Penetration flow into a branch pipe of mixing tee in relation to thermal fatigue prevention

Abstract

Thermal fatigue cracking may initiate at a tee pipe where high and low temperature fluids flow in. The mixing flow causes temperature fluctuation near the pipe wall and may result in fatigue crack initiation. In this study, flow pattern in a branch pipe with low flow velocity was investigated by experiments in order to prevent the thermal fatigue. Visualization of flow was conducted by a test section of transparent acrylic resin and a tracer method. Velocity distributions were also measured by particle image velocimetry. The test section is consisted of a horizontal main pipe with an inner diameter of 60 mm and a vertical branch pipe with an inner diameter of 20 mm. Fluid temperature difference in the inlets was about 30ºC. Visualization tests showed flow in the main pipe penetrated into the branch pipe intermittently. The penetration patterns were categorized by the momentum ratio for the flows in the main and branch pipes. The maximum penetration depth correlated with the momentum ratio. The effect of fluid temperature difference on the penetration depth was insignificant within present test conditions. Thermal fatigue cracking in the branch pipe might be caused, because the flow in the main pipe penetrated into the branch pipe intermittently.