Mechanical seal

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Mechanical seal; the two rings d and a must be as flat as possible
Section through a direction-independent, single-acting mechanical seal.
1) threaded pin / set screw
2) O-ring (secondary seal)
3) dowel pin to prevent rotation of the slide ring (4)
4) sliding ring
5) counter-ring
6) O-ring (secondary seal)
7) housing wall (only indicated)
8) dowel pin to prevent rotation for the counter ring (5)
9) shaft / axis
10) springs

Mechanical seals are dynamic seals , i.e. they seal a rotating shaft against a wall, e.g. B. a machine housing . The main components are two components that slide on each other, the spring-loaded sliding ring (in the upper picture position a) and a counter ring (in the upper picture position d). One of the two rings sits rigidly in the stationary housing ( stator ) (position d in the top picture), the other is attached to the rotating shaft ( rotor ) with the help of anti-rotation pins . The surfaces between these two parts are - depending on the type of the mechanical seal - mostly plane and are usually made of carbon-graphite materials , metal , ceramic , plastic or synthetic resin- bonded carbon .

Classification

Dynamic seals can be divided into two subgroups:

  • The axial seal is the mechanical seal, with the special shapes of the feed head seal (steam head seal) and ball valve seal.
  • the radial seals can be divided into

construction

The opposing axial or radial sealing surfaces rotate relative to one another and form a primary sealing gap. Depending on the state of aggregation , the surrounding medium creates a liquid or gaseous lubricating film between the sealing surfaces . The sealing of the mechanical seal parts against the shaft or housing is usually carried out with secondary seals in the form of additional O-rings or sleeves. In this respect, mechanical seals basically consist of five components: the sliding ring, the counter ring, the spring system and a secondary seal each. Depending on the design of the seals, the number of components can be reduced or increased significantly.

Sliding ring and counter ring are referred to as counter-rotating pairs. The optimal choice of materials for this pairing depends primarily on the type of media to be sealed, the rotation speed and the operating temperature. When it comes to mechanical seals, one speaks of so-called hard-hard or hard-soft pairings. The hard-soft pairing denotes a combination of sliding parts in which one of the sliding partners is significantly softer than the other and therefore more easily worn , but also has better emergency running properties under unfavorable lubrication conditions. A typical combination is a carbon ring against a silicon carbide ring . With hard-hard pairings, both sliding rings are made of hard and wear-resistant material. This pairing is mainly used for abrasive products, but has poorer emergency running properties. In addition to the material pairing, the absolutely flat design of the sliding surfaces and the surface roughness are of decisive importance for tightness and wear. They are therefore usually lapped , polished and possibly given a superfinish .

The selection of the suitable material pairings should be made in direct cooperation and careful consultation with the manufacturer of the mechanical seals or the component manufacturer. The main applications for mechanical seals include pumps for water , feed water , car cooler or air-conditioning systems , agitators for wet and dry running, centrifuges or water turbines . Nowadays, mechanical seals are available for shaft diameters from approx. 5 to 630 mm, pressures from approx. 0.01 to 450 bar, temperatures from approx. −200 to +450  ° C and sliding speeds of up to approx. 150 m / s.

Materials for slip rings

A number of different materials can be used as the sliding ring or counter ring, which can be divided into 4 main groups:

  • carbon
Carbon-graphite - synthetic resin or antimony impregnation, electrographite, synthetic resin-bonded carbon
  • Ceramic materials
Silicon carbide , aluminum oxide , tungsten carbide
  • metallic materials
Stainless steel , cast chrome , special materials
  • plastic
PTFE

Wear of the slip rings

Since hydrazine was banned as a corrosion inhibitor because of its health hazard, the mechanical seals of boiler feed pumps show previously unknown corrosion phenomena. The reason was found to be electrocorrosion , because the relative movement between the sliding ring and the counter ring leads to static charges which are no longer discharged due to the very low conductivity of the boiler water of less than a micro-Siemens per cm [μS / cm] in contrast to earlier times . Within short operating times, sometimes a few hundred operating hours, fingertip-sized pieces can break out of the sliding ring and / or the counter ring and lead to a rapid increase in the leakage current, which is unacceptable on the one hand because of the loss of efficiency and on the other hand because the operational reliability is no longer guaranteed and leads to a repair standstill the associated high costs due to loss of production. To solve the problem z. B. worked with wavy surface profiling of the sliding surfaces or coated the sliding surfaces with crystalline nature-identical, but artificially produced diamond. After 26,680 hours in a long-term test with diamond-coated sliding rings at the Institute for Hydraulic Fluid Machinery at the Graz University of Technology , it was shown that the mechanical seal was in excellent condition with almost no corrosion after such a long period of operation.

See also

literature

Web links