Intake Screen Cleaning at Water Treatment Plants
By Walt Kesler, Power plant maintenance manager, nuclear-qualified. Reviewed by Amanda McCallister, editor.
Underwater Inspection | Commercial Diving Contractors Alabama Last November, I was standing on the catwalk above the intake bay at the plant, watching the number 2 traveling screen stall. The motor started drawing over 12 amps, the shear pin snapped, and then the chain slacked. Down in the water, a mat of dying algae and one very unhappy carp had packed the mesh solid. That's the moment I knew the raw water intake cleaning was going to be a whole weekend job. I've been responsible for the intake screens, condensers, and circulating water systems at this plant for 23 years. Nuclear-qualified, and that means I don't guess. I know the torque specs for every coupling, and I've gone into the water intake bay with a dive team when the bar racks got plugged. It's not glamorous, but when the screens are clean, the plant runs. This post is how we clean raw water intake screens at a water treatment plant, or a power plant, or any facility that pulls from a river or lake. There's a lot of variation between plants, but the fundamentals are the same. Call it preventive maintenance, call it debris management, but it's all about keeping that downstream condenser pressure drop where it belongs.
What I'm Actually Cleaning
Screen systems are not all the same. You've got the old stationary bar racks with a rake that moves up and down, and you've got traveling screens with panels that rotate. At our plant, we have five traveling water screens, each with 18-inch wide panels of 1/4-inch mesh. That mesh stops the stuff that plugs a condenser tube – leaves, twigs, plastic bags, ice, spawning alewives. A lot of what comes down a river is trash, and some of it has teeth. Debris varies by season. In the spring, it's runoff – branches, dead grass, an occasional tire. In the summer, it's algae and zebra mussels. In the fall, leaves, and in the winter, ice chunks. If you're on a coastal plant, you might get jellyfish or seaweed. Each type of debris calls for a different approach. That's why the cleaning method isn't a one-size-fits-all.
The Tools I Rely On
The workhorse is the traveling screen's own backwash system. A high-pressure spray bar shoots water from the outside of the screen panels to wash debris into a trough that carries it away. We run that at 80 psi from two 1,200 gallon-per-minute pumps. When it works, it's simple and constant. But it fails, and that's when you get the high-pressure wand. For the tough stuff – encrusted mussels or heavy silting – we use a 4,000 psi pressure washer and a crew with waders. Sometimes we have to hand-screen. We use a poly-blend brush and a scraping blade. Then there's chemical treatment. Chlorine or a commercial molluscicide injected into the screen wash water can keep mussels from establishing. But that's a whole environmental permit dance.
The Cleaning Sequence
Here's the sequence that works for us, and it's not the only one. First, check the differential pressure across the screen. That's your primary indicator. If it goes above 10 inches of water column, the screen is getting dirty. On our screens, we start the wash cycle once a day, but when the river is running high, we'll run it every six hours. Second, run the traveling screen through a full revolution. That's the only way every panel gets hit by the backwash spray. After that, stop the screen in the park position. Then manually inspect a few panels near the waterline for carryover. If there's still debris, hit the screen with the high-pressure wand. Third, keep an eye on the time – each revolution takes about 10 minutes. Fourth, log it all. Date, time, differential pressure, tone of the drive motor, condition of the panels. You can't manage what you don't measure, and a half-hour of logging saves you a catastrophic plugged screen in a rainstorm.
When You Need To Do It Differently
Every plant is different, so consider the specifics. In warm water, zebra mussels can form colonies in a week. If that's your problem, you might need to rotate the screen continuously, or use a chlorination system. A screen is not a disposal system – you have to remove the debris and dispose of it properly. On the Gulf, dead jellyfish will block the backwash nozzle faster than a kid with a sock. One digression: I had an engineer from corporate once ask me if we could just reverse the screen direction to clean it. I had to explain that it's not a conveyor belt – the panel is the screen, and reversing wouldn't unplug it, just risk jamming the drive. Then he asked if we could use compressed air to blow off the debris. We tried it once. The air just made a nice hole in the debris mat, but the rest stayed locked.
Measuring Success
Success is boring numbers. A clean screen has a low differential pressure. We keep ours under 4 inches of water column at normal river flow. The condenser vacuum stays in spec, and the heat rate doesn't climb. When the intake screens are dirty, the condenser cools less, and you burn more fuel. It's dollars, even in cents per heat rate. I also watch the trash racks upstream. If the trash racks are doing their job, the traveling screens last longer. And I track the debris weight we collect. It's not a scientific metric, but it tells me what the river is doing. Some days it's 50 pounds of leaves; some days it's a waterlogged deer. That's never fun to haul out.
Frequently Asked Questions
How often should raw water intake screens be cleaned? It depends on the debris load and the plant. At a river plant, you might do a scheduled wash every 8-12 hours, plus an unscheduled wash when differential pressure exceeds the setpoint. Some plants with heavy zebra mussel growth run the wash cycle more often. The real answer is to monitor differential pressure and inspect. What is the best way to remove zebra mussels from intake screens? A combination of high-pressure water and periodic chemical treatment. Mussels attach to the mesh, and scraping is required. You can use chlorination at a low level, but permits take time. There are also commercial molluscicides. Mechanical scrubbing works, but it's labor intensive. What safety precautions are needed for intake screen cleaning? Confined space entry comes into play, because the screen well is a confined space. We always follow lockout/tagout, wear harnesses, and have a stand-by observer. Also watch out for slips on algae-covered steel and the high-pressure water itself. If you're doing chemical cleaning, you need the full PPE. What happens if an intake screen breaks or gets jammed? If the screen breaks, debris goes downstream to the condensers, which is a bigger problem. A jammed screen removes the backup. You'd have to shut down or bypass the screen and clean it manually. We've isolated a screen and had divers clear it. It's not something to take lightly.