Power Plant Intake Diving: Screens, Bays and Traveling Screens
By Walt Kesler, Power plant maintenance manager, nuclear-qualified. Reviewed by Amanda McCallister, editor.
Underwater Inspection | Commercial Diving Contractors Alabama The call came at 2:30 in the morning on a Tuesday. Unit 2 loss-of-cooling alarm, and the circulating water pumps were screaming. The traveling screen rotated about four feet before the motor overloaded, and I knew we had something big wrapped around the bottom of the bay. I'm Walt. For twenty-six years, I've been the maintenance manager for the intake structure at this plant — trash racks, traveling screens, condenser water boxes, the whole bay system. When you can't shut down the unit, you call a diver. This is how intake diving works in a real power plant.
Know the Intake Structure
First, you need to know what you're working with. The intake structure has three parts: the approach bay, the trash rack, and the traveling screen. The approach bay is the concrete channel between the river and the pump suction pipes. Water should slow to about 0.5 ft/s through here. That's not an accident — it keeps fish from getting pinned against the screens, and it keeps the head loss across the structure manageable. If you try to pull harder, you're impinging shad and bass, and the EPA isn't going to smile on you. After the approach bay, the water passes through a trash rack — steel bars on 3-inch centers, or 4 inches in an older plant. This catches the big stuff: logs, tires, the occasional deer. Most plants have a mechanical rake that chains up along the bars, but the rake misses plenty on the edges and under the sill. You still end up sending a diver down at least once a year to clear what the rake can't reach. Downstream of the trash rack is the traveling screen. It's a continuous chain of wire mesh panels, typically 3/8-inch mesh, with a drive motor and a washing spray. The screen rotates on demand, lifting debris to the top where a high-pressure spray knocks it into a trash trough. Differential pressure is the key number to watch. When it climbs above six inches of water column, the screen's auto-start kicks in. If you let it sit at a foot, you're courting disaster.
When You Need a Diver
You don't dive just because you're curious. Most of the time, the screens do their job. But there are three situations that put a person in the water. First, after a major storm or runoff event — the river brings down more than the rake can handle, and you'll find branches, mats, and plastic sheeting plastered against the traveling screen. Second, when the screen differential stays high even after the wash cycle runs, or when the screen itself stalls. That usually means something's wrapped around the bottom of the chain or wedged against the screen bay sill. Third, when you have a biological fouling problem. Zebra mussels are the classic example. They'll fasten to the concrete floor and the screen frame, and they'll build up enough to choke the approach flow. We've also seen freshwater clams. If the mussels are your issue, the diver's job is scraping and suctioning — and you need a disposal plan that meets your state's aquatic nuisance species rules. Call it experience: we pulled nine shopping carts out of the bay one winter. People ask how they get there. I don't know. But they do.
What a Dive Actually Looks For
Before a diver enters the water, you tag out the traveling screen drive and the pump in that bay. No exceptions. If you have two pump bays, you isolate one and let the plant run on the other. The diver works in the isolated bay, with the water level held at the normal basin elevation. He goes in with a dry suit, a harness, and a dive station supplying breathing air. The checklist is simple but thorough. He checks the trash rack for bent or missing bars, broken anchor bolts, and sediment drifts around the sill. On the traveling screen itself, he looks for missing or damaged panels, loose mesh wires, misaligned chain shoes, and sprockets. A 3/8-inch mesh panel is about three feet wide by six feet tall. They bend pretty easily when they catch a heavy branch, and a bent panel means debris can slide past into the condenser water box. He also checks the seal between the screen frame and the wall — a common leak path. The seal is usually a rubber or neoprene wiper that can tear and allow fish or debris to pass. That's a compliance issue under EPA Section 316(b) for impingement, and it also means your downstream equipment is getting grit it shouldn't have. We use a dive team that carries a video camera and underwater lights, but a good diver will feel more than he sees. We used to have a guy named Darrin who could find a void in a grout joint just by running his fingertips over the concrete.
Running the Dive Job
It's not like sending a guy to the bottom of a swimming pool. We hire a commercial dive company with power plant experience. They stage from a flat-bottomed workboat in the intake, or from the trash rack deck if it's high enough. We set up a confined-space permit for the pump bay — it feels open at the surface, but there's deep water, entanglement hazards, and hydraulic suction if the clearances aren't followed. The diver wears a dry suit because river water in April is still around 40°F, and he breathes from a surface-supplied dive station. We don't tie into plant breathing air; too many ways to get oil or debris into the line. While the diver is down, we maintain a dedicated radio link to the control room. The control room operator knows the exact position of the tagouts, and on no condition will he start a pump or rotate the traveling screen until we give the all-clear. We also keep a standby diver fully dressed, ready to go on a moment's notice. This is basic commercial diving practice, but in a plant environment, you're dealing with operators who are used to flipping switches. One miscommunication and you're sending someone into a spinning screen. We treat the pump bay like an oven with a pilot light: everything stays locked out. We've tried using a remotely operated vehicle for the visual part of these inspections. It's fine for taking a look, but it can't clear a jammed branch or peel off mussel colonies. So the diving stays. You can't beat a man with a crowbar and a sawzall — he's still the most flexible tool in the box.
Lessons from the Screen Bay
If you're smart, you schedule these dives before the screen problems hit. We do a full intake dive on a rotating basis every three years, and we always do an emergency dive after a flood event or a trash jam that overwhelms the screens. It's impossible to get a shutdown just to poke around the intake; the plant has to run. So you budget the dive now and you prevent a forced outage later. Finally, keep records. We have a map of the intake with segment numbers painted on the concrete, and the diver calls out findings by segment. That way, when the performance engineer asks why condenser pressure is creeping up, you can look back and see that the seal in Segment 3 was patched two years ago and is due for a retake. Call it a fifty-fifty proposition: a big repair will uncover an even bigger problem nearby. It's never a tidy one-and-done job.
Frequently Asked Questions
What does a power plant intake dive cost? Depends on the plant, water depth, visibility, and what you need cleaned. For a routine inspection, most utilities budget somewhere between $5,000 and $20,000, give or take, but that's a guess — commercial dive quotes are contract-specific. You're paying for the dive team, the boat, the permit, and the insurance, not just the person underwater. Can an ROV replace a diver for intake screens? For a visual survey, yes. We use an ROV with a camera and sonar to screen the tunnel and the trash rack. But when you have to clear a debris jam, feel the seal, or scrape mussels off concrete, you need hands. An ROV can't carry a pry bar. Do I need to shut down the plant to do an intake dive? Not usually. We isolate one pump bay and run the unit at reduced load on the other bay. If you only have one bay or one screen, then yes, the unit has to come off line. It depends on the plant's redundancy. What about zebra mussel cleanouts? That's a specialized dive. Divers scrape or suck the mussels off the concrete and screens, and the debris is collected and disposed of onshore. You have to check your state's aquatic nuisance species plan—you don't return live mussels to the river. And you should dry or chlorinate the equipment before the next dive.