Devices for controlling flow of fluid under pressure

A system for controlling flow of fluid along a conduit from a source of fluid under pressure to a consumer device comprises a primary throttle for connection in the conduit and controllable between a first condition in which it is at least partially closed and a second condition in which it is more fully open, and a secondary throttle defining at least one orifice for connection in the conduit and including a member which is displaceable to vary the area of the orifice. A first force dependent upon the pressure drop across the secondary throttle and a second force dependent upon the time integral of the pressure drop across the primary throttle are applied to the member, the first and second forces being oppositely directed, whereby the member is displaced towards a position in which the pressure drop across the secondary throttle bears a predetermined relationship to the time integral of the pressure drop across the secondary throttle.

This invention relates to devices for controlling flow of fluid under 
pressure, and is particularly concerned with a device for controlling flow 
of liquid fuel to a jet engine. 
As might be expected, the graph of steady state jet engine speed against 
fuel flow is a generally upward curve. When the engine is operating at a 
steady fuel flow below that which is associated with maximum engine speed, 
and the fuel flow is then increased, the engine speed does not immediately 
increase to the value associated with the higher fuel flow, but there is 
rather some delay before the engine speed reaches the higher value. If the 
fuel flow is suddenly increased by an excessive amount, the engine is 
overfueled and malfunctions. It is essential that overfueling of the 
engine be avoided. 
Overfueling can be avoided by interposing a servo control between the 
operator of the jet engine (usually an aircraft pilot) and the fuel flow 
control valve, so that the fuel flow never increases by such an amount and 
so rapidly as to cause overfueling. However, in such a system the operator 
has no direct mechanical link to the valve, and in addition the control is 
effective not only upon increase in fuel demand but also upon decrease in 
fuel demand. 
According to a first aspect of the present invention there is provided a 
system for controlling flow of fluid along a conduit from a source of 
fluid under pressure to a consumer device, comprising a primary throttle 
means and a secondary throttle means for connection in the conduit, said 
primary throttle means being controllable between a first condition in 
which the primary throttle means is at least partially closed and a second 
condition in which the primary throttle means is more fully open, means 
for detecting the pressure drop across the primary throttle means, means 
for detecting the pressure drop across the secondary throttle means, and 
servo means for controlling the secondary throttle means to establish a 
predetermined relationship between the pressure drop across the secondary 
throttle means and the time integral of the pressure drop across the 
primary throttle means. 
According to a second aspect of the present invention there is provided a 
system for delivering fluid at a controlled rate from a source of fluid 
under pressure to a consumer device, comprising first and second conduit 
which are connected in parallel, a primary throttle means for connection 
in the first conduit and controllable between a first condition in which 
it is at least partially closed and a second condition in which it is more 
fully open, a control throttle means for connection in the first conduit, 
a controlled throttle means for connection in the second conduit, means 
for detecting the pressure drop across the primary throttle means, means 
for detecting the pressure drop across the control throttle means, servo 
means for controlling the control throttle means to establish a 
predetermined relationship between the pressure drop across the primary 
throttle means and the control throttle means and the time integral of the 
pressure drop across the primary throttle means, and means for slaving the 
controlled throttle means to the control throttle means. 
According to a third aspect of the present invention there is provided a 
system for controlling flow of fluid along a conduit from a source of 
fluid under pressure to a consumer device, comprising a primary throttle 
means for connection in the conduit and controllable between a first 
condition in which it is at least partially closed and a second condition 
in which it is more fully open, and a secondary throttle means defining at 
least one orifice for connection in the conduit and including a member 
which is displaceable to vary the area of the orifice, and the system 
further comprising means for applying to said member a first force 
dependent upon the pressure drop across the secondary throttle means and a 
second force, directed oppositely to said first force, dependent upon the 
time integral of the pressure drop across the primary throttle means, 
whereby the member is displaced towards a position in which said pressure 
drop bears a predetermined relationship to said time integral. 
