Extract from rules and regulations passed and approved Feb. 25, 1885,
by the United States Board of Supervising Inspectors of Steam Vessels:
SECTION 24. “Lever safety valves to be attached to marine boilers shall
have an area of not less than one square inch to two square feet of the
grate surface in the boiler, and the seats of all such safety valves
shall have an angle of inclination of forty-five degrees to the centre
line of their axis.
“The valves shall be so arranged that each boiler shall have one
separate safety valve, unless the arrangement is such as to preclude
the possibility of shutting off the communication of any boiler with
the safety valve or valves employed. This arrangement shall also apply
to lock-up safety valves when they are employed.
“Any spring-loaded safety valves constructed so as to give an increased
lift by the operation of steam, after being raised from their seats,
or any spring-loaded safety valve constructed in any other manner, or
so as to give an effective area equal to that of the aforementioned
spring-loaded safety valve, may be used in lieu of the common
lever-weighted valve on all boilers on steam vessels, and all such
spring-loaded safety valves shall be required to have an area of not
less than one square inch to three square feet of grate surface of the
boiler, and each spring-loaded valve shall be supplied with a lever
that will raise the valve from its seat a distance of not less than
that equal to one-eighth the diameter of the valve opening, and the
seats of all such safety valves shall have an angle of inclination
to the centre-line of their axis of forty-five degrees. But in no
case shall any spring-loaded safety valve be used in lieu of the
lever-weighted safety valve, without first having been approved by the
Board of Supervising Inspectors.”
The following size “Pop” Safety Valves are required for boilers having
grate surfaces as below:
2 inch “Pop” Valve for 9.42 square feet of grate surface.
2-1/2 inch “Pop” Valve for 14.72 square feet of grate surface.
3 inch “Pop” Valve for 21.20 square feet of grate surface.
4 inch “Pop” Valve for 37.69 square feet of grate surface.
5 inch “Pop” Valve for 58.90 square feet of grate surface.
6 inch “Pop” Valve for 84.82 square feet of grate surface.
PROFESSOR RANKIN’S RULE.—Multiply the number of pounds of water
evaporated per hour by .006, and the product will be the area in square
inches of the valve.
The U. S. Steamboat Inspection Law requires for the common lever valve
one square inch of area of valve for every two square feet of area of
grate surface.
United States Navy Department deduced from a series of experiments the
following rule: Multiply the number of pounds of water evaporated per
hour by .005, and the product will be the area of the valve in square
inches.
Rule adopted by the Philadelphia Department of Steam Engine and Boiler
Inspection:
1. Multiply the area of grate in square feet by the number 22.5. 2. Add
the number 8.62 to the pressure allowed per square inch. Divide (1) by
(2) and the quotient will be the area of the valve in square inches.
This is the same as the French rule.
The maximum desirable diameter for safety valves is four inches, for
beyond this the area and cost increase much more rapidly than the
effective discharging around the circumference.
There should not be any stop valve between the boiler and safety valve.
The common form of safety valve is shown in Fig. 96.
Here the load is attached to the end _B_ of the lever _A_, _B_, the
fulcrum of which is at _c_. The effective pressure on the valve, and
consequently the blowing off pressure in the boiler can be regulated
within certain limits, by sliding the weight _W_ along the arm of the
lever. In locomotive engines, as well as on marine boilers, the weight
would on account of the oscillations, be inadmissible and _a spring_ is
used to hold down the lever.
In the calculations regarding the lever safety valve, there are five
points to be determined, and it is necessary to know four of these in
order to find the fifth. These are: (1) The Steam Pressure, (2) The
Weight of Ball, (3) The Area of Valve, (4) The Length of Lever, (5) The
Distance from the Valve Centre to the Fulcrum.
[Illustration: Fig. 96.]
