Originally published in the November 2012 issue of Pumps & Systems.
Dry gas sealing has become a catch-all term for a variety of seal face lubrication mechanisms, but not all dry gas seals are created equal.
An effective dry gas seal offers non-contacting, liquid-free operation, eliminating friction that contributes to excess heat and hampers system performance. Over the past several decades, dry gas seals with spiral groove technology have proven to be a preferred method for delivering the non-liquid lubrication and lift needed to eliminate friction and contribute to equipment reliability. While end users can find several dry gas seal methods on the market, including semi-contacting wavy-faced seals, T-slot depressions and deep grooved face patterns, these less effective methods have diluted the definition of dry gas sealing, or gas-lubricated sealing, and often serve to confuse reliability engineers and equipment operators.
When selecting a dry gas sealing system, the correct sealing arrangement is also an important aspect of reliability. By choosing a spiral groove seal with an inside diameter to outside diameter (ID to OD) gas pumping arrangement, end users benefit from the latest spiral groove developments and lock in dependable, long-lasting equipment operation.
Dry Gas Mechanical Seals
Unlike typical, liquid-lubricated seal faces, spiral grooved, dry gas seal faces run without face contact. Non-contacting operation eliminates wear, seal face friction and the heat that results from friction.
The most costly component in a liquid-lubricated sealing system is often the seal support system. Liquid-lubricated seals generate significant heat because of face friction and liquid churn from the seal components running in the captive barrier fluid. Along with providing an adequate volume of seal barrier liquid, the removal of seal-generated heat is the goal of the seal support system. By eliminating friction, an effective dry gas seal can greatly reduce the amount of time and resources associated with system operation and maintenance.
Dry gas seals are ideal for sealing volatile fluids in chemical and petrochemical processes, reactive or temperature-sensitive media, oxygen-sensitive processes, and cold applications. Because the seals do not require barrier liquid, they are also well-suited for sealing extremely pure materials with virtually no process contamination.
Spiral Groove Technology
Developed more than 40 years ago, the spiral groove dry gas seal remains the standard for effective dry gas sealing. This type seal face features micro-machined spiral grooves. In spiral groove seals that feature an ID to OD gas pumping arrangement, the face pair is pressurized on the ID. When the pump shaft turns, the spiral grooves compress the pressurized barrier gas and create a lifting force on the opposing face ring. The result is non-contacting operation.
With barrier gas at the ID of the face pair, the spiral grooves are designed to pump gas toward the OD. The stiff ring of compressed barrier gas exiting the face pair creates a highly effective dry seal. Exiting barrier gas passes into the seal chamber at the OD of the inboard seal. Exposed to a large volume of pumpage, the barrier gas entering the seal chamber is unable to dewater the pumpage. This allows the inboard seal to run in a liquid environment and remain free-moving, eliminating inboard seal hang-up and possible seal failure.
Some spiral groove gas seals also feature reverse pressure capability. In the event of a pressure reversal, in which seal chamber pressure exceeds barrier gas pressure, the secondary seal shifts, closing the inboard seal and preventing product release.
Other types of dry gas seals, such as wavy-faced seals, T-slots and deep grooved face patterns, fall short compared with spiral groove technology. While they may impart some gas movement and provide some lubrication and lift, they do not achieve predictable and efficient lift — and the consistent non-contacting operation — that spiral groove dry gas seals offer.
Spiral groove dry gas seals are provided in cartridge form and are simple to install. Non-contacting operation provides a multitude of operating benefits — including zero emissions, extended mean time between repairs, lower sealing system investment and increased product purity. Gas-lubricated, non-contacting seals require a simple gas delivery system. With no seal face contact, the seal does not generate heat from friction, eliminating the need for system cooling water. In addition, non-elastomeric designs allow the use of the gas seal at elevated temperatures.
Dry gas seals with spiral groove technology provide effective sealing for volatile, corrosive and pure processes.
Dry gas seals use less energy than a typical liquid-lubricated seal, providing additional cost savings to end users. They are also a good alternative to magnetic drive pumps, which typically require major capital expenditures to retrofit to equipment, are expensive to operate and repair, and are not well-suited for certain types of corrosive processes or processes in which temperatures exceed the Curie limit of the magnets. When end users factor in the reduction in maintenance time, costs and increased reliability, life-cycle costs are typically lower for dry gas seals than for liquid-lubricated seals or magnetic drive pumps.
Other Important Factors
Once end users have chosen their seals and arrangements, selecting the right gas control system is another step toward increased reliability. These systems regulate gas flow and pressure to seals, filter barrier supply gas, and monitor seal performance, contributing to better performance. Certain systems also feature check valves that can prevent the backflow of process fluid and alarms that alert the user if the system is losing pressure or if too much gas is flowing from the seal.
End users should also ensure that they install all equipment correctly, choose quality pump bases, and follow best practices for eliminating pipe strain and providing hydraulic support on the equipment’s suction and discharge sides.
An Effective Match
Volatile, corrosive and pure processes demand a dry gas seal that provides effective non-contacting, liquid-free sealing. By choosing spiral groove dry gas seals and following an ID to OD sealing arrangement, end users can rest assured that they are providing the best sealing environment possible for their process.
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