According to a fourth aspect of the present invention there is provided a 
system for delivering fluid at a controlled rate from a source of fluid 
under pressure to a consumer device, comprising first and second conduits 
which are connected in parallel, a primary throttle means for connection 
in the first conduit and controllable between a first condition in which 
it is at least partially closed and a second condition in which it is more 
fully open, and a control throttle means defining at least one orifice 
connected in the first conduit and including a member which is 
displaceable to vary the area of the orifice, and the system further 
comprising a controlled throttle means for connection in the second 
conduit, means for slaving the controlled throttle means to the control 
throttle means, means for applying to said member a first force dependent 
upon the pressure drop across the control throttle means and a second 
force, directed oppositely to said first force, dependent upon the time 
integral of the pressure drop across the primary throttle means, whereby 
the member is displaced towards a position in which said pressure drop 
bears a predetermined relationship to said time integral and said 
controlled throttle means is slaved accordingly. 
By stating that the first throttle means is at least partially closed in 
the first condition it is meant that the first throttle means is not fully 
open; the first throttle means might be fully closed in the first 
condition, i.e. at the closed end of the open-to-closed range, although 
even in the fully closed condition there might be flow through the first 
throttle means. In the second condition, the first throttle means might be 
fully open.

In the different figures, like reference numerals designate like elements. 
The system illustrated in FIG. 1, which is designed to control flow of fuel 
to an aircraft jet engine, comprises two throttles 2 and 4 which are 
connected in series in a conduit 6 extending between a fuel pump (not 
shown) and burners (not shown). The throttle 2 is under direct pilot 
control, whereas the throttle 4 is servo controlled. 
The pump operates to drive fluid fuel under pressure along the conduit 6 
through the valves 2 and 4 to the burners. The pressure of fluid in the 
conduit 6 is tapped of by means of three lines 20, 22 and 24 connected to 
the conduit 6 upstream of the two throttles, between the two throttles and 
downstream of the two throttles respectively. 
The servocontrol for the throttle 4 comprises a piston and cylinder device 
coupled to the throttle 4. The piston and cylinder device comprises a 
piston 8 having a large portion 10 moving in a large bore of the cylinder 
and a small portion 12 moving in a small bore of the cylinder. The two 
portions 10 and 12 may be rigidly connected together. The piston portions 
and the bores define three chambers 14, 16 and 18. 
The area of the large piston portion 10 is chosen in this example to be 
twice the area of the small piston portion 12. Consequently, the area of 
the rear face of the large portion which bounds the chamber 16 is equal to 
the area of the small portion. The lines 20, 22 and 24 are connected to 
the chambers 18, 14 and 16 respectively, the connection of the line 20 to 
the chamber 18 being through a rate restrictor 26. The throttle 4 is 
controlled in dependence upon the position of the piston. As a consequence 
of the two-to-one relationship between the areas of the piston faces of 
the portions 10 and 12, and the connections which are made to the chambers 
14, 16 and 18 respectively, the piston 8 will always tend to move to a 
position of force balance in which the pressure drops across the throttles 
2 and 4 are equal. 
