There are many process considerations other than abrasion that might prevent a cool, clean lubricating film at the sealing interface. If the sealing fluid has a low vapor point, for example, flashing can result. Flashing occurs when the sealed fluid changes from liquid to gas at the sealing interface, expanding quickly and forcing the sealing elements apart until the pressure and temperature are relieved, only to have the sealing elements collapse back into contact.
Mechanical damage to the sealing contact surfaces quickly results in seal failure. No lubricating film is established. Hence, operators must incorporate process controls and ensure proper mechanical seal selection to prevent such upsets. There are many other process conditions that require special attention such as fluids that are toxic, must be kept anaerobic .
Seal environmental controls are often overlooked, resulting in surprisingly short mechanical seal life. Many seal failures of this type happen in cool water applications. Cool water is an effective sealing fluid for creating a stable lubricating film at the sealing interface, but failure to apply proper seal environmental controls can lead to mechanical seal failure.
Many cool water applications fail prematurely because they are vertical, with the mechanical seal installed at a high point in the system where air is trapped. Without properly venting the air out of the seal chamber area, the mechanical seal seals air, not cool water. This is a dry-running condition that generates heat and quickly degrades the materials at the sealing interface.
A common environmental control used in vertical applications is a recirculation line from the seal chamber to pump suction, but in some cases the seals run dry for too long before the fluid replaces the air in the seal chamber.
Poor equipment conditions, caused by bad bearings, cavitation, excessive impeller loads and misaligned shafts, will result in excessive motion, vibration and mechanical shock to the mechanical seal. These conditions cause greater stresses, more heat and more opportunity for abrasives to enter the sealing interface.
Mechanical seals are designed to handle a range of motions and conditions, but they are just one machinery component in a larger system. Understanding the basics of mechanical seals and how they may be adapted for different application requirements is critical for choosing the best seal for the job and ensuring optimal system reliability.