- Published on Tuesday, 15 April 2008 18:00
- Written by Joseph C. Parker
Manufacturers of mechanical seals today are constantly working to engineer longer-lasting, safer, easier-to-use and lower-emission seals for the benefit of seal users, the products they produce and the environment. Technology can be an enabling key to mechanical seal reliability, but it is not the only key. Without knowing the true process conditions, understanding interrelated effects of the larger system, and operating the system properly, good technology applied badly may not ultimately improve reliability.
Successful manufacturers of mechanical seals today must apply critical investigative skills into selecting the right technology for each application, and then instill a practical, working knowledge of proper operating and maintenance techniques essential for success. A close relationship between the seal supplier and plant personnel increases the likelihood of achieving high-reliability objectives.
Increasing the Lifecycle
To understand what the manufacturers are doing to increase the life cycle of seals, look directly at the primary mechanism in a mechanical seal: the small separation between parallel-sliding stationary and rotating seal faces. Striking the proper balance of loads, heat generation, surface finish and parallelism will define the amount of leakage that passes through and rubbing wear that limits life. Every component of a mechanical seal—from the stationary gland to the rotating sleeve—must be designed to contribute positively toward maintaining this micro-fine parallel interface.
Heat generation caused by rubbing friction at the seal interface can lead to shorter seal life if the mechanical seal is not designed to tolerate such conditions. For example, seals that are designed for liquid services count on the fluid to provide cooling and lubrication. If this fluid is no longer available—such as if a valve is unexpectedly closed—no lubrication plus no cooling equal high heat generation and rapid seal face wear. There are many other ways to cause dry running of seals and a special case involves the fluid itself vaporizing on or near the seal faces. Light hydrocarbons and hot water will boil on seal faces when the vapor pressure is breached by the temperature or absolute pressure.
If the seal faces run without contacting, heat from friction can be eliminated. However, non-contacting seal faces allow leakage, and depending on the fluid type, this can be a concern for environmental or safety reasons. If the fluid type is a gas, having non-contacting seal faces is a good way to eliminate frictional heat and wear, when gas leakage is acceptable. Again, the seal manufacturer’s goal is to manage the parallel-sliding interface appropriately for the service conditions. The challenge is to engineer a mechanical seal that performs reliably under constantly changing and often unknown conditions, where mere micro-inches (micrometers) define the difference between excessive emissions or heat generation problems.
One of the most important areas on which technology is being applied today is in optimizing the relationship between the two seal faces by introducing some very fine structures or patterns on the faces in order to create a special environment in the parallel-sliding interface.
Surface features on the seal face, or face topography, create supportive forces to counteract the forces trying to crush the faces closed. These engineered patterns and grooves create a pressure profile that ranges from fully neutralizing all the closing loads, resulting in complete seal face separation, or a pressure profile that reduces most of the closing loads, resulting in light seal face contact. Seals destined for dry gas service use a topography that develops full-face separation, and seals designed for wet or marginally wet service use a topography that acts to reduce contact loads.
Some topographies for gas media have a series of sharp-edged grooves that draw gas into a narrowing area. This narrowing causes the internal pressure to increase to a higher level than outside the seal face. The net result is the higher interface pressure overcomes the pressures that work to close the seal faces, and the seal faces separate and stay open with a very thin film of gas. At the point of seal face separation, the interface pressure decreases due to lower effectiveness over the separation, and force equilibrium of the seal faces is satisfied at a specific separation. Whole product lines exist for dry gas applications that use seal face topographies to generate lift and cause seal face separation.
Another pattern is a smooth wave design that involves a sinusoidal series of peaks and valleys that allows the processing fluid to enter and then exit from the same direction it entered. When the fluid is squeezed back out of the wave valley at the wave peak, the pressure increases in a similar fashion to the topography for gas seals. Waves are especially useful in providing enough interface pressure to relieve a measured portion of the closing loads so that the net result is face contact at a reduced rate.
In addition to the grooved and wavy seal faces, there are many varieties of micro-surface features that have been introduced to optimize reliability and performance for particular problems.
In various process industries and especially by fluid type, zero emissions to the environment are allowed. Many volatile, toxic and hazardous fluids are regulated with penalties against emitters or the fluid may pose an inherent safety risk. In these situations and others, dual seals are typically used to prevent any process leakage. As the name implies, two sets of mechanical seals are combined and filled with a liquid or gas that provides a barrier against process leakage. The seal faces then operate on this barrier liquid instead of the process fluid, which alone or with seal face features can contribute to further reliability gains. Dual gas seals are good examples of applied seal face technology that address dry running, zero emissions, process contamination and energy consumption issues all at the same time.