5.4 Mechanical seal

Mechanical seal

When the sealing of a rotating shaft against a pump casing is provided by a mechanical seal, the seal is in means of rotating an optically (see surface flatness below) flat ring, that is rubbing against a stationary ring with a fluid film in between, see figure 5.4a. Sealing between the rotating ring and shaft, and the stationary ring and gland, is performed by means of some sort of elastomer such as O-rings, V-rings, etc. The primary purpose of the spring is to provide an initial pressure between the parts to be Sealed. One of the great advantages with this type of sealing is that it does not cause wear to the shaft or the sleeve.

 

 

 

 

 

 

 

 

 

Figure 5.4a Single mechanical seal.

1. Stationary seal ring sealed with an O-ring.
2. Rotating seal ring sealed with an O-ring which allows axial movement to compensate for wear.
3. Sealing takes place here, axially, between pan 1 and part 2.
4. The rotating spring, the purpose of which is to maintain contact between the seal faces and to drive the rotary seal ring.

The theory as to what happens between the seal faces has been dealt with schematically in figure 5.4b. The seal shall be considered as drip-free but not leak-free. The very low leakage, described in technical terms as diffusion, will escape in the form of vapour on the atmospheric side of the seal.

Figure 5.4b The appearance of the fluid film and pressure distribution p_s – p_o

Leakage through the seal faces in the case of mechanical seals amounts to 0.05 to 2 cm³/hour depending on operating conditions, dimensions and choice of seal face materials. No dripping, however, should be observed, only the formation of vapour which can lead to a certain amount of crystal formation. A general and very important rule is that a mechanical seal should never be run dry there must always be a fluid film between the faces. Dry running can occur even when the pump is full of fluid if the temperature is so high that the fluid vaporizes between the seal faces. Only specially designed seals can be run at the vapour phase of a fluid.

At high pressures and high speeds the sliding surfaces must be relieved so that the fluid film is kept stable. The frictional heat must be limited at temperatures approaching the vapour temperature. In such cases, the face pressure is  reduced hydraulically. This is done by balancing the ratio between the sealing diameter “S” and the inner diameter of the stationary ring. The pressure between the seal faces is thus varied according to the fluid pressure. Dependent upon the relationship between the two surfaces A and B in figure 5.4c, the seal is termed, balanced or unbalanced.

Figure 5.4c Hydraulic balancing of mechanical seals.

The limit between balanced and unbalanced seals is determined by the pv-factor and the temperature conditions. Balanced standard seals can be used for pressures up to 10 MPa. In the case of higher pv-values > 100 MPa • m/s
so called hydrodynamic seals having a controlled leakage must be used.

Mechanical seal surface flatness

The flatness of the axial seal faces must be of a very high quality. The leakage is approximately proportional to the third power of the thickness of the fluid film which normally is in the order of 0.001 mm. Flatness is normally expressed
in so-called light bands, since checking normally takes place using an optical flat and monochromatic light, usually sodium light. A deviation of one light band corresponds to an out of flatness of 0.25μm (μm 0.001 mm).

Normal standards of flatness are better than 1-2 light bands with variations up to approximately 5 light bands in the case of shaft diameters over 100 mm or in the case of non-homogeneous face materials. In addition to the number of  light bands the light pattern must also be considered, see figure 5.4d. Large differences in seal tightness occur, for example, between a spherical surface (light pattern 3) and a wavy surface (light pattern 5). A check of seal faces is  recommended as a standard measure prior to the installation of a mechanical seal.

Figure 5.4d Light pattern flatness checks using an optical flat.

• Light pattern 1
  The surface is flat within 1 light band = 0.25μm. The distance between the bands does not tell us anything about flatness only about the size of the air gap.
• Light pattern 2
  The surface is almost flat but the edges have been rounded, probably in connection with hand polishing
• Light pattern 3
  The surface is concave or convex to 3 light bands. This can be decided by pressing on the centre of the glass. If the rings then move outwards the surface is convex.
• Light pattern 4
  The surface is not completely flat. The tangent is drawn to the wavy bands. Half the number of light bands which are bisected by the tangent is a measurement of the flatness.
• Light pattern 5
  The surface goes in waves. It is difficult to achieve tightness. The cause can be residual tension after tightening in a chuck. The flatness fault is evaluated by comparison with light pattern 1.
• Light pattern 6
  The surface goes in waves. The same phenomena as light pattern 5 but 4 high points.

