Failure analysis and improvement of mechanical sea

  • Detail

Failure analysis and modification of the mechanical seal of the alkali liquor circulating pump because all the electromechanical devices are exposed outside

[Key words] alkali liquor circulating pump, mechanical seal, failure analysis, economic benefits

[Abstract] through the analysis of the reasons for the frequent failure of the mechanical seal of the alkali liquor circulating pump in the ethylene plant, the sealing structure and friction pair materials have been improved, and good sealing effect and economic benefits have been achieved

the concentrated alkali liquor circulating pump of the compression system of the ethylene plant is the bottom circulating pump of the alkali washing tower, which transmits the alkali liquor washed by the cracked gas to the top of the tower, so that the alkali liquor can be recycled. During the actual operation of the pump, leakage occurs frequently, and the mechanical seal begins to leak slightly after installation and operation for about a week. The leakage increases within 1 ~ 2 months, and it cannot be used anymore. It has seriously affected the environmental protection standard and long-term stable operation of the device

1 basic information:

the pump is a single suction cantilever horizontal centrifugal pump. The main technical parameters are as follows:

the medium composition contains colloidal 20% NaOH liquid of particle impurities

medium temperature 45 ℃

medium viscosity 1070cp

flow 30m3/h

lift 18m

suction pressure 1.078mpa

discharge pressure 1.272mpa

shaft power 2.7kw

motor revolution 2950r/m

the failure of the crane mechanical seal will be disassembled, and its damage is as follows:

(1) moving ring and moving ring.Ring seat The clearance between the shaft sleeves is blocked

(2) the moving ring and the stationary ring have annular wear marks

(3) there are impurities and crystals such as coke like polymers and particles in the sealing cavity

(4) the compilation and publication of recommendations for the use of catalog springs are seriously rusted, jammed by impurities and crystals, and have insufficient elasticity

2 failure analysis:

generally, the main causes of seal failure are end face liquid film failure and material and medium incompatibility, And manufacturing and installation problems. The specific pressure of the end face is an important performance parameter of the mechanical seal. The specific pressure of the end face should be within the design range (generally 0.2 MPa ~ 0.4 MPa for the built-in mechanical seal), so as to ensure the good coordination of the end faces of the mechanical seal and form an effective liquid film between the sealing faces

fb (end force) = ft (spring force) + FP (medium action pressure) -fm (liquid film action pressure). The calculation formula of end specific pressure is:

pb = Pt + PP - PM

pb = Pt + (k- λ) P

where: D2 = 50mm D1 = 32mm D2 = 38mm

spring specific pressure Pt = ft (spring force)/[π/4 (d22-d02)] = 0.3MPa (manufacturer's design value)

load coefficient k = (d22-d02)/(d22-d12) = 0.715

membrane 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 mechanical seal end face is 0.28 MPa, which is in line with the design specifications and can meet the production requirements under normal working conditions. This is consistent with the actual situation. At the initial stage of use, the mechanical seal does not leak. What is the reason for its failure? This requires a comprehensive analysis of the working environment of mechanical seals

since the pump is located at the bottom of the alkali washing tower, the rust of pipes and equipment, impurities and heavy polymers in the cracked gas are deposited at the bottom of the tower. At the same time, NaOH in the tower reacts with H2S and CO2 components, and the generated Na2S and Na2CO3 will produce crystals. The filter at the inlet of the pump is 40 mesh, which is often blocked in actual production, which makes the pump empty. We changed the filter to 20 mesh, which basically solves the problem of inlet blockage. At the same time, it also causes the alkali liquor entering the pump body to contain a lot of impurities and crystals. Due to the self flushing method, these impurities and crystals are inevitably deposited in the sealing cavity. The gap between the moving ring, the moving ring seat and the shaft sleeve is blocked, resulting in the movement of the moving ring in the axial direction is not flexible or even stuck, the spring is corroded and jammed, and the elasticity is insufficient, which cannot provide the original design end face specific pressure of 0.28 MPa, resulting in the sealing surface can not be well attached, and the liquid film balance is broken. At this time, the seal will have micro leakage. As the crystals and impurity particles enter the end face of the friction pair, the sealing surface will be gradually worn, In addition, the axial compensation of the moving ring is not effective, resulting in an increase in the leakage of the seal, which eventually leads to the complete failure of the seal

3 improvement measures

according to the failure causes of the seal, we have carried out technical transformation from the following aspects:

(1) change the seal structure form

since the medium is alkaline liquid containing impurities, from the perspective of the seal structure, the static ring compensation is more suitable than the original design of the dynamic ring compensation. According to the specific working conditions of the pump, we selected egger 3055 card type single end mechanical seal (cartridge type), whose main features are as follows:

L precise centering is achieved through automatic locking ring

l the adjustable on cell products of Shanghai Tianma and in cell products of Wuhan Tianma can be used directly with common bolts, and the cartridge structure is convenient for installation

l the clearance between the stationary ring and the shaft sleeve is not on the medium side, which ensures the flexibility of the axial compensation of the stationary ring

l the spring is located outside the auxiliary seal ring and completely separated from the medium, avoiding the corrosion and jamming of the spring

(2) change the material of the friction pair and remove the seal flushing

tungsten carbide is used as the material of the movable ring and the stationary ring of the cartridge seal. Tungsten carbide has extremely high strength and hardness, good wear resistance and corrosion resistance, which makes it difficult for impurities and crystals to damage it. At the same time, tungsten carbide friction pair has excellent thermal conductivity and small linear expansion coefficient, so that it can maintain good sealing at high temperature. The auxiliary sealing ring is made of fluororubber fpm2461, which is corrosion-resistant and elastic, and the maximum working temperature is 250 ℃

due to the impurities and crystals contained in the alkaline solution, the self backwashing form adopted by the original mechanical seal is difficult to achieve the expected purpose, and there is no other medium suitable for washing solution on the production site. With the above conditions as guarantee, and the medium working temperature is not high (45 ℃), the 3055 cartridge mechanical seal can operate normally without self flushing, which can not only reduce the energy and material consumption losses caused by flushing, but also simplify the daily maintenance of the mechanical seal

e-ga203 two pumps have used cartridge mechanical seals since January 2001. From the field situation, the sealing effect is very good. There is still no sign of leakage after operation, and the service life has reached more than 2 years. The price of each set of original 8b1-p mechanical seal is 960 yuan, and the cost of spare parts of mechanical seal alone will save more than 10000 yuan in two years. The flushing water flow of the original design e-ga203 pump is 6l/min. By removing the flushing water, more than 3000 tons of circulating water will be saved every year, and the economic benefit is obvious

4 conclusion

through the reasonable selection of the mechanical seal structure of the alkali pump, combined with the selection of appropriate friction pairs and auxiliary O-ring materials, the service performance of the mechanical seal is improved, so that it can still work stably under the condition of removing the seal flushing. The cost of equipment maintenance and repair is reduced, and the continuous and stable production of the device is ensured


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