Since the mid-1950s, mechanical seals have become increasingly popular as shaft seals over the traditional stuffing box. Compared to the stuffing boxes, the mechanical seals offer the following advantages:
- They hold tight to the shaft in the event of minor displacements and vibrations.
- You don't need to be hired.
- The sealing surfaces have less friction and therefore minimal power losses.
- The shaft does not slide along the sealing components and is therefore not damaged by wear (lower repair costs).
The mechanical seal is the part of the pump that separates the medium from the atmosphere. Figure 1.3.1 shows different types of pumps that are equipped with mechanical seals. The majority of mechanical seals are manufactured in accordance with the European standard EN 12756. To select a specific mechanical seal, the following information about the properties of the pumped medium and the resistance of the seal to the pumped medium must be determined:
- Type of medium to be pumped
- Pressure to which the mechanical seal is exposed
- Speed to which the mechanical seal is exposed
Installation dimensions The following pages explain how a mechanical seal works, the various types of seals, the materials used for mechanical seals and the factors that affect the performance of a mechanical seal.
Components and functionality of the mechanical seal
The mechanical seal consists of two main assemblies: a rotating assembly and a stationary assembly. These assemblies consist of the parts listed in Figure 1.3.2. Figure 1.3.3 shows how the various parts of the mechanical seal are arranged.
- The stationary part is built into the pump housing. The rotating part of the seal sits on the pump shaft and rotates during pump operation.
- The two primary sealing surfaces are pressed against each other by the spring pressure and the pressure of the pumped medium. During operation, a film of lubricant from the pumped medium forms in the narrow gap between the two sealing surfaces. This film evaporates before it is released into the atmosphere, making the mechanical seal fluid-tight (see Figure 1.3.4).
- The secondary seals seal the mechanical seal against the shaft.
- The sealing surfaces are mechanically pressed together by the spring.
- The driver transfers the torque from the shaft to the seal. With bellows seals, the torque is transmitted directly through the bellows.
During operation, the pumped medium forms a lubricating film between the sealing surfaces. This lubricating film consists of a hydrostatic and a hydrodynamic element.
- The hydrostatic element is generated by the pumped medium, which is pressed into the gap between the sealing surfaces.
- The hydrodynamic lubricating film is created by the pressure generated by the rotation of the shaft.
The thickness of the lubricating film depends on the pump speed, the liquid temperature, the viscosity of the pumped medium and the axial forces of the mechanical seal. The constant exchange of the pumped medium in the sealing gap is guaranteed by two effects:
- the evaporation of the pumped medium into the atmosphere
- the circulating movement of the pumped medium.
Figure 1.3.5 shows the optimal relationship between good lubrication and low leakage rate. The optimum ratio is achieved when the lubricating film wets the entire seal gap, except for a very narrow evaporation zone close to the atmospheric side of the mechanical seal.
Often leaks occur due to deposits on the sealing surfaces. When using coolants, deposits quickly build up through evaporation on the atmospheric side of the seal. When the pumped medium evaporates in the evaporation zone, microscopic solid particles from the pumped medium remain as deposits in the sealing gap and cause wear there. These deposits are seen with most types of fluids. Conveying media with a tendency to crystallize are problematic. In order to avoid excessive wear, it is best to choose sealing surfaces made of hard material (e.g. tungsten carbide (WC) or silicon carbide (SiC)).
The narrow sealing gap between these materials (approx. 0.3 μm) minimizes the risk of solid particles penetrating the sealing gap and thus also minimizes deposits.
Balanced and unbalanced seals
To achieve a balanced pressure between the primary sealing surfaces, two types of seals are offered: balanced and unbalanced seals.
Figure 1.3.6 shows a relieved seal and the forces acting on it.
Figure 1.3.7 shows a non-balanced seal and the forces acting on it.
Several forces act on the sealing surfaces in the axial direction. Spring force and hydraulic force of the pumped medium compress the seal, while the force of the lubricating film in the seal gap counteracts this. At high pressure of the pumped medium, the hydraulic forces may be so great that the lubricating film in the sealing gap cannot prevent contact between the sealing surfaces. Since the hydraulic force is proportional to the area on which the pressure of the pumped medium acts, the load in the axial direction can only be reduced by reducing the pressure area.
The load factor (K) of a mechanical seal is defined as the ratio between area (A) and area (B): K = A / BK = load factor A = area that is exposed to hydraulic pressure B = contact area of the sealing surfaces Load factor around K = 0.8, for those not relieved around K = 1.2.
Mechanical seal types
This section describes the main types of mechanical seals: O-ring seal, bellows seal, and cartridge seal.
In an O-ring seal, the seal between the rotating shaft and the rotating sealing surface is achieved by an O-ring (Figure 1.3.9). The O-ring must be able to slide freely in the axial direction in order to be able to absorb the axial displacement due to temperature changes and wear. Incorrect position of the stationary seat can lead to abrasion and unnecessary wear on the O-ring and shaft. O-rings are made of different elastomers according to their operating conditions (e.g. NBR, EPDM and FKM).
Advantages and disadvantages of an O-ring seal
- Suitable for hot
- Fluids and high
- Deposits on the shaft (e.g. rust) may be a hindrance
- the movement
- the O-ring seal in the axial direction
Fig. 1.3.9: O-ring seal
A common feature of bellows seals is the rubber or metal bellows that act as a dynamic sealing element between the rotating ring and the shaft.
Rubber bellows seals
The bellows of a rubber bellows seal (see Figure 1.3.10) can be made from different elastomers (e.g. NBR, EPDM and FKM) - depending on the operating conditions. Two geometrical principles are used in the construction of rubber bellows:
Fig. 1.3.10: Rubber bellows seal
Metal bellows seals
In a conventional mechanical seal, the spring generates the force required to close the sealing surfaces. In the case of a metal bellows seal (Figure 1.3.11), the spring is replaced by a metal bellows with the same force. The metal bellows acts as a dynamic seal between the rotating seal ring and the shaft and as a spring. The bellows has several folds with which the required contact pressure is generated.
Advantages and Disadvantages of Metal Bellows Cartridge Seals
- Insensitive to deposits (e.g. rust and lime) on the shaft
- Suitable for hot media and high pressures
- Low load factor leads to less wear and a longer service life
- Possible fatigue failure of the mechanical seal if the pump is not correctly aligned
- Possible fatigue from excessive temperatures or pressures
With the cartridge mechanical seal, all parts form a compact unit, which is attached to a protective shaft sleeve ready for installation. A cartridge seal offers many advantages over a conventional mechanical seal (Figure 1.3.12).
Advantages of the cartridge seal:
- Easy and quick service
- Construction protects the sealing surfaces
- Pre-tensioned spring
- Safe handling
"With the kind permission of GRUNDFOS GMBH"