t amount of steam room, an
inadequate area of water level, an insufficient width between the flues or
tubes for the ascent of the steam and the descent of water to supply the
vacuity the steam occasions, and the use of dirty water in the boiler. New
boilers prime more than old boilers, and steamers entering rivers from the
sea are more addicted to priming than if sea or river water had alone been
used in the boilers--probably from the boiling point of salt water being
higher than that of fresh, whereby the salt water acts like so much molten
metal in raising the fresh water into steam. Opening the safety valve
suddenly may make a boiler prime, and if the safety valve be situated near
the mouth of the steam pipe, the spray or foam thus created may be mingled
with the steam passing into the engine, and materially diminish its
effective power; but if the safety valve be situated at a distance from the
mouth of the steam pipe, the quantity of foam or spray passing into the
engine may be diminished by opening the safety valve; and in locomotives,
therefore, it is found beneficial to have a safety valve on the barrel of
the boiler at a point remote from the steam chest, by partially opening
which, any priming in that part of the boiler adjacent to the steam chest
is checked, and a purer steam than before pusses to the engine.

296. _Q._--What is the proper remedy for priming?

_A._--When a boiler primes, the engineer generally closes the throttle
valve partially, turns off the injection water, and opens the furnace
doors, whereby the generation of steam is checked, and a less violent
ebullition in the boiler suffices. Where the priming arises from an
insufficient amount of steam room, it may be mitigated by putting a higher
pressure upon the boiler and working more expansively, or by the
interposition of a perforated plate between the boiler and the steam chest,
which breaks the ascending water and liberates the steam. In some cases,
however, it may be necessary to set a second steam chest on the top of the
existing one, and it will be preferable to establish a communication with
this new chamber by means of a number of small holes, bored through the
iron plate of the boiler, rather than by a single large orifice. Where
priming arises from the existence of dirty water in the boiler, the evil
may be remedied by the use of collecting vessels, or by blowing off largely
from the surface; and where it arises from an insufficient ar