🎧 Podcast Link and Guest Information
This blog post is inspired by a deep-dive conversation on the implementation, benefits, and challenges of this crucial technology in a recent podcast.
- Podcast Link: https://feeds.acast.com/public/shows/6696a51513ea555fe3519605
- Podcast Guest: The expert guest for this insightful episode was Ken Tacon, Rotating Equipment Consultant at Ormond Rotating Equipment
Dry gas seals are crucial in modern process industries, replacing older “wet seals” over the past four decades. These sophisticated shaft seals, found in centrifugal, axial, and screw compressors, prevent gas leakage where the compressor shaft exits the casing. Their design involves a rotating and a stationary seal ring, creating a minuscule leakage path of about four microns, significantly reducing gas leakage.
While widely adopted recently, the technology has a longer history. John Crane is associated with pioneering the spiral groove concept, though the first dry gas seal was patented by Caden Ring and Seal Company in 1951. John Crane was instrumental in making them a viable alternative.
Dry gas seals offer several advantages:
- Enhanced Safety: Tandem seal arrangements significantly reduce gas leakage risks, unlike wet seals where gas accumulation in the oil reservoir can create an explosive atmosphere.
- Cost Efficiency: Despite complex support systems, dry gas seals have lower capital costs and comparable or lower energy consumption than wet seals, making them more economical long-term.
- Environmental Responsibility: Wet seals have much higher leakage rates and require disposal of contaminated oil, issues eliminated by dry gas seals.
However, dry gas seals present challenges:
- Seal Gas Quality: They are highly sensitive to clean, dry seal gas. Particulate matter or liquid droplets can cause damage or failure. API 692 recommends a one-micron coalescing filter.
- Maintaining Dryness: The seal gas must remain gaseous, requiring detailed phase diagram analysis and a minimum 20-degree superheat margin to prevent liquid dropout.
- Pressurized Hold Conditions: During shutdowns, raw process gas can migrate into the seal, causing liquid dropout and seal hang-up.
- Separation Seal Integrity: Failure of the separation seal can lead to unrevealed secondary seal failure, necessitating adequate instrumentation.
- Reverse Pressurization: Dry gas seals are designed for positive pressure gradients; back pressure valves are used to prevent issues from reverse pressurization.
- Startup and Commissioning Risks: High risk of contamination and maloperation during initial phases.
- Comprehensive Operating Procedures: Well-defined procedures are vital for all operating conditions.
Dry gas seals are remarkably reliable, with some running over 15 years. This longevity depends on a consistently clean and dry seal gas supply. The seal gas support system is critical, and seals are most vulnerable during transient conditions. With proper maintenance, the life-limiting component is typically the elastomers, with eight years of operation being reasonable.
Early dry gas seals were unidirectional, posing challenges with interchangeability. Major manufacturers developed symmetrical groove patterns for bidirectional operation, which are now widely accepted.
Terminology can be confusing:
- Primary Seal: Closest to the process.
- Secondary Seal: Furthest from the process.
- Buffer Gas: Supplied to the process side of a double seal to keep untreated process gas away from seal faces (not applicable to tandem seals).
- Seal Gas: Supplied to the high-pressure side of a primary seal, flowing through the seal faces.
- Secondary Seal Gas: Supplied between primary and secondary seals of a tandem dry gas seal, typically nitrogen.
- Separation Gas: Air or inert gas supplied to the separation seal, creating a barrier between the bearing housing and the dry gas seal.