Originally published in the April 2012 issue of Pumps & Systems
After more than five years of planning, the American Petroleum Institute (API) is preparing to release the 4th edition of API Standard 682 (ISO 21049:2011). The API 682 standard, which dates back to 1994 and is formally known as Shaft Sealing Systems for Centrifugal and Rotary Pumps, offers specifications and best practices for mechanical seals and systems to pump end users.
The standard’s latest edition began to take shape in 2006, when API formed a 4th edition task force to respond to end users’ questions and comments about previous editions. The task force soon realized that major changes, including reorganization and editing, would be necessary. While addressing every aspect of the resulting 4th edition (which is more than 250 pages long) would be impossible, this article summarizes the standard’s main points.
Those who use API 682 should understand the standard’s scope and remember that the standard does not include specifications for equipment outside that scope, such as engineered seals or mixers. Another important but often misunderstood point is that API 682’s figures are illustrative and not normative in their entirety.
For example, one of API 682’s figures shows a fixed throttle bushing combined with a rotating Type A seal, but seal manufacturers do not always have to combine these two components. The standard provides normative details in clauses and tables to help purchasers distinguish between requirements and suggestions.
Mechanical Seal and System Designs
The 4th edition continues to divide seals into three categories, three types and three arrangements. For all practical purposes, seal manufacturers can combine a seal’s component parts into nearly any orientation or configuration. Each orientation and configuration has advantages and disadvantages with respect to certain applications, performance and system disturbances.
Before the 4th edition, API 682 did not specify a minimum clearance between the inside diameter of a stationary seal part and the outside diameter of a rotating seal part. The 4th edition specifies this minimum clearance—typically the clearance between the sleeve and the mating ring. The specified clearances are representative of standard clearances that end users have used for decades. End users should not consider seal components to be “shaft catchers” to restrict shaft movement. The minimum clearance specified in API 682 also applies only to equipment within the standard’s scope. Equipment outside that scope, such as non-cartridge seals, older pumps, non-API 610 pumps and certain severe services, might benefit from larger clearances.
The new standard also updates the default bushings for the gland plate for the three seal categories. Fixed throttle bushings are now the default for Category 1 only, while floating bushings are the default for Categories 2 and 3.
Seal Selection Procedure
While the 4th edition features the recommended seal selection procedure from the standard’s first three editions, it adds an alternative selection method in Annex A. Proposed by task force member Michael Goodrich, this alternative method recommends using material data sheet information to select a sealing arrangement.
The 4th edition of API 682 provides specifications for piping plans such as the one shown here, Plan 53C.
Once again, seal codes have changed. The new code, described in Annex D, is probably the best to date. It uses eight fields:
- Seal category
- Seal arrangement
- Seal type
- Containment device
- Gasket material
- Face material
- Approximate shaft size (in millimeters)
- Piping plan
For example, based on the 4th edition codes, a seal code of 31B-LIN-075-53A indicates:
- 3 — Category 3 seal
- 1 — Arrangement 1 seal
- B — Type B seal
- L — Floating bushing
- I — Perfluoroelastomer (FFKM) secondary seals
- N — Carbon (versus reaction-bonded silicon carbide)
- 075 — Installed on a nominal 75-millimeter shaft
- 53A — Plan 53A
Standard Piping Plans and Auxiliary Hardware
Annex G of the 4th edition is generally reorganized and includes six new piping plans:
- Plan 03 gives purchasers the option of using the seal chamber’s special features to enhance circulation or venting in the machinery.
- Plan 55 is an externally circulated buffer (unpressurized) fluid for Arrangement 2 seals.
- Plan 65 is now subdivided into 65A and 65B. End users can use Plan 65A to detect an excessive leakage flow rate and Plan 65B to detect a certain amount of cumulative leakage.
- Plans 66A and 66B are new to the standard, although end users have used them previously in pipeline applications. These plans detect and restrict excessive leakage rates in case of an Arrangement 1 seal failure.
The 4th edition now requires Plan 52, 53A, 53B and 53C systems to have a sufficient working volume of buffer or barrier fluid for at least 28 days of operation without refilling. As a point of reference, the default reservoir for Plans 52 and 53A has a three-gallon capacity, or pot, for pump shafts smaller than 2.5 inches and a five-gallon pot for larger shaft sizes. Plan 53C must have the same working volume of fluid as Plan 53A. For Plan 53B, the default bladder and accumulator sizes are five gallons and nine gallons, respectively. The design of Plan 53B systems can be complex, especially when ambient temperatures vary widely, and purchasers should become familiar with the calculations and procedures in the 4th edition’s Annex F tutorial. The new edition also discusses the option of adding a pressure gauge and isolation valve to check the accumulator or bladder’s integrity in a Plan 53B system.
The change from switches to transmitters with local indicators is the most obvious and significant change to the systems’ instrumentation in the 4th edition. End users requested this change, stating that it is consistent with evolving best practices.
Nonetheless, the change from simple switches to transmitters is likely to be controversial and expensive. If purchasers want to continue to use switches, they now have to request them in their specifications or data sheets.
Generally, the updated standard’s approach to accessories is more pragmatic than in previous standards. For example, the standard notes that some instruments may not be exposed to the pump’s maximum allowable working temperature and that it is acceptable for those instruments to have a lower design temperature. The updated standard now also addresses natural draft air-cooled heat exchangers but does not address forced draft systems.
Data Transfer and Data Sheets
The 4th edition has revised the data sheets in Annex C extensively to make them the same for all seal categories. Only two data sheets are included in the 4th edition—one in metric units and one in U.S. customary units. The new edition also folds Annex J into Annex E.
Inspection, Testing and Preparation for Shipment
In an organizational change, Section 10 of the standard—“Inspection, testing and preparation for shipment”—is now divided into two sections. In light of the additional orientations and configurations included in this edition, the standard has revised the qualification test procedures and moved them into Annex I. Section 10 still includes all other requirements.
Section 10 also features a clearer version of the assembly integrity test, or air test. This simple, yet often misunderstood, test is based on two decades of practical experience and is intended to show whether the cartridge is assembled correctly.
Previous editions of API 682 required metal plugs and anaerobic sealants when shipping new or repaired cartridges. After much debate, the task force decided that threaded connection points should be protected with plastic plugs for shipment. These plastic plugs should be red and have center tabs that operators can pull easily to distinguish the plugs from metal plugs. Shippers should also attach yellow warning tags to the plugs to indicate that end users need to remove the plugs before operation.
Technical Tutorials and Illustrative Calculations
Although tutorial notes are scattered throughout API 682, this edition expands the tutorial section, Annex F, from seven pages to 42 pages. The expanded annex includes illustrative calculations. In particular, users interested in systems such as Plan 53B will find Annex F to be useful.
A Comprehensive Guide
The 4th edition of API 682 is the product of more than 20 years of discussion, debate, usage and peer review. It includes a strong set of defaults and is by far the best and most logical starting point for mechanical seal and systems use. Equipment operators should take the time to familiarize themselves with API 682 to get the most out of this comprehensive standard.
Des centaines d’experts en ingénierie. Plus de 200 sites répartis sur la planète. Offrir l’innovation et les services pour vous aider à réussir.