API SPEC 12K: A Guide to Indirect Type Oilfield Heaters
Indirect type oilfield heaters are devices that use a firetube to heat a fluid bath, which then heats a coil carrying oil, gas, or other fluids. They are commonly used in the production of oil and gas to prevent hydrate formation, reduce viscosity, or increase recovery.
This article will provide an overview of the API SPEC 12K standard, which covers the minimum requirements for the design, fabrication, and testing of indirect type oilfield heaters. The standard was first published in 1972 and has been revised several times, with the latest edition being issued in 2008.
Scope and Application of API SPEC 12K
API SPEC 12K applies to oilfield indirect type fired heaters that are used in the production of oil, gas, and their associated fluids. They are usually located at some point on the producing flow-line between the wellhead and pipeline. The standard does not cover steam and other vapor generators, reboilers, indirect heaters employing heat media other than water solutions, direct fired heaters, shell-and-tube bundles or electrical heating elements, and coils operating at temperatures less than -20ÂF.
The standard specifies the minimum requirements for the following components of indirect type oilfield heaters:
The pressurized coils, which carry the fluid to be heated.
The shell, which contains the heater bath and supports the coil.
The heater bath, which is a water solution that transfers heat from the firetube to the coil.
The firetube, which is a metal tube that contains the combustion gases from the firing system.
The firing system, which provides the fuel and air for combustion.
The standard also defines the termination of a heater coil as the first bevel when coils are furnished beveled for welding, or the face of the first fitting when fittings are furnished as the inlet or outlet connection to the coil. All fittings and valves between the inlet and outlet of the coil are considered within the coil limit.
Design Requirements of API SPEC 12K
API SPEC 12K specifies the design requirements for indirect type oilfield heaters based on their operating conditions, such as pressure, temperature, flow rate, specific gravity, viscosity, and heat duty. The standard also provides formulas and tables for calculating the dimensions and capacities of various heater components.
Some of the design requirements specified by API SPEC 12K are:
The coil shall be designed for a minimum working pressure of 500 psi or 1.5 times the maximum operating pressure, whichever is greater.
The shell shall be designed for a minimum working pressure of 15 psi or 1.5 times the maximum operating pressure of the heater bath, whichever is greater.
The heater bath shall have a minimum specific heat capacity of 0.9 Btu/lb-ÂF and a maximum freezing point of -20ÂF.
The firetube shall have a minimum inside diameter of 6 inches and a maximum outside diameter of 14 inches.
The firing system shall have a minimum turndown ratio of 3:1 and a maximum excess air ratio of 1.5:1.
Fabrication Requirements of API SPEC 12K
API SPEC 12K specifies the fabrication requirements for indirect type oilfield heaters based on their material selection, welding procedures, inspection methods, and marking standards. The standard also provides guidelines for handling, storing, and shipping of heater components.
Some of the fabrication requirements specified by API SPEC 12K are:
The coil shall be made of carbon steel or low alloy steel that meets ASTM A106 Grade B or equivalent specifications.
The shell shall be made of carbon steel or low alloy steel that meets ASTM A285 Grade C or equivalent specifications.
The heater bath shall be made of water with corrosion inhibitors and antifreeze additives as required.
The firetube shall be made of carbon steel or low alloy steel that meets ASTM A53 Grade B or equivalent specifications.
The firing system shall be made of materials that are suitable for high temperature and corrosive environments.