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RELIEF VALVES
Some fluid power systems, even when operating normally,
may temporarily develop excessive pressure;
for example, when an unusually strong work
resistance is encountered. Relief valves are used
to control this excess pressure. Relief
valves are automatic valves used on system
lines and equipment to prevent over-pressurization.
Most relief valves simply lift (open) at
a preset pressure and reset (shut) when the pressure
drops slightly below the lifting pressure. They
do not maintain flow or pressure at a given amount,
but prevent pressure from rising above a
specific level when the system is temporarily overloaded.
Main system relief valves are generally installed
between the pump or pressure source and the
first system isolation valve. The valve must be
large enough to allow the full output of the hydraulic
pump to be delivered back to the reservoir.
In a pneumatic system, the relief valve controls
excess pressure by discharging the excess gas
to the atmosphere.
Figure 6-10.—Hydraulic and pneumatic globe valve (rising stem).
Smaller relief valves, similar in design and operation
to the main system relief valve, are often used
in isolated parts of the system where a check valve
or directional control valve prevents pressure from
being relieved through the main system relief valve
and where pressures must be relieved at a set
point lower than that provided by the main system
relief. These small relief valves are also used
to relieve pressures caused by thermal expansion
(see glossary) of the fluids. Figure
6-11 shows a typical relief valve. System pressure
simply acts under the valve disk at the inlet
to the valve. When the system pressure exceeds
the force exerted by the valve spring, the valve
disk lifts off of its seat, allowing some of the
system fluid to escape through the valve outlet until
the system pressure is reduced to just below the
relief set point of the valve.
All relief valves have an adjustment for increasing
or decreasing the set relief pressure. Some
relief valves are equipped with an adjusting screw
for this purpose. This adjusting screw is usually
covered with a cap, which must be removed
before an adjustment can be made. Some type
of locking device, such as a lock nut, is usually
provided to prevent the adjustment from changing
through vibration. Other types of relief valves
are equipped with a handwheel for making adjustments
to the valve. Either the adjusting screw
or the handwheel is turned clockwise to increase
the pressure at which the valve will open. In
addition, most relief valves are also provided
Figure 6-11.—Relief valve.
with an operating lever or some type of device to allow
manual cycling or gagging the valve open for
certain tasks.
Various modifications of the relief valve shown
in figure 6-11 are used to efficiently serve the
requirements of some fluid power systems; however,
this relief valve is unsatisfactory for some
applications. To give you a better under-standing of
the operation of relief valves, we will discuss
some of the undesirable characteristics of this
valve.
A simple relief valve, such as the one illustrated
in figure 6-11, with a suitable spring adjustment
can be set so that it will open when the
system pressure reaches a certain level, 500 psi
for example. When the valve does open, the volume
of flow to be handled may be greater than the
capacity of the valve; therefore, pressure in the
system may increase to several hundred psi above
the set pressure before the valve brings the pressure
under control. A simple relief valve will be
effective under these conditions only if it is very large.
In this case, it would operate stiffly and the valve
element would chatter back and forth. In addition,
the valve will not close until the system pressure
decreases to a point somewhat below the opening
pressure.
The surface area of the valve element must be larger
than that of the pressure opening if the valve
is to seat satisfactorily as shown in figure 6-12.
The pressure in the system acts on the valve element
open to it. In each case in figure 6-12, the
force exerted directly upward by system pressure
when the valve is closed depends on the area
(A) across the valve element where the element
seats against the pressure tube. The moment
the valve opens, however, the upward force
exerted depends on the horizontal area (B) of
the entire valve element, which is greater than area
A. This causes an upward jump of the valve element
immediately after it opens, because the
Figure 6-12.—Pressure acting on different areas.
same pressure acting over different areas produces forces
proportional to the areas. It also requires a
greater force to close the valve than was required to
open it. As a result, the valve will not close until the
system pressure has decreased to a certain point
below the pressure required to open it. Let
us assume that a valve of this type is set to
open at 500 psi. (Refer to fig. 6-12.) When the valve
is closed, the pressure acts on area A. If this area
is 0.5 square inch, an upward force of 250 pounds
(500 ~ 0.5) will be exerted on the valve at
the moment of opening. With the valve open, however,
the pressure acts on area B. If area B is
1 square inch, the upward force is 500 pounds, or
double the force at which the valve actually opened.
