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.
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 . 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.
• The seal chamber must be an open-frame plate liner with vortex breakers or a closed-frame plate liner designed to prevent excessive erosion.
• A mechanical seal support system must be provided as a pre-engineered turnkey system; it must include all instrumentation and fittings necessary for site installation.
• The tank must have a minimum capacity of 25 liters (6.6 U.S. gallons) and be self-filling. Inboard seal face integrity must be visually confirmable at the support system with a flow indicator.
• The distance between the seal supply and return port should be a minimum of 15 inches to maximize residency time for barrier-fluid heat dissipation.
• The system at all times must deliver barrier fluid at pressure differentials 15 psig (minimum) above the process pressure in the pump stuffing box.
• The seal system must include inline filtration of plant seal water to 1 micron. An internal standpipe on the supply leg, a three-way valve on the return leg, and a blowoff valve at the bottom of the tank must be included to allow clearing the system of any contamination after the initial installation and during its service life.
• As part of the initial supply package, documentation must include a heat generation report for each installation. The report must refer to the operating conditions for the intended shaft diameter, speed, process/barrier pressure, temperature and induced flow. The data must provide the input for a thermal equilibrium estimation and result in a calculation of the heat generated by the specific seal supplied in each case.
As regulatory legislation issues persist, a thoughtful compliance strategy will drive sealing solutions that truly optimize reliability of slurry pumps in virtually all industries.
HEINZ P. BLOCH, P.E., is a Westminster, Colo.-based process machinery consultant. TOM GROVE is executive vice president of AESSEAL, Inc., Rockford, Tenn. E-mail them at email@example.com and firstname.lastname@example.org.
1. "American National Standard for Rotodynamic (Centrifugal) Slurry Pumps for Nomenclature, Definitions, Applications and Operation," pp. 75–77, Hydraulic Institute, Parsippany, N.J. (2011), available via www.pumps.org/standards.
2. "Resource Adequacy Implications of Forthcoming EPA Air Quality Regulations," pp. 1, 6–13, U.S. Department of Energy, Washington, D.C. (Dec. 2011), posted at http://energy.gov/sites/prod/files/2011%20Air%20Quality%20Regulations%20Report_120111.pdf