Ethylene compression system concentrated lye circulation pump is alkali washing tower bottom circulating pump, it will be washed by dissociated alkali lye transported to the top of the tower to make lye recycle use. In actual operation, the pump was in frequent leakage, and mechanical seal began small leaking after one week after well installation and operation, 1 to 2 month, the leakage was increased, and could not be used anymore. It seriously affected the environmental protection standard and long-term stable operation of the device.
1: Basic situation:
The pump is a single suction cantilever horizontal centrifugal pump; the main technical parameters are as follows:
Medium composition: 20% NaOH liquid with gelatinous, granular impurities;
Medium temperature: 45 ℃
Medium viscosity: 1070CP
Flow rate: 30m3 / h
Head of delivery:18m
Suction pressure: 1.078MPa
Discharge pressure: 1.272MPa
Shaft power: 2.7KW
Motor speed: 2950r / m
Disintegrating the NaiSiTe mechanical seal which was in failure, and the damage situation as follows:
(1) The gap between the rotating ring, rotating ring holder and sleeve was blocked.
(2) There was wear traces in the rotating and stationary ring.
(3) There were coke-like polymers, particles and other impurities and also crystals in seal space cavity.
(4) Spring was badly corrosive and also jammed by impurities and crystalline, and lack of flexibility, too.
2 Failure Analyses:
Seal failure is generally caused by the end of the liquid film failure, incompatibility of materials and media, as well as manufacturing and installation issues. The end face specific pressure is the important performance parameter of the mechanical seal, the end face specific pressure should be within the designed range (the built-in mechanical seal is generally 0.2 MPa ~ 0.4 MPa to ensure the mechanical seal face is well matched, so that the sealing surface could form the effective liquid membrane.
Fb (end seal face force) = Ft (spring force) + Fp (media action pressure) -Fm (liquid membrane pressure), the end seal face of the specific pressure is calculated as:
Pb = Pt + Pp-Pm
Pb = Pt + (K -?) P
then: d2 = 50mm d1 = 32mm d2 = 38mm
Spring specific pressure Pt = Ft (spring force) / [π / 4 (d22-d02)] = 0.3MPa (factory design value)
The load factor K = (d22-d02) / (d22-d12) = 0.715
The membrane pressure coefficient λ = 0.742
Medium pressure P = 0.45 MPa
It can be calculated the original 8B1-P mechanical seal end seal face pressure Pb is 0.28 MPa, and it is in line with design specifications, under normal working conditions it is able to meet the production requirements. This is consistent with the actual situation, in the early use of mechanical seals there is no any leakage, so what is the reason to make the failure? It requires working from the mechanical seal working environment, and doing comprehensive analysis.
As the pump is in the bottom of the caustic scrubber, the rust of the pipeline and equipment, as well as impurities and heavy polymer in the cracked gas are deposited to the bottom of the tower, and the NaOH reacts with H2S and CO2 to generate Na2S, Na2CO3, this will produce crystals. Pump inlet filter original was 40 mesh, during the actual production, it usually was blocked so we changed the mesh to 20, it basically solved the problem of inlet blockage, but it also caused the alkali liquid which into the pump contains a lot of impurities and crystals, which are unavoidably deposited in the seal space cavity due to its self flushing method. So that it occurred that the gap between the rotating ring, rotating ring holder and sleeve was blocked to lead that the rotating ring in the axial ring is not flexible or even stuck, and the spring was badly corrosive and lack of flexibility, could not provide the original end pressure design 0.28 MPa, and then result that the sealing surface could not be a good paste and liquid film balance was destroyed, then the seal appeared micro-leakage, with the crystallization of particles and impurities into the friction seal face surface, the sealing surface was gradually worn, coupled with rotating ring axial compensation was not working, resulting in increased leakage seal, eventually leading to complete failure seal.
3 Improvement Measures
According to the failure reasons of sealing, we provide following aspects technical transformation:
(1) Change mechanical seal structure
As the medium in the impurity lye, from the seal structure to use stationary ring compensation is more suitable than the original design with the rotating ring compensation. According to the specific working conditions of the pump, we use the Egger 3055 type single mechanical seal (cartridge type), and its main features are as bellows:
Precise centering is achieved by an automatic locking ring.
Adjustable gland can be used directly with the common bolt, cartridge-type structure is easily to install.
Stationary ring and sleeve gap is not in the media side to ensure the flexibility of the stationary ring axial compensation.
The spring is located on the outside of the secondary seal ring and is completely separated from the medium to prevent corrosion and jamming of the spring.
(2) Change the seal face material, remove the seal flushing
Both rotating ring and stationary ring are used tungsten carbide, tungsten carbide has a very high strength and hardness, good wear resistance and corrosion resistance, which makes it is difficult to be damaged by impurities and crystalline, while tungsten carbide seal face has excellent thermal conductivity, linear expansion coefficient is very small, it can maintain good sealing at a higher temperature. Auxiliary sealing ring material, we use fluorine rubber FPM2461, it is corrosion-resistant, good elasticity, and the maximum working temperature is around 250 ℃.
As the lye contains impurities and crystals, the original washing method of the mechanical seal is difficult to achieve the desired purpose, and also at the production site there is no other suitable media rinse. With the above conditions, coupled with the medium working temperature is not high (45 ℃), 3055-type mechanical seal can operate normally in removal of self-flushing conditions,, which not only can reduce energy consumption flushing, but also simplify the routine maintenance of mechanical seals.
Through the rational selection of the mechanical seal structure of the pump, combined with the right choice of the appropriate material for seal face and auxiliary O-ring to improve the mechanical seal performance and ensure it can be stably work after removing seal self flushing conditions, it not only reduce equipment maintenance and repair cost but also ensure the continuous and stable production of the device.
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