BOILER ACCESSORIES: FEED APPARATUS Perhaps the most important of all auxiliaries connected with the boiler is the feed apparatus. This is vital; for, if the feed is interrupted and the water runs low in the boiler, not only is there danger of damaging the boiler itself, but a disaster may follow of far greater concern. For marine purposes—and the same is true to a considerable extent in stationary work—at least two independent feed systems should be provided. In marine work, the main feed-pump draws water from the filter box or feed-water heater, and pumps it into the boilers under ordinary conditions. There should be a by-pass around this pump, and the feed line should be connected by means of a valve to what is known as the donkey pump, which may be used for auxiliary feed purposes in case the main pump is damaged or needs repairs in any way.
Both these pumps draw from and discharge into the same feed line; but, to provide against emergencies, there is usually a crossconnection to the sea, so that sea water may be had if necessary. While in port, when the main engines are not running, and consequently when the feed-water cannot be heated economically, an injector is almost invariably used. On land it is usually considered sufficient to install an injector in addition to the feed pump, although in large plants an auxiliary feed pump should be installed as well. In a small plant the fireman usually attends to the water; but on board ship and in large plants, a water tender is usually provided, whose business it is to keep the water in the boiler at the proper level. His task may be materially lessened by some automatic arrangement, so that if the water discharged into the hot well from the condenser rises above the normal level, a float will open the valve leading to the feedpump and increase the rapidity of its stroke. This will reduce the level of the hot well or filter box, as the case may be.
Such an arrangement as this will keep a fairly uniform level of water in the boilers; and if a surface condenser is employed, and all the condensation is pumped back into the boilers, the water-level will remain constant except for slight leakages of steam and for the possibility of improper action of the feed-pump. Leakage of steam can be made up from the supply of fresh water. At sea, salt water may have to be used for this purpose although its use is objectionable.
There is a considerable difference of opinion as to where the feedwater should be introduced into the boiler, although the consensus of opinion seems to be that it should enter not far from the water-line.
In stationary practice, the feed-water is introduced at the rear of the boiler near the bottom; but this is open to grave objections, for the feed-water, being comparatively cool and being introduced into the coldest part of the boiler, naturally tends to become dead water and to retard proper circulation which is essential to economical steaming . and often essential to the safety of the boiler itself.
The best place for introducing the feed-water will naturally depend upon the type of boiler, and the service for which it is intended. If the entering water is of high temperature, it might enter near the bottom of the boiler. But if the feed-water is comparatively cold and it is always colder than the water in the boiler and the surrounding steam if the circulation is good—great care must be taken that it does not strike directly against the hot boiler-plates, as it might thereby cause local contraction and possibly a serious leak, and it should be introduced in such a way as to make sure of its aiding the natural circulation of the boiler.
The higher the steam pressure in the boiler, the more difficult becomes the problem of feed, and the more danger there is of injury to the boiler by the comparatively cold feed-water striking hot plates. It is a universal practice in marine work,.and a common practice on land, especially for internally-fired boilers, to cause the feed to enter above the water-level near the center of the boiler; then branching off into two pipes, one leading to each side through the steam space until the side of the boiler is reached ; and then running downward toward the bottom. The feed-water, which very likely has been previously heated by a feed-water heater, is still further heated by its passage through this feed-pipe, which is in direct contact with the live steam of the boiler. This internal feed-pipe, turning down at the sides, causes the water to strike the outer shell of the boiler which is the most remote from the fire, and this downward motion materially assists the circulation in the boiler. When this arrangement of feed is adopted (see Fig. 50), care must be taken that the lower end of the feed-pipe is well below the low-water level. If the end of the pipe is alternately immersed in water and then exposed to steam, violent explosions in the pipe are likely to follow, although they are likely to do nothing more serious than break an elbow or frighten the attendants.