Sealing Requirements In The Petroleum Refining Industry

The API Standard 682 is an industrial standard developed by users with input from equipment and seal manufacturers. The goal of the standard was to create a specification for seals that would have a good probability of meeting emission standards defined by the U.S. Clean Air Act of 1990 and have a life of at least three years. The implementation of this specification indicates not only an increase in emissions control, but also a major increase in equipment reliability.

The following contacting, liquid-lubricated seal designs were identified by API 682,1st edition (October 1994) as solutions to sealing refinery services. These have been verified by seal manufacturer tests under simulated refinery conditions. These are as follows:

• Type A A single, pusher-type seal mounted inside the seal chamber with a rotating flexible element. This is a balanced cartridge design with multiple springs and an O-ring as a secondary seal (see Figure 27). This seal is preferred for all refinery services except non-flashing hydrocarbons above 300°F (150°C). It is considered to be the standard for temperatures up to 500°F (265°C).

• Type B A single, low-temperature, non-pusher, inside-mounted seal, with a rotating metal bellows flexible element. The secondary static seals for this nickel allow metal bellows design are fluorocarbon elastomer O-rings. This low-temperature seal design is a standard optional selection for non-flashing hydrocarbon services up to 300°F (150°C).

• Type C A single, high-temperature, non-pusher, inside-mounted seal with a stationary metal bellows flexible element. The secondary static seals for this high-temperature bellows design are flexible graphite. This seal is the standard selection for non-flashing hydrocarbon applications when temperatures are above 300°F (150°C) and pressures are less than 250 lb/in2 absolute (17 bar).

Each of the previous seal types is also available as a dual seal arrangement (see Figure 30). When the space between the inboard and outboard seals is pressurized with a barrier fluid, the seal arrangement is referred to as a pressurized dual seal. When the

space between the inboard and outboard seals is unpressurized with a buffer fluid, the seal arrangement is referred to as a nonpressurized dual seal. This is the only seal cartridge that can function as a double or tandem seal with the same individual seal parts. The terms barrier fluid and buffer fluid refer to the same fluid lubricating the seal. When the fluid is pressurized, it is a barrier fluid. When the fluid is non-pressurized, it is a buffer fluid.

FIGURE 31 API 682 qualification test rig (John Crane Inc.)

The dual seal design shown in Figure 30 is without a pumping ring on the outside diameter of the outboard seal. This figure also represents a successful installation on an HF alkylation unit. In this design, isobutane is circulated at a pressure greater than at the pressure at the outside diameter of the inboard seal. The isobutane is then flushed over the inboard seal to keep the hydrofluoric acid away from the inboard seal. Seal life has been significantly increased with this improved sealing technology.

API 682 requires qualfication testing for all seal designs by the seal manufacturer. To meet these requirements, seal manufacturers constructed new testing facilities that allow testing at simulated refinery conditions for common process fluids. Figure 31 shows an API qualification test rig with instrumentation installed. Each seal type from each seal application group is required to be tested in four different test fluids that model fluids that model fluids from the application groups. These fluids include water, propane, 20 percent NaOH solution, and mineral oil. Each qualification test for each test fluid consists of three phases:

a. the dynamic phase at constant temperature, pressure, and speed b. the static phase at 0 rpm using the same temperature and pressure as the dynamic phase c. the cyclic phase at varying temperatures and pressures, including start-ups and shutdowns. For flashing hydrocarbons, the cyclic test phase includes excursions into vapor and back to liquid.

The seal is expected to perform within the regulated emissions limits after being exposed to qualification testing and upset conditions and demonstrate a capability of at least three years life in service. The result of this effort is not only an improvement in emssions control but also a major improvement in seal reliability. This naturally results in substantially lower life-cycle cost for the user.

The success or failure of a seal installation can often be traced to the selection of the proper piping arrangement. A piping arrangement or plan defines how a seal installation will be cooled or, in some cases, heated. Commonly used systems have been defined by API and are shown in Figure 32.

Performance testing to qualify seal designs to API 682 has resulted in an improved seal flush required for cooling. The mating ring, chamfered at the outside diameter, enables the flow of the flush liquid not only around the circumferential groove, but also directly to the

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