In making these calculations it is necessary to take into account the
load on the valve due to the weight of the valve-stem and lever. The
leverage with which this weight acts is measured by the distance of its
centre of gravity from the fulcrum. The centre of gravity is found by
balancing the lever on a knife edge, and the weight of the valve-stem
and lever can be found by actual weighing. This load can also be found
by attaching a spring balance to the lever exactly over the centre of
the valve stem when they are in position. The following examples will
be computed under these conditions: (1) Steam Pressure, 120 pounds; (2)
Weight of Ball, 100 pounds; (3) Weight of Valve and Lever, 60 pounds,
weighed in position; (4) Length of Lever, 45 inches; (5) Length of
Distance from Valve Centre to Fulcrum, 5 inches; (6) Area of Valve, 8
square inches.
_To find the area of the valve:_
RULE.—Multiply the length of the lever by the weight of the ball,
and divide the product by the distance from the valve centre to the
fulcrum, and to the quotient add the effective weight of the valve and
lever, and divide the sum by the steam pressure.
_Example._
45 inches, length of the lever,
100 pounds, weight of the ball,
----
Fulcrum, 5 in. )4500
----
900
60 pounds, weight of valve and lever,
----
Steam pressure 120 lbs. )960 (8 square inches, area of valve.
960
_To find the pressure at which the valve will blow off:_
RULE.—Multiply the length of the lever by the weight of the ball;
divide this product by the distance from the valve centre to the
fulcrum, and to the quotient add the effective weight of the lever and
valve, and divide the sum by the area of the valve.
_Example._
45 inches, length of lever,
100 pounds, weight of ball,
----
Fulcrum, 5 in. )4500
----
900
60 pounds, weight of valve and lever,
----
Area of Valve 8 ) 960
----
120 pounds, pressure at which valve will blow.
_To find the weight of ball:_
RULE.—Multiply the steam pressure by the area of the valve, and from
the product subtract the effective weight of the valve and lever, then
multiply the remainder by the distance from the valve centre to the
fulcrum, and divide the product by the length of the lever.
_Example._
120 pounds, steam pressure,
8 inches, area of valve,
----
960
60 pounds, weight of valve and lever,
----
900
5 inches, fulcrum,
----
Length of lever, 45 in. )4500
----
100 pounds, weight of ball.
_To find the length of lever:_
RULE.—Multiply the steam pressure by the area of the valve, and from
the product subtract the effective weight of the valve and lever, then
multiply the remainder by the distance from the valve centre to the
fulcrum, and divide the product by the weight of the ball.
_Example._
120 pounds, steam pressure,
8 inches, area of valve,
----
960
60 pounds, weight of valve and lever,
----
900
5
----
100)4500(45 length of lever.
Every boiler should be provided with two safety valves, one of which
should be put beyond the control of the attendant.
Safety valves that stick will do so even though tried every day, if
they are simply lifted and dropped to the old place on the seat again.
_If a boiler should be found with an excessively high pressure, it
would be one of the worst things to do to start the safety valve from
its seat unless extra weight was added_, for should the valve once
start, it would so suddenly relieve the boiler of such a volume of
steam as would cause a rush of water to the opening, and by a blow,
just the same as in water hammer, rupture the boiler.
Such a condition is very possible to occur of itself when a safety
valve sticks. The valve holds the pressure, that gets higher and
higher, until so high that the safety valve does give way and allows so
much steam to escape that the sudden changing of conditions sets the
water in motion, and an explosion may result.
The noise made by a safety valve when it is blowing off may be
regarded in two ways. First, by it is known that the valve is capable
of performing its proper function, and that there is, therefore, a
reasonable assurance that no explosion will result from excessive
pressure of steam or other gas, and on the other hand too much noise of
this kind indicates wasted fuel.
The hole of the safety valve may be 2, 3 or 4 inches; that does not
say that the area is 3.1416, 7.06 or 12.56 square inches, but the area
is that which is inside of the joint. The valve opening may be, say
2 inches, but _the circle of contact of valve to seat_ may be of an
average diameter of 2-1/8 inches, if so, all the close calculations
otherwise will not avail. In the first place, the area of 2 inches
equals 3.1416; that of 2-1/8 diameter equals 3.5466, showing a
difference of .4 square inches.