1.3.4 Seal face material combinations 1.3.5 Factors affecting the seal performance
From the middle of the 1950s mechanical shaft seals gained ground in favour of the traditional sealing method - the stuffing box. Compared to stuffing boxes, mechani- cal shaft seals provide the following advantages:
• They keep tight at smaller displacements and vibrations in the shaft
• They do not require any adjustment
• Seal faces provide a small amount of friction and thus, minimise the power loss
• The shaft does not slide against any of the seal’s components and thus, is not damaged because of wear (reduced repair costs).
The mechanical shaft seal is the part of a pump that separates the liquid from the atmosphere. In figure 1.3.1 you can see a couple of examples where the mechanical shaft seal is mounted in different types of pumps.
The majority of mechanical shaft seals are made according to the European standard EN 12756. Before choosing a shaft seal, there are certain things you need to know about the liquid and thus the seal’s resistance to the liquid:
• Determine the type of liquid
• Determine the pressure that the shaft seal is exposed to
• Determine the speed that the shaft seal is exposed to
• Determine the built-in dimensions
On the following pages we will present how a mechanical shaft seal works, the different types of seal, which kind of materials mechanical shaft seals are made of and which factors that affect the mechanical shaft seal’s performance.
1.3.1 The mechanical shaft seal’s
components and function
The mechanical shaft seal is made of two main components: a rotating part and a stationary part; and consists of the parts listed in figure 1.3.2. Figure 1.3.3 shows where the different parts are placed in the seal.
• The stationary part of the seal is fixed in the pump housing. The rotating part of the seal is fixed on the pump shaft and rotates when the pump operates.
• The two primary seal faces are pushed against each other by the spring and the liquid pressure. During operation a liquid film is produced in the narrow gap between the two seal faces. This film evaporates before it enters the atmosphere, making the mechanical shaft seal liquid tight, see figure 1.3.4.
• Secondary seals prevent leakage from occurring between the assembly and the shaft.
• The spring presses the seal faces together mechanically.
• The spring retainer transmits torque from the shaft to the seal. In connection with mechanical bellows shaft seals, torque is transferred directly through the bellows.
Seal gap
During operation the liquid forms a lubricating film between the seal faces. This lubricating film consists of a hydrostatic and a hydrodynamic film.
• The hydrostatic element is generated by the pumped liquid which is forced into the gap between the seal faces.
• The hydrodynamic lubricating film is created by pressure generated by the shaft’s rotation.
Fig. 1.3.4: Mechanical shaft seal in operation Lubrication film Liquid force
Spring force
Vapour Evaporationbegins
Fig. 1.3.3: Main components of the mechanical shaft seal
Rotating part Stationary part Shaft Primary seal Secondary seal Primary seal Secondary seal Spring Spring retainer
Mechanical shaft seal Designation
Seal face (primary seal) Secondary seal Spring
Spring retainer (torque transmission) Seat (seal faces, primary seal) Static seal (secondary seal) Rotating part
Stationary part
1.3.2 Balanced and unbalanced shaft seals
To obtain an acceptable face pressure between the primary seal faces, two kind of seal types exist: a balanced shaft seal and an unbalanced shaft seal.Balanced shaft seal
Figure 1.3.6 shows a balanced shaft seal indicating where the forces interact on the seal.
Unbalanced shaft seal
Figure 1.3.7 shows an unbalanced shaft seal indicating where the forces interact on the seal.
Several different forces have an axial impact on the seal faces. The spring force and the hydraulic force from the pumped liquid press the seal together while the force from the lubricating film in the seal gap counteracts this. In connection with high liquid pressure, the hydraulic forces can be so powerful that the lubricant in the seal gap cannot counteract the contact between the seal faces. Because the hydraulic force is proportionate to the area that the liquid pressure affects, the axial impact can only be reduced by obtaining a reduction of the pressure-loaded area.
The thickness of the lubricating film depends on the pump speed, the liquid temperature, the viscosity of the liquid and the axial forces of the mechanical shaft seal. The liquid is continuously changed in the seal gap because of
• evaporation of the liquid to the atmosphere
• the liquid’s circular movement
Figure 1.3.5 shows the optimum ratio between fine lubrication properties and limited leakage. As you can tell, the optimum ratio is when the lubricating film covers the entire seal gap, except for a very narrow evaporation zone close to the atmospheric side of the mechanical shaft seal. Leakage due to deposits on the seal faces is often observed. When using coolant agents, deposits are built up quickly by the evaporation at the atmosphere side of the seal. When the liquid evaporates in the evaporation zone, microscopic solids in the liquid remain in the seal gap as deposits creating wear.
These deposits are seen in connection with most types of liquid. But when the pumped liquid has a tendency to crystallise, it can become a problem. The best way to prevent wear is to select seal faces made of hard material, such as tungsten carbide (WC) or silicon carbide (SiC). The narrow seal gap between these materials (app. 0.3 µm Ra) minimises the risk of solids entering the seal gap and thereby minimises the amount of deposits building up.
Pressure Liquid
Pump pressure Stationary
seal face Rotating seal face
Vapour Atmosphere Entrance in seal Exit into atmosphere Start of evaporation 1 atm Fig. 1.3.6: Interaction of forces on the balanced shaft seal
Fig. 1.3.7: Interaction of forces on the unbalanced shaft seal
A
Spring forces Hydraulic forces
Contact area of seal faces
B A B
Hydraulic forces
Contact area of seal faces
Fig. 1.3.5: Optimum ratio between fine lubrication properties and limited leakage
The balancing ratio (K) of a mechanical shaft seal is defined as the ratio between the area A and the area (B) : K=A/B K = Balancing ratio
A = Area exposed to hydraulic pressure B = Contact area of seal faces
For balanced shaft seals the balancing ratio is usually around K=0.8 and for unbalanced shaft seals the balancing ratio is normally around K=1.2.