Slack Flow – To Be or Not To Be

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). 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. 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.

We didn’t even need to run the hydraulics…the profile just screamed “SLACK FLOW”. 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. The fluid then vaporizes which results in the pipeline flowing partially full under the influence of gravity. 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).

Figure 1: Slack Flow Schematic

So the question becomes: “who cares”. 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.

My P&C team quickly assessed the situation and figured out a solution that would 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!). 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 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. 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 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.

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 quickly reversed course and said “yeah, that will be really hard to operate; how ‘bout we just let the slack flow happen, protect the pipe as best we can against the high velocities, and you won’t have to do anything”. Thor took his hands off my neck, grunted, and slowly nodded his head in approval. When the oxygen returned to my brain, the following image of my concept suddenly appeared in my head.

Figure 2: Slack Flow Pipeline Dream

On the trip back to the office, I was attempting to convince myself that my newly invented “slack flow pipeline” concept, developed under extreme duress, was going to be viable. I got back to the office and met with the P&C design team. Luckily we don’t have any Thor’s working in our office, so no physical harm was inflicted while I explained that we were just going to let slack flow happen.

The team quickly got to work and figured out the slack flow velocity, the estimated wear rate, and the duration of slack flow exposure based on the dam raise schedule and the deposition plan. The results looked promising…some extra wall thickness on the pipe coupled with a pipe rotation schedule would give us the life we needed without breaking the bank. And the real beauty for Thor – other than some periodic maintenance, no special operating procedures required to adjust to changing flows and changing discharge locations…just set it and forget it.

Slack flow is an issue that presents itself in many pipeline applications. My advice: don’t automatically assume that the best solution is one that prevents the slack flow from happening. A long-distance concentrate pipeline with consistent flow rates, a fixed discharge point, a 20-year life, and an off-property slack flow area in a remote location is an obvious candidate for a choke station to prevent the slack flow. Contrast that against a tailings application where you have constantly changing flow rates, multiple discharge points spread out over great distances, a dam crest that is continuously rising, and a short duration of slack flow exposure – you should certainly consider the possibility of a slack flow pipeline…Thor might even thank you for it.

Figure 3: Slack Flow Pipeline in Tailings Application

Final consideration: if you have a downstream process that will not perform well with copious amounts of air entrained in your fluid, do not install a vent at the top of the slack flow pipe. The hydraulic jump that occurs inside the pipeline at the end of the slack flow will gulp huge quantities of air if you provide a vent and the air will get sent downstream with the fluid. If you do not vent the pipe, just be sure the pipe and any internal linings can handle full vacuum pressure conditions.