There are two common forms of industrial seal: braided packing seals and mechanical seals. Braided packing seals have been around for several thousands of years; however mechanical seals are a modern invention from the days of the invention of the automobile. Mechanical seals have quickly found a wide range of applications outside of the car industry, including paper and pulp industry, petrochemicals and utility sectors and many other industrial applications where chemical processing is carried out in some form or other.
Mechanical seals are used when a braided packing seal is not able to meet the design specification requirements. The design requirements may mandate a mechanical seal on cost or performance grounds, and even both. Mechanical seals are more expensive than braided packing seals, however they have the ability to be more economical because they are more efficient in terms of reducing leakage or delivering higher performance compared to packing seals. Designers must finely balance the use of either seal against the need for environmental, performance and cost factors of the overall pumping system’s operational performance.
Mechanical seals are capable of operating in high temperature conditions, up to 1,800 F and pressurized environments between near complete vacuum and up to 5,000psi; in addition, shaft speeds up to 50,000 revolutions per minute (RPM) and where the seal must deal with corrosive chemical properties, or where contamination of the fluid being handled is a factor.
The cost factor is crucial when choosing a mechanical seal: typically mechanical seals can add up to $ 1,400 per diameter inch, and there is also the installation and maintenance costs to be assessed also. Mechanical seals typically require more specialized operation and maintenance staff to oversee them in both the installation and operational phases of a pump deployment, whereas braided packing seal alternatives do not.
Mechanical seals work using two sealing surfaces; an inner surface operates within the pump shaft and rotates around the inner shaft wall because it is fixed directly to the shaft. The second surface is mounted to the shaft wall and is fixed, usually on the gland plate. The rotating, inner seal makes contact with the shaft wall against the fixed counterseal providing continuous seal integrity and contact using hydraulic axial force or mechanical force. In addition, springs and elastomeric addendums are utilized to maintain and stabilize the seal connection.
The materials which are used in the construction of the two main seals will involve consideration of the operational environment first. If there is a need to operate within certain temperature, pressure and other operational factors, then the material must be able to withstand them. When the pump is operating, can the materials used withstand the forces involved, and there is also the issue of interaction with the fluid itself. Cross contamination of the fluid may not be tolerated, and the relative inertness of the material will play a factor in both contamination and the impact on the integrity of the seal itself.
Cooling of the seal is essential, as friction creates heat due to the rotating parts and continuous contact. A mechanical seal will use less water than a braided packing seal for cooling purposes; however a mechanical seal will typically require water treatment to deal with the discharge.