When the engine is operating in the steady state at a speed of, say, 60% of 
maximum speed, and the pilot suddenly moves the throttle 2 to its fully 
open position, the pressure in the line 22 is suddenly increased 
substantially to the pressure in the line 20. The resulting increase in 
pressure in the chamber 14 will urge the piston to the right of FIG. 1, 
opening the throttle 4 (and consequently reducing the pressure drop across 
the throttle 4 and between the chambers 14 and 16). However, by virtue of 
the presence of the restrictor 26, resisting displacement of fuel from the 
chamber 18, the piston is not displaced instantaneously to a new steady 
state position, but rather its rate of movement is limited, and 
consequently the rate of opening of the throttle 4 is also limited. Since 
the smaller throttle dominates the rate of flow of fuel through the 
conduit 6, according to the law of the reciprocal of the sum of 
reciprocals for series throttles the pilot's sudden opening of the 
throttle 2 is overridden by the slower response of the throttle 4 and 
overfueling is limited. The piston ultimately attains a new steady state 
position, in which the pressure drop across the throttle 2 is equal to the 
pressure drop across the throttle 4. If then the pilot suddenly closes the 
throttle 2, the piston and the throttle 4 respond similarly as before, so 
that the rate of closing of the throttle 4 is limited, but the throttle 2 
(being more fully closed) dominates the throttle 4 and therefore the fuel 
flow is in fact reduced abruptly, substantially in accordance with 
operation of the throttle 2 alone. It will thus be seen that the 
illustrated system responds in retarded fashion to a command for increase 
in fuel flow while responding promptly to a command for decrease in fuel 
flow. If desired, a check valve could be connected in parallel with the 
restrictor, permitting flow into the chamber 18, in which case the piston 
and the throttle 4 would respond promptly to closing of the throttle 2. 
However, this would not affect substantially the response of the entire 
system to a command for increase in fluid flow. 
The rate at which the pressure in the chamber 18, and thus the force acting 
on the small piston portion 12, varies depends on the rate at which fluid 
flows through the restrictor 26, and this rate depends upon the pressure 
difference between the chamber 18 and the line 20 and the time for which 
such pressure difference has persisted. 
One or more additional throttle, such as a throttle controlled by a 
propeller pitch input, could be connected in series with the throttle 2 
between the connections to the lines 20 and 22. 
FIG. 2 illustrates a practical construction of the system illustrated in 
FIG. 1. This practical construction is designed as a back-up control for 
the normal electronic fuel flow control system employed for an aircraft 
jet engine. The aircraft pilot's manual fuel control is a lever mounted on 
a rotary shaft, which is directly coupled mechanically to the throttle 2 
and to a manual shutoff valve 38 and a change-over flow control valve 40. 
The valve 38 is in a conduit which by-passes the throttles 2 and 4 and the 
valve 40. The valve 40 is connected in parallel with the throttle 2 in 
order to provide a certain minimum flow for keeping the engine flame 
alight when there is a change over from electronic control to manual 
control or vice versa. 
In the FIG. 2 construction, the throttle 4 is incorporated in the piston 
and cylinder device. To this end, the piston 8 is formed with passageways 
28 providing communication between the chambers 14 and 16. The passageway 
28 extending from the chamber 14 is axial of the piston and has an orifice 
30 at its upstream end. A profiled needle 32 is mounted axially in the 
chamber 14 and projects into the orifice 30. Therefore, the conduit 6 is 
formed partially by the chambers 14 and 16 and the passageways 28, there 
being no separate lines 22 and 24 for communicating the pressure in the 
conduit 6 to the chambers 14 and 16. When the piston moves to the left, 
the needle 32 penetrates more fully into the orifice 30 and consequently 
the rate of flow of fuel is reduced, whereas when the piston is moved to 
the right the needle penetrates less into the orifice 30 and the rate of 
flow of fuel is increased, as in the case of FIG. 1. The profile of the 
needle can be selected to provide desired characteristics. For example, 
instead of a straight taper needle as shown in FIG. 2, a barreled needle, 
as shown in FIG. 2 A, can be used in order to reduce the response to 
displacement of the piston when the piston is at the left, around idle, 
while increasing the response to displacement of the piston when the 
piston is at the right, around maximum power. 
Since the amount of fuel flowing through the restrictor 26 is very small, 
it is important to avoid leakage around the small piston portion 12. Such 
leakage would cause the rate of displacement of the piston to vary with 
temperature and life. Therefore a seal is provided around the piston 
portion 12. In view of the small variations in pressure in the chamber 18 
and the consequent small changes in force effective on the small piston 
portion, the seal should have low friction and stiction characteristics to 
ensure pressure is correctly shared between the two throttles. The need 
for a seal about the large piston portion is not so critical, in view of 
the much larger flow rates affecting the chambers 14 and 16. Nevertheless, 
it is desirable to provide a seal about the large piston portion to 
minimize leakage past the servo throttle. Friction might be further 
reduced by use of low friction coatings, e.g. Teflon. This allows a 
reduction in the size of the piston and cylinder device without increasing 
the rate, since the rate of the device is a function of the size of the 
device and of the frictional forces involved. 
