Fire Hydrant Maintenance in Cold Weather
Conclusion of Articles—Question of Thawing Simplified Where Alternating Current Is Available—Apparatus May Be Mounted on Motor Truck or Trailer—Time Required for Thawing
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Thawing with Alternating Current
Where alternating current is available, the question of arranging apparatus for thawing is somewhat simplified, though a factor of danger is introduced through stepping down high voltage current. Distribution lines in alternating current systems may carry a voltage of from 2,200 to 6,600, and it is necessary to bring this down to 110 or 55 volts, depending on the size of transformers employed. With the higher voltage a longer line of pipe can be thawed at one time, but where the apparatus is to be used chiefly for hydrants, the 55 volts is entirely sufficient.
Apparatus for alternating current thawing may he mounted either on a motor truck or trailer, the latter being possibly due to the comparatively light weight of A. C. equipment as compared with D. C. The apparatus necessary to the thawing out includes: transformers, water rheostat, ammeter, watt hour meter, should the current be paid for at meter rates; 500 feet of 400,000 C. M. extra flexible, insulated copper cable, in 100-foot lengths. Cable for connecting the transformers to high voltage distributing lines would, of course, be furnished by the power company.
A typical layout of equipment of A. C. apparatus is shown in Fig. 9. In this case the line voltage is 2,200 and the transformer ratio is 20 to 1, giving 110 volts on the secondary side of the transformer. The total transformer capacity for thawing hydrants and mains should be from 30 to 55 kilowatts, the lower figure being for hydrants and short stretches of pipe while the larger, for long stretches. In some cases single transformers will do while in others two connected together may be necessary.
Fig. 10 gives connections for both 30 and 55 kilowatt transformers on 2,200 and 6,600 volt lines, indicating amperage in both primary (high tension in all cases shown) and secondary, the low voltage side.
Time Required for Thawing
Five hundred amperes will thaw a 10-inch main as well as a 6-inch line, but the time required for the larger pipe will be much greater. For instance, the New York Edison Company used 416.6 amperes for thirteen minutes to thaw out 52.3 feet of 2-inch pipe, while in the proceedings of the American Water Works Association figures show that 400 amperes were required for two hours to thaw out 800 feet of 10-inch pipe. The local conditions also govern the time required for thawing, such as moisture in earth, whether or not pipe line passes through salt water bodies, etc.
The following data is given to show the drop in voltage along pipe lines of different diameter, where joints are well made and electrical contact is good.
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Actually, conductivity of pipe joints varies widely, making necessary the use of rheostats to regulate the current to a safe amount, both for the generating or transforming apparatus, and the pipe joints themselves. While transformers may be overloaded 50 per cent for fifteen minutes or so in cold weather, or 25 per cent, for an hour, still the danger is present of burning them out if they are called upon for an excessive amperage.