During the planned outage, the unit rested quickly after the turning gear failed to engage. The unit cooled while stationary as the rotor could not be turned manually. Help was needed to diagnose turning gear and unit-turning issues. Attempts to roll the rotor failed; something internal to the steam path was preventing the rotor from performing as designed.

One potential culprit was failed J-seals. The fact that the rotor wouldn’t turn was an indication that material may have migrated downstream, and at this point, plans were made to inspect the downstream steam path for damage. In this case, there was concern that broken J-seal material may have traveled into the LP system.

Casing drawings of the unit identified the location of four borescope inspection ports. The first port was opened and provided access to the last stage IP exhaust to the LP inlet crossover pipe (Figure 2).

2. Spotting J-seal liberation. A borescope inspection provides images of J-seal liberation inside the turbine. Here, the arrows point to J-seal issues, with J-seal and caulking wire liberated from the casing groove, likely sending material downstream. Courtesy: HRST

Liberation risks include reduced sealing efficiency, vibration from steam whirling instability, and potential hot spot rubs due to material balling. In the worst case, material migration can create substantial downstream steam path damage, and foreign material freely flowing through a steam path wreaks havoc on seal interfaces. Even the smallest material can lodge between mating seal faces, and be picked up by the rotating seal teeth, compromising the tolerance between rotating and non-rotating components and damaging the seal teeth.

The seal teeth grab the loose material and then swipe away the stationary seal face, creating a ball of seal material and a running high spot that picks up heat and causes vibration. In severe cases, the high spot impacts the rotor as it coasts down to the turning gear. As speed decreases, the ball of material cools, but if the rotor stops for any reason, the material in effect welds the stationary casing to the rotating blades, and the rotor locks up (Figure 3).

3. J-seal material can lock up a rotor. A ball of material swiped from the stationary seal face, such as that shown here, can weld itself to the blades and lock up the rotor, causing damage and necessitating repairs. Courtesy: HRST

A detailed review of the system drawings (Figure 4) found a threaded pipe plug located at the LP steam piping elbow entering the LP system. Scaffolding was erected, insulation removed, and a borescope inserted into the crossover pipe elbow. This provided access to the LP steam path flow divider and first-stage LP nozzles.

4. The steam path arrangement. A review of the system drawings for the steam path in this case located a strategically placed inspection plug that enabled a borescope inspection of the low-pressure (LP) steam path flow divider and first-stage LP nozzles. Courtesy: HRST

A J-seal caulking wire was discovered, lodged in the LP stage 1 nozzle (Figure 5). Due to the flow of the steam path, the material could only have come from upstream. This proved that the failed J-seals identified at the IP exhaust had migrated downstream and were very likely the cause of the locked rotor condition.

5. One of the culprits. A caulking wire that had migrated from upstream in the steam path was found in the LP stage 1 nozzle, providing more evidence of upstream J-seal failures. Courtesy: HRST

After discovery of the migrated caulking wire, it was recommended that the LP crossover pipe be removed to provide manual access to the flow divider for closer inspection and material removal. The plant also decided to uncouple the HP/IP rotor from the LP rotor to verify which rotor was locked; the HP/IP rolled, the LP did not. Access to the LP flow divider and stage 1 nozzles allowed the borescope to proceed farther back in the LP steam path, and showed a migrated J-seal at the inlet of the stage 2 nozzle.

Open-casing Inspection

It was recommended the LP rotor be removed from the casing in order to fully inspect the steam path for damage and remove the liberated material.

With the turbine open (Figure 6), an attempt was made to roll the unit with a brass bar placed through a coupling bolt hole. The rotor would not roll, so more force was applied using the overhead crane; the rotor rolled, but slowly. The rotor was clearly locked up on a high spot; the sound of metal scraping the casing was heard as the rotor turned. Once the high spot passed the casing split joint, the rotor rolled smoothly without noise.

6. Looking inside the LP turbine. The LP rotor is shown here after the upper half of its casing was removed. Once open, the rotor was manually turned, which led to the determination that swiped J-seal material (shown in Figure 3) was responsible for the binding in this case. Courtesy: HRST

The material shown in Figure 3 had welded itself to the casing, and was determined to be the swiped seal material responsible for binding the rotor. With proof that the migration of J-seal material into the steam path was creating restriction, the decision was made to open the HP/IP rotor to inspect for more J-seal damage.

The top half of the HP/IP casing was removed, and the stationary seals were inspected, which allowed for failure mapping and a closer look at failures across the system at each stage. The failure analysis did not point to a rub event as the cause of failure as the seal teeth were in good condition, with no signs of “mushroom” as would be expected during a rub.

Further inspection determined the caulking wire was backing out of the retaining groove at five stages. Based on the understanding of the installation process, it was determined the same operator likely installed all the seal sets that were failing. The customer’s insurance investigator reviewed the findings independently on-site without HRST’s involvement and came to the same conclusion.

A complete structural inspection of the steam path was conducted to look for any other suspected J-seal failures. Both the HP/IP and LP rotors were removed from the casing to allow access, and two inspectors were tasked with looking at hundreds of J-seals. The inspection included identification and removal of foreign material, visualization of all caulking wire and J-seals, and measurement of all caulking wire butt gaps and peened fits.