FIG. 2 B illustrates in detail a modified form of the piston and cylinder 
device. It will be seen that in FIG. 2 B the piston 8 is formed as a 
dumbbell, comprising a waisted portion between two enlarged end portions. 
Each end portion is provided with a seal. The needle has a profile with 
three triangular flats, instead of a taper. 
FIG. 3 illustrates diagrammatically an alternative to the construction 
illustrated in FIG. 1. In FIG. 3 the servo throttle 4 is disposed upstream 
of the pilot's throttle 2, and the line 24, being disposed immediately 
downstream of the throttle 2, is provided with the flow restrictor 26. The 
operation of the system will be evident from the foregoing explanation of 
the operation of the system of FIG. 1. 
FIG. 4 illustrates diagrammatically a system which operates on the same 
principle as that of FIG. 3 except that the retardation in response takes 
place both on a reduction in demand and on an increase in demand. In order 
to achieve this, the conduit 6 is used only for a servoflow of fuel, there 
being a second conduit 34 connected in parallel with the conduit 6 and 
taking the main flow of fuel, and an auxiliary servo throttle 36, which is 
ganged to the throttle 4, is arranged to control the main flow through the 
conduit 34. 
When the manual throttle 2 is suddenly closed, the pressure in the line 22 
increases and the force driving the piston to the right increases 
correspondingly. Movement of the piston to the right is retarded by the 
restrictor 26, connected in the line 24, and accordingly the throttles 4 
and 36 close more slowly than the throttle 2. Because the throttle 2 is 
connected in the conduit 6, and not in the main flow conduit 34, the 
throttle 2 is dominated by the slower-closing throttle 36, and 
consequently the main flow is reduced more slowly than the servoflow. If 
the throttle 2 is suddenly opened, on the other hand, the throttles 4 and 
36 also open, but more slowly, and consequently the rate of the flow in 
the conduit 34 increases more slowly than the increase in the command for 
supply of fluid. Thus, this system operates bidirectionally as opposed to 
the unidirectional (increase only) systems of FIGS. 1, 2 and 3. The FIG. 4 
system could be modified to retard the servo throttle 36 only upon 
decrease in demand by connecting a check valve in parallel with the 
restrictor 26 to permit fluid to flow into the chamber 16 without passing 
through the restrictor. Such a system could be used to eliminate water 
hammer by limiting the rate of closing of the throttle 36. 
A modification of the system of FIG. 4 is shown in FIG. 5. The system of 
FIG. 5 differs from that of FIG. 4 in a similar way to that in which the 
system of FIG. 1 differs from that of FIG. 3. The system of FIG. 5 is 
preferred to that of FIG. 4, and similarly that of FIG. 1 is preferred to 
that of FIG. 3, because in each case, the restrictor is connected to the 
fuel line upstream of the throttles, and therefore any air present in the 
chamber to which the restrictor is connected is compressed to 
insignificance. A second, although less important, reason for preferring 
FIGS. 1 and 5 is that, in each case, if flow past the large piston portion 
can be ignored, only a single seal is required to avoid leakage from the 
chamber to which the line with the restrictor 26 is connected. In FIGS. 3 
and 4, the line with the restrictor is connected to the chamber 16 and 
accordingly two seals, with attendant friction, are required in order to 
avoid leakage. 
