What is the most likely cause when a feedwater pump becomes vapor bound?

When a feedwater pump becomes vapor bound, it means that steam or vapor has entered the pump, preventing it from transporting liquid effectively. The most likely cause of this condition is the temperature of the water being too hot. When water exceeds its boiling point at a given pressure, it begins to vaporize, forming steam bubbles within the pump. This vapor can create a situation where the pump is unable to generate sufficient pressure to push liquid water through, leading to vapor binding.

In the context of boiler operation, maintaining an appropriate feedwater temperature is crucial. If feedwater becomes too hot, it not only risks phase changes but can also accelerate the wear and tear on pump components. A combination of high temperature and insufficient pressure can exacerbate the likelihood of vaporization occurring within the pump.

Other options do not directly lead to vapor binding. While very cold water might create different operational challenges, and incorrect pump pressure could affect performance, it is primarily the temperature exceeding safe limits that leads to vapor formation and the associated binding issue. Additionally, a blocked suction line typically causes cavitation rather than vapor binding, as it restricts the flow of liquid into the pump.