Performance Enhancement and Improved Availability in Primary Coolant Pumps After Modification in Seal Cooling Flow Instrumentation

Primary coolant pumps are a prime source of core heat removal in nuclear reactor. The performance and availability of Primary coolant pumps in Research Reactor are monitored by various electrical and process parameters. One such important process parameter is cooling water flow through mechanical seal of Primary Coolant Pumps (PCPs). Adequate seal cooling flow is a vital parameter to ensure healthiness of mechanical seal and thereby avoid damage to the seal and carbon bearings of the pumps. The pump shaft is sealed by a metallic seal followed by labyrinth seal. The mechanical seal is cooled and lubricated by heavy water tapped from pump discharge with required flow monitoring instrumentation. Earlier instrumentation consisted of turbine flow meters which provided continuous monitoring and flow low alarm. In order to avoid noise effects on pulse output, the turbine was designed to rotate at very high speeds so that pulse frequency in much higher the noise frequencies arising from line voltage. Due to high rotational speed, rotor and rotor bearings of the turbine sensor were observed to be failing frequently if some minor suspended particles are found in the seal cooling line. Such failures lead to improper flow readings from the sensor and thereby hampering actual flow condition of Primary coolant pump seal cooling line. Due to the operational constraints, for carrying out any preventive or break down maintenance job on the turbine flow sensors Primary coolant pumps must be stopped. This had an adverse effect on availability and performance of Primary coolant pumps and thereby the Reactor. To address these issues, integral orifice sensors with Differential Pressure (ΔP) transmitters were designed and installed in the seal cooling line of Primary Coolant line. A full scale mockup of the integral orifice and seal cooling flow line was prepared to determine the changes in Differential pressure generated by the orifice and flow through Primary coolant mechanical seal in event of leak or breakage in instrument impulse tubing. Based on the test set-up data, all possible impulse tubing failure modes were addressed in the modified instrumentation by providing high and low flow alarms. After modification, performance of seal cooling flow instrumentation was observed and no failures were found for a period of 2 years. Moreover, due to non-moving parts and sufficient orifice diameter, clogging or wear/tear of sensor is not expected thereby reducing the failure rate and ensuring seal cooling flow in Primary coolant pumps. Thus, overall performance and availability of the Primary coolant pumps have increased.