_a._ The total lift of suction valves must not exceed 1/2-inch.
_b._ The net suction valve port area and the total suction valve outlet
area under valves lifted 1/2 inch high must not be smaller than the
figures given in the table below.
====================================+=====+========
(1) Length of Stroke |=12= | 16
(in inches) | |
------------------------------------+-----+--------
(2) Greatest No. revolutions per |=70= | 60
minute. | |
------------------------------------+-----+--------
(3) Corresponding Piston travel |=140=| 160 ft.
per minute. | |
------------------+-----------------+-----+--------
Approx. |(4) Feet | 308 | 352
actual max. Piston| per minute. | |
velocity at full +-----------------+-----+--------
speed, per row |(5) Feet | 5.1 | 5.9
(3) × 2.2. | per second. | |
------------------+-----------------+-----+--------
(6) Net Suction Valve-port area |=56%=| 64%
speed regarded necessary for | |
this per cent. of Plunger area. | |
------------------------------------+-----+--------
(7) Total Suction Valve Outlet |=56%=| 64%
Area under Valves lifted | |
1/2 in. high. | |
------------------------------------+-----+--------
(8) Discharge Valve Area. |2/3 of Suction
|Valve Area.
------------------------------------+--------------
By “valve-outlet area,” we mean the vertical cylindrical surface over
the outer edge of the valve ports, _i. e._, the distance L multiplied
by the circumference at the outer edge of the valve ports _C_, Fig.
8. Thus for a 4-inch valve, with ports inscribed in a 3-1/2-inch
circle, whose circumference is 3·5 × 3·1416 = 11 inches; the valve
“outlet area” for 1/2-inch lift would be 5-1/2 inches.
[Illustration: FIG. 8.]
The actual velocity of piston during the middle portion of stroke is
from 2.0 to 2.4 (average 2.2) times as great as the piston travel per
minute (as determined in experiments by Mr. J. R. Freeman on several
duplex pumps of different manufacture). This is because each piston
stands still nearly half the time, or while its mate is working,
and, moreover, moves more slowly near start and finish of stroke.
The words “piston speed” are commonly incorrectly used and refer to
“piston travel.” A clear understanding that the actual piston speed
is _more than twice as great_ leads to more generous valve design.
Large aggregate valve areas are necessary for pumps designed to run
as fast as these, and experience has shown that to prevent valve slam
at high speed and to accommodate high suction lifts, it is just as
important to have a large “valve outlet area” as to have a large area
of valve port.
It is valve slam or water hammer which commonly limits the highest
speed at which a pump can be run. This water hammer may originate
from the pulsations in a long or small suction pipe. The vacuum
chamber lessens it, but there is commonly some point of high water in
the vacuum chamber that will give much smoother action than any other.
Valve slam in this style of pump is caused chiefly by the short
rebound of the steam piston against the elastic steam cushion at the
end of the stroke. This in turn snaps the valves down with a jump
when the speed is high. Dividing this impact or slam on numerous
valves of low lift, tends to break up and lessen the shock, therefore
with valves of the size and style used in fire-pumps, other things
being equal, the less they have to rise and drop to let the water
through them, the less will be the valve slam. This height of rise
and drop is governed by the circumference rather than the port area.
Experience and practice has shown that a 1/2-inch limit of lift is
reasonable and does ensure a smooth working pump under all ordinary
conditions.
_c._ The following table gives minimums for aggregate valve port area
and aggregate valve outlet area, for the different size plungers,
figured on a basis of 56% of plunger area for a 12-inch stroke, and 64%
for a 16-inch stroke.
===+=======================+=========+========+=======+========
| =Size of Pump.= | =500= | =750= |=1000= |=1500=
| | =Gal.= | =Gal.= | =Gal.=| =Gal.=
+-----------------------+---------+--------+-------+--------
=1=| Diameter of plunger. | | | |
| Inches | 7-1/4″ | 9″ | 10″ | 12″
+-----------------------+---------+--------+-------+--------
=2=| Area of plunger in | | | |
| sq. inches | 41·28 | 63·62 | 8·54 | 113·10
+-----------------------+---------+--------+-------+--------
| 56% of plunger area, | | | |
| or Minimum aggregate | | | |
=3=| valve port area | | | |
| allowed per section. | | | | 64% =
| Square inches | 23·11 | 35·63 | 43·98 | 72·38
+-----------------------+---------+--------+-------+--------
| Minimum aggregate | | | |
=4=| valve port | | | |
| circumference, allowed| | | |
| per section. Inches | 46·22 | 71·26 | 87·96 | 144·76
+-----------------------+---------+--------+-------+--------
| Minimum aggregate | | | |
| valve outlet area | | | |
=5=| allowed per section | | | |
| for valves lifted | | | |
| 1/2 inch high. | | | |
| Square inches | 23·11 | 35·63 | 43·98 | 72·38
---+-----------------------+---------+--------+-------+--------
_d._ If we consider using any one of the three sizes of valves below,
whose port areas may be assumed approximately as
=========+================+===========+================
Diam. | Diam. of Valve | Circ. of | Valve Port
Valve. | Port. Circ. | V. C. | Area (Net).
| | Circle. | Square inches.
---------+----------------+-----------+----------------
3″ | 2-1/2″ | 7·85″ | 3·5
3-1/2″ | 3″ | 9·42″ | 4·7
4″ | 3-1/2″ | 10·99″ | 6·3
---------+----------------+-----------+----------------
given, then the necessary number of valves per section will be as in
the table following:
=================+============+============+============+=============
=Size of Pump.= | =500= | =750= | =1000= | =1500=
| =gal.= | =gal.= | =gal.= | =gal.=
-----------------+--+------+--+--+------+--+--+------+--+--+------+---
=Size of Valves.=|3″|3-1/2″|4″|3″|3-1/2″|4″|3″|3-1/2″|4″|3″|3-1/2″|4″
-----------------+--+------+--+--+------+--+--+------+--+--+------+---
Necessary number | | | | | | | | | | | |
of valves | | | | | | | | | | | |
to satisfy | | | | | | | | | | | |
(4) under _c_ | 6| 5 | 5| 9| 8 | 7|11| 10 | 8|19| 16 |14
-----------------+--+------+--+--+------+--+--+------+--+--+------+---
Necessary number | | | | | | | | | | | |
of valves | | | | | | | | | | | |
to satisfy | | | | | | | | | | | |
(3) under _c_ | 7| 5 | 4|10| 8 | 6|13| 10 | 7|21| 16 |12
-----------------+--+------+--+--+------+--+--+------+--+--+------+---
The exact number and size of valves will, however, not be insisted upon
provided the aggregate valve area and the aggregate valve outlet area
for each section is not less than that given in the table under _c_ for
the limiting lift of 1/2 inch.
Manufacturers will note that with the established lift of 1/2
inch, the 3-1/2-inch valve will permit a valve outlet area more
nearly equal to its port area than will either the 3-inch or 4-inch
valves, and a _relatively_ less number of valves will satisfy the
specifications.