, the sealing surface is gradually worn, coupled with the axial compensation ring Insufficient, resulting in increased leakage seal, eventually leading to complete failure of the seal. 3 improvement measures According to the sealing failure reasons, we have targeted from the following aspects of technological transformation: (1) change the structure of the seal Since the medium is a lye containing impurities, the static ring compensation is more suitable than the original design of the moving ring compensation from the viewpoint of the sealing structure. According to the specific conditions of the pump conditions, we chose the Iger 3055-type single-end cartridge mechanical seal (cartridge), the main features are as follows: Precise centering with automatic locking ring. l Adjustable gland can be used directly with the commonly used bolts, container-mounted structure easy to install. The gap between the stationary ring and the sleeve is not on the medium side, which ensures the flexibility of axial compensation of the stationary ring. l The spring is located outside the auxiliary seal, completely separated from the media to avoid corrosion and jamming of the spring. (2) change the friction material, remove the seal flushing Tungsten Carbide is used as the material for the ring and the stationary ring of the cartridge seal. The tungsten carbide has extremely high strength and hardness, good abrasion resistance and corrosion resistance, which makes it difficult for the impurities and the crystal to cause damage to it. At the same time, the tungsten carbide Friction pair has excellent thermal conductivity, linear expansion coefficient is small, so that it can maintain good sealing at higher temperatures. Auxiliary sealing ring material We use fluorine rubber FPM2461, it is corrosion-resistant, good elasticity, the maximum operating temperature of 250 ℃. As the lye contains impurities and crystals, the original machine seal used in the form of their own backwash difficult to achieve the desired purpose, the production site and no other media suitable for washing liquid. With the above conditions as a guarantee, coupled with medium temperature is not high (45 ℃), 3055-type mechanical seal in the removal of self-flushing under the conditions of the fully operational, which can reduce flushing bring energy consumption, material consumption Loss, but also simplify the routine maintenance of mechanical seals. E-GA203 two pumps since January 2001 began using cartridge mechanical seal, from the field point of view the sealing effect is very good, the operation is still no signs of any leakage, the service life of up to 2 years. The original 8B1-P machine seal price of 960 yuan each set, only two years, the mechanical seal spare parts cost savings of more than 10,000 yuan, the original design E-GA203 pump flushing water flow of 6L / min, by removing the flushing water annual savings of circulating water 3000 I tons, the economic benefits are obvious. 4 Conclusion Through the rational selection of alkali pump mechanical seal structure, combined with the selection of a suitable pair of friction, auxiliary O-ring material, improve the performance of the mechanical seal so that the seal can be removed under the conditions of washing, still stable work. Reduce the cost of equipment maintenance and repair, and ensure the continuous and stable production of the device.