Dry Gas Seal: An Introduction
Dry gas seal provides sealing system so it allows the centrifugal compressor shaft (and all connected part) to rotate without leakage of the process gas along the shaft in the atmosphere. For this purpose, seal gas is injected in to the compressor to provide a barrier between the process gas and dry gas seal. At dual seal configuration inert gas i.e., nitrogen may be injected to the secondary outboard seal.
Fig 2 – Groove design, Unidirectional (Left) and Bidirectional (Right)
The sealing mechanism is comprised of two rings called static and shaft-mounted rotating face. Static face is flat and the rotating face has groove pattern which is during rotation the groove will create a thin gap (lift-off) between faces. The gap or clearance between rotating and static part is designed to be around 2.5-5 microns and the seal gas is injected in to this gap to provide lubrication and prevent leakage of the process gas from the casing along with the shaft.
Single seal with inboard labyrinth usually applied in nitrogen, air, CO2 or other nonhazardous services
It is two seals in line with the same orientation. The second outboard seal is designed for full sealing pressure
Tandem Seal with Intermediate Labyrinth
It is two seals in line with an intermediate labyrinth to safeguard and reduce sealing pressure
of the secondary outboard seal. It is commonly used in hydrocarbon applications.
It is two opposite, face-to-face seals with clean gas injected in between at higher than the sealing pressure. Used in applications where absolutely no leakage to atmosphere is acceptable (e.g., toxic gases).
Seal Gas Requirement
Seal gas which is injected in to the dry gas seal (which has around 2.5-5-micron clearance) shall be clean and dry and the specific requirement will be specified by the manufacturer. The seal gas requirement is critical to the seal safety and life. Therefore, in the compressor package there is seal gas conditioning system either it is provided by compressor manufacturer or supplied by the purchaser. In the case seal gas conditioning system is purchased by the purchaser, the requirement shall be from the compressor vendor as this is specifically serve the compressor.
The following is typical requirement of the seal gas just to provide general visualization only. Specific requirement and value shall refer to the manufacturer.
- Seal gas must be filtered process gas or nitrogen. No particle larger than 2 microns.
- It must be free liquid and no residue
- The pressure shall be certain pressure above compressor suction pressure but shall be below pressure rating of the seal gas system
- Seal gas temperature shall be certain temperature as required in the manufacturer document.
- No hydrate or solid formation when expanding to atmospheric pressure
Source of Seal Gas
A normal source of seal gas is from the compressor discharge and it is subject to seal gas conditioning to ensure the supplied gas is sufficiently clean and dry over the complete range of operating condition (also to fulfill above typical requirement).
However, this gas source from the compressor discharge is only available when the compressor is running. During start up, shut down and pressurized hold, for example, seal gas will be not available from the compressor discharge either it is isolated from the seal gas conditioning package or does not have sufficient pressure.
So it is shall be part of the design that alternative source of supply need to be provided during abnormal operating condition.