Specify the Right Slurry Seal

Consider new dual-seal and water-management options.

By Heinz P. Bloch, consultant, and Tom Grove, AESSEAL Inc.

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Leading processors have had success with hybrid solutions whereby Plans 53 and 54 are combined and water comes from a self-contained water management system (Figure 3). The system is designed to control pressure and cool the seal faces; it uses a regulator and a backflow preventer to set the correct water-barrier pressure for the seal faces. The water is recirculated, reducing actual consumption to just a few gallons per year. An inline filter connected to the continuous source of water filters the barrier fluid to 1 micron absolute. A three-way valve in the line returning from the seal to the reservoir enables the operator to inspect the condition of the barrier fluid in the seal without compromising seal performance. Should any particles cross the inboard seal face, the three-way valve is activated to flush the seal. An internal standpipe on the supply line to the seal protects the seal from contaminants. By connecting a valve and drain line to the bottom of the tank, an operator can purge contaminants from the reservoir while the connected water source automatically replenishes the system with clean water. If process air bubbles accumulate at the seal face, the secondary liquid provides sufficient cooling to ensure consistent seal performance. The size of the seal pot and the positioning of the inlet and outlet ports determine the level of heat dissipation by the support system. Independent control of the seal environment broadens the success margin for the seal.

A recent U.S. Environmental Protection Agency rule tightened the caps on sulfur and nitrogen oxide emissions. As a consequence (although appeals are pending), processors are giving ever-closer attention to equipment reliability and efficient use of existing pollution-control technologies. In late 2011, the U.S. Department of Energy outlined "near-term compliance pathways," highlighting the need for increased utilization and reliable performance of wet and dry flue-gas desulfurization (FGD) units [2]. Of course, selecting reliable mechanical seals is of critical importance, not only in FGD, but also in the majority of other slurry applications. New dual-seal and flush-water management options allow users to upgrade from maintenance-intensive packing to highly reliable mechanical seal alternatives. Moreover, in large pump sizes, designs that allow seal installation from the wet end of the pump, which will minimize the cost of overhaul, deserve to be considered.

Plant reliability professionals should consider bridging the distinct operating parameters of numerous slurry-containing processes with existing industry standards for slurry sealing. To incorporate the options outlined above requires important amendments to current equipment standards. The add-on wording should state:

• The mechanical seal must be a heavy-duty dual-cartridge mechanical seal suitable for slurry duty and designed to operate at all times at a higher pressure than the process pressure.
• Seal internal cross-sections must have large radial clearances; the inboard face set must be hydraulically balanced to the barrier fluid.
• Tungsten carbide (TC) and/or silicon carbide (SiC) faces matched with solid TC faces must be used when the pH is greater than 5; solid SiC must be used when the pH is 5 or less. Pin drives must be designed to minimize face fracturing.
• Wetted alloys must be super duplex for abrasion resistance.
• Mechanical seals must perform equally with or without impeller back-vanes; the user requests that back-vanes be incorporated in the equipment impellers.

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