A steam generating plant is a maze of pipes, valves and fittings. Rapid or intermittent fluid flow during start-up or shut-down can cause unwanted vibration and movement of liquid-filled piping. This can induce significant levels of stress in the pipe and its mounting hardware. How can the stress in the piping be measured during power plant operation?

The movement of the piping during vibration can be measured with two types of displacement sensor. For very small movements, just a few thousandths of an inch, an LVDT displacement sensor is best. An LVDT requires an AC excitation signal and returns an AC signal that is proportional to the amount of movement. This AC displacement signal must be converted to a high level DC signal before it can be sent to a data acquisition system.

For larger displacements, up to 12 inches, a lanyard pot is used. A lanyard pot is a device consisting of a multi-turn potentiometer connected to a spring-loaded cable. As the cable extends, the potentiometer turns and produces a DC voltage signal. Lanyard pots require a DC power source and need to be scaled for the displacements to be measured.

The stress in the piping is measured using strain gages. A strain gage is a device that is bonded to the pipe and changes resistance when stress causes the pipe the change shape slightly. Strain gages require precision excitation and high amplification before their signals can be fed into a data acquisition system.

The pressure and flow rate of the fluid in the piping is also measured so that the displacements and stresses observed can be correlated to the relevant operating levels of the power plant.