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6
Steam Boilers Part1 - The Hartford Loop
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Hartford Loop
During the years 1879 and 1891, there were 2,159 reported steam boiler explosions in the United States. These explosions killed 3,123 people and seriously injured 4,352.
We all know that when water boils and becomes steam, it increases in volume 1700 times and this sudden and massive release of steam from water is as destructive as dynamite.
Rolla Carpenter in his classic 1895 book, Heating and Ventilating Buildings, tells how a Cornish cylinder boiler under 30 psig has the ability to leave a basement (steam explosion) with an initial velocity of 290 miles per hour and rise to a height of 3,431 feet!
All it took in the old days to blow a boiler sky-high was a leaking return line. The water level dropped, the boiler dry-fired, the feeder came on; 5-4-3-2-1 lift off!
The first low water cutoffs were introduced in 1923 and they weren't very effective with coal-fired boilers.
The Hartford Steam Boiler Insurance and Inspection Company was, at the time, one of America's larger insurers of steam boilers. They got tired of all the claims that kept coming in so in 1919 they came up with an idea to keep steam boilers from lifting off. It was an arrangement of return pipes which kept the water from siphoning out of the boiler should a return line spring a leak.
The loop was revolutionary when Hartford introduced it because it effectively reduced the rash of boiler explosions. Today, we have reliable low water cutoffs and automatic water heaters, so is the Hartford loop still required? Yes
With this system water is heated to its boiling point (212ºF) and steam rises by convection through pipes to radiators located throughout the house. The air in the pipes and radiators is dissipated into the rooms through air-vent valves that are located either on the radiators or at the end of the steam main. The steam gives up its heat at the radiators by conduction to the air; condenses and the water flows back to the boiler by gravity for re-heating. This system is less efficient than a hot-water system since there is no force convection (like a hot-water system with a circulating pump) so it not used in new residential construction but may still be found in older homes.
It is similar to a hot-water boiler but operates at a higher temperature and pressure. Unlike a hot-water system, there is no pressure-reducing valve or expansion tank. Instead there is a automatic safety control and pressure gauge. A glass water-level gauge shows the water level in the boiler and the low-water cutoff automatically shuts down the boiler should the water level drop too low.
A steam system is a gravity system that is inefficient and slow to respond to changing demands for heat. It works on the principle of convection – steam rises and cooler water descends. To retain heat, most steam pipes are covered with asbestos insulation. This covering should not be disturbed. Inspect the condition of the pipe insulation on an annual basis. If it looks “friable,” call for professional service. Boilers were typically made with heavier-gauge steel and may last about 50 years – some as old as 80 years! The automatic pressure-reducing valve reduces household water supply from 30 to 60 psi (pounds per square inch) to 2 to 10 psi. The boiler is also equipped with a pressure-relief valve that discharges steam if the pressure exceeds 30 to 45 psi.
In a one-pipe system, the same pipe is used to distribute steam to various radiators and carries the condensate back to the boiler. The radiators used in this system must be sloped so the condensate flows back through the supply valve. Each radiator has a manually operated vent valve and supply valve. With a two-pipe system, the steam is supplied in one pipe and the condensate returned by gravity to the boiler by another. The radiators do not have an air-vent valve, instead they are equipped with steam traps that allows the air bound in the radiator and condensate to flow in the return pipe but closes on steam contact. The air is then vented by a main vent. A two-pipe steam system can be converted to a forced hot-water system.
In a dry return system, the return line in the boiler room is above the boiler water level. With a wet return the return line is below the water level. The piping in the latter must include a Hartford loop which prevents water from draining out of the boiler if a leaks occurs in the return line.
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