A separation system including a horizontal separation vessel having a gas separation chamber defined by an upper wall of the separation vessel, a gas dome extending above the upper wall, a baffle plate disposed within the separation vessel, and a level of a lower surface of the baffle plate. The lower surface of the baffle plate is disposed below a level of an upper surface of a liquid weir disposed within the separation vessel. The upper surface of the liquid weir may set a liquid level within the separation vessel. The gas separation chamber may include a hydrostatic pressure seal zone between the level of the upper surface of the liquid weir and the level of the lower surface of the baffle plate. Liquid within the hydrostatic pressure seal zone may prevent a gas disposed within the gas separation chamber from escaping into a remainder of the separation vessel.