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API 682 provides guidance on selecting mechanical seals for specific applications. This is an informative annex which means that it only provides guidance and is not a requirement of the standard. Although chemical and petrochemical industries are now covered by API 682, this selection procedure does not attempt to cover every application due to the myriad of process fluids involved in these industries. The procedure is a series of steps that directs the user to collect information on the application, regulations, and local requirements. This information is used to select the seal category, type, arrangement, and piping plan.
This article is an excerpt from the paper, "Advancements in mechanical sealing - API 682 Fourth Edition", presented by Michael B. Huebner of Flowserve Corporation, Gordon S. Buck of John Crane Inc., and Henri V. Azibert of Fluid Sealing Association at the 3rd Middle East Turbomachinery Symposium.
While there are some limitations to this approach, it has provided users with solid advice on considerations which must be made while selecting a seal. The Fourth Edition of the standard keeps the current selection procedure but also adds an alternative selection process. One of the primary reasons why a user selects a specific seal arrangement is to mitigate process fluid leakage to atmosphere.
Seals which pump relatively benign process fluid can easily be sealed with a single seal because leakage to the atmosphere is not critical or can be easily controlled. Arrangement 2 seals can provide addition leakage control by capture leakage across the single seal and collecting it for proper disposal. Some small amounts of process fluid may leak to the atmosphere. If no leakage is allowed, a user will often select an Arrangement 3 seal which prevents leakage by virtue of the high pressure barrier fluid.
The larger question is however, when should I use each of these options? How does a user know when leakage is considered hazardous?
In Fourth Edition, an alternative method was presented based on methodology proposed by Michael Goodrich. This alternative method is primarily directed towards selection of the sealing arrangement using Material Data Sheet information. The alternative method takes into account the toxicity of a process fluid and not just its physical properties. The selection is based on the fluid hazard code according to the United Nations Globally Harmonized System of Classification and Labeling of Chemicals (GHS) and the European Union Regulation on the classification, and packaging of substances and mixtures. The substances are categorized in “H” statements and “R” codes.
Tables are provided placing the H statement or R code into a one of four groups. A Seal Selection Logic is then provided based on these groups to select the seal arrangement including the additional need for a floating carbon bushing for Arrangement 1 seals.
A test seal cartridge is specified by the parameters in this column and the representative materials and geometry of its core seal components. In dual seals combinations of face material pairs, types and flexible element positions are possible. b For a specific service, a seal vendor's commercial product only needs to be tested in the representative test fluid. c Default d 2CW-CS and 2NC-CS shall be tested as inner seal, arrangement and containment seal in accordance with I.4.1 and I.4.5. e 3NC-BB, 3NC-FB and 3NC-FF shall be tested as arrangement in accordance with I.4.1 and I.4.6. f Commercially available petroleum based diesel fuel. This seal selection takes into account exposure limits for hazardous or toxic chemicals and mixtures of these chemicals, and is thus a benefit to a broader audience, not just petroleum refining based processes. It is important to note that a hazard assessment is only one criterion which must be considered.
Other consideration such as the fluid properties, dry running of the equipment, seal leakage detection strategies, leakage disposal options and process contamination must also be considered before making a final selection.