Cooling Tower and Spray Pond Cleaning
By Walt Kesler, Power plant maintenance manager, nuclear-qualified. Reviewed by Dana Whitcombe, technical reviewer.
Underwater Inspection | Commercial Diving Contractors Alabama Last Tuesday, I stood on the catwalk over Unit 1's cooling tower basin and watched a diver pull a mat of roots and silt off the suction screen. Nothing spectacular. Same job that's been done since the first cooling tower went up, just with better wetsuits now. We run two 500-megawatt units, and each tower holds about 400,000 gallons. When that basin gets dirty, the condensers feel it fast. The guy in the water was a cooling tower basin cleaning diver. That's not a job title you see on a business card often, but it's exactly what it sounds like. Someone who drops into the basin, wades or swims through water that's not exactly swimming-pool clear, and pulls out everything that shouldn't be there. It's a niche trade, but every plant that runs a closed-loop or open-loop cooling system needs it sooner or later. I've been in power plant maintenance for twenty-two years, nuclear-qualified, and I've spent a good chunk of that time staring at the underside of the water box and the inside of a cooling tower. This article is the practical side of that. If you're considering hiring a diver or thinking about doing the job in-house, here's what I know.
Why the Basin Gets Nasty in the First Place
The basin is the sump at the bottom of the cooling tower. It collects the water that cascades down through the fill, and it's the reserve for the circulating water pumps. It's open, warm, and sits there 24/7. So you get silt, sand, leaves, bird droppings, and if you're near a river, occasionally a fish. And in the last ten years, zebra mussels. Those little bastards can cover the flash screen in a month. We pump from the river into the basin, and the strainer catches most of the big stuff, but it doesn't catch everything. The draw pulls silt and fine particles through, and they settle out in the basin. Give it a summer and you've got six inches of sludge across the bottom. That's not just a housekeeping problem. That's a heat transfer problem, because the circulating water picks up that sediment and carries it to the condenser tubes. And if you're not careful, you're back flushing tubes every week. The debris also clogs the fill, which cuts the airflow and drops the tower's efficiency. We had to replace a section of Brentwood fill last year because a soggy mattress somehow got in—don't ask me how.
What a Diver Actually Does
The diver's job is to get the sludge, debris, and mussel shells out of the basin without draining it. You could drain the basin, but that means taking the unit offline, and a day of lost generation costs more than the diver does. So they go in wet. They work in a wetsuit or dry suit with a helmet that's connected to surface air, or a full surface-supplied system if the water's deep enough to count as confined space. They carry a suction hose like a giant Shop-Vac, and a couple of guys on deck handle the hose and a debris bag. They'll also use a bar or a scraper to knock mussels off the screens. We use a local outfit that brings their own Godwin pump for the sludge transfer. They drop the pump on the floor of the basin, and it pumps the slurry into a dump truck. It's not glamorous. But they do it right. They start at one end and work in lanes, like cutting a lawn. For a 400,000-gallon basin, it takes two to three days. The diver'll surface every hour to check the intake truck, and they'll move the Godwin to the next grid. The crew on deck is just as important as the diver—they're the ones who keep the hose from kinking and the pump from cavitating.
Planning and Safety: The Stuff You Don't See
Before a diver goes in, we've got a lot of prep to do. Our plant is nuclear, so we've got procedures for everything. But even at a fossil plant, you'd want the same. The basin is a confined space, period. That means air monitoring, a standby watchman, and a rescue plan. The water itself will have biocide and sometimes chlorine. If you're not treating it right, you're putting a diver in a soup of chemicals. We flush the system with fresh water before the diver enters, and we adjust the pH to something livable. Also, lockout/tagout. The circulating water pumps are a hazard if they start while a diver's in the basin. We physically lock the pump breakers open and put a tag on them. And we check the screens. If a pump starts, the suction could pull a diver right against the screen. That's a conversation I never want to have with a widow. A digression: I remember one plant, they hired a guy who said he'd done a dozen jobs, but he showed up with a snorkel and a pair of flip-flops. We sent him home. No certification, no proof of confined-space entry, nothing. You get what you pay for. That's the one time I'd say spend the extra money.
How Often, and When to Call a Diver
Frequency depends on the plant, the source water, and how much crud you pull in. Call it every three to five years for a basin that's fed by a river. If you're on a well or a municipal supply, maybe longer. But check the basin floor every year. If you see more than a few inches of sludge, or if the differential pressure across your inlet screens starts climbing, it's time. And if you notice your condenser backpressure creeping up in the summer, that's often the same problem showing up two steps downstream. The pricing model for a diver is usually a day rate or a per-job quote. It's not cheap, but it's a lot cheaper than a forced outage. A day of lost generation on a 500-MW unit is somewhere around hundreds of thousands of dollars, depending on the market. So a two-day cleanout every four or five years is a rounding error. Just get it in the budget. The worst time to find a diver is when the plant is already in trouble.
Frequently Asked Questions
What does a cooling tower basin cleaning diver cost? It's usually a day rate or a fixed bid. Depends on the size of the basin, how deep it is, and what they're pulling out. You'll see quotes from a few thousand to tens of thousands, but the real cost of not doing it is a condenser that runs hot. Do I need a certified diver? What qualifies? Look for commercial diver certification (e.g., ADCI accredited) and confined-space entry experience. Ask for references from other plants. A dive company that does underwater construction might not know cooling tower ops. Ask specific questions about sludge handling and screen cleaning. Can I clean the cooling tower basin myself with a vacuum truck? You can skim the surface, but a vacuum truck won't get the bottom sludge or the mussels off the screens. For a deep clean, you're better off with a diver or draining the basin. Draining means downtime, which is expensive. If you've got the time, sure, drain it and shovel. Otherwise, call a diver. How often should we clean? Every 3-5 years for river-sourced cooling water, depending on your basin design and debris load. Check the floor annually if you can - use a pole with a clear jar to sample. If the sludge is over 3 inches, it's probably time.