Dry gas seals in API 682 and API 617 Standards
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Dry gas seals are generally non-contacting, dry-running face seals, mainly used in high speed applications. Natural gas compressors in a 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 a danger of seal faces overheating at high speed, especially if high speed is combined with high pressure. A common 6500 rpm 70 bar application is very tough for a wet seal to handle. At such high pv factors, a mechanical seal must have certain allowed leakage of the sealed product to prevent overheating and failure. In this case it would be mineral oil at pressure several bars higher than that of the transferred gas as per API 682 flush plan 54. The normally leaking oil is directed back to the oil tank, but not all of it may be collected. Part of the leaked oil will get “into the process“, that is the transferred gas contaminating it. In some industries 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 in an oil refinery.
In a dry gas seal normally 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. The buffer gas, usually 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. There certain applications where these seals are desirable. A good example would be sealing a cold (sub-zero) fluid with an inboard wet seal and an outboard non-contacting seal. Nitrogen would prevent icing of the inner parts of the seal and also would eliminate problems associated with arranging a buffer fluid seal system at low temperatures. Still, there are not many applications where dry gas seals are used for sealing pump shaft while sealing centrifugal compressor shafts with dry gas seals has become an industry standard.
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Author Resource:-
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By :
Jai Prakash Srivastava
Submitted
2012-03-29 17:25:38 |
Article From Article Mayhem
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