Portable machine tools expedite in-situ valve and pump repairs at power plants.

When power plant pumps, valves and piping systems become worn or corroded due to the amount of steam that passes through them, operations managers seek the best tools and processes to get the equipment repaired in the safest, most cost-effective, long lasting, and efficient way possible. Among the technologies increasingly being adopted by both in-house machinists and service companies are portable machine tools, including boring bars, flange facers and valve repair machines.

These portable machines make it possible for experts to repair and maintain heavy equipment in-situ. They offer many advantages over the traditional, more costly method of disassembling equipment and transporting it to an off-site machine shop. This article discusses some prime examples of how these machines made an in-situ repair of valves and pumps easier and more cost-effective.

Feed Water Control Valve Repair

A portable boring machine, facing head and threading attachment expedited the work of machining a corroded steam by-pass valve and rethreading a water control valve, according to Wayne Kirkpatrick, a journeyman machinist who spearheaded the project at The Palo Verde Nuclear Generation Station near Phoenix, Ariz.

The plant's engineers inspected the steam by-pass valves for corrosion every 18 months during the routine refueling cycle. Very hot water flows through the 20-inch bypass valve and steam pipes that circulate water back to the turbine feed water pump, converting the water back into steam.

The valves have a seat-ring on the bottom, and due to the piping configuration and design, water flows under the valve seat and causes considerable water turbulence within the valve. Because the valve seat is made of harder material than the valve body, thinning and erosion occurs on the valve body.

The steam bypass valve repair work was located inside the valve that has a 15.5-inch diameter pressure seal bore with the basket area in question located approximately 16.5 inches from the bonnet surface.

To repair it, plant engineers first preheated the A105 base material to 200 degrees and then conducted a weld buildup with a filler material in the throat of the valve at the seat. This left a small space that could be registered to.

Machinists were then brought in to machine the valve seat. To do the work, a portable boring machine was assembled and mounted to the valve body using the bearing housing and existing four valve studs, and it was registered into the valve body.

They then machined the weld to the proper bore diameter specification set by the valve manufacturer and the Palo Verde engineering department. See Figure 1.
Once the valve was bored, they worked to reface the taper at the bottom of the valve using a form tool. A measuring tool was used to get a clear reference for the depth of the valve.

Boring machine repairing threads on steam bypass valve

Figure 1. A boring machine repairing the threads on a nuclear, high-pressure steam bypass valve.

A tool carrier was also set up to bore the diameter of the valve body. The same boring machine was used because it was able to take an additional tool carrier which was used as a reference for the machining depth.

By putting this second tool carrier on the machine, the micro-switch plate could then be used to gauge the stop limits and measure travel. The light and buzzer plate, which was developed by the plant's machinist, was mounted on the second carrier. It illuminated the interior of the valve and sounded when the tool reached its machining limit.

Next, the feed water control valve needed threads machined into the body of the valve. (See Figure 2) This work was located approximately 24.0 inches inside the valve from the bonnet surface. The thread pitch diameter was 16.9459 and 12 threads per inch. After putting a 45 degree seat in the valve's body, the machinists cut a 1/8-inch-wide thread relief.

Machining threads into the body of the valve
Figure 2. Machining threads into the body of the valve

Once that was established, they put a threading attachment onto the boring machine's tool carrier and zeroed the tool on the bore of the valve body and made a cut.

Another tool carrier was positioned on top of the machine with a buzzer and battery that could be set with a micro-switch.

A tool carrier with a buzzer plate and battery to assist with blind boring
Figure 3. A tool carrier witha a buzzer plate and battery to assist with blind boring


This let them know when they were in the relief. Since they were blind boring, they had to stop within one revolution of the thread relief or risk damaging the valve seat, and the micro-switch made it possible to stop in that thread relief. The boring machine had a dial indicator and thread gauge built in that enabled the machinist to tell how far he was advancing the tool. After the bore was machined to size for the threads, they set the threading attachment and achieved 17/12 threads per inch. Once the machine work and thread check was completed, the seat ring was torqued to 12,000 ft. lbs.

On a previous project, Kirkpatrick's team had used another company's machine tool that had made its own threading attachment to do the work but provided no way to use a thread gauge to tell when the threading was to depth.

Once team took the machine, the index was lost, and the machinists had to machine the valve seat ring to accommodate the undersized thread in the valve body.
This caused more than two extra days' work because the valve seat ring then had to be removed, brought to a machine shop, tested and reassembled several times.

