A specialty classification that involves installation, maintenance, repair, alteration or extension of a system of piping, tubing, pumps, apparatus and appurtenances in connection with pressure piping used for circulation, transporting, holding or processing of gas, vapor, fluid, liquid or any combination thereof. This classification includes transmission and distribution of gas, but does not cover boilers, boiler piping, piping used to convey potable water, sanitary sewage, liquefied petroleum, manufactured or natural gas, air conditioning and comfort heating piping.