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ACCUMULATORS
An accumulator is a pressure storage reservoir
in which hydraulic fluid is stored under pressure
from an external source. The storage of fluid
under pressure serves several purposes in hydraulic
systems.
In some hydraulic systems it is necessary to
maintain the system pressure within a specific
pressure range for long periods of time. It is very
difficult to maintain a closed system without some
leakage, either external or internal. Even a small
leak can cause a decrease in pressure. By using
an accumulator, leakage can be compensated for
Figure 9-5.–Cross-section view of a piston-type accumulator with a tailrod.
and the system pressure can be maintained within
an acceptable range for long periods of time.
Accumulators also compensate for thermal
expansion and contraction of the liquid due to
variations in temperature.
A liquid, flowing at a high velocity in a pipe
will create a backward surge when stopped
suddenly by the closing of a valve. This sudden
stoppage causes instantaneous pressures two to
three times the operating pressure of the system.
These pressures, or shocks, produce objectional
noise and vibrations which can cause considerable
damage to piping, fittings, and components. The
incorporation of an accumulator enables such
shocks and surges to be absorbed or cushioned
by the entrapped gas, thereby reducing their
effects. The accumulator also dampens pressure
surges caused by pulsating delivery from the
pump.
There are times when hydraulic systems
require large volumes of liquid for short periods
of time. This is due to either the operation of large
cylinders or the necessity of operating two or more
circuits simultaneously. It is not economical to
install a pump of such large capacity in the system
for only intermittent usage, particularly if there
is sufficient time during the working cycle for an
accumulator to store up enough liquid to aid the
pump during these peak demands.
The energy stored in accumulators maybe also
used to actuate hydraulically operated units if
normal hydraulic system failure occurs.
Four types of accumulators used in Navy
hydraulic systems are as follows:
1. Piston type
2. Bag or bladder type
3. Direct-contact gas-to-fluid type
4. Diaphragm type
PISTON-TYPE ACCUMULATORS
Piston-type accumulators consist of a
cylindrical body called a barrel, closures on each
end called heads, and an internal piston. The
piston may be fitted with a tailrod, which extends
through one end of the cylinder (fig. 9-5), or it
may not have a tailrod at all (fig. 9-6). In the latter
case, it is referred to as a floating piston.
Hydraulic fluid is pumped into one end of the
cylinder and the piston is forced toward the
opposite end of the cylinder against a captive
Figure 9-6.—Floating piston-type accumulator.
charge of air or an inert gas such as nitrogen.
Sometimes the amount of air charge is limited to
the volume within the accumulator; other
installations may use separate air flasks which are
piped to the air side of the accumulator. Piston
accumulators may be mounted in any position.
The gas portion of the accumulator may be
located on either side of the piston. For example,
in submarine hydraulic systems with tailrod
pistons, the gas is usually on the bottom and the
fluid on top; in surface ships with floating pistons,
the gas is usually on the top. The orientation of
the accumulator and the type of accumulator are
based upon such criteria as available space,
maintenance accessibility, size, need for external
monitoring of the piston’s location (tailrod
indication), contamination tolerance, seal life, and
safety. The purpose of the piston seals is to keep
the fluid and the gas separate.
Usually, tailrod accumulators use two piston
seals, one for the air side and one for the oil side,
with the space between them vented to the
atmosphere through a hole drilled the length of
the tailrod. When the piston seals fail in this type
of accumulator, air or oil leakage is apparent.
However, seal failure in floating piston or
nonvented tailrod accumulators will not be as
obvious. Therefore, more frequent attention to
venting or draining the air side is necessary. An
indication of worn and leaking seals can be
detected by the presence of significant amounts
of oil in the air side.
