Editor's Note: This is the first in a six-part series on seals. For other articles in this series, click here.
Find a reliable dry gas seal system for any application.
As manufacturers continue to increase production capabilities to meet customer orders, the demand for compressors with higher speeds, higher pressures, lower and higher temperatures and larger sizes is also growing. A properly functioning compressor is critical to optimal plant operation. Compressor failure can bring down an entire plant or bring production to a halt, costing a company $250,000 an hour or destroying millions of dollars of catalyst. Therefore, a reliable, well-designed dry gas seal system included with a compressor can make a difference to a plant’s bottom line.
Typically, compressors with dry gas seals are used in applications in which they are critical parts of the entire process. A well-designed dry gas seal system will prevent extensive damage to the dry gas seal, compressor and process. Compared to pump operations, dry gas seals and compressors usually operate at higher speeds and pressures. Every component of a dry gas seal must be designed to manage exposure to extreme conditions.
Five factors influence the reliability of dry gas seals:
- The seal
- Separation and process seals
- Seal selection
- The dry gas seal panel
- Auxiliary systems
Each factor must be analyzed to identify what is required for the application. When selected correctly, they will provide the best dry gas seal system and deliver the highest reliability achievable from dry gas seals. Not only will this provide reliable dry gas seals with a minimum of six years mean time between repairs (MTBR), it will also reduce utility, maintenance and repair costs.
Improvements Increase Reliability
Over the years, features in dry gas seals have changed, improving the technology. Many small adaptations have taken place to correct the flaws or deficiencies of past dry gas seal systems. These changes range from adjusting the hardness/durometer of an o-ring for handling high-pressure dry gas seals to using polymer gaskets, which have no depressurization limits and will not take a set such as elastomers do. The question is, “Although these are small improvements, how do they affect the reliability of dry gas seals?”
Since the first dry gas seal was developed, tested and installed 32-years ago, the knowledge gained from upgrading oil seals to dry gas seals, analyses of dry gas seal failures and troubleshooting dry gas seal failures at site have identified the need for auxiliary systems and the necessity for improvements in the dry gas seal.
Through all this, many dry gas seal designs were identified as unsuited for the applications in which they were installed—including the use of improper materials of selection that resulted in seal failures, incorrectly installed seals that limited seal life and poor dry-gas-seal panel designs that caused unnecessary compressor shutdowns. In many instances, when designing dry gas seal panels, little consideration has been given to piping configurations, auxiliary systems and utility costs even though all these affect the reliability of dry gas seals.
Choosing Seal Features
Because dry gas seals operate under extreme conditions, choosing the right seal features can substantially increase the reliability by ensuring that they maintain a gap between the rotating seat and the stationary face. To understand this, let’s start with the basic principles of a dry gas seal (Figure 1).
A dry gas seal creates a film of gas (3) between a rotating seat (1) and a stationary face (2). This film of gas (or gap) between the seal faces must be generated and maintained at 150 to 200 micro-inches so the seal faces do not touch and the smallest amount of gas is allowed to leak through the seal.
This gap can be generated in two ways
When applying pressure to the seal
When the rotating seat is turned fast enough to pump gas between the rotating seat and the stationary face
The rotating seat is fixed to the compressor rotor, and the stationary face is connected to the compressor case. The stationary face must adjust axially to maintain the gap as the rotating face/compressor rotor moves and as the compressor case expands and contracts from changes in heat and pressure. To manage axial movement, the stationary face must move freely when required, and as a result, the dynamic secondary sealing design is key to attaining a response to axial changes without resistance or delay. Therefore, a reliable dry gas seal will have a parallel gap of 150 to 200 micro-inches between the rotating seat and the stationary face even when axial movement occurs.