Pressure Control Valves
Fluids that are under pressure are not safe or useful unless they are controlled.
The focus of this unit is to explain how and where pressure control valves are used.
It should be noted that the location of the valve is critical in determining its
function as related in a schematic diagram.
Basic Valve Symbol
In
Out
The above symbol is representative of both hydraulic and pneumatic
pressure controls. The system power source determines the fluid
nature of the valve. All pressure controls are interpreted the same
way. These valves have two ports, one an inlet and the other an
outlet, as indicated by the directional arrow. The “dashed” line
represents a pressure sensing capability.
Hydraulic and Pneumatic Pressure
Control Valves
Pictured above are two pressure controls most often discussed in
fluid power. On the left is a relief valve used in hydraulics systems
and on the left, a pressure regulator used in pneumatics. Their
schematic symbols are similar .
Hydraulic and Pneumatic Pressure Control
Valves
At a glance the two valve symbols above look the same but they are
very different. The one on the left is normally not passing and the
one on the right normally passing. Also, the sense lines are located in
different ports. You can see that failure to identify these small
differences can cause confusion.
Pressure Adjustment
In general pressure control valves that can be adjusted have the
arrow through the spring diagonally as shown but if the arrow is not
present then the valve is considered not adjustable. Usually, not
adjustable valves have a lock nut or some other such device to
prevent hand adjustment.
Uses of a Normally Closed (not
passing)Pressure Control Valve
In the circuit above, the pressure control ensures that fluid reaches
the actuator at maximum pump flow until a resistance is
encountered that exceeds the tension of the spring. When the
tension of the spring, due to pressure build up, has been reached, oil
will move over the relief valve back to tank. System pressure is
equal to the tension of the spring. This is a relief valve function.
Sequence Valve
In the circuit pictured above, a second pressure control valve
appears in series between the DCV and one of the actuators.
This arrangement ensures that the drill cylinder cannot move
until the clamp has fully extended.
Counterbalance Valve
In the circuit above, gravity acts on the platen to generate a pressure
on the rod side of the cylinder. If there was nothing to resist the
movement of oil from the rod side, the platen could speed the piston
away from incoming oil and slam into the work surface below. By
using the incompressible nature of oil we can slow a heavy platen by
the use of back pressure.
Counterbalance Valve
Back pressure can also be used to slow a motor whose shaft
is being driven by the inertia of a flywheel. The back
pressure generated is relative to spring tension as is the
stopping speed.
Pressure Reducing Valve
In this circuit the clamp cylinder must hold a part that
could be damaged by full system pressure. The pressure
reducing valve senses pressure downstream and closes
when the limit of the spring is reached. Fluid must be
able to enter a volume in order for pressure to increase
so by cutting off flow to the volume, a cylinder in this
case, the pressure and clamping force are controlled.
Drains
Drains are only found in hydraulic pressure control valves. Since
hydraulic pressure control valves use an internal member that allows
some bypass leakage, a way of releasing that leakage oil is needed.
As the diagrams above indicate, there exists an area above the spool
that could fill with oil and “lock” up the valve. To prevent the lock
up we provide one of two ways for leakage oil to escape the valve. If
the valve does not have pressure on the secondary port, it may be
drained internally to the secondary port. In the case of all pressure
control valves whose secondary port is under pressure, the valve
must be drained externally through an additional port. Pressure
reducing valves and sequence valves are always externally drained.
Direct and Remote Operation
A valve that senses pressure only from its inlet or out port
is said to be directly operated as opposed to one that
senses pressure from another location which would be
called remotely operated. Usually, remote operated valves
are referred to as unloading valves.
Unloading Valve
Unloading valves are used in hydraulic circuits to
momentarily “dump” the flow of the pump back to tank
during periods of machine idle time. This reduces heat
and saves energy since the load on the prime mover is
reduced.
Hi-Lo System
Note position of
unloading valve.
The purpose of the Hi-Lo system is to provide momentary high flow
for traverse action but then reduce it for feed rate. A ram, for
example, with a long stroke could be moved at a high rate of speed to
approach a work piece and then slowed to a crawl before contact
thus increasing the cycle time of the machine. Both pumps are used
for high flow and the large one “dumped” for low flow.
Remotely Operated Counterbalance
Valve
In a previous slide a counterbalance valve was shown as directly
operated. In actual practice a counterbalance valve might be
remote operated as above. The reason for remote operation is
reduce lost energy as a result of back pressure.
