When discussing power plants, the common applications for LVDTs and derivative products like LVRTs are measuring valve positions in steam turbines and gas turbines. There are also some applications for quarter-turn rotary valve position. In addition, there are applications within power plants for measuring case or shell expansion and bearing vibration/expansion for predictive maintenance (PM) purposes. Macro Sensors supplies LVDTs to Bently Nevada, the best-known name for case and shell expansion and other PM measurements. We are an OEM supplier to Siemens-Westinghouse and General Electric of high temperature LVDTs and LVRTs for their steam power plants and we are in the process of developing LVDT products for GE's gas turbines.

In a typical power plant, the diameter of the steam pipe coming into a turbine is 8-12 inches (200-300 mm), and the valve strokes vary anywhere from about 6 inches (150 mm) for some very short ones to 20 inches (500 mm) for the big ones. A typical power plant has a minimum of four valves on each steam turbine, sometimes as many as seven valves. Normally there's a reheat stop value, an interceptor valve, a governor valve, and a throttle valve. These are the four standard valve functions that are used with a steam turbine, but sometimes there will be more than one governor valve or more than one stop valve and so on. Of those valves being used, for two of them plant operators have to know the position all the time and for the other two, the reheat stop valves and the interceptor valves, they want to know the position of the valves at start-up and at shut-down. Ultimately, they are fully open or fully closed depending on the start/stop status of the turbine.

The governor and throttle valves are modulating valves that might be 25 or 40 percent open depending on how much power is being generated. The positions of these valves depends on the control schemes for the plant. Typically, the plant has very highly developed control algorithm, the whole purpose of which is to increase operating efficiency and save fuel. For a medium sized plant, a 2% efficiency improvement translates into a million dollars fuel savings annually. That's a big, big incentive to update and rehab a power plant. Today, turbines that date back fifty years or more are still perfectly good if they've been well maintained. This is why so many of our customers buy LVDTs for doing these rehab jobs.

LVDTs for shell expansion, bearing vibration, and shaft end play are exclusively used in predictive maintenance activities. Bently-Nevada is the number one name worldwide and we sell to Bently as an OEM-qualified supplier. In Bently-Nevada applications like shell expansion, plant operators want to make sure that the turbine isn't going to come apart, and they want to make sure that there's not undue amounts of vibration in the bearings which would indicate that the bearings were wearing out. If the bearing starts to wear, they want to be able to take the turbine out of service during a scheduled outage and do the maintenance on it rather than having the turbine trip off and shut down because there's too much vibration, puting the plant out of operation for a while. For the most part, the LVDTs involved in vibration or case expansion measurement tend to be moderate ranges: a few inches down to one-half an inch (10-100 mm).

Macro Sensors offers Westinghouse custom LVDTs and GE LVRTs plus custom designed position sensors for system integrators to work in their multiple-redundant control systems. These systems have become the watchword of the day... multi-redundant systems so that if there is a failure, the system can stay running and also so that utilities don't necessarily have to stock spares. These systems operate on a polling basis, and if one sensor's reading disagrees with others in the system, the system disregards that LVDT's readings. In these redundant systems, LVDTs typically measure position of steam valves, water level in boilers, and lubricant levels in the bearings.

However, not withstanding the redundant systems, LVDT failures in reality are extremely rare. Any failures that we have had have not been electrical. Because we provide LVDTs for the power industry with core connecting rods, where there has been a purported failure of an LVDT, invariably there have been installation mistakes that cause overstressing or bending of the point where the core is brazed to the core rod. Mechanical failures due to improper installation have been documented time and time again.