Design principles of mechanical seals

The generation of the initial pressure, i.e. the initial face load is provided by using different types of springs. Single springs, multiple springs and wave springs are the most common, see figure 5.4e. The drive, i.e. transmission of the frictional torque of the seal can, when single springs used, be carried out by the spring itself, separate driving elements are used for other types. At least two driving points are necessary in order to guarantee a satisfactory loading of the seal faces. Single springs should generally be chosen in preference to multiple spring types. The reason for this is that driving and face loading can be combined in one element, that the single spring is robust and withstands corrosion, as well as the fact that the risk for blocking is less than in the case of small springs. In addition, the single spring is easier to handle.

Figure 5.4e Examples of face loading and drive.

The appearance of the rotary seal rings varies with the method of driving, engagement, and the loading, as well as the materials of which the seal faces are made. In order to take up any vibrations in the shaft, parallel deviations in the O-ring grooves as well as any effects of the driving arrangement, the stationary ring should be mounted as flexible as possible. Figure 5.4f shows various methods of attachment.

Figure 5.4f Some examples of how a stationary seal ring can be attached and sealed, a provides the greatest flexibility. (Source Mayer E: Axiale Gleitringsdichtungen)

Static seals are produced in a great many varieties by different manufacturers. O-rings dominate, see figures 5.4g, for temperatures up to 250°C. Wedge shaped seal rings as c in figure 5.4g, are often used for high temperatures.

Figure 5.4g Some examp!es of sections of static seal rings. (Source Mayer).

Temperature aspects in mechanical seals

In order to achieve acceptable temperature conditions at the seal faces, frictional heat must be dissipated in order to avoid dry friction. This takes place by means of heat transfer in the rings and by convection to the surrounding medium, see figure 5.4h.

Figure 5.4h Heat dissipation in a seal.

Each seal and combination of materials has what is called a minimum ΔT in order to be able to function, see figure 5.4i. ΔT is the difference in temperature between the boiling point of the liquid at sealed pressure and the actual temperature of the liquid. Usually, ΔT should be at least 20°C. (The seal manufacturer can provide information for each type of duty). By means of various installation options, internal and external circulation or cooling, the available ΔT can be increased. The required ΔT of a seal can also be reduced by using materials with good thermal conductivity or with low friction, etc. In such special cases the ΔT required can be reduced to approximately 5°C.

 

Figure 5.4i Pressure and temperature relationships at three different speeds for a balanced seal. The operation range of the seal is to the left of each respective speed boundary. The liquid is water. The distance between the vapour curve and the seals limit is the required ΔT for each speed (Source Lymer).

Different mechanical seal arrangements

Single seal without circulation

A seal without circulation, see figure 5c,  can be used in favorable cases in the case of clean media and moderate temperatures. Cooling or heating can take place in direct connection to the seal surfaces if cooling channels are provided, see figure 5.4k.

 

 

 

 

 

 

Figure 5.4k Seal without circulation but with cooling.

Single seal with circulation of the pumped liquid

The purpose of the circulation is:

• to remove the frictional heat from the seal rings
• to remove particles and crystals from the area around the seal faces
• to cool or to heat the seal area in relationship to the pump in general

This is the most common seal arrangement and by varying the circulation system and the choice of material most media can be handled, both cold and hot, clean and contaminated, see figure 5.4l. There are certain exceptions and they are treated as follows:

Figure 5.4l Seal with circulation, flushing and/or quenching.

Single seal with flushing external liquid (dilution)

Used for strongly contaminated liquids and suspensions where dilution in the process is permissible, see figure 5.4l.

Single mechanical seal for slurries without flushing

If dilution cannot be allowed due to process technical or economic reasons, a balanced reversed seal can feasibly be used with hard-wear surfaces, e.g. tungsten carbide versus tungsten carbide, see figure 5.4m. The spring is located outside the medium and all the parts in contact with the medium are so designed that clogging is prevented. Quench for cooling and keeping the atmospheric side of the seal clean is also recommended.

Figure 5.4m Balanced reversed seal for contaminated fluid.

Single seal with quench

The quench, see figure 5.4l, can be used to:

• remove dangerous liquids or gases which leak out
• cool or heat the seal from the outside
• rinse away any wear powder and crystals
• function as dry-running protection

Double seals

A double seal, see figure 5.4n, consists of two seals usually mounting back-to-back and having a special barrier liquid in between. This sealant should have a pressure which exceeds the pressure inside the inner seal by 0.1-0.15 MPa so that the lubricating film for both sides of the seal is comprised by the sealant. A double seal involves extra equipment in order to pressurize, Circulate and possibly cool the sealant. Due to these facts, double seals are only recommended where regarded as being necessary.

