Naphtha vapours are observed at manway lid of roof top when its opened . Mild naphtha vapours are also observed at roof drain of the storage tank when its opened. Generally roof drain of the storage tank is kept in open condition only . The storage tank is of Internal Floating Roof type . Tank safe height is 10.75m . Generally tank height is maintained at 9 m or lower. Tank outlet Temperature is between 36 to 42deg. Let me know why naphtha vapours are emanating from roof tanks and roof drains. I dont think there is swivel joint leak .