Dry gas seals are generally non-contacting, dry-running face seals, mainly used in high speed applications. Compressors in a natural gas transfer and distribution network is a common example where dry gas seals are routinely used.
The problem with an ordinary contacting mechanical seal or “wet” seal is the risk of seal faces overheating at high speed, especially if high speed is combined with high pressure. A common 6500 rpm 70 bar application would be very tough for a wet seal to handle. At such high pv factors, a mechanical seal must have bigger allowed leakage of Plan 54 barrier fluid to prevent overheating and failure. The normally leaking oil is directed back to the oil tank, but not all of it may be collected. Some quantity of the barrier fluid will get into the process (through compressor and into the pipeline), contaminating the transferred gas. In some applications it is “just” loss of oil, easily 1-3 barrels per day. In other industries, contamination with oil involves other cost, for example, contamination of catalyst by a hydrogen recycle compressor at an oil refinery would be to expensive.
Contrary to "wet" mechanical seals, in dry gas seals there is no contact between the faces so overheating, or actually, any frictional heat, does not occur. There is a very small gap that separates the rotary and stationary faces. How does it happen? The rotary face or faces have shallow, often spiral, grooves, which “catch” gas and under the action of the generated pressure the faces lift one against the other. In a double seal barrier gas, often nitrogen, also gets into the process. Most often it does not create a problem and double seals are used then. If nitrogen is not allowed to get into the process then tandem or triple gas seals are used.
All manufactured dry gas seals are subjected to a dynamic performance test. During the test the seals are running under normal (design) and abnormal conditions, as per API 617 Standard.
Since dry gas seals are used mainly in compressors, they are described in API 617 Standard – a standard for compressors. But because dry gas seals have found some use in pumps as well, they are also described in API 682 Standard, which is a standard for mechanical seals for pumps.
Speaking of use in pumps, dry gas seals or non-contacting seals (or a combination of contacting wet and non-contacting faces) for pumps are still considered a luxury while sealing centrifugal compressor shafts with dry gas seals has become the industry standard.
