About mechanical seals
The safest and most efficient way to seal an axle inlet with a gas or fluid under pressure on one side and atmosphere on the other, is to use a mechanical seal.

The principle is two seals, very even lapped, bearing against each other. One of them, is fixed to the wall through which the shaft is passing and the other seal is spring loaded fixed on the shaft.It is also statical sealed against the shaft.

The flatness of the seal surfaces is optical measured in helium light which refraction in a quarts glass disc is stated in light bands. The flatness deviation is less than 0.0025 mm.

The product pressure and the capillary force is pressing in a fluid film between the seal surfaces providing the lubrication. If the seal is running dry it will crash.

When choosing a seal the rotation part is determined first and then the type of seat. There are various material alternatives for the components for the different seal types.

Seal selection
Following factors effect the seal selection:

Following seal components and functions are particularly critical:
- The primary seal i.e. material and design of the sealing surfaces.
- The secondary seal, carrying axial movements.
- Torque transmission
- Spring force and movement.

Pressure
The higher hydraulic pressure behind the seal the harder the seal surfaces i.e. sealing ring and seat are pressed against each other.

The hydraulic force to the seal surface can be influenced by the seal design with the hydraulic load factor "HBF". This factor is the relation between the surface of the total force which works close to the seal and the sealing ring surface e.g.:

HBF 1,3
"general seal" media pressure x 1.3 working towards the sealing surface
HBF 0,8
"unloaded seal" media pressure x 0,8 working towards the sealing surface

For resp. seal the HBF- factor is stated below the built-in drawing.

Number of revolutions
The rotation speed of the sealing ring against the seat, the hydraulic force compressing the sealing rings plus the spring force by the friction coefficient will cause a heat generation. If the heat removal is less the seal surfaces will reach their critical temperature and crash. Various material combinations can cope with different high heat generation. The calculation of the power need is stated in watt.

To determine what temperature the sealing surfaces will reach the PV-number i.e. the force that has a closing effect on the seal by the speed- expressed MPa*m/s - is used. Following guiding values can be given for different material combinations:

See App.
3.2.1 Method of calculation max. pressure/ Number of revolutions
3.2.2 Diagram for max. pressure/ Number of revolutions

Maximum number of revolutions
For rotation speeds more than 15 m/s standard seals generally cannot be used as the vibration risk in the seal can cause leakage and mechanical damages.

Temperature
Primarily the secondary seal is making the limitation in which temperature field the seal can operate.
See App. 3.1.2 Material specification

Friction
The friction is determined by the structure of the sealing faces, the vicosity in the fluid film between these and the dry friction coefficient in present materials.

The lapped sealing faces have a surface structure implying about 90% contact between the surfaces in dry form. The capillary force and the pressure from the pump media resp. sealing liquid keep a fluid film between the surfaces so that most of the surface will run dry. The friction coefficient is varying between 0,05-0,3.

Leakage through the seal surfaces
The leakage rate is determined by the contact pressure between the sealing faces, the surface structure, the fluid pressure, viscosity and the rotation speed. A leakage of 0,005 l/h is normally accepted. Shaft vibrations and movements have a great impact on the leakage rate.

Evaporating temperature
When hot water for example 5 bar and 110 degrees C is pumped, the pressure is quite a bit above the evaporating pressure, which is 1,3 bar.

The fluid film between the surfaces is holding the same pressure at the edge of the sealing surface. Further in the fluid film has a somewhat higher pressure, about 8 bar, as the heat generation between the surfaces does increase the temperature and pressure. Towards the inside diameter of the sealing ring the pressure will drop gradually to the atmospheric pressure.

If the product pressure is not large enough or if the cooling of the sealing surfaces is insufficient there is a risk of boiling in the fluid film.

See App. 3.4.1 Resistant tables.

Product - Chemical structure
Many products are aggressive against the materials in the sealing surfaces, steel parts and secondary seal. A check-up on how the materials in different sealing parts withstand product and temperature must be made by resistant tables.

See App. 3.3.1 Resistant tables.

Crystallization
Some products e.g. chlorides and sugar solutions are forming crystals between the seal surfaces and the atmosphere side of the sealing rings. In order to disolve these crystals a sealing arrangement with flushing must be used. Generally a flushing without pressure is enough but best result is obtained when the flush media has a higher pressure than the product.

Sanitary seal built-in
Within the Food Manufacturing and Pharmaceutical Industries it is important to use enclosed seals without gaps and hidings where the product can form colonies of bacteria.

Food approved material
For assembling within the Food Manufacturing Industry all components are manufactured in approved materials.

Particles and fibres in product
Blocking
These are a common average cause as they after a while cake in the seal and block the moveability. When this has occured a smaller leakage will arise allowing particles to pass through sealing surfaces and quickly wear them down. It is therefore important to use a completly enclosed seal preventing particles and fibres to enter springs and parts.

Wear
It is often stated that the product must not contain particles over 50 micron to avoid a premature wear of the sealing surfaces. The seal gap however is an effective filter preventing larger particles to pass, but very small ones are able to go eith the fluid film and form severe abrasive grains which work upon the sealing surfaces.

Safety extent
Depending on the product, temperature and pressure, the factor of safety of the seal built-in must be considered. If the operating conditions are anti-environmental and unhealthy extra security must be arranged to prevent the product from spurt out of the machine in case of a seal breakdown.

Seal configuration
In clean media a simple mechanical seal without any extra arrangements will work. Higher temperatures, crystal forming and fibre containing products need a flushing of the atmosphere side. This can be made up of a low- pressure flushing with a more simple seal type as secondary seal for the flushing media. For more severe applications a high-pressure-flushing might be necessary - a double mechanical seal is then to be used. For hot water a "quench" is often used, i.e. a return from the pressure side to reduce the temperature over the seal through a cooler.

See App. 3.5.1 Different seal configurations.

Rotation dependency
Some seals are transmitting the torque by the pressure spring. These seals are dependent of rotation. The spring winding must be the same as the direction of rotation.

Service life
It is very difficult to estimate the durability of a built-in seal. We have example of seals running more than 10 years. Normally we calculate a durability of 3-5 years when recommending a seal. When testing e.g. carbon materials a wear of 0,0001 mm/h is considered acceptable.

Built-in directions
In order to avoid unnecessary damages at the assemblage is it important to follow given instructions.

See App. 3.6.1 Built-in directions.