Tube inlet orifice design of a once-through steam generator for flow stabilization

Performance Analysis and Tube Inlet Orifice Length Evaluation of a Once–Through Steam Generator

“…The minimum orifice length ranges from 183.8 mm to 198.9 mm for 100% power level, and from 285.1 mm to 288.0 mm for 20% power level. As expected, the low power level results in a higher minimum orifice length because flow instability tends to be more severe at a low feedwater flow rate [6] and this accordingly brings forth a higher orifice loss coefficient [4]. Thus, the secondary coolant flow rate for the minimum power level should be chosen as the orifice design condition; this implies that the orifice length is determined by the lowest power level considered in plant power operation.…”

“…The orifice has a narrow spiral flow channel that provides high flow resistance. Introducing the hydraulic resistance ratio, orifice pressure drop, and geometry formulas gives an orifice length criterion for flow stabilization as follows [4]…”

“…For this reason, a screw-type tube inlet orifice was introduced and detailed orifice design was implemented for the MRX OTSG (JAERI-developed integral reactor [4]. In Reference 4, the secondary coolant outlet pressure was controlled to maintain constant thermal power of the OTSG and the corresponding orifice length was evaluated.…”

“…Here, the design margins for uncertainties and tube plugging are taken into account by applying an area utilization factor in each control volume and by changing the number of tubes. In the present study, the design data of MRX OTSG [4] is used, and the thermal-hydraulic performance of the OTSG is investigated in terms of the thermal power, outlet degree of superheat, and secondary coolant pressure drop. The steam generator condition is divided into five steps according to the plugging ratio, as defined in Table 1.…”

“…Here, SG condition-1 represents the initial stage of the OTSG operation without tube plugging, whereas SG condition-5 denotes the final stage when the OTSG experiences tube plugging of α = 10.3%. The average tube length of the OTSG is 47.0 m with a tube plugging ratio of α = 10.3% and 5% design uncertainty (a = 0.95) [4].…”