Designed to fit on and around view-through visual flow indicators, bolt-on heating jackets allow direct observation of high-temperature or temperature-sensitive process fluids without creating cold spot on process piping. Jackets are composed of clamshell halves consisting of carbon or stainless steel pressure chamber within aluminum casting, and are capable of maintaining operating temperatures up to 750°F. Heating media includes hot water, steam, hot oil liquid, and vapors.