The tailings superintendent (we will call him Thor), scowled at me, spat some chewing tobacco on my boot and threatened to bury me in the tailings impoundment. I explained that if he has too many chokes engaged, he won’t get the flow rate he needs, the pumps won’t be able to keep up, and he will overflow his tailings tank. ![]() I explained that if he has too few chokes engaged, a portion of the pipeline will operate in slack flow and damage the pipe. I told him that he will need to constantly monitor the pressures in the pipeline and adjust the number of chokes engaged in the station from 3 chokes to 12 chokes depending on the flow rate and the discharge location along the spigot header. We stood on the existing TSF crest and I explained to him that we were going to have a slack flow issue and that we could put in a choke station at the bottom of the steep slope to prevent the slack flow from happening. I set up a meeting with the hard-as-nails tailings superintendent who had been operating the TSFs at this particular mine for the last 40 years (every mine has one of these individuals!). My P&C team quickly assessed the situation and figured out a solution that would prevent the slack flow from happening. The answer: if you are transporting a tailings slurry in slack flow down a 10% decline, your velocity is upwards of 25 feet per second…and that is going to wear out your pipe in a hurry. This hydraulic phenomenon is precisely the reason why you can’t siphon a fluid over anything higher than about a 30-foot-high hill anything higher and the fluid vaporizes at the apex and causes the siphon to fail (ultimately depends on your elevation above sea level and the friction losses in your siphon pipe…but you get the point). The fluid then vaporizes which results in the pipeline flowing partially full under the influence of gravity. Slack flow occurs when a pipeline hydraulic grade line passes sufficiently below the pipeline profile to cause the absolute pressure in the fluid to drop below the fluid’s vapor pressure. We didn’t even need to run the hydraulics…the profile just screamed “SLACK FLOW”. ![]() At the bottom of the profile drop, the tailings deposition spigots started and continued along the crest of the new facility for thousands of feet. The new TSF was adjacent to the existing TSF and the tailings pipeline was planned to run along the crest of the existing TSF and then drop down to the crest of the new facility. I was working with my P&C teammates on a brownfields tailings project where the owner was planning to construct a new tailings storage facility (TSF).
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