Deterioration of mechanical seals in case of chemical effect
Deterioration of seals in case of chemical effect.
The materials of friction pairs should be selected taking into account the chemical aggressiveness of the condensed medium. A slight erosion of materials with a liquid to be sealed can often be foreseen by tolerances and compensated by an appropriate allowance for the wall thickness. However, it must be taken into account that corrosion at elevated temperatures can increase several times. It is known, for example, that sliding surfaces, which are heated much more strongly than the liquid to be condensed, are oxidized particularly strongly.
Forecast of durability of mechanical seals.
Answer the question: "What kind of service life can be expected from face seals in practice?" - it is very difficult. If the wear is mainly due to adhesion, and the known operating conditions are constant, then, based on the wear values obtained under similar conditions, the service life L = a/A can be determined with sufficient accuracy. The amount of wear of the seal can not, of course, be increased arbitrarily, since it also affects the mechanical and thermal deformations. Under normal conditions, the amount of wear: a = 0.05 - 0.5 cm. In general, we can say that the service life of mechanical seals over the past several decades has increased compared to the previous values several times, which can be explained by improving the quality of materials and modernizing the structure.
|Picture 1. Wear of ring A of end seals of circulating pumps operating on warm water for a long service life: h (P1 = 3.5 - 5 kgf/cm2, t = 55 - 90°C, n = 3000 rpm , hardness about 16° on the German scale, wear of the carbon ring a = 2 mm)|
Picture 1 shows that under the specified operating conditions, the average life of the seals can be estimated to be about 100,000 hours. In practice, this corresponds to a period of about 10 to 15 years. Often, the service life of the mechanical seal is determined not only by the wear of the end surfaces, but also by the stability of the elastomers used to the aging processes. When the monoethanolamine was sealed at a temperature of 50°C, p1 = 5.8 kgf/cm2 and vg = 5.4 m/s, the O-rings became brittle after about 40,000 operating hours, while the wear of coal during this period was only about 0.35 mm (A = 0.009 μm/h).
Changing the operating mode, as well as any increase in pressure, sliding speed, friction coefficient and temperature, shorten the life of the seal. However, abrasive, corrosive and jet wear can occur along with adhesive wear, in addition, vibration, temperature and material parameters influence the service life of the seal, therefore the prediction of the service life of mechanical seals is always associated with a low reliability factor. Operational experience in recent years shows that at a sealable medium pressure of up to 25 kgf/cm2 and a sliding speed of up to 10 m/s in the chemical industry, the expected service life L of end seals in pumps is on average 1 to 2.5 years, in agitators 2 - 5 years. In the oil industry and in mechanical engineering, the service life of the seals reaches 2 to 4 years or more. In addition, in many cases, the service life for half a year is considered as very good.
Table 1. Estimated service life L of mechanical seals working at high speeds and low pressures of the medium to be compressed.
[negative pressure equal to vacuum pressure; hardened tool steel with λ = 35 kcal/(m*h*°C), chromium steel with λ = 20 kcal/(m*h*°C)]
|Picture 2. Service life of seals with sliding rings that do not carry loads (Dw = 100 mm, vg = 5 m/s, feed water temperature of boilers 65°C)||Picture 3. Coefficient of friction f and wear A of the unloaded mechanical seal depending on the pressure of the medium to be compressed p1 (cold water, vg = 5 m/s, coal/metal)||Picture 4. Expected service life of thermohydrodynamic seals with grooves for circulation of coolant in pumps for feeding water to boilers and reactors for continuous operation, depending on internal pressure p1 (Dw = 75 - 120 mm, vg = 13 - 15 m/s, net boiler feed water, temperature 40 - 60°C)|
The coefficient of friction f decreases with increasing p1 to 5 kgf/cm2 and then remains constant to about p1 = 40 kgf/cm2, since the heat released in the gasket is still managed to be removed. With increasing contact pressure, heat removal from the sealing gap becomes more complicated. With the appearance of the effect of evaporation of the lubricant dry friction begins, at which the coefficient of friction is characterized by instability with a strong overall increase in pressure. With a sharp increase in wear A, the danger of formation of thermal cracks increases, and the service life of the seals is sharply reduced.
Using under these conditions (picture 3) thermohydrodynamic mechanical seals with grooves for coolant circulation allows several times to increase the average life of these seals (picture 4). Due to a significant reduction in friction coefficients in thermohydrodynamic mechanical seals, their service life and operational reliability are significantly increased.
In connection with the use of high-quality materials for sliding pairs, the expected life of mechanical seals has increased significantly in recent years. For example, according to study of the performance characteristics of seals produced in a large series and installed in several hundred thousand pumps for pumping hot water, worked more than 10 years at a temperature of 80 - 120°C, p1 = 5 kg/cm2, vg = 5 m/s, the average service life was 2 to 4 years, and the purity of water was crucial. Average service life mechanical seals working under high loads, for example, in thermal and nuclear power plants, reached several tens of thousands of hours, while it should be noted the optimal selection of materials for sliding pairs, seal design, cooling and safety systems.
Suitable additional ancillary equipment, for example, magnetic filters, rinsing devices from an alien source, cyclones for separation of contaminants and additional cooling, can significantly extend the life of mechanical seals, even under severe operating conditions.