BLADDER-TYPE ACCUMULATORS
Bladder- or bag-type accumulators consist of
a shell or case with a flexible bladder inside the
shell. See figure 9-7. The bladder is larger in
diameter at the top (near the air valve) and
gradually tapers to a smaller diameter at the
bottom. The synthetic rubber is thinner at the top
of the bladder than at the bottom. The operation
of the accumulator is based on Barlow’s formula
for hoop stress, which states: "The stress in a
circle is directly proportional to its diameter and
wall thickness." This means that for a certain
thickness, a large diameter circle will stretch faster
than a small diameter circle; or for a certain
diameter, a thin wall hoop will stretch faster than
a thick wall hoop. Thus, the bladder will stretch
around the top at its largest diameter and thinnest
wall thickness, and then will gradually stretch
downward and push itself outward against the
walls of the shell. As a result, the bladder is
capable of squeezing out all the liquid from.
Figure 9-7.—Bladder-type accumulator.
the accumulator. Consequently, the bladder
accumulator has a very high volumetric efficiency.
In other words, this type of accumulator is
capable of supplying a large percentage of the
stored fluid to do work.
The bladder is precharged with air or inert gas
to a specified pressure. Fluid is then forced into
the area around the bladder, further compressing
the gas in the bladder. This type of accumulator
has the advantage that as long as the bladder is
intact there is no exposure of fluid to the gas
charge and therefore less danger of an explosion.
DIRECT-CONTACT GAS-TO-FLUID
ACCUMULATORS
Direct-contact gas-to-fluid accumulators
generally are used in very large installations where
it would be very expensive to require a piston-or
bladder-type accumulator. This type of
accumulator consists of a fully enclosed cylinder,
mounted in a vertical position, containing a liquid
port on the bottom and a pneumatic charging port
at the top (fig. 9-8). This type of accumulator is
used in some airplane elevator hydraulic systems
where several thousand gallons of fluid are needed
to supplement the output of the hydraulic pumps
for raising the elevator platform. The direct
contact between the air or gas and the hydraulic
fluid tends to entrain excessive amounts of gas
in the fluid. For this reason, direct contact
accumulators are generally not used for pressures
over 1200 psi. The use of this type of accumulator
with flammable fluid is dangerous because there
is a possibility of explosion if any oxygen is
present in the gas, and pressure surges generate
excessive heat. For this reason, safety fluids are
used in this type of installation.
DIAPHRAGM ACCUMULATORS
The diaphragm-type accumulator is
constructed
in two halves which are either screwed
or bolted together. A synthetic rubber diaphragm
is installed between both halves, making two
chambers. Two threaded openings exist in the
assembled component. The opening at the top,
as shown in figure 9-9, contains a screen disc
which prevents the diaphragm from extruding
through the threaded opening when system
pressure is depleted, thus rupturing the diaphragm.
On some designs the screen is replaced
by a button-type protector fastened to the center
Figure 9-8.—Direct-contact
gas-to-fluid accumulator.
Figure 9-9.—Diaphragm accumulator.
of the diaphragm. An air valve for pressurizing
the accumulator is located in the gas chamber end
of the sphere, and the liquid port to the hydraulic
system is located on the opposite end of the
sphere. This accumulator operates in a manner
similar to that of the bladder-type accumulator.
FILTRATION
You have learned that maintaining hydraulic
fluids within allowable limits is crucial to
the care and protection of hydraulic equipment.
While every effort must be made to prevent
contaminants from entering the system, contaminants
which
do find
their way into the system must be removed. Filtration devices are installed at key points in fluid power systems to remove
the contaminants that enter the system along with those that are generated during normal
operations.
Filtration devices for hydraulic systems differ somewhat from those of pneumatic systems.
Therefore, they will be discussed separately.
The filtering devices used in hydraulic systems
are commonly referred to as strainers and filters.
Since they share a common function, the terms
strainer and filter
are often used interchangeably.
As a general rule, devices used to remove large particles of foreign matter from hydraulic fluids
are referred to as strainers, while those used to remove the smallest particles are referred to as
filters.
STRAINERS
Strainers are used primarily to catch only very
large particles and will be found in applications
where this type of protection is required. Most
hydraulic systems have a strainer in the reservoir
at the inlet to the suction line of the pump. A
strainer is used in lieu of a filter to reduce its
chance of being clogged and starving the pump.
However, since this strainer is located in the
reservoir, its maintenance is frequently neglected.
When heavy dirt and sludge accumulate on the
suction strainer, the pump soon begins to cavitate.
Pump failure follows quickly.
