Various embodiments herein pertain to apparatus and methods that utilize the water and existing chemicals to generate a foam. The foam can be introduced at that gas-path entrance of the equipment, where it contacts the stages and internal surfaces, to contact, scrub, carry, and remove fouling away from equipment to restore performance. Various embodiments include operating a gas turbine engine; measuring the performance of the engine during operation; determining that the engine should be foam washed based on the measurements; mixing pressurized gas with pressurized liquid and creating a supply of foam; and streaming the supply of foam into the engine.