Document: NRC Regulatory Guide
Document ID: 6f0a99f2-d25a-44e3-b7f2-3286449a9752
Document Type: regulatory_guide
Title: Water Sources for Long-Term Recirculation Cooling Following a Loss-of-Coolant Accident (Rev. 5)
Source: NRC Regulatory Guide Division 1
Source URL: https://www.nrc.gov/docs/ML2126/ML21266A185.pdf
Revision Date: 2023-05
Chapter: 
Section ID: RG-1.82
CFR Part: 
CFR Title: 

Content:
nts. The staff proposes that the NPSH margin be calculated as NPSHa − NPSHreff, where NPSHreff is the NPSHr3% value with uncertainties included. This calculated NPSH margin should be greater than or equal to zero. NPSHr as a function of flow rate is typically obtained by testing the pump in question or a similar pump at the pump vendor’s facility in accordance with ANSI/HI 14.6-2016. Operating experience shows that the NPSHr of a pump installed in the field is greater than the NPSHr obtained by testing at the pump vendor’s facility. This is because the following factors may differ between the field and the vendor’s tests, introducing additional uncertainty: a. pump speed (because of motor slip) b. water temperature c. suction piping configuration d. air content of water (which may be lower in the vendor’s test than in pumped water in the field) DG-1385, Appendix B, Page B-6 These sources of uncertainty are discussed in greater detail below. a. Pump speed The NPSHr varies as the square of the pump speed, which changes with change in the motor slip. Operation at less than full rated motor power or with high-efficiency motors tends to reduce motor slip. Motor slip can cause the pump to operate at slightly higher speeds in the field compared to a factory test speed with the factory-calibrated motor. b. Water temperature The NPSHr decreases as water temperature increases. Pump vendor tests are mostly run with water at lower temperatures than in the field. At higher water temperatures, the saturated vapor pressure and vapor density increase, which reduces the NPSHr, resulting in higher apparent NPSH margin. However, this could be offset by the effect of an increase in the vapor pressure on the NPSHa, which would reduce the apparent margin. Therefore, the effect of water temperature should not be included when determining the NPSHr in the field. c. Suction piping configuration For acceptable pump operation, it is important that the flow entering the pump inlet be