Brake Valve
A real brake valve is more complicated than a simple pressure
control. It has two sense lines that compare inlet and outlet
pressures. It is assumed by design that when a pressure is present at
the inlet port that the motor should turn. The pressure at the inlet
causes a “piston” to move against a spool and to hold the valve open
for free movement of the motor. The moment the pressure drops at
the inlet port the piston allows the spool with its spring to take over
and begin to apply back pressure to slow down the motor shaft and
its load.
Reverse Flow
As you may have noticed, the sequence valve, like many others, can
only sense pressure from one port. For fluid to get around the valve
on return, a check valve is added. The check valve allows normal
operation to take place as fluid is moving toward the cap end of the
actuator. However, when fluid is leaving the cap end the cylinder is
in retraction and the function of the sequence valve is not needed.
Without the check valve, fluid would not be able to get around the
valve and the cylinder would lock up.
Pressure Control Valves in
General
1. If a pressure control valve has pressure on its
secondary port, it must be externally drained.
2. If the secondary port of a pressure control
valve is not under pressure, it will usually be
internally drained.
3. Check valves are used to bypass flow around
all pressure control valves.
Completed Valve Symbols
In the following slides, schematic valve symbols appear
in their completed form. Their simple form was used to
introduce them in a non-intimidating manner. In the
industry, this is how they really appear. Study each in
detail.
Relief Valve
Unloading Valve
Sequence Valve
Counterbalance Valve
Brake Valve
Pressure Reducing Valve
Pressure Regulator
Pressure regulators are normally found at the beginning of branch
line circuits. Typically, the pressures required by branch line circuits
are much lower than the air pressure at the receiver tank. A venting
type regulator is one that can exhaust air from its secondary port if
pressure exceeds the spring setting.
Two Stage Pressure Control
Valve Operation
At first glance the diagram would appear to show three different
valves but the box surrounding the two schematic symbols is a
component enclosure meaning that there is only one component and
it includes everything seen.
Direct vs. Pilot Operated
A direct acting pressure control valve is one whose
sealing member is forced to open solely by the
pressure of fluid. Direct acting pressure control
valves are common in hydraulics as a relief valve.
Pilot operated pressure control valves are actually two
valves made into one. Sometimes called a “two stage”
pressure control, pilot operated pressure control valves
have a primary and secondary stage. When the
primary stage activates
Override-Direct Operated
Pressure Control Valves
Override is a characteristic of a valve where the valve will
momentarily create a pressure beyond its setting. This is
caused by the compression of a spring. High override can
damage sensitive components as well as waste energy.
Override-Pilot Operated Pressure
Control Valves
Pilot operated valves don’t have as much override because they use a
light spring and fluid pressure to hold the valve in its normal state.
The moment the fluid is released, the valve opens very fast because
the light spring has little resistance.
Direct Acting Pressure Control
In the illustration above, a only spring tension holds the
ball against the seat. The adjustment knob, shown at the
bottom, is for changing tension on the spring.
Pilot Operated Pressure Control
In the illustration above both fluid and spring force hold the main
poppet(main stage) closed. After the pilot relief poppet opens from
system pressure, oil begins to vent from the spring side creating a
pressure differential and reducing the total force holding down on
the main poppet. When the pressure differential is great enough the
main poppet opens quickly. It is this rapid opening capability that
makes this design more efficient than the direct acting type.
Pilot Operated Pressure Control Valve
Operation
Initially at start up the hydraulic system would be at low pressure,
for a couple of seconds, and the relief valve shown above would be
closed. The orifice ensures that system pressure is “felt” on both
sides of the spool. Study the illustration to become familiar with the
individual components.
Pilot Operated Pressure Control
Valve Operation
In the illustration, the port subjected to 1000psi is the
inlet port and is directed attached to the system. The
port down below is connected to tank.
Pilot Operated Pressure Control Valve
Operation
Since the dart is biased by a stiff spring, it will take more pressure to
overcome it than the spring of the main spool which is light by
comparison. The action of the dart opening starts the venting
process by creating a pressure differential across the main spool.
Pilot Operated Pressure Control Valve
Operation
Once the dart has begun to open, it will take an additional pressure
build up of 25psi before the main spool opens fully. It should be
noted that the main spool “throttles” in a relief valve. Main spool
positioning is based on the demand for flow.
