Tallow is sometimes forced into a boiler fed with salt water to stop priming, by means of a syringe that is screwed into a tallow cock provided upon the boiler below the water level. If the boiler is fed with fresh water, tallow is apt to cause priming.
Angle irons are used in boiler construction to be riveted to plates that require supporting or strengthening, or for gusset stays to be riveted to. Flanged plates are used in the construction of the furnaces, flame, boxes or combustion chambers, boiler ends and tube plates or tube sheets.
Division plates are fitted in some boilers to prevent the water from passing from one side of the boiler to the other when the vessel rolls heavily. This prevents some of the tubes from being left uncovered by water, and thereby getting injured from undue heat.
These division plates are neither steam nor water tight, and stand fore and aft of the ship. Similar division plates are sometimes used, however, to prevent the tops of the combustion boxes from getting overheated from the motion of the ship leaving them uncovered with water, their location being subserved to this end and varying with the position of the boiler.
The superheater of a marine boiler is provided with a safety valve, and sometimes with a pressure gauge to enable the comparing of the steam pressure with that in the boiler, and should also be provided with a gauge glass, to show when heavy priming is going on.
The main stop valve is upon the superheater, as is also the blast pipe.
Priming is a lifting, into the steam space of the boiler, of a part of the water, and may arise from heavy firing, from the safety valve blowing off, from too little steam space, and from other causes.
Priming[70] often occurs when the boiler feed is changed from salt water to fresh water, or from fresh to salt water.
[70] See [page 370] Vol. II., on priming.
A separator or interceptor is a device fitted to either the superheater or to the steam receiver, for separating entrained water from the steam. It consists of a rectangular box or chamber with a partition plate extending from the top half down into the box.