Modern Gasworks Practice

ELECTRICAL PLANT IN GASWORKS 351 lay earthenware pipes in the concrete floor, and to draw all cables in. Wlicn the cables have to cross the gasworks yard, the safest method is to lay armoured cables in wooden troughing rather than to run V.I.R. cables in stoneware duets. The motor is fed by two cables, one carrying the current to it, and the other carrying the return current. The size of cable necessary for any given motor has to be calculated out by the füll load current of the motor. The cables are usually referred to as of certain sectional area, and the size should be chosen so that the ■current does not exceed a density of 1,000 amps per square inch ; 2,500 megohm grade cable is the best that can be obtained and is very suitable for use in gasworks. There are various classes of protection available for cables, which may be either armoured or lead-covered, or supplied with an ordinary vulcanized india-rubber covering. For the majority of positions V.I.E. or vulcanized india-rubber ■can be used. This may in many instances be carried overhead by means of stranded Steel suspension wires and leather suspenders, for example, from the generating station to the corner of the retort house. and may be run along the sides of the walls if a position where it is not likely to be damaged can be found. Where the cable is taken inside the retort house and put within reach of or near the motors it should be run in screwed steel tube. This tube should be run continuously right into the switchgear and from the switchgear direct into the motor terminal box. CONSTRUCTIONAL POINTS The retort house itself is a most unsuitäble place in which. to instal generating plant. It is very advisable that the generating plant should be ins talled in a separate building which can be kept reasonably clean and well ventilated. A thin brick or reinforced concrete building entails but small expense, and permits of the house being designed to suit the lay-out of the plant, and not the reverse, as is so often the case. Ventilation of the generating plant house is of importance, as discoloration of the commutators and the metal work on the switchgear is common if there is a leakage of gas. In no case should generating plant be installed near the purifiers or in the same house as the exhausters. Any trace of sulphur in the atmosphere will aflect the insulation of generators and may eventually lead to trouble. The inclusion of some or all of the following stipulations is advisable when specifying electrical plant for gasworks. (1) Generators must be capable of devcloping their füll load output on con-tinuous rating with a temperature rise not exceeding 75° F. They must be capable of meeting an overload of 20 per cent, for two horns. (2) Generators must be approximately level compound from no-load to füll load. (3) Temperature rise of semi-enclosed ot protected type motors should not exceed 75° F. after six hours full-load run. Temperature rise on totally-enclosed motors should not exceed 90° F. or 95° F. after six hours full-load run. Temperature rise on pipe-ventilated motors should not exceed 85° F. after six hours full-load run.