Water Induction Detector

The Water Induction Detector is an inexpensive, easy to install, in-line instrument for the continuous monitoring of steam systems. It can also be used for liquid induction for other fluids. It can help prevent water hammer, high thermal stresses, and damage of valves and turbine blades and indicate malfunction of steam traps.

Background

Water induction through the steam piping of utility and industrial steam cycles is a low frequency high impact event. In some operating units, main, reheat, and extraction steam pipes get partially or completely filled with liquid water that then moves downstream and damages turbines, steam traps, feedwater heaters, and other equipment. In susceptible units, this usually occurs during startup and other transient operation, however, water induction or re-entrainment can occur during all modes of operation.

Water induction into steam turbines occurs so frequently that the American Society of Mechanical Engineers formed a special committee in order to study this phenomenon. Early detection of the increase in water content and rapid corrective actions can significantly reduce or eliminate damage to process equipment.

Water hammer is an extreme case of water induction where a column of water is forced down a steam pipe at a high velocity. The slug of water impacts elbows, valves and other flow obstructions with significant force, in some cases resulting in significant damage.

Description

Each Water Induction Detector consists of a Detecting Probe and a Controller/Alarm. The Detecting Probe is a cartridge heater with a thermocouple installed at the tip of the heater. The Probe is inserted into the pipe to be monitored and attached using a threaded connection. The power for the heater is supplied by the Controller/Alarm, which is a digital temperature controller with a low level alarm. The Controller/Alarm supplies a constant flow of electrical current to the Detecting Probe to maintain the temperature of the heater approximately 10°F above the saturation temperature of the steam in the pipe being monitored. The Detecting Probe can be installed on any pipe size and wall thickness and on steam systems up to 2000 psia.

When water induction occurs, the exposed section of the Detecting Probe is covered by water, increasing heat transfer from the heater to the fluid. As a result, the temperature at the tip of the heater will be rapidly reduced. The Controller/Alarm monitors the temperature of the thermocouple in the Detecting Probe and activates an alarm when the temperature drops below a preset value. The temperature and digital alarm are output from the Controller/Alarm for recording by a datalogger or data acquisition system (DAS) or for display in a control room. The output can also be used to control a valve or other process equipment to automatically reduce or eliminate the water induction event.