.Dry gas seals have really gained popularity over the last 20 years. Being the seal system of choice for the process gas centrifugal compressors thus increasingly replacing the traditionally complex oil fill seals. It is estimated that almost 80% of the process gas centrifugal compressors manufactured today are equipped with dry gas seals, implying acceptance within the users of this comparatively new technology.
One factor that plays a role in the increasing popularity of the dry gas seal is the seal gas heating and conditioning system. The system ensures that seal gas is heated to a temperature that eliminates the moisture and conditions to a specification free of contaminants. Thereby improving the longevity and reliability of the seal. Contamination is a major issue in the dry gas seal failure. So it is a priority when designing the compressor seal system.
Centrifugal compressors which compress process gas like methane or natural gas in a booster role or for any process gas compression, need a seal system to prevent the gas from escaping along the shaft into the atmosphere. Sealing along the shaft is critical because some of these process gases are flammable, corrosive, poisonous and their escape into the atmosphere contributes to global warming.
In a multistage compressor with a beam-type configuration there is a requirement for two seals, one at each end. Whereas in an “overhung” configuration only one cartridge type seal is necessary.
Dry gas seals are essentially like wet mechanical seals but with one fundamental difference. They have shallow gas grooves which generate aerodynamic lift-off force. This creates a “film” of gas between the rotating face and stationary face.
Another important element of the seal is the secondary sealing element. This can be an O-ring or a PTFE-filled sealing gasket. When the compressor is rotating, or when pressure is applied, the force keeps the seal faces together. Dry gas seals, however, use a design with grooves on the face. This generates the liftoff force. So there is no contact between the rotating seal and stationary face and there is a gap of 3 µm to 5 µm between the two faces. The grooves on the seal face can be in a spiral pattern. But, usually, the pattern and location are dependent on the seal supplier and varies between the designs.
The biggest advantage of dry gas is the removal of the seal oil which is needed in the wet gas seal application. This system is a complex assembly in standalone configuration and becomes more complicated when it is part of the seal and lube system of the process gas centrifugal compressor.
With wet gas seals the seal oil which was also used for lubricating the compressor bearings would find its way into the gas stream resulting in a costly disposal process for the oil and oil separation system that had to be installed downstream of the centrifugal compressor. Although the net loss of oil is less, the seal loses gas both in the form of gas in oil (liquid dilution) and gas leakage into the environment.
One of the limitations of a dry gas sealing system is its dependence on the buffer gas, typically nitrogen. The gas ensures the seals work properly so a reliable supply of buffer gas is important. A loss of supply even for a short duration can result in seal damage. Technically, a seal can operate in a zero-differential pressure, but manufacturers prefer little differential pressure.
Another limitation is the supply of clean buffer gas free of moisture and contaminants. Typical dry gas seals have a tandem configuration where the first seal is the primary seal and a second acts as the secondary or backup seal. It is a requirement for reliable operations of these seals that high purity gas is provided. The buffer gas must be conditioned to remove impurities and heated to prevent moisture from entering the seal as well as prevention of condensation when the buffer gas enters the seal. if the gas condenses the pressure in the seal is lost and the process gas can escape.
Heating of the dry seal gas is important for maintaining the reliability of the seal. This is usually accomplished by an immersion heater with either direct heating by circulation or indirectly by immersion heater immersed in the oil bath or aluminum casting with the process coil.
These immersion heaters can operate at voltages up to 690V and power rating <50 KW. For operational safety and operation in the explosive environment they come with CSA C/US, ATEX, IECEX, UL, and EAC certifications. All flow wetted parts are manufactured commonly of SS 304 stainless steel for indirect heaters. Although different other metallurgies (Inconel, SS 316) are available for direct application.
Due to the ability of electric immersion heaters to control the temperature precisely there is an integral temperature sensing element at the outlet to control the temperature through a feedback loop.
A dew point analysis of the seal gas is necessary if the seal gas is a mixture of gases. Maintaining the dew point margin at operating ranges is important, especially pre-startup when the compressor is cold. The seal gas passes through the electric immersion heater. There it is heated to temperature maintaining margin from the dew point.
Once heated, the seal gas piping is also heat traced and insulated so that there is minimum heat loss across the piping.
The advantages of dry gas seals surpass their limitations and in the presence of a reliable conditioning and heating system with electric immersion heaters most of the limitations can be overcome. Nowadays dry gas seals are favored more for new installation and for retrofit of the older process gas centrifugal compressors because of higher oil consumption in the wet gas seals which also can result in intrusion of oil in the process gas stream resulting in costly preventive maintenance programs and ability to simply by eliminating the complicated lube and seal oil circuits.
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