The Steam Injector
A theoretical and practical treatise on the design and operation of injectors and on the flow of fluids through and the design of nozzles.

OVERFLOW ARRANGEMENTS. 47 CHAPTER V. Overthrow Arrangements. The object of the overflow arrangements of an injector is to provide for the bye-passing or release of the jet within the combining nozzle, when said jet is unable from any cause to overcome the pressure in the vessel being fed and so travel from the injector through the delivery pipe. The causes which may operate to prevent delivery against the pressure in the boiler being fed are numerous, but may be briefly summed up by the statement that the jet leaving the combining nozzle has not both the proper density and velocity to eft’ect delivery. Thus the ratio of steam to water passing into the combining nozzle may be too great or too small. In the former case the velocity is high enough, but the density of the jet is too low. In the latter case the density may be right, but the velocity is too low. The importance of the overflow arrangements is par- ticularly emphasised when the injector is fitted above, and therefore has to lift its feed water. In this case there may be steam only passing into the combining nozzle for a short period. If no means are provided for the escape of the contents of the combining nozzle when delivery from the injector is not taking place. the entering steam will, in the absence of a non-return valve, force its way down the feed-water pipe and heat the feed water. If such a non-return valve is provided, then the appliance simply remains inoperative. In tlie earliest injectors, as illustrated at fig. 1, a gap was left between the combining and delivery nozzles to form the overflow passage. It will be noticecl that this gap is placed at the portion of the injector where the nozzles are of smallest diameter. When this type of injector is to lift its feecl water, the steam supply has to be reduced for starting purposes (by the spindle valve shown in fig. 1), otherwise the steam would be throttled in