PBW stands for Powder Brewery Wash, which is a non-hazardous buffered alkaline brewery cleaner. PBW is the cleaner of choice for many breweries, where it out performs more hazardous caustic chemicals without removing the flesh off your bones. PBW also will not corrode soft metals.
When mixing hot water and PBW in an air or water tight space, it will create pressure. Please use caution when using PBW in enclosed equipment.