Horizontal heater treaters are a type of vessel used in the oil and gas industry to separate oil, water, and gas produced from wells. They are a crucial part of production facilities, helping to process and treat crude oil before it is transported or stored. Horizontal heater treaters use a combination of heat, gravity, and chemical processes to separate oil from water and other impurities, ensuring that the oil meets quality standards and that water is sufficiently clean for disposal or reinjection.