SEALING SENSE
What are the considerations in applying
mechanical seals to abrasive slurry applications?
Many process industries are confronted with pumping abrasive slurries of various types. This article
will focus on one that deals with some of the most
aggressive, high solids content slurries. Continuing growth
in worldwide demand for industrial metals has increased the
value of nickel, aluminum (bauxite), copper, and other key
ores. As a result, the mineral and ore processing industry
(M&OP) is investing in more efficient processes for extracting and processing these ores to maximize the output of
their operations.
Because many of the operations associated with the
M&OP industry are located in remote regions of the world
where the ore deposits exist, issues like power consumption
and flush water consumption are significant considerations
in selecting the equipment used in these processes.
Pumps are a key component in M&OP processes, serving to move slurries which contain ore particles throughout
the process. In order to reduce their power and flush water
requirements, many mineral and ore processing operations
are considering shifting from packing, their traditional sealing method, to the application of mechanical seals for their
process pumps. Total life cycle cost calculators have received
increasing acceptance in the M&OP market as tools to
quantify the full economic benefit of this shift to mechanical
seals.
In remote locations where water is scarce, flush water can
cost as much as $0.016 (U.S.) per gallon. Because packing
relies heavily on substantial flush rates to maintain adequate
cooling, lubrication, and sealing performance, annual flush
water costs for a single packed pump can be in the range of
$50,000 (U.S.).
Additional costs are incurred to remove the effluent
water from the slurry through evaporation and other processes. Many mechanical seals that are designed for slurry
applications require no seal flush water, resulting in flush
water savings of the total flush water and water removal
costs. In addition to these cost savings, environmental issues,
including water conservation, are growing in prominence
and also becoming key drivers in the use of flushless mechanical seals.
However, the abrasive slurries that are handled by many
M&OP operation pumps require special mechanical seal
designs to ensure reliable performance in these harsh environments. Key design considerations include:
• Face materials
• Adaptive hardware metallurgy
• Seal geometry
• Dynamic O-ring and spring designs
• Drive collar design capable of working with hardened
pump shafts
Face Materials
Most abrasive slurry applications require hard versus hard face
material combinations to avoid premature wearing of the seal
faces. The most common face material in these applications
is direct sintered silicon carbide, because this material offers
abrasion resistance and broad chemical resistance. Tungsten
carbide is also applied in some mining applications, but care
must be taken to ensure that the binder material (typically
nickel or cobalt) will not be leached out of the seal rings by
chemicals used in the processing of the ore.
