In the 1990s, slurry-sealing trends moved strongly from braided packing toward single-type heavy-duty mechanical seals. Still, mechanical seals often are among the first components to fail; whenever fine abrasive slurries can migrate into the seal faces, the performance of mechanical seals becomes especially unpredictable. Yet, virtually all modern process plants place great importance on reliable mechanical seals. Facilities experiencing repeat failures may find that better slurry seals now available enable effective remedial action. For mild slurries, single-type heavy-duty mechanical seals with springs located away from the pumpage (Figure 1) often suffice. For more difficult slurry services, best-in-class companies typically specify dual seals (Figure 2).
Today, top reliability professionals select these dual seals by invoking and further amplifying the American National Standards Institute (ANSI)/Hydraulic Institute (HI) Rotodynamic (Centrifugal) Slurry Pump Standard ANSI/HI 12.1-12.6-2011 . Section 12.3.8 of this standard describes general arrangement details for mechanical shaft seals. It states that dual pressurized seals have the advantage of providing enhanced lubrication to the faces with a pressurized barrier fluid. This arrangement prevents process fluid leakage to the atmosphere and so improves safety. The standard further notes that dual pressurized seals are used when the limits of heavy-duty single mechanical seals are exceeded, when air potentially can be entrained in the slurry, or when large volumes of air can be introduced into the pump. Experience shows that both the seal environment and seal face materials must be carefully selected for the service. Buffer fluid pressurization requirements and associated controls are important as well.
Figure 2 depicts a dual pressurized seal design. The inboard set of seal faces contains the process slurry or impure pumpage; a secondary barrier fluid (clean water) is pressurized higher than the process stream. An outboard set of seal faces confines the clean barrier fluid. The higher pressurization means the secondary barrier forms the inboard-seal-face fluid film. Seal face failure risks normally originating with micron-size-range particle contaminants are mitigated because the seal-face operating environment is clean water at a stable pressure.
Delivery of the water barrier fluid is important to application success. Traditional piping configurations are API Plan 53-A and API Plan 54. Plan 53-A is limited by a fixed volume of barrier fluid; a fluid-containing vessel or "seal pot" is externally pressurized by air or nitrogen. During process upset conditions, the pressurized volume of fluid crosses the inboard seal face, and the seal pot must be recharged during operation. This recharging process is not operator-friendly — so there's high likelihood the seal will run dry. Plan 54 is a centralized water-barrier distribution system, usually through multiple pumps. This means the circulating system always must be pressurized 15 to 30 psig above any seal chamber pressures to avoid cross contamination of the barrier fluid.