Piping analysis for the life extension of Heavy Water Plant, Kota

Heavy water production in India has achieved many milestones in the past. One of the most successfully running heavy water plant situated at Kota (Rajasthan) is on the verge of completion of its design life in near future. Heavy Water Plant, Kota is hydrogen sulfide based plant. Various exercises have been planned with an aim to assess the fatigue usage for the various components of these plants in order to extend their life. Considering the process parameters and past history of the plant performance, 25 process critical nozzle locations and connected piping systems are identified. Analyses have been carried out for these critical piping systems for mainly two kinds of loading, viz. sustained loads and the expansion loads. The static analysis has been carried out to find the induced stress levels due to sustained as well as thermal expansion loading as per the design code ANSI B31.3. Due consideration is given to the design corrosion allowance while evaluating the stresses due to sustained loads. At the locations where induced stresses (S1) due to the sustained loads are exceeding the allowable limits (Sh), exercises have been carried out considering the reduced corrosion allowance value. This strategy is adopted due to the fact that the corrosion measurements carried out at site at various critical locations show a very low rate of corrosion. Where it is found that system is getting qualified with reduced corrosion allowance values, it is recommended to keep that location under periodic monitoring. The strategy adopted for carrying out the analysis for thermal expansion loading is to qualify the system as per the code allowable value (Sa). Where it is found that the stresses are more than the allowable value, credit of liberal allowable value as suggested in the code i.e., with the addition of the term (Sh-S1) to the allowable stress (Sa) value, has been taken. If at any location, it is found that the problem of high thermal stress still persists, the strategy adopted is to qualify the system via fatigue analysis route. This is done using the provisions of ASME Code Section VIII, Div. 2 by evaluating the cumulative fatigue usage factor. Results of these exercise reveal that the critical piping systems of the plant are in a very healthy state. With these exercises, it has been concluded that the file of the plants can be safely extended further with enhanced in-service inspection provisions. (author)