Trash Rack Cleaning and Repair at Hydro Plants
By Walt Kesler, Power plant maintenance manager, nuclear-qualified. Reviewed by Dana Whitcombe, technical reviewer.
Underwater Inspection | Commercial Diving Contractors Alabama Last October, I watched our crane operator pull the #2 intake trash rack at the Lakeland Falls plant. The rack came out like a couch somebody'd been hiding in the basement – leaves, sticks, a kid's bicycle, two shopping carts, and about three hundred pounds of dead smallmouth bass. That's not the worst I've seen, but it's a good reminder of what trash racks do: they keep the big junk out of the turbine, and they take a beating for it. If you let a rack plug, the differential pressure across it climbs. You'll see it on your plant SCADA – forebay level goes up, tailrace drops, and you're generating less while sending more water over the spillway. You don't recover that lost head until you clean the rack. That's why this article exists. I've been doing this for twenty years, nuclear-qualified and all, and I've cleaned more racks than I can count. Here's my field manual for trash rack cleaning and repair at hydro plants – the real-world version.
Know What You're Pulling
Before you touch a rack, you need to know what you're working with. Most intake trash racks are made of steel flat bars, 3/4 inch by 4 inches or so, spaced 4 to 6 inches on center. They're set in a steel frame that slides into vertical guides at the intake opening. A typical section might be 10 feet wide by 20 feet high and weigh 4 tons, give or take. The guides are grease-lubricated and they have a lip on the bottom. If a section has been sitting in the water for months, the guides can corrode and bind. We've had to use hydraulic jacks to break a rack loose from the guides. You'll want to check the coating condition, too. Coal tar epoxy or a zinc-rich primer are common. If the coating's peeling, you're going to have corrosion and pitting on the bars, which is where cracks start. One thing I've learned to respect is the environmental side. Our plant has a fish passage requirement, so we coordinate cleaning work with the state fish and wildlife agency. That means we don't flush accumulated debris during certain migration windows. It costs us a little scheduling flexibility, but it keeps the fish people happy.
Cleaning Methods That Work
For light debris – leaves, grass, small sticks – a hydraulic trash rake is often enough. That's a bar with tines that goes up and down the intake screen, mechanically scraping the rack. But for the heavy stuff – whole trees, tires, shopping carts – you need a crane with a clamshell bucket or a grapple. We use a 50-ton Link-Belt crane with a 4-tine grapple. It takes two guys: one on the crane, one in the pit with a radio. If the rack is too plugged to even see through, you might need divers. We've used a local dive service for underwater inspections and for clearing debris with high-pressure water at 3,000 psi. The divers work in a dry suit and a full-face helmet with comm. It's slow and expensive, but sometimes that's the only way when you can't pull the rack because of head pressure or because the guides are damaged. Don't forget the basics: lock out the trash rack hoist if you have a mechanical rake. Tag out the intake gate, and follow your confined space entry plan if anyone needs to go into the trash rack slot. We've had guys try to skip the safety steps to save time. Doesn't work. It only takes one moment to ruin a career or a life.
Repair and Weld-Up
Cleaning is only half the job. Every time we pull a rack, we lay it on the deck and go over it bar by bar. We're looking for bent bars – we've had debris bend a 3/4-inch bar by 6 inches. That changes the trash rack's flow characteristics and can let debris through. Bent bars need to be straightened or replaced. Straightening works if the bar isn't cracked. Heat it with an oxy-acetylene torch to cherry red, then bend it back with a hydraulic jack. But if you see cracks, cut the bar out and weld in a new one. The steel is usually A36, and the rod we use is E7018. Preheat the area to around 100°F if it's cold out – welding on wet, cold steel gives you porosity and hard spots. After the weld cools, we grind it smooth and check it with a magnetic particle inspection. Then we touch up the coating with a cold-galvizing compound. It sounds simple, but I've seen plants skip the inspection and just send the rack back down. Then a crack runs and you get a whole section let go. That'll cost you more than a week of turbine downtime and a trip through the turbine with a piece of steel. If you don't have a weld inspection procedure, call a local NDT shop. They'll bring a yoke and a can of contrast paint – couple hundred bucks, worth every penny.
Scheduling and Avoiding the Hard Lessons
When do you clean? Depends on your plant. At Lakeland Falls, we clean every two to three weeks during fall leaf drop, and once a month in the winter. In the spring, during high water, we check weekly if debris is coming down the river. A good rule of thumb: if the differential pressure across the rack climbs above 2 feet of water, it's time to clean. That's plant-specific, but it's a safe starting point. For repairs, don't wait until a bar breaks free. We keep one spare rack section on the slab, fully welded and coated, ready to swap in. That way we can take a damaged rack out and send it to the shop without a long shutdown. The swap takes about four hours with two crews. And document everything. We record the date, the differential pressure, the debris volume in cubic yards, and what we found on the rack – bent bars, coating wear, bolt corrosion. That's how we've built a maintenance schedule that actually works. It's not glamorous, but it keeps the lights on. And that's the job.
Frequently Asked Questions
How often should trash racks be cleaned at a hydro plant? It varies, but a good rule: clean whenever differential pressure across the rack exceeds 2 feet of water. During leaf drop, that may be every 2-3 weeks. In winter, monthly. In spring runoff, weekly. Depends on your plant's river conditions and the rake system you have. What equipment is used for trash rack cleaning at hydro plants? Mechanical trash rakes, cranes with grapples or clamshell buckets, high-pressure water (3000 psi from divers or deck-mounted washers), and for heavy plugs, sometimes a dragline or excavator. We use a 50-ton Link-Belt crane with a 4-tine grapple. Can trash racks be repaired underwater? Yes, underwater repair is possible for minor damage, usually by divers using hydraulic or pneumatic tools. But for major cracks or bent bars, it's safer and more cost-effective to pull the section and weld in the shop. Underwater welding exists, but it's expensive and needs certified welder-divers. How do you know when a trash rack needs repair? Look for bent bars (anything beyond 1/2 inch deflection), cracks in welds or bar steel, corrosion pits deeper than about 1/8 inch, and loose or missing fasteners. Magnetic particle inspection after cleaning will reveal surface cracks. If you see any of these, plan a repair.