“With the Climax BB6000 machine,” said Kirkpatrick, “the threading machine comes out easily as a unit, so we can check the seat ring, test the threading for depth and then keep machining if needed. It was very easy to do and saved a lot of time.”

As a result of using a portable boring bar, facing and threading attachment, the valve was re-machined and threaded to its proper dimensions. Start to finish, the in-situ machining took 12 hours, considerably less time than it would have taken machine it with the conventional method of removing the valve.

MSIV Valve Repair

Machining the surface of a main steam isolation valve's (MSIV) poppet used to be done in a machine shop capable of handling contaminated materials because tools for in-situ machining of a valve's interior was almost non-existent until a new machine was developed to resolve these MSIV maintenance issues for the River Bend boiling water reactor.

MSIVs often fail the low leak rate test due to wear and corrosion between their internal surfaces. This is one of the top causes of nuclear plant time loss. Wear is most often seen in the valve seat and the vanes that guide the poppet in and out of the valve. Machinists worldwide now use this valve repair machine in MSIV's with an inside diameter of 16- to 28-inches (406.4 to 711.2 millimeters) and to a depth of 40 inches (1,016 millimeters) to restore valve seats, guide ribs and pressure seal areas to their proper specifications. It fits on the valve with a mounting sleeve.

The cutting head of the machine is mounted on the bar and feeds automatically radially or at a pre-set angle. The expendable spider chucks are adjusted for size and concentricity once inside the valve. It uses a single-point cutting head, and a grinding head can be inserted in the chuck while the machine is inside the valve to be used for finishing the metal surfaces. The machine is lightweight and all controls are placed atop the machine for easy operator access to enable machinists to correct leakage using a simple and efficient process.

Circulating Pump Repair

Circulating pumps' traditional packing is converted to mechanical seals to prevent wear on pump shafts and to ensure better leakage control. During one such conversion, the pump casings were severely corroded around the stuffing box faces, which would be used as a gasket surface for the mechanical seal. The stuffing box face had to be realigned and machined to provide a flat, true surface for the gasket before the new mechanical seals could be installed.

The size of the pump — more than 11 feet deep — posed another challenge. For this project, a portable boring machine was used with a 12-foot boring bar and a heavy-duty facing attachment. The portable boring machine is capable of line and blind boring even in cramped places in which other tools won't fit, and its inside-diameter bearing mount brackets have a wide mounting range. Its heavy-duty facing head attachment has a high metals removal rate to handle large-diameter facing jobs and can achieve a flatness of 0.003 inches. See Figure 4.

Circulating pump seal replacement
Figure 4. Circulating pump seal replacement

To position the 12-foot bar from one end of the pump to the other, a machinist got inside the pump to set up the inside diameter (ID) bearings. With the top half of the pump casing off, the bar was set in the bore with ID bearings. After the top was put back on, mounts were made to put the adjustable bearing on the outboard ends of the pump, allowing the bar to be adjusted, and the drive and feed hooked up. The bar was set up and aligned with a dial indicator so that it was perpendicular to the shaft and parallel with the installed bearings.

The surface was then cut with a facing attachment. This set-up avoided misalignment of the mechanical seal. Typically, converting pumps to mechanical seals can take up to 80 hours, but because machinists were able to conduct the repair on site, the project was completed within 25 hours, saving a substantial amount of time and labor.

Portable Machine Tool Customization

A select few portable machine tool designers and builders can custom-design these machines to fit specific repair applications such as re-facing worn flanges on heat exchangers, valve bases or piping systems. Moreover, the capability now exists to integrate Computer Numerical Control (CNC) software technology into these machines. With a CNC-equipped portable machine tool, control information can be programmed using 3D models to define geometric information for boring, aligning and re-facing contoured pipes or other fixtures. Maintenance teams have found they can perform more intricate repairs in situ as well as reprogram them to expand their use for more complex geometries—such as refurbishing corroded funnel -shaped, contoured and curved valves on turbines.

Conclusion

As the demand grows to reduce power plant equipment downtime, plant owners are being driven to find more innovative and cost-effective methods to safely streamline repair and maintenance processes.

 As these examples clearly demonstrates, portable machine tools have been steadily evolving to facilitate on-site repair, offering significant advantages over traditional machine shop methods and enabling plants to get back online faster.