D type water tube, steam boilers consist of a mud drum and an upper steam drum. The design is a well proven and reliable design for high pressure and high capacity steam producing systems. The large and sufficient combustion chamber allows the use of different types of oil and gas fuel efficiently. The structure of the combustion chamber consists of membrane wall type water tube sections. The flue gas travels inside the tube bundle convection heat transfer section and then leaves the boiler from the side smoke exit channel. Super heater or economizer can be placed in the boiler system on customers’ request. The steam drum design is very important in water tube steam boilers. Saudi Boiler has developed its own cyclone separator design with the help of computer aided analyzing. The heat transfer area of the boiler is indicated definite and it is the net area along where, the heat is transferred inside the boiler. D type water tube model boilers can be produced for an operating pressure range of 0,5 bar – 40 bar.Boiler is supplied with all necessary accessories, control devices and auxiliary equipment on customer’s request. Isolation of the boiler is specially made to minimize the thermal energy loss. Special covering plates are used on the surface, to protect the shell of the boiler from outdoor conditions and to prevent tearing. D type water tube boiler consists of: Mud drum, steam drum and water tubes at proper thickness, according to the boiler’s operating pressure. EN12952 is taken as reference for design calculations where applicable.