Flush-less Slurry Seal Applications
I have been involved with mechanically sealing slurry seal applications for the past 20+ years. It was somewhat of a mystery, in the beginning, to seal a slurry. Do you use a typical pusher or metal bellows seal with an outside flush? Do you use a conventional mechanical seal if the solids content is not too high?
What constitutes a slurry? In my experience, it can be anything from 5%-50% solids by weight. The solids can be suspended or dissolved. Slurry applications for mechanical seals typically possess suspended solids. This can be determined by taking a sample of the slurry and placing it in a jar on your desk; if the solids separate from the liquid in an hour or less, you have a workable application where a “flush-less” slurry seal can work. Mechanical slurry seals operate on a liquid fluid film between the rotating and stationary seal faces. As long as this film is maintained, the typical hardness of the toughest slurry will not impact the silicon carbide faces.
Solids are defined by their (1) Hardness, (2) Body, and (3) Body Abrasion. (1) Hardness is a comparative scale, diamond being the benchmark followed by silicon carbide. (2) Body encompasses a situation where the particles abrade the mechanical seal in such a way that the seal wear is consistent from face to face. (3) Body Abrasion is seen where the particles (such as fly ash) are hard, small, and sharp enough that they migrate between the rotating and stationary faces to actually break and chip portions of the silicon carbide or tungsten carbide faces out. The face fragments then join the abrasion party.
Anyway, back to sealing slurry.
My first experience featured limestone slurry in a large power plant wet scrubber application. They had large, slow-moving slurry pumps with softer limestone slurry up to 30% solids by weight. The solution was a stationary rubber spring component seal with CD4MCu (ferrillium 255) metallurgy and hard faces (silicon carbide vs silicon carbide). The pump seal chamber was modified to an open 15% taper bore with rubber lined seal chamber, liners, and impellers.
The first seal ran for about five minutes because the operators never opened the suction valve. After that slightly expensive mistake, the first “flush-less” mechanical seal ran for 90+ days. Other “flushed” seals, had not lasted that long in this application. Purchase of an additional 29 seals was made by the client in 20 years past. Current mean time between repairs (MTBR) for pumps and seals is 12-15 years; references available.
Criteria for successful flush-less mechanical seals (in no particular order):
- Quality materials of construction
- Quality engineered products
- Proper pump/seal design, including adequate taper to the seal chamber and locking the bearings to prevent excessive axial thrust
- Operational training to prevent operation of the pump sans fluid
- Customers committed to thinking outside the box to reduce flush water requirements, prolong intervals for required maintenance, eliminate packing issues, and reduce overall costs
There is more to discuss, possibly in a future article, regarding the sealing of very abrasive slurries. The slurry outlined in this piece was a very elementary example of how successful mechanical seals can be given the right opportunity to perform.
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