The primary component of a Hartford loop diagram is the location of a Hartford loop top and its distance from the boiler water top. Standard Hartford loop diagrams generally place the top of the loop 2 inches below the steam boiler's water line.
Other components of a Hartford loop diagram include the steam main, condensate return, wet return and close nipple. All components except the close nipple are also present on standard boiler diagrams without Hartford loops.
The purpose of the Hartford loop is to prevent the steam boiler from losing all of its water in the event of a leak. The loop is a square bend in the boiler piping, forcing the water to rise up a pipe and back down again. A close nipple connects the loop to the condensate return above the water line. If the main wet return line breaks, the Hartford loop is still able to siphon water from the boiler. As a result, only a minimal amount of water is lost during a pipe leak.
The Hartford loop was invented in 1919 by the Hartford Insurance Company to protect itself from recurring steam boiler damage claims. Due to its overall safety, many boiler makers now require a Hartford loop to be added to most steam boiler designs.