Mechanical Seals for Rotary Pumps (With PDF)

WHY TO USE MECHANICAL SEALS?

Major advantages of using Mechanical seal are as follows:

• No visible leaks
• No shaft/sleeve wear
• Less maintenance required
• Less power consumption
• Less bearing wear
• Less corrosion of equipment
• Pressure and vacuum
• Shaft speed
• Flushing water etc
• Vertical pumps
• Less wasted product
• Downtime is greatly reduced
• Long life

HOW A SEAL WORKS? 

Refer Fig. 1. The lapped face of a  mechanically loaded rotating component (sealed to the shaft) rubs against the lapped face of a fixed component (sealed in a housing).  Sealing is achieved across the seal faces because of their incredible degree of flatness. The faces are initially held together by a mechanical load applied to those faces.

Evaporation & Solids Formation (Fig. 2):

THE KEY TO SUCCESS IS “Maintaining A Stable Fluid Film”

Fluid Film & Seal Faces (Fig. 3):

Mechanical Seal Technology:

Face Flatness / Lapping-The primary seal is the basis of all mechanical seal design.  This seal consists of two flat faces, one fixed, one rotating, and running against each other, with a liquid film between them providing lubrication.

The width or thickness of the lubricating film is dependent upon a number of variables but the distance between the two faces is constant in that this has the greatest influence, the closer these are together, the thinner the fluid film and the least likelihood of leakage across the faces.

It therefore follows that the flatter these surfaces are within practical manufacturing constraints, the better.

The process of achieving face flatness adopted by the mechanical seal industry is called lapping.

In this process a lapping machine is used, which provides one or a number of rotating ‘plates’ on to which the face or seat of a mechanical seal is placed.  The surface to be finished is in contact with the lapping plate.

Following lapping the workpiece surface is then polished to a reflective finish.  Final surface finish must be polished to a degree of light reflection, sufficient for its flatness to be checked.

Final surface finish must be polished to a degree of light reflection, sufficient for its flatness to be checked.

The degree of face flatness is checked by what is known as the optical interference fringe method.

Here an optical flat (a quartz or Pyrex disc, with surfaces finished to within 0.000001/0.000005-0.025/0.125 microns) is placed over the lapped and polished face and placed under a helium light source, from this a pattern of fringe interference is produced.

This pattern is then translated into a measurement of flatness by comparison of the pattern obtained with various patterns (usually in chart form) which indicate the flatness accuracy.

Figure 4 shows a helium light source (wave length approximately 0.0006mm/0.00002) hitting the optical flat (A) it then passes through the optical flat. When it reaches the other side, some is reflected (Point B) and the rest reflects from the specimen (Point C). When the light hits the optical flat again (Point D) it passes back through the optical flat.  The two reflections (E and F) can then be observed from above the optical flat.

Flatness is measured in light bands, one light band = 0.000011 or 0.0003mm.  There are 85 light bands to one thousand (0.001) of an inch.

Small diameter seal faces (<4” Diameter) are lapped to two light bands or less, larger (>4” Diameter) faces less than five light bands.

The additional sheet shows how face patterns are interpreted, the process is similar to reading the contours on a map.

Figure 5 shows a typically flat surface, a series of straight, parallel equi-shaped lines shows that a face is flat to within one light band.

Seal Faces under a Microscope (Fig. 6):

Fluid Film Condition Relates To:

• Fluid Viscosity
• Face Closing force
• Surface Finish
• Surface Speed
• Contract Face Parallelism

WHAT HAPPENS WHEN A SEAL RUNS (Fig. 7)?

FLUID FILM:

Think about Bearings!

Seals & bearings are both lubricated by a hydrodynamic fluid film

FLUID FILM PROVIDES LUBRICATION

• Prevents Dry Face Contact
• Frictional Heat Build Up
• Solids Contamination

WHAT CAN HAPPEN?

• Dry Face Contact / Dry Running
• Excessive Wear
• Frictional Heat Build Up
• Solids Contamination

CONCERNS WITH HEAT:

• Product Vaporisation
• Corrosion Rate Increase
• Damage to Components
• Product Vaporization / Corrosion Rate Increase
• Change in State of Fluid Film / Solids Deposition
• Reduction / Loss of Fluid Film                                                                                                                               …………..SEAL FACES RUB AGAINST EACH OTHER…HEAT GENERATED! THIS HEAT MUST BE REMOVED…