The Isolation Check That Should Happen Before Every Intake Dive
By Walt Kesler, Power plant maintenance manager, nuclear-qualified. Reviewed by Dana Whitcombe, technical reviewer.
Cathodic Protection & Anodes | Offshore & Subsea Diving October 2017, intake bay #3, Waterford, CT. The water was the color of coffee, and the current was pulling the lifting bag sideways. I was in the drysuit, ready to drop over the edge, when the dive supervisor stopped me. 'Walt, you check the valves?' I waved the clearance order. He shook his head. 'That's paperwork. Go verify.' He was right. A clearance order is only a request. The real proof is a valve stem that doesn't turn, a pipe that stays cold, and a drain that stays dry. In the years since, I've made that check a checklist. It's not paranoia. It's physics. Here's every intake dive, before the ladder goes in, we run the same five checks. It takes forty minutes. It has never once been wasted.
1. Walk the line — don't trust the tag
I've seen a clearance tag on the wrong valve. Twice. In a maze of intake and discharge lines, the drawings don't always match the paint. So the first check is a slow walk. Take the clearance order, a flashlight, and your hands. Find every single isolation point listed on the form. Physically touch the valve stem. Watch for any signs of leaking, any active condensation. If the order says 'closed' but the stem has a few turns of open exposed, you stop. You question. A tag is just a slip of paper. The metal tells the truth.
2. Feel for heat or vibration
Intake water in a coastal plant runs cold. Discharge runs warm. If you put your palm on a pipe that's supposed to be isolated and it's warm, that's a leak path. Vibration is another warning. A closed valve should be still. If the line hums or kicks, there's flow somewhere. I once caught a butterfly valve that was slamming open under load. The noise was the only sign. The actuator was tagged closed, but the disc had sheared off the stem and was spinning. You can't see that from the control room. You can feel it from the pipe.
3. Bleed a tell-tale downstream
Every intake system has a drain or a vent on the downstream side of the isolation valve. Open it. That's the tell-tale. If water comes out hard, or if air hisses for more than a few seconds, you don't have positive isolation. If nothing comes out, close the drain and walk away. But do not do this alone. Have a partner watch the outlet. Residual water is normal. A pressurized stream is not. One time we found a 2-inch drain that was still weeping clear water twenty minutes after closure. We let it drip. The valve was passing. A new seat solved it. The dive went ahead the next day.
4. Verify the actuator — and the breaker
Motor-operated valves have position indicators, but the indicator can lie. There's a mechanical actuator stem that shows the actual position. Look at it, not the LED. Then kill the power. Rack out the breaker, put your own lock and tag on it. If the valve is air-operated, isolate the air supply and bleed the line. The goal is that nobody, from the control room, can change the valve state without physically removing your lock. And if they try, you'll see their handprint on your tag. That's a conversation you want to have before the dive, not during.
5. Make control room watch the gauges
Finally, establish a direct channel with the operator on shift. Put a radio on a dedicated channel. Tell them: 'I am in the bay, valve closed, tell-tale drained. Do not start anything.' Then ask them to watch the flow and pressure instruments for ten minutes. If the intake pressure rises, or the screen differential changes, they call you. That's the last net. The entire dive, you want someone at the desk who knows you're under the water. That check isn't just a good practice. In a nuclear plant, it's part of the clearance procedure. It's the difference between a routine job and a casualty.
Frequently Asked Questions
What if there's only one valve between me and the water? Then you don't have positive isolation. You need a physical break—a blind flange, a spool piece removed, or an installed gate. A single valve is not enough. It can fail, and you have nothing behind it. If you can't get a physical break, you have to treat the system like it's still live. That's a different dive, with different protection. How long does this add to the job? Forty minutes to an hour. That's a small price when you consider the consequence of a pump starting with a diver in the bay. I've never had a manager argue with that. The only people who argue are the ones who haven't met the diver's family yet. Can I trust a remotely operated valve's position indicator? No. The stem can shear, the coupling can strip, the limit switch can get water inside. The indicator says what it wants to say. The stem, the drain, and the feeling on the pipe are the proof. Always verify mechanically.