Pilot Operated Pressure Control
Valve Operation
While venting, the dart is completely unseated and passing oil at
high pressure. With the dart open, a pressure differential exists
because the oil leaving the area above the spool is greater than the oil
that can flow in through the tiny orifice. When pressure drops the
dart closes and oil pressure builds on the spool, closing it.
Sequence Valve
Simply put, the purpose of a sequence valve is to cause a series of
operations to occur in order. This normally not passing valve blocks
oil to the “secondary” actuator until a preset limit of pressure is
reached. When pressure is correct, the sequence valve opens and
allows flow to reach the secondary actuator for movement. Unlike
the relief valve, the sequence valve will open completely. Usually, a
bypass check valve is included for reverse flow.
Counterbalance or Unloading
Valve
This valve is remotely operated. Remember that the valve is
watching the sense line for pressure, not its inlet, so we can control a
tremendous load but allow it to move at relatively low pressure by
use of the remote pilot.
Pressure Reducing Valve
The pressure reducing valve is the only normally passing pressure
control used in hydraulics. Just like the pneumatic pressure
regulator, the reducing valve controls pressure in hydraulic branch
line circuits. Recall that we can control the clamping force of a
cylinder with this valve.
Methods of Remote Control
Any time the oil on top of the main spool is vented, the
main spool will open. All methods of remote control
work on this principle.
Remotely Operated Pressure Control
Valve
In the example above, the remote pilot valve serves to control the
relief valve by venting the oil on top of its main spool. The remote
pilot might be located some distance from the relief valve, maybe on
a control panel. The other dart on the relief valve could be used for
maximum pressure control of the valve.
Remotely Control of Relief Valve
Schematic Representation
There is no component enclosure symbol because these two
valves are not physically together.
Alternative Remote Control
When examine the above illustration, please remember that any
time the oil is released from above the main spool, this type of
valve will actuate. You could even use two way hand valve to do
the job.
Differential Unloading Relief Valve
This valve, unlike the others, gives us a “cut in” and “cut out”
range for unloading a pump. This valve is usually used with an
accumulator so that the pump may be unloaded during periods
where the system is being powered by the stored volume of the
accumulator.
Differential Unloading Relief Valve
In a Circuit
Notice the two sense lines. This valve is watching the pressure at the
pump outlet as well as the accumulator. The check valve isolates the
system from the pump during the unloaded periods.
Pilot Controlled Pressure
Regulator
Some of the control methods used in hydraulics are also used in
pneumatics. Here, pilot pressure, in this case air pressure, is used to
push the piston down while pressure from the secondary port pushes
up on the piston. Whenever the air on top is exhausted, the valve will
close.
Review Part One
1.
What is override?
2.
What is a “two stage” valve?
3.
What is pilot pressure?
4.
What is the sequence of operation within the pilot operated valve?
5.
Which of the two stage valves opens first?
6.
As above, which of the two flows the most?
7.
What is the function of the control orifice?
8.
What is remote operation?
9.
Which of the two valves is “direct acting”?
10.
What is the normal state of a pressure regulator.
11.
What is the name given to the hydraulic pressure control valve that is normally passing?
12.
What is the purpose of the pressure regulator?
13.
From which port does the regulator sense pressure?
14.
What does it mean if a regulator is venting air and it has not been adjusted?
15.
Explain some of the differences between the relief valve and the pressure regulator.
Review Part Two
16.
What acts on the internal member of pressure control valves to help them achieve a normal
condition?
17
Give at least two different functions for normally closed pressure control valves?
18.
What determines the name given to a pressure control valve?
19.
What is the normal state of a relief valve and to what is it’s outlet port connected?
20.
What is the function of an un-loading valve?
21.
Give the function of a sequence valve.
22.
Give the name of a normally not passing pressure control valve used in hydraulics to
counteract or to balance a weight attached to a load?
23.
Describe what “internal” pilot pressure refers to in a counterbalance valve?
24.
Describe the function of a brake valve.
25.
Give the name of the only normally passing pressure control valve used in hydraulics.
26.
Explain why a pressure control valve would need an external drain.
27. What would be the draining arrangement for a pressure control valve whose secondary port
is connected to tank?
28.
What is the function of a check valve in relation to a pressure control valve?
29.
What is the normal state of the pneumatic pressure regulator?
30.
What does the term “vented” refer to in regard the pressure regulator?