exchangers connected to fresh seawater or divertible to recirculate through tanks of seawater in the engine room. Both supplies draw heat from the engines via the coolant and oil lines. Heat exchangers are plumbed in so that oil is represented by a yellow mark on the flange of the pipes, and relies on paper type gaskets to seal the mating faces of the pipes. Sea water, or brine, is represented by a green mark on the flanges and internal coolant is represented by blue marks on the flanges. Thrusters In addition to this array of equipment is the ship's thruster system (on modern vessels fitted with this equipment), typically operated by electric motors controlled from the bridge. These thrusters are laterally mounted propellers that can suck or blow water from port to starboard (i.e. left to right) or vice versa. They are normally used only in maneuvering, e.g. docking operations, and are often banned in tight confines, e.g. drydocks. Thrusters, like main propellers, are reversible by hydraulic operation. Small embedded hydraulic motors rotate the blades up to 180 degrees to reverse the direction of the thrust. A variant on this is the azipod, which are propellers mounted in a swiveling pod that can rotate to direct thrust in any direction, making fine steering easier, and allowing a ship to move sideways up to a dock, when used in conjunction with a bow thruster. Safety Fire precautions Engine rooms are noisy, hot, usually dirty, and potentially dangerous. The presence of flammable fuel, high voltage (HV) electrical equipment and internal combustion engines (ICE) means that a serious fire hazard exists in the engine room, which is monitored continuously by the ship's engine department and various monitoring systems. Ventilation If equipped with internal combustion or turbine engines, engine rooms employ some means of providing air for the operation of the engines and associated ventilation. If individuals are normally present in these rooms, additional ventilation should be available to keep engine room temperatures to acceptable limits. If personnel are not normally in the engine space, as in many pleasure boats, the ventilation need only be sufficient to supply the engines with intake air. This would require an unrestricted hull opening of the same size as the intake area of the engine itself, assuming the hull opening is in the engine room itself. Commonly, screens are placed over such openings and if this is done,