Pressure Sensing Line Diagnostics in Nuclear Power Plants

Abstract

Nuclear power plants (NPPs) have been designed to attain safe and reliable functioning through the monitoring and analysis of various critical operational parameters. Data obtained from monitoring systems provide for the control of feedwater flows, recirculation flows, reactor water levels, etc. and can be used to initiate emergency procedures, such as water injection into the reactor coolant system. Therefore, it is crucial for sensing equipment to precisely convey neutron flux, temperature, pressure, level, and flow of plant processes to assure the continued safe and reliable operation of a NPP. A differential pressure transmitter is used to measure fluid flow and level while a nondifferential transducer is utilized to measure absolute and gauge pressure. A NPP generally uses about 200 to 800 pressure and differential pressure sensors to measure the process pressure, level, and flow in its primary and secondary systems (Hashemian, 2006). Pressure transmitters are usually located away from the process to protect them from the adverse effects of ambient temperature, radiation, and vibration on the operability and qualified life of the sensor (Hashemian, 2006). For example, high ambient temperatures will have effects on the mechanical components of the transmitter and shorten the life of its electronics. Pressure sensing lines, also referred to as impulse or instrument lines, are employed to couple a pressure transmitter to the process piping, reactor vessel, or primary flow elements to convey a pneumatic or hydraulic signal from the process to the sensors. In some industrial plants, the pressure sensors are generally installed near the ground using long sensing lines so that personnel can easily access the transmitter for replacement or maintenance purposes. However, process connections using sensing lines may be blocked by accumulations of impurities from the fluid. It is important for the sensing lines to be periodically purged, or blown down, in order to remove any foreign fluid and impurities that can degrade the accuracy of pressure transmitters. Instrument lines can encounter a number of problems that can influence the accuracy and response time of a pressure sensing system. Sensing line problems that have been noticed in NPPs include  blockages due to sludge, boron, or deposits,  air or gas entrapped in low-pressure sensing lines,  frozen sensing lines, and 7