B.O.G(Boil-off gas) is generated while LNG is being transported, and this gas is utilized as fuel in steam turbine for sailing of LNG Carrier. Therefore, water-feeding shall be maintained very strictly on LNG Carrier. Meanwhile, the general-purpose-steam is supplied from the main boiler, and utilized to operate various auxiliary components of the ship. However, if those auxiliary component are defaulted or being operated incorrectly, feed-water could be mixed with boiler water, and eventually the life-time of boiler could be shorten. In this reason, the steam for mail boiler and general-purpose-steam are separated.