"Fire pumps are an essential part of many fire protection systems. They are, however, vulnerable to damage when certain conditions occur in the water supply. Water emits a vapor over its surface, which produces a certain amount of pressure. This vapor pressure increases as the liquid temperature rises. Water boils when the vapor pressure is equal to or greater than the pressure surrounding the liquid. Vapor pressure is important because of the way it affects pump operation."
Coffee Break Training - Fire Protection Series No. FP-2013-34
United States Fire Administration: http://www.usfa.fema.gov/