Disclosed is a bypass valve assembly for a fluid device, such as a heat exchanger having an inlet and an outlet. The assembly defines a bypass passage communicating with the inlet at one end. A tubular structure communicates with the outlet and has a bypass aperture in a sidewall thereof which communicates with the other end of the bypass passage. A semi-cylindrical spring flapper is fitted in the tubular structure and opens to allow bypass flow when the pressure in the bypass passage exceeds a predetermined limit.