Manufacturing method of porous glass base material for optical fiber and manufacturing apparatus

A manufacturing apparatus of porous glass base material for optical fiber comprising:

• a raw material tank storing a liquid raw material, which is organic siloxane in a liquid state and filling the remaining space with inert gas;
• a liquid feed pump to pump the liquid raw material from the raw material tank;
• a circulating pipe that returns part or all of the liquid raw material pumped by the liquid feed pump to the raw material tank;
• a first supply pipe branched from the circulating pipe;
• a pressure loss unit provided downstream of the first supply pipe;
• a second supply pipe provided downstream of the pressure loss unit;
• a mass flow controller provided downstream of the pressure loss unit via the second supply pipe to control the flow rate of the liquid raw material to a predetermined flow value;
• a third supply pipe provided downstream of the mass flow controller;
• a vaporizer provided downstream of the mass flow controller via the third supply pipe to vaporize the liquid raw material;
• and
• a burner that combusts the raw material gas vaporized by the vaporizer to deposit silica fine particles,
• wherein
• when the internal pressure of the raw material tank is P1, the internal pressure of the circulating pipe is P4, the internal pressure of the second supply pipe is P3, and the internal pressure of the third supply pipe is P2, P1≤P2<P3≤0.6 P4 is satisfied, and
• a distal end of the circulating pipe extends to a lower region of the raw material tank so as to be positioned below a liquid surface of the liquid raw material in the raw material tank.