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:
ater 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 as uniform as possible and free of swirl (prerotation, before entering the impeller) and vortices. For improved pump performance, the suction piping should be short and straight. This is not always possible in piping configurations in the field. The pressure drop in the piping should be minimized to obtain the maximum NPSHa. d. Air content of pumped water The NPSH margin is affected by the release of noncondensable gases (such as air or nitrogen) dissolved in the water as the minimum pressure in the pump approaches the saturation pressure. The air has several effects: (1) it dampens the effect of cavitation by lessening the shock due to implosion of the condensing vapor bubbles, which causes cavitation erosion damage, (2) it increases the NPSHr, and (3) it may interfere with the water cooling of pump seals. Figure B-2 (taken from Budris, “Technical Report on Task #4 Findings,” dated November 14, 2009 (Ref. B-7)) shows an example of the effect of air on the NPSHr. The “knee” of the curve with high air occurs at a higher value of NPSHr. The solubility of air and nitrogen in water decreases with increasing temperature. This tends to decrease the gas entrained in the pump flow. Figure B-3 (taken from Ref. B-7) shows the effect of air coming out of solution on the erosion rate. The figure is a plot of cavitation noise (due to bubble collapse and measured with acoustic instrumentation) as a function of the NPSH margin ratio. Cavitation noise is a measure of the intensity of cavitation occurring in the pump; it is correlated with the extent of cavitation erosion. 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,