Alliance Sensors Group’s PG Series LVDTs Prove Their Durability Once Again

The PG series of heavy duty LVDT position sensors from Alliance Sensors Group have become the defacto standard of the power gen industry for measuring steam valve position on power plant turbines, having demonstrated outstanding performance, reliability, and durability over many years of operation.
These LVDTs ’ robust construction is clearly illustrated by the following pictures from a power plant in which the failure of the anti-rotation mechanism on the shaft of the hydraulic actuator operating a modulating steam valve while the turbine was operating resulted in extreme misalignment of the LVDTs’ extension rods, as shown in Figure 1 below.

Figure 1 Extension rods of LVDTs severely misaligned after anti-rotation device on cylinder shaft failed.

Because the LVDT extension rods are relatively large in diameter and utilize a ball joint coupling to the anti-rotation arms, as shown in Figure 2 below, the rods did not fracture at their coupling threads, as might otherwise have been expected. Instead, the ball joints conformed to the extension rods’ angular misalignment, relieving bending stress on the extension rods’ coupling threads that can lead to fracture.

Figure 2 Close-up of the ball joint couplings on the LVDT core extension rods at the anti-rotation arms.

Plant maintenance personnel straightened out the LVDT extension rods by correcting the anti-rotation problem after rotating the mechanism back into position WITHOUT SHUTTING DOWN THE TURBINE, as shown in Figure 3 below. Keeping the turbine operating during this repair avoided a costly shutdown.

Figure 3 Extension rods re-aligned only by adjusting the anti-rotation mechanism back into position.

Features enhancing the reliability and durability of ASG’s PG LVDTs in power gen applications:

• A large shaft which totally captivates the LVDT’s core so it never can break off and fall way
• Double shaft seals to prevent the ingress of contaminants that could compromise reliability
• Ball joint couplings to overcome shaft misalignment which can lead to coupler joint failures
• Screw clamp terminals that totally eliminate potential connector contact and sealing issues
• ½ / ¾ NPT hub for easy connection of cord grips or liquid tight and sealtite flexible conduit