Essential Knowledge About Air Compressor Seals
1. Functions of Seals
For centrifugal compressors to achieve satisfactory operation, a certain clearance must be maintained between the rotor and stator to avoid friction, abrasion, collision and damage accidents. Meanwhile, such clearances inherently cause inter-stage and shaft-end leakage. Leakage not only reduces compressor efficiency, but also leads to environmental pollution and even explosion hazards. Therefore, leakage must be prevented.
Seals are effective components to avoid inter-stage and shaft-end leakage while maintaining appropriate clearance between the rotor and stator.
2. Classification of Sealing Devices by Structural Features & Selection Principles
Different structural types of sealing devices are adopted according to compressor operating temperature, pressure and hazards of process gas.
Sealing devices are categorized into five types by structure: extraction type, labyrinth type, floating ring type, mechanical type and spiral type.
Floating ring seals, mechanical seals, spiral seals and extraction seals shall be selected for toxic, hazardous, flammable and explosive gases.
Labyrinth seals can be used for non-toxic, harmless gases under low pressure rise conditions.
3. What Is a Gas Seal?
A gas seal is a non-contact seal that uses gas medium as lubricant. Through elaborate structural design of sealing components and performance optimization, leakage can be minimized.
Characteristics and Sealing Principle
(1) The seal housing and rotor are relatively fixed.
Sealing blocks and sealing dams are arranged on the end face of the seal housing opposite the primary ring (primary sealing face). The sealing blocks vary in size and shape. When the rotor rotates at high speed, pressure is generated in the injected gas, pushing the primary ring open to form gas lubrication. This reduces abrasion on the primary sealing face and minimizes medium leakage. The sealing dam prevents gas escape during shutdown.
(2) This type of seal requires a stable sealing gas supply. The gas can be process medium gas or inert gas. In either case, the gas must be filtered and purified.
4. How to Select Dry Gas Seals
Tandem dry gas seals with intermediate gas injection are adopted under working conditions where neither process gas leakage to atmosphere nor barrier gas ingress into the compressor is permitted.
Conventional tandem dry gas seals are suitable for conditions allowing a small amount of process gas leakage to atmosphere. The primary seal on the atmospheric side serves as a backup safety seal.
5. Main Function of Primary Seal Gas
Primary seal gas prevents unclean gas inside the centrifugal compressor from contaminating the primary sealing faces. As the compressor rotates at high speed, gas is pumped through spiral grooves on the primary sealing faces into the primary seal vent flare chamber. A rigid gas film forms between sealing faces to lubricate and cool the faces.
Most of the gas flows back into the compressor via the shaft-end labyrinth, while only a small portion passes through the primary sealing faces into the flare vent chamber.
6. Main Function of Secondary Seal Gas
Secondary seal gas prevents a small amount of medium gas leaking from the primary sealing faces from entering the secondary sealing faces, ensuring safe and reliable operation of the secondary seal.
Most secondary seal gas mixes with leaked gas from the primary seal and flows to the flare pipeline via the primary seal flare chamber. Only a small volume passes through the secondary sealing faces into the secondary seal vent cavity for high-point venting.
7. Main Function of Barrier Gas (After Seal Isolation Gas)
Barrier gas prevents lubricating oil from the compressor bearings from contaminating the secondary sealing faces.
Part of the barrier gas flows through the internal labyrinth of the after seal, mixes with minor leakage gas from the secondary seal and is discharged via high-point vent. The other part flows through the external labyrinth of the after seal and vents at the bearing lubricating oil drain port.
Precautions Before Commissioning of Dry Gas Seal System
(1) Switch on barrier gas supply 10 minutes prior to starting the lubricating oil system. Similarly, barrier gas can only be cut off 10 minutes after the lubricating oil system stops. Once oil circulation starts, barrier gas supply must not be interrupted; otherwise, seal damage will occur.
(2) When putting a filter into service, slowly open the upper and lower ball valves. Rapid opening may cause instantaneous pressure shock and damage the filter element.
(3) Slowly open the upstream and downstream ball valves of the flowmeter during commissioning to maintain stable flow.
(4) Check whether the supply pressures of primary seal gas, secondary seal gas and barrier gas remain stable, and verify if any filter is clogged.
