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American Society of Heating and Ventilating Engineers Guide, 1928 IIIChapter --Steam Heating Systems and Piping Fig. 6. Connecting Two Boilers Using Check Valves and Equalizers in Returns upon the available pressure drop through the pipe. Where no water is present or where a limited quantity flows by gravity in the same direction as the steam the available pressure drop only need be considered. Where water and steam flow counter to each other the velocity of the steam must not exceed certain values above which disturbance between the counter flowing steam and water may produce objectionable sounds, TRAP Fig. 7. Connecting Two Boilers Using the Approved Return Connection 89