Valve wedges and seat rings that have been exposed to radiation in nuclear power plants can now be safely lapped in areas with low radiation, using a valve grinding system fitted in a stainless steel cabinet with negative air pressure. The solution has been developed by LarsLap in cooperation with test bench manufacturer EMCS.
The grinding cabinet helps minimise exposure to contaminated parts during service. This enables maintenance workers to lap valves close to the affected pipeline, eliminating unnecessary transport of parts and cutting the time for the procedure.
Negative air pressure inside the cabinet prevents escape of contaminated matter. The movement of grinding dust is controlled by a two-stage filtration system compliant with nuclear industry standards, comprising an electrostatically charged HEPA pre-filter and a secondary VHE filter, as well as a radiation monitoring system, keeping the outside atmosphere free from radioactive dust.
Grinding time is minimised by the use of a LarsLap valve grinding system with individually driven grinding heads. This cuts the time required for the grinding process. The time can be further reduced by using diamond grinding heads.
The individually driven grinding heads provide fast and accurate grinding, creating an optimal sealing surface with crosshatch pattern.
The grinding machine has a range of 100 to 400 mm diameter. Rotation and grinding pressure are controlled from the outside of the cabinet.
From start to finish in six hours
During a test conducted by EDF, a contaminated wedge was cleaned, inspected, lapped and returned by the end of the afternoon. The complete time of the service was six hours, with radiation controlled throughout.
Normally, valves are removed and transferred to an intermediary workshop for inspection, via a decontamination workshop. This would normally involve a large number of staff and take considerable time, frequently outside normal working hours, as the risk of exposure to radiation needs to be monitored at all times.
Using the protective cabinet, the grinding machine can be used much closer to the site, reducing the need to move contaminated parts between areas. This minimises downtime and costs. It also simplifies the job, while lowering the risk to radiation during transport and servicing.
The GC400VB protective cabinet operates on 240V electricity supply. The radiation monitoring system indicates radiation in the HEPA filter. A digital display shows radiation inside the cabin. Operation stops automatically if the filtration system is not operating correctly. The cabinet door cannot be opened unless the atmosphere inside is safe.
The cabinet is manufactured by LarsLap, while the filtration and radiation monitoring systems are manufactured by EMCS, a company specialising in test benches for the nuclear industry.
The LarsLap valve grinding system with individually driven grinding heads was originally developed to meet the needs of the nuclear power industry, where the time available for servicing parts is limited due to risk of radiation. It is designed to provide excellent results in the shortest time possible. The system has since found use in many other industries to meet ever increasing requirements for productivity improvements.