Accessories to mechanical seals

Barrier fluid system with bladder accumulator by Eagle Burgmann[/caption]

Seal accessories can be defined as any piece of hardware which is required to support the mechanical seal or the seal piping plan. These include such items as an orifice, seal cooler, or seal barrier fluid reservoir. API 682 has historically defined design characteristics of accessories and, over time, these have increased in scope to cover more accessories. In addition, the level or detail and specificity of the requirements have increased. The Fourth Edition of API 682 carries over most of the requirements from previous editions but has added these new accessories.

This article contains excerpts 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 Middle East Turbomachinery Symposium in Feb 2015.

Air Coolers

Air cooling is a useful alternative for piping plans when external utilities (such as cooling water) are not available. Air coolers are also often the only solution when high temperatures fluid must be cooled due to the potential of fouling in water cooled seal coolers. The standard places many of the same requirements on water cooled and air cooled seal coolers such as tags (venting of Plan 23 systems), tubing (minimum 0.500”, 0.065 wall 316 stainless steel), and over pressurization protection. In addition, seal cooler sizing is now based on application conditions and not the pump shaft size as was done in previous editions. Fins may be aluminum or stainless steel.

Strainer

While strainers have been used sparingly in most seal applications, they are supported in the defined piping plans. Strainers are limited to minimum mesh size 125um.

 Reservoirs

Reservoirs have specific requirements which have been carried over from previous editions. These include materials of construction, port locations, instrumentations, and dimensions. The standard specified that the working volume of the barrier or buffer fluid must be either 3 gallons (for shaft sizes smaller than 2.500”) or 5 gallons (for shaft sizes greater than 2.500”). In the Fourth Edition, the size requirement is expanded to require that working volume is sufficient for a minimum of 28 days of operation without the need to add additional barrier or buffer fluid.

Bladder Accumulators

Bladder accumulators are used to provide pressurization of the barrier fluid in Plan 53B systems. The expansion of the bladder allows the system to make-up lost barrier fluid leakage while providing feedback on seal performance through a pressure drop in the system. One of the challenges in selecting a bladder accumulator is selecting a size which allows for longer periods of time without operator intervention while not experiencing a large fluctuation in pressure. Annex F in the Fourth Edition is an excellent tutorial on how to size, pre-charge, and operate a Plan 53B system in operation. To support these efforts, the standard will define some basic characteristics of an accumulator. Standard sizes are 20 L [5 gal] and 35 L [9 gal] depending upon shaft size. These sizes were selected to provide a minimum of 28 days of operation without operator intervention. The shell of the accumulator shall be carbon steel and the bladder material will be recommended by the manufacturer based on available options and operating conditions. Because Plan 53B pressures can vary with ambient temperatures, a pressure alarm with a temperature bias is recommended. Tags and labeling requirements are also included.

 Piston Accumulator

A piston accumulator is used to provide barrier fluid pressurization in Plan 53C systems. This consists of a piston with different hydraulically loaded areas which provides pressurized barrier fluid based on a reference pressure in the pump. The accumulator is defined in two sizes: maximum 2,8 L [0.7 gal] for shaft sizes 60mm or less and maximum 5,1 L [1.28 gal] for shaft sizes larger than 60mm. The metallic material should be the same as the seal gland and the gasketing elements (O-rings, lip seals) shall be suitable for exposure to both the process and barrier fluid.

Collection Reservoir for Liquid Leakage

Liquid leakage which leaves the seal gland can be collected with a Plan 65 and Plan 75. The Condensate Collection Reservoir, used with a Plan 75, had been defined in previous editions. Even though the Plan 65 has been defined and used extensively in some industries, there has been no attempt to create a definition for a standard Plan 65 detection vessel. The Fourth Edition defines that the Plan 65 system is considered part of the pressure boundary and is subject to pressure requirements of the rest of the seal support system. The reservoir shall have a capacity of at least 3 L [0.75 gal] and be equipped with a locally indicating level transmitter. The collection reservoir should be constructed from schedule 40 pipe.