The mechanical seal is an assembly used to seal the parts of the pump where the shaft passes through the cover. Sufficient density is formed by strong pressing on the surfaces of two elements - a rotating and a stationary one. Parts must have high precision, it is achieved by lapping and grinding.
Mechanical seals for dry type pumps prevent liquid from entering the surrounding space, and for submersible pumps they do not allow water to enter the motor, thus 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, cartridge or split design. Fluid pressure, fluid type and temperature affect the hard or soft materials used. Aluminum oxide, impregnated graphite, and carbide have become widely used. Rubber can be used as a secondary seal.
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 pumps are associated with a breakdown of the named device. The service life is reduced due to many reasons, for example, due to the action of 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 suits the tasks.
Emulsions and oils are not suitable for rubber as it is not resistant to refined products. In this case, Viton is used for sealing. Dry running, which causes overheating, may damage the device. Requires systematic venting through installed pump outlets.
Proper setting of automation is of particular importance. There is also the possibility of sticking, in which the planes stick together after downtime or the rubber is soldered to the shaft. Spin the shaft occasionally to prevent sticking.
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 liquids. Single seal is used for liquids that are not explosive and flammable, as well as for explosive substances of categories T1, T2 and T3.
Double seal is used for liquids wheresingle will not work. These are environmentally hazardous, toxic, caustic or abrasive substances. It is worth noting that under difficult working conditions, this design lasts five times longer than others. In addition, its internal metal part is resistant to environmental influences, and thermosetting, viscous masses quickly seal at no additional cost.
In accordance with GOST R 52743-2007, a double mechanical seal is used for explosive liquids of category T4. Damaged emission rates, usage period and cost of double sealing make the final choice.
Among the general requirements for mechanical seals, it is worth noting compatibility with the landing and overall dimensions of pumps. The general structure should provide for the previous mode of operation at a low pressure of the mass in the tank in comparison with the medium being sealed. The use of a double seal eliminates the possibility of leakage. The material for the body elements must fully comply with the technological characteristics of the liquids to be transferred.
Gas barrier
Similar to a double seal cartridge, using inert gas as cooling and lubricating agent, and instead of water jet flushing or coolants. This system was developed due to the fact that some of the barrier compounds used in the past cannot be used due to non-compliance with the updated emission standards.
Seals as a gas barrier useharmless air or nitrogen to prevent substances from escaping into the environment and to comply with established emission standards. In situations requiring increased reliability, or when transporting hazardous or toxic formulations, gas double barrier designs should be used.
By the way, the average price (mechanical seal with a gas barrier) is about 2 thousand rubles.
Tandem
Environmental and he alth regulations govern the use of the tandem for pumping substances such as hydrocarbons, carbon monoxide, vinyl chlorides and a wide range of other carcinogenic or volatile compounds.
Such a device helps to 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 buffer characteristics. Also, the tandem increases the degree of reliability. When a conventional structure fails, the external component takes over maintenance tasks.
Without pusher
To maintain the contact of the sealing planes, it is undesirable to move the structure along the sleeve or shaft. Among the main advantages, it is worth highlighting the performance over a wide temperature range and the absence of the need for additional sealing.
The downside is cross-section upgrades for heavy duty use.
Pusher
To ensure contact between the surfaces of the nodes, it is required to connect a secondary mechanical seal that moves along the axis of the sleeve or shaft. Due to this property, damages on the front plane of the device are compensated. 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 displacement of the secondary seal.
Balanced devices
Seal balancing involves a simple design transformation that reduces the hydraulic force that closes the mechanical shaft seal. Balanced assemblies are distinguished by the release of less heat, low stress on the sealing surfaces and increased pressure. This makes them the best choice for transporting formulations with high vapor pressure and poor lubricity.
Unbalanced elements
They have great cavitation and alignment stability, low leakage and low cost. A sufficiently low pressure stage is a disadvantage of such designs. When the pressure limit of the resultant reinforcement acting on the sealing parts is exceeded, the film between the surfaces is squeezed out, and the work becomes dry.
Standard
An example is Grundfos - endseals that require mounting and alignment on the sleeve or shaft. Despite the simple installation method of the device, reducing maintenance costs is currently a priority, leading to more widespread cartridge designs.
Internal single
This type of Wilo mechanical seal is the most popular. There is the possibility of easy change to form a buffer system for flushing, balancing to withstand the increased pressure of the environment. Recommended for use with aggressive and non-aggressive fluids with sufficient lubricity.
Cartridge
This category includes mechanical seals for pumps mounted on the sleeve, including the shaft sleeve and stuffing box cover, which has a tight fit to the shaft. Their main advantage is that there is no need to use connecting elements for their installation. These designs reduce the possibility of errors during installation and reduce operating costs.
External single
This mechanical seal becomes a cost-effective alternative to the expensive metals needed to provide corrosion resistance to internal seals, provided that the aggressive fluid has high lubricity. True, it is worth noting among the disadvantages the susceptibility to hydraulic pressure and the influence of shocks, which is why seals of this type have small pressure limits.
