Clean Water Act 316(b): Intake Screens and the Diver's Role
By Walt Kesler, Power plant maintenance manager, nuclear-qualified. Reviewed by Amanda McCallister, editor.
Cathodic Protection & Anodes | Offshore & Subsea Diving Last February the river gave us a tree. The tree jammed the number three traveling screen and tore a hole in the wire mesh. I found out the way I find out most screen problems: a diver went down in the intake bay and came back with video that made us cancel a week of planned maintenance. That dive also kept us on the right side of 316(b). This plant is 550 MW, and we pull between 22 and 30 million gallons a day out of the river for cooling — call it 26. The Clean Water Act's 316(b) rule for existing facilities covers anyone with a cooling water intake of 2 million gallons a day or more, using at least 25 percent of that for cooling. So we're in it. Our permit says our through-screen velocity can't exceed 0.5 feet per second. That's the impingement standard, and it's the number I think about when the trash racks start to load up. But the rule doesn't just care if the screen turns. It cares if the screen actually works. And proving that means someone has to look at the mesh underwater, measure the gaps, check the seals, and tell me the condition in plain numbers. That's the diver's real job. Let me walk you through what that involves.
What 316(b) Actually Demands
The 2014 existing facility rule under 40 CFR Part 125 Subpart J gives you a few ways to comply. The straightforward path is meeting performance standards. For impingement, that's typically a maximum through-screen velocity of 0.5 fps, though you can also get site-specific approval based on your actual screen design. For entrainment, the standard is often linked to closed-cycle recirculation, like a cooling tower. But many of us don't have that luxury, so we rely on a site-specific compliance plan. That plan has to be documented. We document it by measuring everything. We have six traveling screens with 3/8-inch mesh. Each panel's clear opening gets checked with a feeler gauge or a caliper during a dry overhaul, but the underwater condition is what changes between overhauls. Zebra mussels, debris, ice, and just plain corrosion can close up openings or stretch the wire. The diver's job is to measure those openings while the screen is still in the bay and flag anything that would push the velocity above our permit limit. That's not a nice-to-do. That's a compliance record.
Diver Is a Sensor System
A diver in the intake bay is more than a pair of hands. He's a sensor system with a GoPro. We run a specific dive scope: inspect the screen face, check the center seals, look at the fish return trough, and clean any debris off the trash racks. The diver also runs a straight edge across the screen panels to find bent or bowed sections. Numbers matter. We log the width of any gap bigger than 1/16 of an inch. We log the number of panels that are leaning. All of that goes into our annual report. One dive last spring found a section of angle iron wedged between the screen and the seal wall. The differential pressure reading had gone up but not enough to alarm anyone. The diver took a photo, we pulled the screen, and the angle iron turned out to be a piece of our own trash rack bracket. That's the kind of thing that becomes a fish kill if it goes unnoticed. So when I budget for dive time, I don't call it maintenance. I call it insurance.
Delta P Is a Safety Rule, Not a Suggestion
I want to be absolutely clear about the hazard because I've seen a lot of contractors get this wrong. The danger on an intake screen is not the velocity of the water pulling you in. It's the static head difference across a sealed panel. Water pressure builds with depth — about 0.433 psi per foot. If the bay has ten feet of head across that screen, that's about 4.3 psi acting on the area your body covers. A human torso might be 1,500 square inches. That's over 6,000 pounds of force. You are not fighting that. So the only safe control is positive isolation. We lock out the screen drives. We physically blank the flume if we can, or we remove the screen section before any diver enters. I also require a standby diver and a topside air supply — no scuba cylinders at depth in those confined spaces. OSHA's 29 CFR 1910 Subpart T covers commercial diving, and the Association of Diving Contractors International (ADCI) is the industry consensus body. Some folks will say 'I'm PADI certified, I can do it.' That's a hard no.
The Report Is the Only Lasting Product
Every dive ends with a report. That report has the date, the diver's name, the conditions, the videos, the stills, and the measurements. It goes into a binder that sits next to my desk, and it's been pulled out in more inspections and more pre-permit meetings than I can count. When a state inspector asked how we know the screen meets the 0.5 fps limit, I didn't give him a theory. I gave him a video of a clean screen and a log of panel openings. That's the part people don't expect. Compliance isn't just mechanical. It's evidentiary. A dive crew that documents their work is worth twice what you pay them. And one more thing — that same binder saved our bacon a few years back when a citizens' group filed a complaint about fish impingement. The dive videos from the previous two seasons showed our screens in spec. The complaint went nowhere. You can't buy that.
Frequently Asked Questions
Can a diver inspection replace the need to meet 316(b) performance standards? No. A diver can document condition and help support a site-specific compliance approach, but you still have to meet the permit's impingement and entrainment requirements. The dive is evidence, not a substitute. What exactly is the 0.5 feet per second impingement standard? It's the maximum through-screen velocity that prevents fish and other aquatic life from being pinned against the screen. We maintain it with variable-speed drives and by keeping the mesh clean. The diver checks for blockages and wear that could raise local velocity. Do I need a commercial dive company, or can I use a local scuba instructor? Use a commercial dive company that operates under OSHA 1910 Subpart T. A scuba instructor's instructor rating doesn't count. We also require ADCI accreditation in our contracts. How often should a plant inspect its intake screens underwater? There's no fixed federal interval for existing facilities, but a reasonable baseline is annual or biennial, depending on biofouling and debris loads. We do an annual wet dive and a dry overhaul of each screen every five to seven years. The key is to document whatever interval you choose.