By analyzing the reasons of the frequent failure of the mechanical seal of the lye circulation pump in the ethylene plant, the sealing structure and the friction pair material are improved, and the good sealing effect and economic benefit are achieved. 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: Medium composition with gelatinous, particulate impurities, 20% NaOH Liquid temperature 45 ℃ Media viscosity 1070CP Flow 30m3 / h Head 18m Suction pressure 1.078 MPa discharge pressure 1.272MPa shaft power 2.7KW continued use. The number of revolutions of the motor is 2950r / m. After the failure, the mechanical seal of Crans will disintegrate. The damage is as follows: (1) The clearance between the moving ring and the moving ring seat is blocked. (2) The moving ring and the static ring have circular wear marks (3) There are coke-like polymers, particles and other impurities and crystals in the sealed chamber (4) The spring is seriously corroded, which is jammed by impurities and crystallites and has insufficient elasticity. Failure Analysis: The main causes of seal failure generally are: Failure, incompatible materials and media, as well as manufacturing and installation issues. End pressure is an important mechanical seal performance parameters, the end surface pressure to be 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 end face with a good fit, so that the seal surface between the formation of effective solution membrane. Fb (spring force) + Fp (medium acting pressure) -Fm (liquid film 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 (manufacturer's design value) Load factor K = (d22-d02 ) / (D22-d12) = 0.715 Membrane compression stubber 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 face is 0.28 MPa, which is in accordance with the design specifications, Under working conditions is able to meet the production requirements. 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 targeted from the following aspects of the technical transformation: (1) change the structure of the seal structure As the medium is a lye containing impurities from the sealing structure point of view using the static compensation ratio The original design of moving ring compensation is more suitable. According to the specific conditions of the pump conditions, we chose the Iger 3055 cassette single-end mechanical seal (cartridge), the main features are as follows: l through the automatic locking ring to achieve precise centering. 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) to change the friction material, remove the seal flushing seal ring and stationary ring materials are used tungsten carbide, tungsten carbide with high strength and hardness, good wear resistance and corrosion resistance, which makes impurities and It is difficult for the crystal to cause damage to it, meanwhile, the tungsten carbide friction pair has excellent thermal conductivity and a small linear expansion coefficient, which enables it to maintain a good sealing property at a higher temperature. 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.