Circulating Water Pump Repairs Underwater

By Walt Kesler, Power plant maintenance manager, nuclear-qualified. Reviewed by Amanda McCallister, editor.

Underwater Inspection | Commercial Diving Contractors Alabama I remember the number 3 pump throwing its impeller key at 2 a.m. On a Sunday. The lake temp was 43°F and the unit was down to 300 MW because of it. I've been doing this for twenty-six years, and I still get a knot in my stomach when I have to send a dive team into a pump bay at the end of a long outage. Underwater circulating water pump repair is a specific skill that not many people talk about. It's not like diving on a ship hull. You're working in a confined screen well or volute, with zero visibility, intake suction that can kill, and parts that weigh tons. But it's often the cheapest and fastest way to avoid pulling a pump and losing another week of generation. The plants that do it well treat it like a surgical procedure, not salvage work. This is how we handle circulating water pump diving repair at my plant—from planning and inspection to the actual bolts and hack saws. I'm going to walk you through the process, the tools, and the things that go wrong, so you don't have to learn them in the water.

First, Stop the Water and the Motor

Before the diver goes in, we turn everything off. We isolate the pump, close the discharge valves, stop the motor, and engage the brake. We set the water level to about the mid-point of the pump shaft, but that depends on the plant. Sometimes you can pull the water down to the pump suction elbow. We use two independent divers, a standby diver, and a supervisor. We have a written procedure with a step-by-step for every cut, bolt, and torque. In a nuclear plant, we have extra review—the NRC, the operations staff, and a hold point before each major step. It's paperwork, but I've seen a key come loose twice, and those holds saved us. If the discharge valve won't hold, we install a blank and brace it. We test the isolation by dropping the water level, then watching for 30 minutes. If it stays, we send the diver. We always have the diver on a hard-wired intercom and a rope line. Video is nice, but you don't need it for every move. What you need is discipline. Everyone knows the exact hand signals and the words that mean 'get me out now.'

Inspection with Wet Hands

The diver's first job is to find the damage and measure it. We use a guide wire and a camera, but the main tool is a set of gauges. The diver runs a finger over the impeller edge and wear ring, looking for grooves and steps. If the clearance between the impeller and the casing is more than one-sixteenth over the manual's spec, we plan to machine or replace the ring. But that's rare on a quick repair. We did an inspection last fall on a pump that had eaten a piece of wood. The diver found a decent chunk missing from the trailing edge of the vane. We grinded the edge smooth and applied a trowel-grade epoxy that filled the void. That repair ran for four years until the next outage allowed a proper replacement. I'm not saying it's always that good, but it works when the stress is low. We record all the underwater video and review it with the engineering group. If we suspect a crack, we use a dye penetrant kit that works underwater. The diver scrubs the area, applies the dye, then wipes and flashes with a UV light. You can detect hairline cracks that way.

The Repair: Cut, Fit, Torque, and Epoxy

For a key repair, we make a new key from 416 stainless. We put a slight taper on it and drive it in with a lead hammer. Then we torque the impeller nut to 2,500 foot-pounds with an Enerpac hydraulic wrench. That sounds simple, but underwater you can't see the torque marks. We use a witness plate and a torque bushing that leaves a distinctive gouge when you're in the range. Underwater welding is rare for us. We do it for sealing rings and some overlays, using a wet-stick electrode with a waterproof coating. But the water quenches the weld, so you get high hardness and risk of cracking. We prefer mechanical fasteners and cold-curing epoxy between fit areas. We use a two-part, marine-grade, fast-cure epoxy that sets in 30 minutes and reaches full strength in 24 hours. It won't hold the impeller, but it can restore a seal face. When we have to replace a wear ring, we cut it with a hydraulic band saw and then split it with a chisel. We install the new ring as a segmented piece and hold it with socket-head screws. Then we lap it true with a hand grinder. Depends on the plant, but that gets us back to service.

Tools That Go in the Water

Here's the list we keep in the water-side tool trailer. A hydraulic torque wrench with a pump and gauge calibrated for water depth—you lose about 5% torque per 10 feet of depth because of the hose and friction. A submersible grinder with a 4.5-inch diamond wheel. A hydraulic band saw that runs off the same power unit. An impact wrench with a welded cage so the socket doesn't fly off. And copper-beryllium tools for any chance of spark. We never reuse high-tensile bolts. When you cycle a pump 2,000 times a day, old bolts are a nonstarter. The other essential is communication. We use a hard-wired diver intercom powered by 12 volts. It's not fancy, but it's clear. We also have a rope with knots every five feet so the diver can look at their depth without lifting their head. Final thought on tools: keep them tethered. A dropped wrench in a pump bay is a wreck. We learned that the hard way when a socket got sucked into the strainer and took out a downstream pump.

When the Dive Goes Wrong

I remember a dive on a raw water pump at a fossil station where the water level rose two feet because a discharge butterfly valve leaked. The diver felt the current change and got out. Standby diver was in the water in less than a minute. We had a checklist for emergency withdrawal, and it worked. But that's the thing—underwater, you can't see anything, and the only safety is the rigging and the procedure. We never work on a pump with the motor clutch engaged. And we always put a steel guard over the rotating shaft before we start cutting. If the visibility turns to zero, we call it. Mucking around by feel can do more damage than good. We also watch the clock on bottom time, even in fresh water. A diver can be down for an hour, but no more than two with thermal protection, depending on the water temp. The last rule: never patch a pump and forget it. We log every repair, and we track it in the plant's predictive maintenance system. We know the flow and vibration baseline before we start, and we check it after we load the unit. That's how you know it's actually fixed.

Frequently Asked Questions

How long does an underwater circulating water pump repair typically take? Most single-pump repairs take 12 to 48 hours of dive time, but the whole job—outage prep, isolation, setup—runs 3 to 5 days. Depends on the plant and the depth. Can you weld underwater on a nuclear plant pump? We can, but we rarely do. Wet welding creates hydrogen and hardened metal that usually doesn't meet our code. We prefer mechanical fasteners and cold-set epoxy for most repairs. How deep are these pumps? Most circulating water pump bays are 10 to 30 feet below the surface. In a flooded condenser pit, the suction might be 20 feet down. Give or take. How do you know the repair is good if you can't see the result? We use a diver-held camera and a witness plate on the torque wrench. Then we do a controlled start-up and watch vibration and discharge pressure. If they're in spec, it's done.