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 56
AFM 91-19 / TM 5-629 / NAVFAC MO-314
24 May 1989
(a) Nozzles producing a flat, fan-shaped
pump is directly proportional to its speed. Dis-
spray are the most popular for weed control.
advantages of piston pumps are their low deliv-
They produce an even distribution across the
ery rate and high initial cost, but their long
width of the spray. When evenly spaced on
service life usually offsets the latter. The low
spray booms, they distribute the herbicide across
delivery rate, however, means that mechanical
the boom swath. Sometimes they are designed to
agitation is necessary if wettable powders are
produce a lighter spray at the edges of the spray
used.
pattern, resulting in more uniform application
(2) Centrifugal or turbine pumps develop
where nozzle patterns overlap within the boom
pressure through centrifugal force created by
swath.
rapidly rotating blades, fins, or disks. They are
(b) Hollow-cone nozzles produce a cone-
inexpensive and handle abrasive materials well.
shaped spray, with the heaviest droplet distribu-
They provide a high delivery rate with enough
tion on the outer edges of the pattern. Coverage
excess flow to provide hydraulic agitation in the
is less uniform than with the flat-fan nozzle, but
spray tank. To maintain adequate pressure,
cone nozzles operate more satisfactorily at low
however, centrifugal pumps must be operated at
application rates of about 2 to 3 gallons per
higher speed than other pumps. They generally
acre.
are not self-priming, and must be placed below
the spray tank to ensure gravity flow to the
(c) Flooding nozzles deliver coarse drop-
lets of spray under low pressure, thus minimiz-
pump.
ing drift. They produce a wide-angle fan pat-
(3) Gear pumps develop both vacuum and
tern. Because the nozzle orifices are large,
pressure through meshing gear teeth. They may
clogging is also reduced.
be used advantageously for oil emulsions and
other nonabrasive materials, but have a limited
(3) There are a number of nozzles for
life under adverse usage, such as with wettable
special situations. The boomless or cluster noz-
powders. Because they are cheap, gear pumps
zle, for example, is often used on rights-of-way
are usually replaced rather than repaired. Their
where obstructions would interfere with the
delivery rate varies with size, but is usually
operation of a spray boom, and in grasslands
adequate for hydraulic agitation.
where scattered trees would interfere with a
boom. It provides a rather varible distribution
(4) Impeller pumps have a rotor set to one
of spray because its wide range of droplet sizes
side within the pump housing. The rotor has
are affected differentially by wind. It is particu-
flexible rubber vanes or rollers that maintain
larly useful on rough terrain that would be
contact with the wall of the housing. The space
hazardous to spray booms.
between the rotor and pump housing expands
for half of every revolution and contracts for
(4) The pressure in the system determines
the other half, thus creating an alternation of
the rate of flow through the nozzle orifice.
vacuum and pressure. In recent years, the roller-
Increasing or decreasing the pressure affects the
type of impeller pump has become extremely
size of spray droplets. Increasing the pressure
popular. The rollers are made of nylon or
produces a finer spray; to increase volume it is
rubber, and work best with oil emulsions and
more effective to increase the size of the orifice.
nonabrasive materials. Rubber rollers are the
The distance that the spray is projected depends
most satisfactory with abrasive materials. Worn
primarily on the pressure being delivered by the
parts can be replaced in most instances. Impeller
pump.
pumps are moderately priced and provide suffi-
c. Types of Pumps. The four most common
cient volume for most applications.
types of pumps are piston, centrifugal, gear, and
d. Pressure Regulators and Control Valves:
impeller.
(1) Pressure regulators, or relief valves, are
(1) Piston pumps are probably the most
used to maintain a relatively constant pressure at
versatile, and before 1945 they were used on
the nozzle. It generally consists of a ball that is
nearly all sprayers. They can deliver a wide
range of pressures, from 40 to 1,000 lb/in2
held against an orifice by a spring that can be
screw-adjusted to vary the pressure. Variations
making them useful for chemicals other than
include a double-spring type and a combination
herbicides. They are easily repaired, are resistant
of spring and diaphragm. Excess liquid is re-
to wear by abrasive materials such as wettable
leased back into the spray tank through a bypass
powders, and have long service lives. Each
port. A pressure gauge is usually placed on the
stroke of the pump delivers a given amount of
outlet side of the regulator, between the regula-
spray liquid, and, therefore, the output of the
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