Editor's Note: This is the second in a six-part series on seals. For other articles in this series, click here.
Proper seal choice is critical for optimal compressor operation.
The focus of this installment is how to choose the proper seal features to ensure a reliable dry gas seal system for optimal compressor operation. It covers the improvements made to dry gas seals from the original dry gas seal design and how these improvements offer a more reliable dry gas seal.
To guarantee optimal dry gas seal operation, the gap between the rotating seat and the stationary face must be maintained at 150 to 200 micro-inches. To maintain this gap effectively, the rotating seat and stationary face materials, groove geometries and dynamic secondary sealing designs must be considered in the seal design to produce a seal that will operate reliably.
Seal Seat & Face Materials
Choosing the correct material for seal faces and seats is critical to managing the changing conditions in compressor applications and for minimizing damage to the seal and the compressor.
Tungsten carbide was the original material of choice for the rotating seats in dry gas seals. Tungsten is very tough and has the ability to handle high-applied loads without deforming the material. However, tungsten carbide is not problem-free. When it fails, it can cause significant damage to the seal and the compressor’s rotor. Tungsten is also a heavy material. For rotating applications, the hoop stresses applied to the material are substantial, resulting in limitations for applications at higher speeds.
Figure 1. Comparison of synthetic diamond-like carbon and crystalline diamond coating
As the requirements for higher speeds continue to increase for new applications and compressor designs, other materials were considered for optimal dry gas seal operation. Silicon carbide has proven to be a better material for rotating seats in dry gas seals. It exhibits fewer problems when heat is generated by contact or liquid contaminants between the seal face and seat. Silicon is also a lighter material than tungsten and produces lower hoop stresses, so it can be used for higher- speed applications. Since silicon is a brittle material, more caution is required when handling it. When properly applied in the seal cartridge, it has not shown any causes for concern.
Stationary faces were originally made of carbon graphite, which made them more economical to produce, but temperatures and pressures affected the ability of the material to maintain the flatness of the seal face. Again, silicon carbide was a more suitable material for the application. Silicon carbide had the same material properties as the rotating seat, so deflection and changes in thermal growth were similar and enabled higher constancy in maintaining a parallel seal gap.
Figure 2. Groove Geometries: Comparison of (specified-depth) edged groove and (three-dimensional) tapered grooves.
Diamond Seal Material
Since both the rotating seat and stationary face are made of the same materials when using silicon carbide, a diamond-like coating (DLC) of 35 to 50 micro-inches can be applied to the stationary seal face providing lubricity and hardness to minimize damage when the seal face and seat contact one another. This works well for most applications, but because of the thin layer of coating, any hard material larger than the seal gap will damage the seal face or seat as it passes between them.
For slow-roll applications, the secondary seal face and seat can be in contact continuously. When the secondary seal is subjected to this condition, the seal face and seat will generate heat as the compressor turns, resulting in damage to the secondary seal. This means the secondary seal may not provide the required safety back-up if a primary seal failure should occur.
A new technology was developed to combat problems with contact between seal face and seat and features crystalline diamond that is chemically bonded and grown on the surface, 300 to 400 micro-inches thick (see Figure 1).
Because of the thickness and hardness of the diamond material, any contamination between the seal face and seat will not cause damage to them. This crystalline diamond dissipates heat and reduces resistance when the seal face and seat contact each other—one of the major factors contributing to a dry-gas-seal failure. It also allows a dry gas seal application to manage transient operating conditions.
Now that the material has been selected, the grooves on the surface of the rotating seat must be determined. Grooves can be made in several different designs. What makes one groove better than another groove? The groove design should be efficient at generating a gap between the rotating seat and the stationary face. The groove should be effective at managing debris that will pass between the seal face and seat. If the right groove is used, it will increase the robustness of your seal and provide higher reliability.
An efficiently-operating groove compresses the gas, forcing it between the flat surfaces of the seal and into the sealing dam, generating higher film stiffness. Therefore, a groove should have the basic features of an efficient compressor for compressing the gas.