Pressure - Compensated Flow Control Valve:
Introduction:
Any flow control,
with pressure compensation feature, will automatically maintain a rate of flow
regardless of the pressure changes that are caused by the work (load).
If a cylinder is moving 500 N load at the rate of 0.3
m/min, and suddenly it is required to move 1000 N at the rate of original
speed, it means that the volume of oil going to the cylinder must remain the
same. But the pressure must be doubled in order to move twice the load.
Pressure compensated flow control valves are designed to
accomplish this feature.
Working principle:
To analyze the working principle of these valves, it is necessary to recapitulate the
fluid flow law which states that “a fluid rate of
flow will be constant through an orifice of a given size as long as the
pressure drop remains constant”. This means that the pressure can vary
as long as the change is the same.
Flow rate is the measure of flow volume that streams past
a measuring point in a given time. Flow rate is measured in liters / minute (abbreviated lpm) or cubic meters per minute. Flow
rate has a direct bearing on the speed with which a hydraulic actuator moves a
load and is therefore governed by the design concepts of the machine.
Example:
Let p1 = 70 bar, and p2 = 60 bar.
The pressure differential is now 10 bar.
Assume the flow to be 40 lpm.
If the pressure p1 is increased to 90 bar and p2
is increased to 80 bar, the flow is still 40 lpm because the pressure
differential is still the same (10 bar)
This characteristic of fluid flow permits a wide variety of
automatic flow control valves. Pressure compensated flow control valve is one
such type.
In many hydraulic machines, load on the actuators are
fluctuating which results in fluctuating actuator speeds. This is not desirable
and it needs a technique or device which automatically maintains constant
actuator speed irrespective of the variation in the actuator load.
Since load fluctuation leads to pressure variation at the actuator input line, a pressure compensated flow control valve which controls and maintains a constant flow rate in the actuator line will be a proper solution to maintain constant actuator speed.
Pressure compensated Flow Control valve is designed in such a way that it will maintain a constant flow rate over a limited range of pressure drop.
Principle of operation of the valve:
Pressure compensated flow control valves are designed such that the pressure drop across its inlet and outlet is maintained constant, thereby ensuring constant flow rate to maintain constant actuator speed.
Construction features:
Pressure compensated flow control valve has two orifices. One of the orifice is
either fixed or manually adjusted. Size of the other orifice is a hydraulically
controlled variable orifice.
Fixed orifice is used to set the desirable flow rate
of fluid for a particular pressure drop (generally 8 bar). The valve is
designed such that the hydraulically actuated orifice senses the load pressure
and the supply pressure to change the size of the variable orifice to maintain
a predetermined pressure drop across the fixed orifice.
Fig. 1 shows a schematic representation of a pressure
compensated flow control valve.
Flow passes from inlet port A through the pressure
compensator orifice then through the manually adjusted fixed orifice 1 which is
located on the bottom right. The flow leaves the valve outlet port B. The valve
spool, also called as the compensator spool, is located above and it
hydraulically meters the size of the second orifice 2.
Pressure compensation is based on the use of
pressure positioned variable orifice 2 upstream.
In the normal state of working, the hydraulic forces
on the spool will hold the compensator spool in balance, but the bias spring
force will force it to the extreme right, thus providing an unrestricted
passage (holding
the orifice 2 fully open)
from valve inlet port A to the fixed orifice 1.
When the fluid enters the inlet port A, it is
restricted by the fixed orifice, set manually. This results in the pressure
rise in the control chamber C.
The control chamber C is connected to the valve spool
on the right side, while the spring chamber S (at the bottom of the spool) is
connected to the outlet port B.
To understand the working, following conditions are
analyzed:
1.
When the outlet
pressure at B is Zero.
2.
When the exit
pressure at B rises.
When outlet pressure is zero, since the control chamber
C is connected to the right side of the compensator spool, the spring force
keeps the valve spool from moving to the left thereby keeping the orifice 2
fully open.
When the
pressure in the control chamber is high enough to overcome spring force, the
valve spool moves towards left. This leftward movement of the spool reduces the size
of the orifice 2 (between the inlet and control chamber) metering the flow to
the control chamber (i.e.,
the spool takes a position such that the orifice 2 allows the same flow as the
preset orifice 1). In
this condition, there is pressure balance between the control chamber pressure
and the spring force.
When the inlet pressure rises (Upstream pressure
variation), pressure
in the control chamber C also rises, thereby moving the spool to the left to
make the orifice 2 size smaller. This reduces the pressure in the control
chamber to a value equivalent to the spring force.
When the outlet (exit) pressure at B rises due to the
outlet flow moving an actuator (Downstream pressure variation):
The outlet flow from port B when connected to the inlet
of an actuator and make it move against a load, the outlet pressure at B vary.
As the pressure at B rises, this would assist the
spring to move the compensator spool to move to the right resulting in the
increase of the size of orifice 2. The rise in exit pressure at point B will
cause the balanced spool to move to the right (assisting the spring force) thus
opening the orifice 2 to raise the pressure in the control chamber which is
maintained equal with the spring force but the pressure drop across the orifice
1 is maintained equal.
When the outlet pressure at B lowers, the spool moves
to the left to close the orifice 2 and thereby reduces the pressure in the
control chamber.
Since the valve spool orifice 2 control maintains a
constant pressure drop across the fixed orifice 1, the flow control valve
maintains a constant flow.
Hence, a pressure compensated flow control valve is a
variable resistance valve, which, for a given valve setting, maintains a
constant flow across the valve irrespective of the exit pressure at B (the
actuator working pressure to match the load resistance).
Fig. 2 represents the schematic diagram of pressure
compensation principle to prove that the flow rate is depending on the pressure
drop across the control orifice and measuring orifice.
Control orifice A2 and measuring orifice A1 are connected in series. The compensator spool is pressurized on the right by control chamber pressure p2 and by exhaust pressure p3 on the left along with bias spring force FS.
For pressure balancing:
Hence the pressure drop across the flow control valve will be:
SUMMARY OF THE WORKING PRINCIPLE (REFER FIG. 2)
The constant pressure differential is maintained as
follows.
As the pressure increases at B (which is the pressure
required to do the work), the pressure at spring chamber S will also increase,
forcing the orifice 2 to open. This reduces the pressure drop across this
orifice and increases the pressure in area C. If the pressure at area B
decreases, so will the pressure at S. This means that the pressure at point C (which
is now greater) will force the spool against the spring, thus closing orifice 2
and reducing the pressure at C and on the area of spool on the right side. This
results again in equilibrium. In this manner this spool will be actuated by a
difference in pressure at area C and area B and will always take a setting that
will keep the pressure differential the same between C and B, thus providing
the outlet with a constant volume flow.
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