92

MODERN STEAM PRACTICE.

lating valve, which may be constructed to act also as a back-pressurevalve, should always be placed at the front end of the boiler, withinthe reach of the attendant, and where boilers work in connection,each should have a back-pressure valve attached. The feed watershould be delivered a few inches below the surface of the water inthe boiler, and above the level of the tube crowns, and in a horizontaldirection, or by means of a horizontal perforated pipe. Where thefeed water is delivered near or at the bottom of the boiler, it coolsand contracts the lower plates, whilst those of the upper part areheated and expanded by the steam, frequently causing fracture atthe ring seams at the lower part of the shell. The feed water shouldalways be heated before it is forced into the boiler. The blow-outtap at the bottom of the boiler should be so placed that it may beexamined at any time, so that any leakage occurring, it should be atonce noted; valves should never be used, double-gland taps madealtogether of brass are far preferable. Stout seatings with planedjoint faces, suitable for each fitting, should be rivetted to the boiler.All manholes should be strengthened by a faced mouthpiece, rivettedto the boiler, so that the joint may be easily and well made, andleakage and corrosion avoided. Steam domes are unnecessary instationary boilers; a perforated pipe placed in the upper part of thesteam-space is quite as efficient to prevent priming, and the boileris not weakened. Where domes are preferable, they should neverbe of large diameter, and the shell plates inside them should not beall cut away, that is to say, the hole should be strengthened withstrips left in the plate. The setting of stationary boilers shouldalways be intrusted to a man of experience. When boilers areabout to be set, special care should be taken to thoroughly drain theground, that no dampness may exist in the flues to cause corrosionof the plates. All the flues should be quite large enough to allowa man to pass through, so that every part may be accessible forrepairs and examination. Midfeather seatings are very objection-able, and no boiler should be so set, except those of very small dia-meter, and in such cases, thick but narrow iron plates should beplaced on the top of the brickwork to protect the boiler. Cylindri-cal boilers internally fired should be set on side walls, the boilerresting on fireclay blocks made for the purpose, and so shaped thatwhen built in place the bottom of the side flues may be much lowerthan the point where the boiler rests on the blocks. If the blocksbe properly fitted to the plates, that the bearing thereon may be