Boiler, combined cycle plant, and boiler operation method

A boiler operation method of branching water delivered by a condensate pump, and guiding water in one branch line to a drum while guiding water in the other branch line to a water supply pump and pumping the water to an economizer, wherein the boiler comprises:

• the condensate pump;
• a branch line which causes water delivered by the condensate pump to branch;
• the drum which is connected to one of two lines into which the branch line branches;
• the water supply pump which is connected to the other of the two lines into which the branch line branches, the water supply pump being configured to pump water to the economizer;
• a bypass line which bypasses a portion of the branch line and connects the drum and the inlet side of the water supply pump to each other;
• and
• a check valve which is provided in the bypass line and allows the water to flow only from the drum side to the water supply pump side,
• the method comprising:
• guiding a portion of the water in the drum with a pressure higher than a pressure on an inlet side of the water supply pump to the water supply pump side via the check valve by a pressure difference when an intake pressure of the water supply pump has become lower than a predetermined pressure due to a charge pressure generated by the condensate pump decreasing during operation of the condensate pump and the water supply pump.