_a._ These must be of hard close iron with metal so distributed as to
ensure sound castings, and freedom from shrink cracks.
_b._ The design should be along lines best calculated to resist
internal pressure so as to avoid as much as possible the need of ribs
for stiffening.
_c._ They must be capable of withstanding, without showing signs of
weakness, the pressures and shocks due to running under the conditions
mentioned in Chapter “Tests for Acceptance,” Art. 48-54.
The suction chamber should be able to withstand a water pressure of 100
lbs.
Although suction chambers are not regularly subject to a pressure, it
is sometimes desired to connect them to public water supplies, and
where foot valves are used there is a chance of getting pressure on
the suction, so that ample strength is necessary.
Foundry finish may be permitted on the joints at water cylinder heads
and at hand-hole plates, provided surfaces are so true that a rubber
packing not over 1/16 of an inch in thickness is sufficient to secure
perfect tightness.
_d._ Conveniently placed hand-holes of liberal size must be provided
for the ready examination and renewal of valve parts at the yoke end of
water cylinders and in the delivery chamber.
This will necessitate holes not less than 6 × 8 inches, or its
equivalent, for the two largest-size pumps, and holes proportionately
as large for the 500 and 750-gallon pumps. The easy access to
the valve parts is of vital importance, and must receive careful
attention.
_e._ The thickness of metal for cylinder shell, valve decks,
partitions, ribs, etc., will depend largely upon the form of
construction, but, in a general way, to establish safe minimums for
the average water cylinder, of nearly cylindrical form, whose flat
surfaces are stiffly ribbed, we submit the table below:
=========================+==========+==========+============+============
=Size of Pump.= |=500 gal.=|=750 gal.=|=1,000 gal.=|=1,500 gal.=
-------------------------+----------+----------+------------+------------
Thickness of cylinder | Inches. | Inches. | Inches. | Inches.
shell when of nearly | | | |
cylindrical form | 7/8 | 1 | 1-1/8 | 1-1/4
| | | |
Thickness of valve decks | | | |
when well ribbed | 1-1/4 | 1-1/4 | 1-1/4 | 1-1/4
| | | |
| | | |
Thickness of transverse | 1-1/4 | 1-1/4 | 1-1/2 | 1-1/2
partition, depending | to | to | to | to
on ribbing | 1-1/2 | 1-1/2 | 2 | 2
| | | |
Thickness of longitudinal| 1-1/4 | 1-1/4 | 1-1/4 | 1-1/2
partition, depending on| to | to | to | to
ribbing | 1-1/2 | 1-1/2 | 2 | 2
| | | |
Thickness of ribs | 3/4 | 7/8 | 1 | 1
| | | |
Thickness of suction | | | |
chamber | 5/8 | 3/4 | 3/4 | 7/8
| | | |
Thickness of delivery | | | |
chamber | 7/8 | 1 | 1-1/8 | 1-1/4
| | | |
-------------------------+----------+----------+------------+-----------
Lighter construction than herein specified will not be regarded as
satisfactory, and any construction will be finally passed upon on
examination of drawings.
_f._ The bolting of all parts of the water end is to be of such
strength that the maximum stress at bottom of screw thread will not
exceed 10,000 lbs. per square inch (disregarding for the moment the
initial stress due setting up nuts) for a water pressure of 200 lbs.
per square inch, computed on an area out to centre line of bolts.
No stud or bolt smaller than 3/4-inch should be used to assemble parts
subject to the strain of water pressure, as smaller bolts are likely to
be twisted off.
Although these pumps are not expected to be designed for a regular
working water-pressure of 240 or 320 lbs., it is expected that bolts,
shells, rods, etc., will be figured to stand this comparatively
quiet, temporary, high pressure, exclusive of further allowance for
initial strain due setting up of bolts, with a factor of safety of at
least four.
This high test pressure is analogous to the custom of proving all
common cast-iron water pipes to 300 lbs. and all common lap-welded
steam pipes to 500 lbs. per square inch, and common water-works gate
valves to 400 lbs., even though these are to be regularly used at
much less pressure.
We are assured that castings no heavier than at present used by the
best makers will stand this test, _if properly shaped and liberally
bolted_.
_g._ For requirements for stuffing boxes, see Art. 22.