For the valve to close, pressure in the system
would have to decrease well below the point
at which the valve opened. The exact pressure
would depend on the shape of the valve element.
In some hydraulic systems, there is a pressure in
the return line. This back pressure is caused by
restrictions in the return line and will vary in relation
to the amount of fluid flowing in the return
line. This pressure creates a force on the back
of the valve element and will increase the force
necessary to open the valve and relieve system
pressure.
It follows that simple relief valves have a tendency
to open and close rapidly as they "hunt" above
and below the set pressure, causing pressure
pulsations and undesirable vibrations and
producing a noisy chatter. Because of the unsatisfactory
performance of the simple relief valve
in some applications, compound relief valves were
developed.
Compound relief valves use the principles of operation
of simple relief valves for one stage of their
action—that of the pilot valve. Provision is made
to limit the amount of fluid that the pilot valve
must handle, and thereby avoid the weaknesses
of simple relief valves. (A pilot valve
is a small valve used for operating another valve.)
The operation of a compound relief valve is illustrated
in figure 6-13. In view A, the main valve,
which consists of a piston, stem, and spring, is
closed, blocking flow from the high-pressure line
to the reservoir. Fluid in the high-pressure line flows
around the stem of the main valves as it flows
to the actuating unit. The stem of the main valve
is hollow (the stem passage) and contains the
main valve spring, which forces the main valve against
its seat. When the pilot valve is open the stem
passage allows fluid to flow from the pilot
Figure 6-13.—Operation of compound relief valve,
valve, around the main valve spring, and down to
the return line. There is also a
narrow passage (piston passage) through
the main valve piston. This passage connects
the high-pressure line to the valve chamber.
The pilot valve is a small, ball-type, spring-loaded check
valve, which connects the top of the passage
from the valve chamber with the passage through
the main valve stem. The pilot valve is the
control unit of the relief valve because the pressure
at which the relief valve will open depends
on the tension of the pilot valve spring. The
pilot valve spring tension is adjusted by turning
the adjusting screw so that the ball will unseat
when system pressure reaches the preset limit.
Fluid at line pressure flows through the narrow
piston passage to fill the chamber. Because
the line and the chamber are connected, the
pressure in both are equal. The top and bottom
of the main piston have equal areas; therefore,
the hydraulic forces acting upward and
downward are equal, and there is no tendency for
the piston to move in either direction. The
only other force acting on the main valve is
that of the main valve spring, which holds it closed.
When the pressure in the high-pressure line increases
to the point at which the pilot valve is
set, the ball unseats (fig. 6-13, view B). This
opens the valve chamber through the valve
stem passage to the low-pressure return line.
Fluid immediately begins to flow out of the chamber,
much faster than it can flow through the
narrow piston passage. As a result the chamber
pressure immediately drops, and the pilot
valve begins to close again, restricting the
outward flow of fluid. Chamber pressure therefore
increases, the valve opens, and the cycle repeats.
So far, the only part of the valve that has moved
appreciably is the pilot, which functions just
like any other simple spring-loaded relief valve.
Because of the small size of the piston passage,
there is a severe limit on the amount of
overpressure protection the pilot can provide the
system. All the pilot valve can do is limit fluid
pressure in the valve chamber above the main
piston to a preset maximum pressure, by
allowing excess fluid to flow through the piston
passage, through the stem passage, and into
the return line. When pressure in the system increases
to a value that is above the flow capacity of
the pilot valve, the main valve opens, permitting
excess fluid to flow directly to the return
line. This is accomplished in the following manner.
As system pressure increases, the upward force on
the main piston overcomes the downward force,
which consists of the tension of the main piston
spring and the pressure of the fluid in the valve
chamber (fig. 6-13, view C). The piston then rises,
unseating the stem, and allows the fluid to flow
from the system pressure line directly into the
return line. This causes system pressure to decrease
rapidly, since the main valve is designed to
handle the complete output of the pump. When the
pressure returns to normal, the pilot spring forces
the ball onto the seat. Pressures are equal above
and below the main piston, and the main spring
forces the valve to seat.
As you can see, the compound valve over-comes the
greatest limitation of a simple relief valve
by limiting the flow through the pilot valve to
the quantity it can satisfactorily handle. This limits
the pressure above the main valve and enables
the main line pressure to open the main valve.
In this way, the system is relieved when an overload
exists.
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