As users become more familiar with gas seal technology and its advantages, gas seals are being applied to a wider variety of applications and equipment than ever before. Its common to see gas seals in agitators and other equipment where product contamination is an issue, such as units in the pharmaceutical industry. As gas seals become more common, its important that users understand the differences between them and liquid lubricated seals. First, lets start with the basics.
Mechanical seals consist of two seal rings mated together to create a seal at their interface. One seal ring rotates with the shaft and the other is stationary. The mating faces of these seal rings typically require lubrication from either the process fluid, a barrier fluid or, in the case of gas seals, nitrogen gas is used to separate the faces.
A typical gas seal design will use hydrodynamic lift-off to separate the rotary and stationary seal faces. Spiral grooves in the rotary seal face collect the gas. As the seal rotates, gas is compressed towards the end of the groove creating an opening pressure. This pressure exerts an opening force greater than closing force, separating the seal faces. This slight separation allows the gas to flow across the seal faces. Thus, the seal faces ride on a pressurized, gaseous fluid film (Figure 1).
Figure 1. A cutaway drawing of a compact gas seal.
Inexpensive and inert nitrogen gas is typically used with gas seals and gas consumption is a function of differential pressure between the process and regulated gas supply pressure.
Single liquid lubricated seals rely on the process fluid in the equipment to lubricate the seal faces of a mechanical seal. Dual mechanical seals can use an external pressurized fluid to lubricate the seal faces. In chemical plants, seal barrier fluid tanks are used to maintain a constant supply of pressurized liquid.
The faces of liquid lubricated seals operate in a mixed lubrication environment where face loading is only partially supported by the sealed liquid between the faces as rotary-to-stationary face contact also exists during equipment operation. The liquid present serves only to minimize face loading on the seal faces. Therefore, liquid lubricated seals generate heat at the seal faces due to rotary-to-stationary face contact during equipment operation. Liquid lubricated seals are very dependent on the tribological characteristics of the fluid, seal face design and materials at the operating pressures and temperatures of the equipment.
Select the right seal
Although the configuration of the seal is often dictated by regulatory requirements, the type of seal is left to the engineer. So, the question is: liquid or gas? Gas seals are no different than any other mechanical seal in that regard. Gas seals excel in many applications while in others there are other types of seals that exhibit better performance. To understand the application parameters that are ideal for gas seals, fluid characteristics, equipment operation and operating procedures need to be analyzed.
The ideal fluid to seal in process equipment is cool, lubricating, environmentally friendly and free of particulates. However, many chemical plants are dealing with difficult-to-seal fluids that are hot, polymerizing and operating at low viscosities. Some fluids operate close to their vaporization point and additional heat may cause a phase change. Gas seals are increasingly being used to seal a wide variety of applications in plants. Gas seals are replacing liquid-lubricated dual seals and, magnetic and canned pumps. They are replacing single seals as plants look to increase the reliability of their equipment.
Considerations for single seals
To better understand gas seals its important to understand the limitations of other sealing designs. Single seals use the process fluid. When this fluid is hot, corrosive, abrasive, and viscous, in other words, when the fluid is a poor lubricant, a long life may not be anticipated.
Beyond these concerns there are operation and maintenance concerns. Intermittent dry running, intolerable leakage, inadequate or incompetent maintenance, and the remoteness of the location are all issues in selecting a seal. In many processes, especially batch processes, pumps are often inadvertently run dry, which will cause seal failure due to excessive frictional heat.
Even with process fluid present, single seals will conduct heat developed at the seal face by friction directly to the fluid. The temperature of the fluid will rise and the viscosity will decrease. This reduces the lubrication of the process fluid and, in turn, the seals service life. Other physical properties of the process fluid can affect seal life. Some fluids will polymerize with heat. All single seals have a process fluid/air interface that occurs at the seal faces; this is a concern for fluids that form polymers. Solutions with dissolved solids such as sodium hydroxide, caustic, will precipitate at high rates with increasing temperatures. Of course, fugitive emissions also can be a concern with single seals.
Dual seals are next in the hierarchy of mechanical seal types. Dual seals can operate on an externally applied barrier fluid and eliminate the need for process fluid. A seal is sometimes maintained by a barrier fluid tank. This vessel is pressurized by a second, clean fluid, often water. While the system receives regulated pressure, the liquid level in this system is maintained by operators and requires periodic maintenance. If the liquid level isnt maintained both the inboard and outboard set of seal faces will operate dry, causing excessive frictional heat and premature failure.