Typical installations where double seals are required are as follows:

• strongly contaminated liquids and suspensions where dilution cannot be allowed
• liquids or gases which are toxic, radio-active or liable to explode
• when there is a risk of severe crystal formation

Figure 5.4n Double mechanical seal

Single seal of PTFE bellows type

Used for strongly corrosive liquid. The parts in contact with the liquid are made of ceramic and PTFE, see figure 5.4o.

Figure 5.4o Chemical seal of PTFE bellows type.

Materials of construction

The material of the two seal faces can be the same or different. The combination of materials is determined, among other factors, by wear strength, friction conditions, chemical resistance and thermal conductivity. Figure 5.4p shows some common combinations for the two seal faces.

Tungsten carbide and ceramics can either be used in solid form or as a surface coating on, for example, stainless steel. Elastomers of different types, see figure 5.4q, are used as flexible elements and for static seals.

Table 5.4a Price relationships of seal material combinations.

Figure 5.4p Some elastomers for mechanical seals.

Barrier fluid in mechanical seal

The barrier fluid, sealant, should suit the pumped medium (liquid or gas) so that no explosion, chemical reaction or corrosion takes place as a result of the leakage which occurs through the seal faces or within the stuffing box. Figure 5.4q shows some examples of sealants for different temperature ranges. The clearance between the seal faces is from 0.2-1.0 µm. It acts as a filter for particles larger than this clearance gap. The clearance gap for soft stuffing box packings is between 3-10µm.The requirements for the purity of the sealant are therefore determined, to a large extent, by other components in the circulation flow meters, valves and other equipment in the sealant system. It can be estimated that 50-100 µm is sufficient for the majority of applications.

Special attention should be paid to the auxiliary sealant systems with regard to pressure and the consequences of unintentional shutdown or breakdown. The difficulties of achieving sufficient operating reliability in the auxiliary system have led to a certain movement towards special single mechanical seals, which do not require barrier fluid.

Tightness and rate of leakage in mechanical seals

The requirements for tightness depend on many factors and during the past years demands have increased and will increase further. Some consequences of leakage are given below without them being arranged in any specific order:

• the inner working environment can become unacceptable
• the outer environment requires that the leakage be taken care of and cleaned
• leakage can damage machinery in the form of corrosion and bearing failures
• cost of the liquid which leaks out
• leakage into a process can cause damage or interruptions to the process. In some cases there can be considerable economic consequences resulting from the evaporation of inner leakage.

The rate of leakage from different types of seals depends on the pressure at the seal, shaft diameter, speed and, in the case of soft stuffing box packings, also to a very large extent upon maintenance.

The following can be given as guide values:

• Mechanical seal — single 0.2-2 g/h—cm³/h
• Soft packing — functioning well and properly maintained 150-720 g/h—cm³/h
• Soft stuffing box packing- practical values recorded, up to 5,000 g/h—cm³/h

Environment and leakage

The function of the seal in relation to the inner working environment deserves a great deal of consideration with regard to general well-being and the limiting values recommended by the public health and environmental authorities. These values are usually quoted as ppm or mg/m³ air. It is important to be aware that the limiting values are indicated as absolute maximums the normal values being considerably less. Some examples of common liquids within process industries with suggested maximum limiting values are given on the next page, however, recommendations and limits should always be checked with the local Public Health and Environmental Authorities.

Substance / mg/m³ air :

• Chlorine / 3
• Chlorine dioxide / 0.3
• Sodium hydroxide / 2
• Sulphur dioxide / 5
• Sulphuric acid / 1

Using these figures the required capacity of the ventilation system (m³air/h) can be calculated. A pump which pumps a volatile liquid (pumped volume flow m³/h) with a limiting value of 2 mg/m³ air requires a fan with the following capacity, the average leakage is used:

Calculation: Vented air needed = Pumped volume flow * (f / mg/m³)

• Mechanical seal, single – leak factor f =1
• Soft stuffing box packing, well maintained, f=435
• Soft stuffing box packing, not well maintained, f=5000

Example:

A pump with mechanical seal pumps Sodium hydroxide at Q = 1000 m³/h: This gives a ventilation system capacity of 1000 (1/2) = 500 m³of air/h

In the case of liquids which are not so volatile an evaluation must be made of how much of the leakage escapes in the form of vapour or liquid. In order to obtain the total capacity required, the required capacities for all pumps containing dangerous substances are added together. In many cases, double mechanical seals must perhaps be used from the point of view of environmental safety.