With even tighter caps on sulphur and nitrogen oxide emissions coming into place, selecting reliable mechanical seals in the majority of slurry applications is becoming ever more important. This article gives a brief background to slurry sealing trends and an overview of important amendments to current equipment standards for slurry sealing.
A recent EPA rule replaces and even tightened the caps on sulphur and nitrogen oxide emissions. As a consequence and although appeals to the United States Supreme Court are pending, equipment reliability and efficient use of existing pollution control technologies is receiving ever closer attention. In late 2011, the US Department of Energy outlined ‘near-term compliance pathways’ highlighting the need for increased utilization and reliable performance of Wet and Dry Flue Gas Desulphurization (FGD) units (Ref. 1)*. 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.
Historical perspective is of interest
In the 1990s, slurry sealing trends moved strongly from braided packing toward single-type heavy-duty mechanical seals (Ref. 2*). Still, mechanical seals are often among the first components to fail and their performance can be especially unpredictable whenever fine abrasive slurries are allowed to migrate into the seal faces. It is good to know that remedial action is feasible and that best-of class companies, are getting results by specifying dual seas.
Today, top reliability professionals select these dual seal 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 (Ref. 3*). 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 to improve safety. The standard further notes that dual pressurized seals are used when the limits (of heavy-duty single mechanical seals) are exceeded, when there is a potential for entrained air in the slurry, or when large volumes of air can be introduced into the pump.
A dual pressurized seal design eith two sets of seal faces: here the process slurry or impure pumpage is contained by an inboard set of faces and 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-range particle size contaminants are mitigated because the seal face operating environment is clean water at 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 re-charged during operation. This re-charging process is not operator-friendly and there is 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 must always be pressurized 15 to 30 psig above any seal chamber pressures to avoid cross contamination of the barrier fluid.
Leading user companies have success with hybrid solutions whereby Plans 53 and 54 are combined and water is used in a self-contained Water Management System. The system is designed to control pressure and cool the seal faces. The system uses a regulator and a back-flow preventer to set the correct water barrier pressure for the seal faces. The water is re-circulated, reducing actual consumption to just a few gallons per year. An in-line filter connected to the continuous source of water filters the barrier fluid to 1 micron absolute.
A 3-way valve on 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. In the event any particles cross the inboard seal face, the 3-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. Independent control of the seal environment broadens the success margin for the seal.
Plant reliability professionals will consider bridging the distinct operating parameters of numerous slurry-containing processes with existing industry standards for slurry sealing. Their add-on wording will incorporate the options outlined above and these options will constitute an important amendment to current equipment standards:
• The mechanical seal must be a heavy-duty dual cartridge mechanical seal suitable for slurry duty and designed to operate at a higher pressure than the process pressure at all times.
• Seal-internal cross-sections must have large radial clearances and 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. • In large pump sizes consideration must be given to designs that allow seal installation from the wet-end of the pump, which will minimize the cost of overhaul.
• Wetted alloys must be Super Duplex for abrasion resistance.
• Mechanical seals must perform equally with or without impeller back-vanes and 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 turn-key system; it must include all instrumentation and fittings necessary to install at site.
• The tank capacity must be a minimum of 25 l (6.6 US gal) and self-filling. Inboard seal face integrity must be visually confirmable at the support system with a flow indicator.
• The system must deliver barrier fluid at pressure differentials 15 psig (minimum) above the process pressure in the pump stuffing box at all times.
• The seal system must include in-line filtration of plant seal water to 1 micron. An internal standpipe on the supply leg, a 3-way valve on the return leg, and a blow-off valve at the bottom of the tank must be included to allow clearing the system of any contamination after the initial installation and during the life of the application.
• 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.