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CHAPTER 9
Fluid power systems must have a sufficient
and continuous supply of uncontaminated fluid
to operate efficiently. As stated in chapter 3 and
emphasized throughout this manual, the fluid
must be kept free of all foreign matter.
This chapter covers hydraulic reservoirs,
various types of strainers and filters, and
accumulators installed in fluid power systems.
RESERVOIRS
A hydraulic system must have a reserve of
fluid in addition to that contained in the pumps,
actuators, pipes, and other components of the
system. This reserve fluid must be readily available
to make up losses of fluid from the system, to
make up for compression of the fluid under
pressure, and to compensate for the loss of
volume as the fluid cools. This extra fluid is
contained in a tank usually called a reservoir. A
reservoir may sometimes be referred to as a sump
tank, service tank, operating tank, supply tank,
or base tank.
In addition to providing storage for the reserve
fluid needed for the system, the reservoir acts as
a radiator for dissipating heat from the fluid and
as a settling tank where heavy particles of
contamination may settle out of the fluid and
remain harmlessly on the bottom until removed
by cleaning or flushing of the reservoir. Also, the
reservoir allows entrained air to separate from the
fluid.
Most reservoirs have a capped opening for
filling, an air vent, an oil level indicator or dip
stick, a return line connection, a pump inlet or
suction line connection, a drain line connection,
and a drain plug (fig. 9-1). The inside of the
reservoir generally will have baffles to prevent
excessive sloshing of the fluid and to put a
partition between the fluid return line and the
pump suction or inlet line. The partition forces
the returning fluid to travel farther around the
tank before being drawn back into the active
Figure 9-1.—Nonpressurized reservoir (ground or ship
installation).
system through the pump inlet line. This aids in
settling the contamination and separating the air
from the fluid.
Large reservoirs are desirable for cooling. A
large reservoir also reduces recirculation which
helps settle contamination and separate air. As
a ‘‘thumb rule," the ideal reservoir should be two
to three times the pump output per minute.
However, due to space limitations in mobile and
aerospace systems, the benefits of a large reservoir
may have to be sacrificed. But, they must be large
enough to accommodate thermal expansion of the
fluid and changes in fluid level due to system
operation. Reservoirs are of two general types— nonpressurized and pressurized.
NONPRESSURIZED RESERVOIRS
Hydraulic systems designed to operate
equipment at or near sea level are normally
equipped with nonpressurized reservoirs. This
includes the hydraulic systems of ground and ship
installations. A typical reservoir for use with
ground and ship installations is shown in figure
9-1. This type of reservoir is made of hot rolled
steel plates and has welded seams. The ends extend
below the bottom of the reservoir and serve as
supports. The bottom of the reservoir is convex,
and a drain plug is incorporated at the lowest
point.
Nonpressurized reservoirs are also used in
several transport-, patrol-, and utility-type
aircraft. These aircraft are not designed for violent
maneuvers and, in some cases, do not fly at high
altitude. Those aircraft that have nonpressurized
reservoirs installed and that fly at high altitudes
have the reservoirs installed within a pressurized
area. (High altitude in this situation means an
altitude where atmospheric pressure is inadequate
to maintain sufficient flow of fluid to the
hydraulic pumps.)
Most nonpressurized aircraft reservoirs are
constructed in a cylindrical shape (fig. 9-2). The
outer housing is manufactured from a strong
corrosion-resistant metal. Filter elements are
normally installed internally within the reservoir
to clean returning system hydraulic fluid. Some
of the older aircraft have a filter bypass valve
installed to allow fluid to bypass the filter if the
filter becomes clogged. Reservoirs that are filled
by pouring fluid directly into them have a filler
(finger) strainer assembly installed in the filler well
to strain out impurities as the fluid enters the
reservoir.
Figure 9-2.—Nonpressurized aircraft reservoir.
The quantity of fluid in the reservoir is
indicated by either a glass tube, a directing gauge,
or a float-type rod, which is visible through a
transparent dome installed on the reservoir.
PRESSURIZED RESERVOIRS
A pressurized reservoir is required in hydraulic
systems where atmospheric pressure is insufficient
to maintain a net positive suction head (NPSH)
to the pump. There are two common types of
pressurized reservoirs—fluid-pressurized and
air-pressurized.
Fluid-Pressurized Reservoir
Some aircraft hydraulic systems use fluid
pressure for pressurizing the reservoir. The
reservoir shown in figure 9-3 is of this type. This
reservoir is divided into two chambers by a
floating piston. The piston is forced downward
in the reservoir by a compression spring within
the pressurizing cylinder and by system pressure
entering the pressurizing port of the cylinder.
The pressurizing port is connected directly to
the pressure line. When the system is pressurized,
pressure enters the pressure port, thus pressurizing
the reservoir. This pressurizes the pump suction
line and the reservoir return line to the same
pressure.
The reservoir shown in figure 9-3 has five
ports—pump suction, return, pressurizing,
overboard drain, and bleed. Fluid is supplied to
the pump through the pump suction port. Fluid
returns to the reservoir from the system through
the return port. Pressure from the pump enters
the pressurizing cylinder in the top of the reservoir
through the pressurizing port. The overboard
drain port is used to drain the reservoir while
performing maintenance, and the bleed port is
used as an aid when servicing the reservoir.
Air-Pressurized Reservoirs
Air-pressurized reservoirs, such as the one
shown in figure 9-4, are currently used in many
high-performance naval aircraft. The reservoir is
cylindrical in shape and has a piston installed
internally to separate the air and fluid chambers.
Air pressure is usually provided by engine bleed
air. The piston rod end protrudes through the
reservoir end cap and indicates the fluid quantity.
The quantity indication may be seen by inspecting
the distance the piston rod protrudes from the
reservoir end cap. The reservoir is provided with
Figure 9-3.—Typical
fluid-pressurized reservoir.
threaded openings for connecting fittings and
components. Figure 9-4 shows several components
installed in lines leading to and from the reservoir;
however, this may not be the case in actual
installation. The air relief valve, bleeder valve, and
soon, may reinstalled directly on the reservoir.
Because the reservoir is pressurized, it can
normally be installed at any altitude and still
maintain a positive flow of fluid to the pump.
Figure 9-4.—Air-pressurized
reservoir.
Some air-pressurized reservoirs also have
direct contact of fluid to gas. These reservoirs are
installed in large systems and may be cylindrical
or rectangular in shape. They contain an oil level
indicator, a pump inlet or suction line connection,
a return line, a gas pressurization and venting
connection, and a drain line connection or a drain
plug. These reservoirs are pressurized by air from
the ship’s service air system or nitrogen banks.
These reservoirs are found on board aircraft
carriers and submarines.