For centrifugal compressors to achieve satisfactory operation, a certain clearance must be maintained between the rotor and stator to avoid friction, abrasion, collision and damage accidents. Meanwhile, such clearances inherently cause inter-stage and shaft-end leakage. Leakage not only reduces compressor efficiency, but also leads to environmental pollution and even explosion hazards. Therefore, leakage must be prevented.
Seals are effective components to avoid inter-stage and shaft-end leakage while maintaining appropriate clearance between the rotor and stator.
2. Classification of Sealing Devices by Structural Features & Selection Principles
Different structural types of sealing devices are adopted according to compressor operating temperature, pressure and hazards of process gas.
Sealing devices are categorized into five types by structure: extraction type, labyrinth type, floating ring type, mechanical type and spiral type.
Floating ring seals, mechanical seals, spiral seals and extraction seals shall be selected for toxic, hazardous, flammable and explosive gases.
Labyrinth seals can be used for non-toxic, harmless gases under low pressure rise conditions.
3. What Is a Gas Seal?
A gas seal is a non-contact seal that uses gas medium as lubricant. Through elaborate structural design of sealing components and performance optimization, leakage can be minimized.
Characteristics and Sealing Principle
(1) The seal housing and rotor are relatively fixed.
Sealing blocks and sealing dams are arranged on the end face of the seal housing opposite the primary ring (primary sealing face). The sealing blocks vary in size and shape. When the rotor rotates at high speed, pressure is generated in the injected gas, pushing the primary ring open to form gas lubrication. This reduces abrasion on the primary sealing face and minimizes medium leakage. The sealing dam prevents gas escape during shutdown.
(2) This type of seal requires a stable sealing gas supply. The gas can be process medium gas or inert gas. In either case, the gas must be filtered and purified.
4. How to Select Dry Gas Seals
Tandem dry gas seals with intermediate gas injection are adopted under working conditions where neither process gas leakage to atmosphere nor barrier gas ingress into the compressor is permitted.
Conventional tandem dry gas seals are suitable for conditions allowing a small amount of process gas leakage to atmosphere. The primary seal on the atmospheric side serves as a backup safety seal.
5. Main Function of Primary Seal Gas
Primary seal gas prevents unclean gas inside the centrifugal compressor from contaminating the primary sealing faces. As the compressor rotates at high speed, gas is pumped through spiral grooves on the primary sealing faces into the primary seal vent flare chamber. A rigid gas film forms between sealing faces to lubricate and cool the faces.
Most of the gas flows back into the compressor via the shaft-end labyrinth, while only a small portion passes through the primary sealing faces into the flare vent chamber.
6. Main Function of Secondary Seal Gas
Secondary seal gas prevents a small amount of medium gas leaking from the primary sealing faces from entering the secondary sealing faces, ensuring safe and reliable operation of the secondary seal.
Most secondary seal gas mixes with leaked gas from the primary seal and flows to the flare pipeline via the primary seal flare chamber. Only a small volume passes through the secondary sealing faces into the secondary seal vent cavity for high-point venting.
7. Main Function of Barrier Gas (After Seal Isolation Gas)
Barrier gas prevents lubricating oil from the compressor bearings from contaminating the secondary sealing faces.
Part of the barrier gas flows through the internal labyrinth of the after seal, mixes with minor leakage gas from the secondary seal and is discharged via high-point vent. The other part flows through the external labyrinth of the after seal and vents at the bearing lubricating oil drain port.
Precautions Before Commissioning of Dry Gas Seal System
(1) Switch on barrier gas supply 10 minutes prior to starting the lubricating oil system. Similarly, barrier gas can only be cut off 10 minutes after the lubricating oil system stops. Once oil circulation starts, barrier gas supply must not be interrupted; otherwise, seal damage will occur.
(2) When putting a filter into service, slowly open the upper and lower ball valves. Rapid opening may cause instantaneous pressure shock and damage the filter element.
(3) Slowly open the upstream and downstream ball valves of the flowmeter during commissioning to maintain stable flow.
(4) Check whether the supply pressures of primary seal gas, secondary seal gas and barrier gas remain stable, and verify if any filter is clogged.
