Gate valves, which are widely applied in pneumatic conveying systems, are vulnerable to erosion by particles. It is thus important to investigate the erosion in gate valves from the perspective of fluid analysis, and then to predict and improve their lifetimes. The effects of valve geometry and gas–solid flow conditions on valve erosion are investigated. Since a gate valve usually operates fully open to let fluid pass through, the geometry is simplified as a cavity. As gate valves are always placed horizontally in industrial situations, investigated cavities are placed horizontally, and the erosion damage to the bottom half of the aft wall (surface T), which is most likely to be eroded, is studied. A computational fluid dynamics (CFD) based two-way Eulerian–Lagrangian procedue is used to predict the erosion severity. The simulation procedure is validated by comparing the CFD results with those obtained from experiments of a pipe and an elbow, and also with the erosion region of a damaged valve. For convenience, the total erosion ratio, defined as the ratio of the mass eroded on a particular surface to the total particle mass passing through the pipe inlet during the same time, is introduced. The results show that the total erosion ratio of surface T is largely independent of the mass flux ratio, pipe diameter and cavity depth. Meanwhile, the total erosion ratio increases with cavity width and particle diameter, while it decreases with inlet velocity. According to the fitted results, a simple erosion formula is proposed and validated by the CFD results in another 16 orthogonal experiments. Furthermore, the formula is improved for various values of Brinell hardness of carbon steel and sharpness factors of particles.