Efficient purifying material and purifiers shall comply with the following requirements:
(1) The purifying material shall remove phosphorus and sulphur compounds to a commercially satisfactory degree; i.e., not to a greater degree than will allow easy detection of escaping gas through its odour.
(2) The purifying material shall not yield any products capable of corroding the gas-mains or fittings.
(3) The purifying material shall, if possible, be efficient as a drying agent, but the Association does not consider this an absolute necessity.
(4) The purifying material shall not, under working conditions, be capable of forming explosive compounds or mixtures. It is understood, naturally, that this condition does not apply to the unavoidable mixture of acetylene and air formed when recharging the purifier.
(5) The apparatus containing the purifying material shall be simple in construction, and capable of being recharged by an inexperienced person without trouble. It shall be so designed as to bring the gas into proper contact with the material.
(6) The containers in purifiers shall be made of such materials as are not dangerously affected by the respective purifying materials used.
(7) No purifier shall be sold without a card of instructions suitable or hanging up in some convenient place. Such instructions shall be of the most detailed nature, and shall not presuppose any expert knowledge whatever on the part of the operator.
Reference also to the abstracts of the official regulations as to acetylene installations in foreign countries given in Chapter IV. will show that they contain brief rules as to purifiers.
DRYING.--It has been stated in Chapter III. that the proper position for the chemical purifiers of an acetylene plant is after the holder; and they therefore form the last items in the installation unless a "station" governor and meter are fitted. It is therefore possible to use them also to remove the moisture in the gas, if a material hygroscopic in nature is employed to charge them. This should be true more particularly with puratylene, which contains a notable proportion of the very hygroscopic body calcium chloride. If a separate drier is desirable, there are two methods of charging it. It may be filled either with some hygroscopic substance such as porous calcium chloride or quicklime in very coarse powder, which retains the water by combining with it; or the gas may be led through a vessel loaded with calcium carbide, which will manifestly hold all the moisture, replacing it by an equivalent quantity of (unpurified) acetylene. The objection is sometimes urged against this latter method, that it restores to the gas the nauseous odour and the otherwise harmful impurities it had more or less completely lost in the purifiers; but as regards the first point, a nauseous odour is not, as has previously been shown, objectionable in itself, and as regards the second, the amount of impurities added by a carbide drier, being strictly limited by the proportion of moisture in the damp gas, is too small to be noticeable at the burners or elsewhere. As is the case with purification, absolute removal of moisture is not called for; all that is needed is to extract so much that the gas shall never reach its saturation-point in the inaccessible parts of the service during the coldest winter's night. Any accessible length of main specially exposed to cold may be safeguarded by itself; being given a steady fall to a certain point (preferably in a frost-free situation), and there provided with a collecting-box from which the deposited liquid can be removed periodically with a pump or otherwise.