A practical implementation of the FIG. 5 system is shown in FIG. 6. The 
servo throttles are both incorporated in the piston and cylinder device, 
the needle 32 having two profiled regions 56 and 58 cooperating with two 
separate orifices 42 and 44 in the piston. The interior of the piston is 
divided by the orifice 44 into two chambers 46 and 48. The small portion 
12 of the piston is formed with a peripheral recess 50 which communicates 
with the chamber 48 through a port 52, and the chamber 16 communicates 
with the chamber 46 through a port 54. The conduit 6 is formed partially 
by the chambers 14 and 46, the port 54 and the chamber 16, while the 
conduit 34 is formed partially by the recess 50, the chambers 48 and 46, 
the part 44 and the chamber 16. 
It will be seen that when the throttle 2 is suddenly opened, the piston is 
displaced to the right and the effective areas of the orifices 42 and 44 
increases, the rate of movement of the piston depending upon rate at which 
fuel is displaced from the chamber 18 through the restrictor 26 and the 
pressure difference between the chambers 14 and 16, i.e. the pressure drop 
across the throttle 4 constituted by the region 56 of the needle and the 
orifice 42. The throttle 36, constituted by the region 58 and the orifice 
44, is slaved to the throttle 4 and thus the main flow of fuel is varied 
in accordance with variations in the servoflow through the throttle 4. 
The systems of FIGS. 5 and 6 are bidirectional, like that of FIG. 4. The 
system of FIGS. 4, 5 or 6 could be modified to operate unidirectionally by 
connecting a check valve in parallel with the flow restrictor 26. Such a 
solution might be more desirable than the unidirectional system of FIGS. 2 
or 3, for example, where slaving of the main flow control is desired. 
A feature that the systems of FIGS. 1 to 6 have in common is that they 
require only two connections to the fuel flow line, namely an input and an 
output. 
FIG. 7 illustrates an application of the FIG. 1 system to control the rate 
of supply of fuel to a turbo-prop engine, and consequently the power, when 
the propeller speed is kept constant and the pitch of the propeller is 
varied. 
A throttle 61 is connected in series with the throttles 2 and 4 between the 
lines 20 and 22, and the position of the throttle 61 is controlled in 
dependence upon propeller pitch. Use of the throttle 61 ensures that if 
the pilot suddenly opens or closes the throttle 2, the response of the 
throttle 4 depends not only upon the pilot's demand, exerted through the 
throttle 2, but also on the then-existing value of the pitch of the 
propeller, applied through the throttle 61. Therefore the pilot cannot 
make the throttle system demand excessive power when the pitch is very 
coarse or, on the other hand, reduce the demand for power too low when the 
pitch is very fine. 
The system of FIG. 7 includes a gear pump 60 which delivers fuel at a rate 
proportional to engine speed. 
Fuel in excess of requirements is spilled back to the fuel line upstream of 
the pump by way of a line 63. The rate at which fuel is spilled back is 
controlled by a spill valve 64. The valve 64 comprises a body 66 defining 
an interior space which is divided into two chambers 68 and 70 by a 
diaphragm 72. The diaphragm 72 is biased by a spring 74 to establish a 
maximum pressure difference between the chambers 68 and 70. The chamber 68 
is connected in the spill-back line 63, while the chamber 70 is connected 
to the fuel line downstream of the throttle 4 by a line 62. The diaphragm 
72 acts on a plate valve 76, and consequently the valve is maintained at a 
position such that the pressure drop across the series of throttles 2, 61 
and 4 is proportional to the reference force provided by the spring 74. It 
will therefore be seen that the net rate of flow of fuel to the 
pressurizing valve 78 is a function of the opening of all three throttles. 
It is not necessary that a piston and cylinder arrangement should be used 
in any of the foregoing systems, since many equivalent arrangements are 
available. For example, bellows or a rolling diaphragm arrangement could 
be used, with the advantage that there would be no leakage. 
Each of the servosystems described above has a relatively large time 
constant since it is designed to retard the response of the servo throttle 
to movement of the pilot or command throttle. However, where power 
amplification is the primary goal, e.g. in the case of very large valves 
requiring servo assistance, the time constant can be reduced by 
eliminating the restrictor or reducing the flow resistance thereof.