Ice chunks the size of Volkswagens falling from the sky! Sounds like a Hollywood special effects scene in an action movie, right? Unfortunately, this is a real-life danger that can occur just about anywhere, but especially on wet stacks running in cold weather.
It should go without saying that ice falling from a tall stack can have damaging, even catastrophic effects on process equipment and personal safety. But with some situational knowledge and attention to design details, you can prevent ice build-up on wet-stacks before it becomes a problem.
Essentially, to operate an ice-free wet stack system, you need to properly handle the discharge of wet flue gas during prolonged exposure to cold temperatures. Units running at low loads on cold, windy days can see a dangerous icing develop from an effect called plume downwash.
Plume downwash occurs when a cross-wind at the top of the stack deflects the plume from its vertical path. This phenomenon is more likely to happen when flue gas exits at a lower velocity—like, for example, when units aren’t running at full capacity. As the wind impacts the plume, the plume is pushed downward onto the stack, causing the liquid within the flue gas to deposit on the stack’s surfaces.
And what happens when moisture is allowed to build up on cold surfaces? You guessed it—ice forms.
Ultimately, all stacks can experience downwash if wind speeds are high enough. The only questions are:
Thankfully, much of the guesswork can be eliminated by using computational fluid dynamic (CFD) modeling. CFD modeling is extremely well-suited to simulate the stack plume over a range of plant operating and atmospheric conditions to predict the potential for plume downwash. And if you haven't already identified icing with the naked eye, CFD simulations can be used to predict not only if icing can occur, but where it can form on the stack.
If conditions are right for plume downwash, the following areas are most likely to experience problems, including potential ice-buildup:
These areas are exposed directly to plume downwash, and therefore, icing in the right conditions— some more so than others. Heat tracing is often recommended for some of these surfaces to eliminate snow accumulation and excessive ice build-up, but care should be taken to ensure the drainage run-off doesn’t create a secondary icing problem.
More details about the icing potential in these areas can be found in the EPRI Revised Wet Stack Design Guideline, section 1.4.9.
According to the EPRI Revised Wet Stack Design Guideline, the potential for icing can be reduced by employing the following steps:
Any uncertainty in any of these recommendations can be discussed with our Gas Flow and Wet-Stack Design experts.
Icing can occur at below freezing conditions all winter long, every winter, creating potentially dangerous conditions for both people and property. If you're running a wet stack at a low load in cold, windy weather, icing is probably going to be a concern for you. Contact us for details and recommendations in order to ensure proper performance and reliable operation of your wet stack located downstream of a wet flue gas desulfurization system (WFGD).
And in any instances where ice is present, be careful!
Photo 1. Super-cavitating roughness element, before installation at Cabinet Gorge Dam
The flat face plate of the baffle block causes clean flow separation as water travels over and around the block, forming a “cavitation cloud” which envelops the roughness element and prevents damage from flow. In Part 2 we discussed the use of air ramps to ensure that flow is fully aerated as it passes by the roughness elements, further preventing cavitation damage and the formation of horseshoe vortices around the block bases.
The roughness elements are anchored to the spillway surface or underlying rock abutment using post-tension anchors, and are socketed into the spillway to increase bearing capacity. Impact loads from debris or logs traveling down the spillway generally govern the design loads for anchors. Shear keys with additional shear reinforcement can be used if the blocks are designed to be placed very near the spillway lip, where there is not adequate concrete thickness for the required bearing strength.
Photo 2. Roughness elements and air ramps installed at Boundary Dam
Depending on the type of dam and spillway arrangement, post-installation stability analyses may be necessary to ensure that Federal Energy Regulatory Commission (FERC) stability criteria are met. Spillway capacity calculations are also performed to ensure that the spillways are adequate to pass Probable Maximum Flood (PMF) flowrates after modifications. Alden has teamed with several public utilities to model, design, and test super-cavitating roughness elements at their high head dams, and has successfully implemented TDG-reducing measures while meeting all FERC criteria for stability and spillway capacity.
During spill season at hydroelectric dams, more water flows into the upstream reservoir than can be used to generate electricity in the powerhouses. This excess flow must pass through a number of different flow release structures in order to bypass the dam and powerhouse. Spillways, diversion tunnels, and low-level sluice gates are commonly used to route flow past dams. Open channel spillways are one of the most common flow release structures at high head dams, and create a highly aerated, turbulent jet of water that exits the spillway up to 150 feet above the river downstream of the dam. This waterfall of aerated flow can plunge to the bottom of the tailwater pool, where the bubbles of atmospheric gases are slowly dissolved into solution with the water. The deeper the jet plunges, the more pressure is exerted by the water on the bubbles, dissolving them faster and preventing them from rising to the surface. This is why we see a frothy white plume of flow that can stretch up to half a mile downstream of a dam when flow is being released, as shown in the photo of Boundary Dam spillway below.
Once the water has dissolved all the gases it can hold in equilibrium (saturation level), the river can exceed its saturation level and becomes supersaturated with dissolved gasses. This is a function of the pressure exerted on the gas bubbles at depth, and the travel time of the bubbles to reach the surface. The sum of all the gasses dissolved into solution is called the Total Dissolved Gas (TDG) concentration. High TDG is a hazard for aquatic life, especially migratory fish such as salmon and steelhead. When fish come into contact with high TDG concentrations at depth, their tissues absorb the gases. When they later swim to the surface, the gasses come out of solution and form bubbles, which can cause trauma around the gills and fins. This trauma is known as Gas Bubble Trauma and has become a major problem for fish populations that must use fish passage systems to bypass dams. Some good photos showing Gas Bubble Trauma in fish can be found in this linked article in the Billings Gazette:
Click on image for full article
The Environmental Protection Agency (EPA) has introduced regulations that limit TDG production to 110% of saturation. This limit is enforced regardless of the TDG level of the water coming into the reservoir, which may be at or near saturation due to upstream dams, waterfalls, or other conditions. High head dams and hydroelectric projects are required to be relicensed with the Federal Energy Regulatory Commission (FERC), and must prove that they meet the new EPA regulations before being granted a license. The owners of a number of affected projects have reached out to Alden to help them find cost-effective solutions to TDG problems, which we have explored extensively using computational fluid dynamics and physical modeling, as well as structural and operational changes to the projects.
In Part II we’ll explore the use of energy-dissipating devices to reduce spillway plunge depth and bubble transit time – stay tuned!