The mechanical seal is a unit used for sealing in those parts of the pump where the shaft passes through the cover. Sufficient density is formed by strong pressing on the surfaces of two elements - rotating and stationary. Parts must have high accuracy, it is achieved by grinding and grinding.
Mechanical seals for dry-type pumps prevent liquid from entering the surrounding area, and in submersible ones they do not allow water to enter the engine, avoiding a short circuit. To increase accuracy, two seals are located on one shaft using a dielectric (oil or cooling mass).
The sealed assembly can be balanced, have a cartridge or collapsible design. Liquid pressure, type and temperature affect the use of hard or soft materials. Extensive distribution has gained alumina, impregnated graphite, carbide. As a secondary seal, the use of rubber is possible.
How to extend the service life
It is worth noting that such elements are subject to rapid wear and are sensitive to the environment, but despite this, they are the most accurate. Most of the difficulties in the operation of the pumps are associated with a breakdown of the named device. The service life is reduced due to many reasons, for example, due to exposure to abrasive particles or elevated temperatures. It is also affected by misuse. In order for the mechanical seal to last longer, it is important to choose the composition that meets the tasks.
Emulsions and oils are not suitable for rubber, since it is not resistant to oil products. In this case, Viton is used for compaction. Dry running, contributing to overheating, can damage the device. A systematic air vent through the installed pump outlets is required.
The competent setting of automation is of particular importance. There is also a chance of sticking, in which the planes are glued after being idle or the rubber is soldered to the shaft. To prevent sticking, it is sometimes necessary to rotate the shaft.
Double seal
The type, material and design of the seal depends on the conditions in which the work takes place and the characteristics of the pumped fluids. A single seal is used for liquids that are not explosive and fire hazardous, as well as for explosive substances of categories T1, T2 and T3.
Double seal is used for liquids for which single is not suitable. These are environmentally hazardous compounds, toxic, corrosive or abrasive substances. It is worth noting that under difficult working conditions, this design lasts five times longer than others. In addition, its inner metal part is resistant to environmental influences, and thermosetting, viscous masses are quickly sealed without additional costs.
In accordance with GOST R 52743-2007, a double mechanical seal is used for explosive liquids of category T4. Failure emission standards, usage period and double compaction cost allow final selection.
Among the general requirements for mechanical seals, it is worth noting compatibility with the fit and overall dimensions of the pumps. The general structure should provide for the previous mode of operation at low mass pressure in the tank in comparison with the medium being sealed. The use of double sealing eliminates the possibility of leakage. The material for the housing elements must fully comply with the technological characteristics of the transported liquids.
Gas barrier
It is similar to a double-seal cartridge, while inert gas is used as cooling and lubricant, as well as instead of rinsing with a water jet or coolants. Such a system was developed in view of the fact that some of the barrier compositions used earlier cannot be used due to the inconsistency with the updated emission standards.
Seals in the role of a gas barrier use harmless air or nitrogen, which helps prevent substances from entering the environment and comply with established emission standards. In situations requiring increased reliability, or when transporting hazardous or toxic compounds, gas double barrier structures should be used.
By the way, the average price (mechanical seal with a gas barrier) is about 2 thousand rubles.
Tandem
Environmental and health regulations regulate the use of the tandem for pumping substances such as hydrocarbons, carbon monoxide, vinyl chloride and an extensive list of other carcinogens or volatile compounds.
Such a device can prevent icing of light hydrocarbons and liquids, which are characterized by a decrease in temperature below the freezing point of water. Propanol and methanol are typical examples of formulations having buffering characteristics. Also, the tandem increases the degree of reliability. In the event of a breakdown in the conventional design, the external component takes over the maintenance tasks.
Without collider
To support the contact of the seal planes, it is undesirable to move the structure along the sleeve or shaft. Among the main advantages, it is worth highlighting performance over a wide temperature range and the absence of the need for the use of an additional seal.
The negative point is the modernization of cross sections when used in harsh operating conditions.
Pusher
To ensure contact surfaces of the nodes you need to connect a secondary mechanical seal that moves along the axis of the sleeve or shaft. Due to this property, damage is compensated on the front plane of the device. Its advantages include a fairly low cost and an extensive range of pushers, presented in stores in various configurations and sizes. But it should be noted that it can lead to corrosion damage to the sleeve and the movement of the secondary seal.
Balanced Devices
The seal balancing includes a simple structural transformation that reduces the hydraulic force that closes the shaft mechanical seal. Balanced nodes are distinguished by the release of less heat, a low load of the sealing planes and an increased degree of pressure. Thanks to this, they become the best option for transferring compositions with high vapor pressure and insufficient lubricating ability.
Unbalanced elements
They have great stability when changing cavitation and alignment, with a small number of leaks and low cost. A sufficiently low pressure stage is a drawback of such structures. If the pressure limit of the resultant gain acting on the sealing parts is exceeded, the film located between the surfaces is squeezed out and the work acquires a dry stroke.
Standard
An example is Grundfos — mechanical seals that require mounting and alignment on a sleeve or shaft. Despite the simple method of installing the device, reducing maintenance costs is currently a priority, which leads to a greater spread of cartridge designs.
Internal single
This type of Wilo mechanical seal is the most popular. There is the possibility of easy changes to form a buffer washing system , balancing to transfer increased environmental pressure. Recommended for use with aggressive and non-aggressive fluids with sufficient lubricating properties.
Cartridge
This category includes mechanical seals for pumps installed on the sleeve, including the shaft sleeve and the stuffing box cover, which has a tight fit to the shaft. Their main advantage lies in the fact that there is no need to use connecting elements for their installation. Due to such structures, the probability of errors during installation is reduced and operating costs are reduced .
External single
Such a mechanical seal becomes a low-cost alternative to the expensive metals that are needed to give corrosion resistance to internal seals, provided that the aggressive fluid has high lubricating characteristics. However, it is worth noting that among the shortcomings, the susceptibility to hydraulic pressure and the influence of shocks, due to which seals of this type have small pressure limits.