What is a common cause of a feedwater pump becoming vapor bound?

A feedwater pump becoming vapor bound typically occurs when there is a presence of vapor in the pump due to conditions that prevent the proper flow of liquid feedwater. When the boiler water is too hot, it can lead to the generation of steam or vapor within the system, especially if the feedwater temperature is significantly lower than the boiler water. This vapor formation can fill the pump casing and lines, preventing it from effectively drawing in liquid water, thus causing the pump to become vapor bound.

In contrast, a cold water inlet may not contribute to vapor binding as the inlet temperature can help maintain liquid status. An excess in feedwater supply does not directly cause vapor binding; rather, it may inhibit the system's performance in other ways. Insufficient steam pressure typically relates to operational issues but is not a direct cause of the feedwater pump becoming vapor bound. Hence, the correct option highlights the critical relationship between the temperature of the boiler water and the potential for vapor formation in the feedwater system.