IMPROVEMENTS IN THE CONSTRUCTION OF FUSIBLE PLUGS FOR STEAM BOILERS AND OTHER LIKE VESSELS
Year: 1879

page_text: PROVISIONAL SPECIFICATION left by Harry Jones Harman at the Office of
the Commissioners of Patents on the 5th November 1879.
HARRY JONES HARMAN, of Manchester, in the County of Lancaster, Mechanical
Engineer. "IMPROVEMENTS IN THE CONSTRUCTION OF FUSIBLE PLUGS FOR STEAM
BOILERS AND OTHER LIKE VESSELS."
My Invention relates to improvements in the construction of fusible plugs for
steam boilers and other like vessels, whereby they are rendered more efficient and
reliable in their action, and other advantages are obtained.
In carrying out my Invention I construct the casing of the plug in two parts,
viz, an upper and a lower, which are arranged and combined together in the
following manner :-I make the lower part of the casing of a hollow vertical cylinder
form, of gun metal or other suitable material, having an external or male screw at
the lower end and an internal or female screw at the upper end.
The male screw
is for the purpose of attaching the casing to the top of the boiler, flue, or fire box,
a portion of the exterior of the casing being made hexagonal and forming a shoulder
by means of which the casing is firmly screwed into the boiler plate, where it forms
a permanent fixture.
The upper part of the casing is made of gun metal or other suitable material in
one piece, and is attachable to or removable from the lower part, hereinbefore
described. I make the lower portion of this upper part of a cylindrical form
externally, and with a male screw formed thereon, and internally of an hexagonal
shape. The upper portion of this upper part I make of a hollow conical form,
terminating with a stopped end or cap which projects beyond the outside of the
conical portion, and has attached to it a conical lip or flange projecting downwards,
forming an annular space round the outside of the cone. The cone is provided at
the end nearest the apex with holes forming a communication between the interior
of the cone and the annular space surrounding the outside of the cone formed by
the downward projecting lip or flange.
In the interior of the conical portion I fit a conical plug or valve having on its
upper side a small projection so shaped as to be held fast by any metal or material
when cold, with which it may be surrounded when such metal is in a molten state.
This plug or valve may be made of any suitable metal.
The annular space around the upper portion of cone, the holes uniting it to the
interior of the cone, and the space therein surrounding the projection formed on the

page_text: upper side of the conical plug are filled by a fusible metal, such metal being so
composed as to melt by the heat when not sufficiently covered by any fluid.
The upper portion of the casing containing the fusible metal and plug is screwed
into the internal thread of the lower portion of the casing by means of a suitable
key made to fit the hexagonal part, hereinbefore described. This operation may
generally be performed from the interior of the tube or furnace, thus avoiding the
delay generally incurred in removing the manhole cover and waiting while the
boiler or other vessel is cooled down.
The action of this improved fusible plug is as follows:-When the water or other
fluid in the boiler or other vessel becomes sufficiently low to leave the top of the
gun metal or other suitable metal cap uncovered therewith, the heat from the
furnace is communicated rapidly to the fusible metal which as the water or other
fluid recedes lower becomes entirely melted out. The conical plug in the interior
of the cone then drops or is forced out by the pressure of steam within the boiler
or other vessel passing through the annular space and through the holes into the
interior of the cone.
The rush of steam gives timely notice to the attendant, and
permits of the fires being drawn before any serious damage is done to the boiler or
other vessel.
The metal cap, formed as herein before described, serves to protect the fusible
metal from any incrustation of any foreign matters acting within the boiler or other
vessel. By the construction also of the internal conical metal plug or valve the
furnace gases are prevented from deteriorating the fusible metal, whilst by the
arrangement of the annular space between the cone and the lip of cap for holding
the fusible metal no upward surface is presented liable to be covered by
incrustation.
The upper portion of the apparatus generally is moreover so formed that the
fusible metal is relieved from any pressure which can affect its reliability or cause
it to be forced instead of melted out.