Efficient And Safe Operation of Indirect Fired Heaters
Authors: Marlett F.D. 
Published: March 2010

Indirect fired heaters are used to preheat natural gas pipeline flow upstream of gas pressure regulating equipment to lessen the possibility of water ice and natural gas hydrate freeze-offs. Design features of indirect fired heaters will vary among heater manufacturers, and the heat transfer capacity of such units can vary considerably depending upon installation site natural gas process stream heat duty requirements. However units generally include a heat source, which is normally a natural gas fuelled burner firing through a submerged furnace called a firetube, an intermediary heat transfer fluid or bath, and a gas coil conqrining, the high pressure natural gas to be heated, with the fire tube, gas coil and surrounding heater bath enclosed in a shell. This paper describes conventional, natural draft indirect fired heaters having atmospheric burners and non-electric control systems. Discussion topics include heater features, codes and regulations, design sizing and selection, operating and maintaining procedures, inspection and maintenance.

Introduction

The presence of water in natural gas has always been a source of potential trouble in the transportation, measurement and regulation of natural gas. One of the chief difficulties has been natural gas flow reduction or stoppage due to water ice and natural gas hydrate freeze-offs of pipeline gas pressure regulating equipment, arising from associated pressure reduction gas temperature drop. To lessen the possibility of such freeze-offs, natural gas flow may be preheated upstream of pipeline gas regulators using a type of heater referred to in the gas industry as an indirect fired heater, also known as a line heater(1–4).

Northwestern Utilities Limited (NUL) has in excess of 40 years of operating experience with indirect fired heaters installed in regulating and metering stations throughout its natural gas pipeline system in Alberta, Canada(5–8). This paper describes Northwestern Utilities Limited's experience with in-house fabricated conventional, natural draft indirect fired heaters having atmospheric burners and non-electric control systems.

Indirect Fired Heater Design Features

The design features of indirect fired heaters will vary among heater manufacturers, and the heat transfer capacity of such units can vary considerably depending upon installation site natural gas process stream heat duty requirements. However, units generally include a heat source, which is normally a natural gas fuelled burner firing through a submerged furnace called a firetube, a heat transfer fluid or bath, which in northern climates is typically an aqueous ethylene glycol solution, and a gas coil containing the high pressure natural gas to be heated. The fire tube, gas coil and heater bath are enclosed in a shell which is normally a horizontal vessel. These heaters are called indirect fired because the fuel gas heat of combustion in the firetube is first transferred to, and through, the surrounding intermediate heater bath, and then into the cooler natural gas stream circulating through the gas coil. API Specification 12K describes such general features and other components(1). Presented here is a description of the major components of indirect fired heaters designed and built by Northwestern Utilities Limited for installation in Alberta.