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:
. The cavitation noise reaches a maximum value at an NPSH margin ratio greater than 1.0 (NPSHa = NPSHr), then decreases. The decrease occurs because air comes out of solution with the vapor formation, cushioning the effect of the vapor bubble collapse. The cavitation noise (an indication of the cavitation erosion rate) is greatly reduced as more air comes out of solution. The amount of dissolved air varies with the water temperature (Budris and Mayleben, “Effects of Entrained Air, NPSH Margin, and Suction Piping on Cavitation in Centrifugal Pumps,” 1998 (Ref. B-8)). DG-1385, Appendix B, Page B-7 Figure B-2 Impact of dissolved and entrained air on NPSHr DG-1385, Appendix B, Page B-8 Figure B-3 Typical relative cavitation erosion rate versus NPSH margin ratio near best efficiency point flow rate The second effect of noncondensable gas coming out of solution at the low-pressure region within the pump is to increase the NPSHr by increasing blockage. Sufficient gas can interrupt pumping altogether through “gas locking” or “gas binding.” Both entrained and dissolved air/gas will increase the NPSHr of a pump by increasing the blockage of entrained and dissolved air at the low local internal pressures within the pump. (See Figure 4-2 of NUREG/CR-2792, “An Assessment of Residual Heat Removal and Containment Spray Pump Performance under Air and Debris Ingesting Conditions,” issued September 1982 (Ref. B-9).) NUREG/CR-2792, Section 4.2, discusses the effects of air and proposes an “arbitrary” relationship between NPSHr and the fraction of air at the pump suction: NPSHrair/water = NPSHrwater (1 + 0.5 AF), where AF is the air volume fraction in percent. As noted in NUREG/CR-2792, Section 4.2, this guideline is intended for use only for air volume fractions less than 2 percent. DG-1385, Appendix B, Page B-9 The effect of air on required NPSH is included as an uncertainty component of NPSHreff. Licensees should determine a value of NPSHreff applicable to their