There is no part of an oxygen metal cutting equipment which requires to be more thoroughly understood and carefully operated than the acetylene generator. This is perhaps more especially true in the case of small portable generators, now so extensively employed in connection with oxygen cutters in scrap-metal yards. It is false economy to purchase a generator that is too small or only just large enough for the job ; the size of generator should be such that recharging is not required more than twice a day and preferably once a day. A table giving size of generator recommended for various outputs of acetylene will be given by makers. Fig. 53 shows a good type of portable generator which is largely used, but there are others on the market equally reliable if the maker's instructions are complied with.

All persons responsible for the working of acetylene generators, whether fixed or portable, large or small, should insist on being provided with a copy of the maker's instructions for working the generator, and should carefully study its construction and the maker's directions before putting the generator into use.

Acetylene Generator

The following precautions apply to every type of generator :-

1. ;Acetylene generators should only be employed in the open air or in a well-ventilated shed or building, outside any main building and without any communication between them.

2. ;Naked lights or smoking must not be allowed in the vicinity of a generator.

3. ;Where artificial light is required in a generator house or near a generator, electric incandescent bulbs encased in outer gas-tight glass globes should be used. If a light has to be taken into a generator house or near a generator, only an electric torch should be employed. Generators should always, if possible, be cleaned during daylight.

4. ;Electric switches, fuses, etc., in a generator house, or near a generator should be so constructed that risk due to sparking or fusing in an explosive atmosphere is avoided.

5. ;Portable acetylene generators must not be taken into the vicinity of any fire or combustible material.

6. ;A conspicuous notice prohibiting lights and smoking should be displayed on the outside of the generator shed.

7. ;Frozen generators should be thawed by means of hot water and not by pieces of hot metal. When leaving generators idle during frosty weather, the water should be drained off, otherwise its expansion in freezing may cause fractures in the metal parts.

8. ;Generators and their fittings should be maintained in good condition. Before executing repairs care should be taken to see that the bell or other parts are entirely free from gas. Generators in constant use should be overhauled at intervals of not less than three months.

9. ;Basket or tray receptacles for carbide must be thoroughly cleaned before being replaced in generators, and must be filled with fresh carbide.

10. ;Partially spent carbide must never be replaced in generators.

11. ;In the case of "carbide to water " generators fitted with feed motions care must be taken not to use undue force when removing hard pieces of carbide which may have become wedged. Sparks are liable to be produced by such action which might cause an explosion.

12. ;Where a fixed generator is used a copy of the instructions supplied by the makers should be conspicuously mounted near it.

13. ;When not in use portable generators should be kept under cover.

14. ;The generator should be of ample capacity for the continuous production, without heating, of the maximum quantity of acetylene required.

15. ;A " water to carbide " generator should deliver acetylene at a rate not exceeding two cubic feet per hour per lb. of carbide, and the temperature of the gas at the outlet of the generator should not be more than 130° F. For cutting cast iron, the generator should take a charge of not less than 28 lbs. of carbide and be capable of continuous production of acetylene without overheating.

16. ;A good water supply is essential. The generator chambers as well as the carbide receptacles should be thoroughly flushed after each charge.

17. ;The charge of carbide must not exceed the quantity and size specified by the makers of the generator.

18. ;Care must be taken to keep the water seal of the gas holder fully charged.

19. ;Where gas holders are constructed with bell lifts, the bells should not be weighted to give a pressure of acetylene exceeding that for which the generator is constructed.

20. ;A good working pressure should be between 8 and 10 inches of water.

21. ;Charging and cleaning should take place without appreciable loss of gas, and without any appreciable introduction of air.

22. ;No copper pipes or fittings must be employed in connection with generators or acetylene apparatus.

23. ;No acetylene must be employed in conjunction with oxygen without a hydraulic back pressure valve being interposed on the acetylfcne delivery pipe. (See Hydraulic Back Pressure Valve, page 83.)

Carbide

1. Carbide must be stored only in iron or steel drums which are hermetically sealed.

2. ;These drums should be kept in a dry and well-ventilated place.

3. ;No smoking should be permitted, and no naked artificial light should be employed near the drums nor in any room where carbide is stored.

4. ;Carbide can only be stored without a licence in a quantity not exceeding 28 lbs.

5. ;If it is desired to store larger quantities a licence must be obtained from the local authority.

6. ;Carbide should be of a quality to yield not less than 4-8 cubic feet of acetylene per lb., and should be of the size specified by the makers of the generator.

7. ;Drums containing carbide must not be brought near to a flame.

8. ;Drums containing carbide should only be opened in a dry place, and care must be taken to avoid striking a spark against pieces of hard carbide or against the container. Carbide drums should be opened with a special cutting tool similar to the domestic tin opener, as such a tool is less likely to cause sparking than a hammer and chisel.

9. ;Good carbide should be completely converted to sludge in the generator. If lumps are found in the sludge when trays or baskets are withdrawn for recharging, they indicate either bad carbide or an overloaded generator, i.e., a generator from which acetylene is being abstracted too rapidly.

10. ;Partly spent lumps of carbide must never be replaced in a generator, nor returned to a carbide drum. They should be kept with the sludge and allowed to gradually disintegrate in the open air.

11. ;Important Note.-The reason why it is dangerous to return lumps of partly spent carbide to a generator is because such lumps, although externally cool, may internally be heated almost to incandescence, carbide being a bad conductor of heat. Ignition of acetylene in air takes place spontaneously at a temperature of 480° C, and as it is impossible to replace recharged trays or baskets in the generator, without the admission of some air, there is the possibility of spontaneous combustion taking place if unspent lumps of previously used carbide are mixed with the fresh dry lumps put into the receptacle.