[0001] The present invention relates to gas turbine engines, and particularly to a compressor
bleed valve for improving the control of surge in such engines.
[0002] U.S. Patent 3,809,490, Harner, issued May 7, 1974, describes the on-going problem
of trying to avoid surge in gas turbine engines. The solution proposed over the years
has been the provision of bleed valves to bleed off compressor air at different stages
of the compressor. Thus, controls, mechanical or pneumatic, are provided for anticipating
a surge condition by causing the valves to be opened to thereby bleed off air before
a surge condition is to happen. Thus, in high power requirement conditions, the bleed
valves are maintained closed, but during low power, the bleed valves are opened.
[0003] A pneumatic bleed valve is also described in U.S. Patent 5,477,673, Blais et al.,
issued December 26, 1995. This patent describes a bleed valve in the form of a piston
extending radially through a bypass flow path, and operable to bleed compressor air
into the bypass flow path when the piston type valve is open. The valve may be closed
when air from a source downstream of the compressor impeller is fed to the head of
the piston and such air is at a higher pressure than air from a downstream stage of
the compressor. The pneumatic force to close the valve acts against a spring normally
urging the valve to an open position.
[0004] As the engine speed changes from low to high, the bleed valve moves from an open
to a closed position gradually. If during this transition the opening becomes too
small, the engine may be in a surge condition.
[0005] It is an object of the invention to seek to provide an improved pneumatic valve of
the type described in U.S. Patent 5,477,673 that includes a means for maintaining
the bleed valve open with a larger bleed opening during gradual closing of the bleed
valve during pneumatic control of the valve.
[0006] According to the invention there is provided in a gas turbine engine including a
compressor with an axial fluid flow path, a bypass fluid flow path concentric with
the compressor fluid flow path, a bleed valve in fluid communication with the compressor
fluid flow path and the bypass fluid flow path whereby the bleed valve comprises a
piston extending radially, with a piston head radially remote from the compressor
fluid flow path, a pneumatic chamber surrounding a portion of the piston head and
means for introducing compressed fluid into said chamber, the piston including a valving
means and a rigid member extending between the piston head and the valving means whereby
the piston is effective to open or close the communication between the compressor
fluid flow path, the improvement comprising a precompressed spring associated with
the piston to normally urge the piston radially outwardly relative to the compressor
fluid flow, the arrangement being such that the pneumatic pressure in the chamber
surrounding the piston head must overcome the precompressed spring in order to close
said valving means and the precompression spring is such that the closing schedule
of the bleed valve will avoid the surge conditions of the engine.
[0007] Thus in a gas turbine engine including a compressor and a bypass fluid flow path
concentric with the compressor fluid flow path, there is a bleed valve in fluid communication
with the compressor fluid flow path and the bypass fluid flow path whereby a piston
extends radially of the valve and includes a piston head radially remote from the
compressor fluid flow path, a pneumatic chamber surrounding a portion of the piston
head and means for introducing compressed fluid into said chamber, the piston including
a valving member and a rigid member connecting the piston head to the valving means
whereby the piston is effective to open or close the communication between the compressor
flow path and the bypass flow path, and a precompressed spring associated with the
piston to normally urge the piston radially outwardly relative to the compressor fluid
flow path to a valve open position whereby to close the valve, the pneumatic pressure
in the chamber surrounding the piston head must overcome the precompressed spring.
[0008] Thus using the invention it is possible to provide a precompressed spring on the
piston forming the operable portion of the valve.
[0009] In a more specific embodiment of the present invention, the spring is precompressed
to 40 lbs.
[0010] It has been found that by precompressing the spring in the bleed valve described
in U.S. Patent 5,477,673, significant improvement can be obtained in avoiding possible
engine surging by maintaining the bleed valve open longer and especially maintaining
a larger opening of the bleed valve until the surge conditions are passed and the
valve can definitely close.
[0011] Having thus generally described the nature of the invention, reference will now be
made to the accompanying drawings, showing by way of illustration, a preferred embodiment
thereof, and in which:
Fig. 1 is an axial cross-section of a compressor portion of a gas turbine engine shown
in dotted lines and illustrating in cross-section a bleed valve in accordance with
the prior art in an open position;
Fig. 2 is a cross-section taken in a vertical plane of the bleed valve in accordance
with the present invention in an open position;
Fig. 3 is a cross-section, similar to Fig. 2, showing the bleed valve of the present
invention in a closed position; and
Fig. 4 is a graph illustrating the operating schedule of the bleed valve in accordance
with the present invention compared with a prior art bleed valve.
[0012] Referring now to the drawings and particularly to Fig. 1, a bleed valve 10 is shown
mounted in a compressor section 14 of a gas turbine engine having a bypass fluid flow
path 12. The bleed valve 10, shown in Fig. 1, is according to U.S. Patent 5,477,673,
Blais et al., which is herewith incorporated by reference. As shown in Fig. 1, the
compressor section includes a fluid flow path 16 which is somewhat concentric with
the bypass fluid flow path 12. The compressor includes a downstream compressor stage
outlet port 22 in shroud 18, adjacent the centrifugal impeller 24.
[0013] The bleed valve 10 is a piston type bleed valve having a closed casing with a piston
26 and a guide rod 28 fixed to the upper chamber housing 27 which defines a closed
chamber 32. A piston head 34 slides within the chamber 32 in sealing relationship.
The piston 26 includes a sleeve 30 and 30a which slides on the rod 28. The rod 28
is connected to the chamber housing 27 by means of a nut 31. The rod 28 is connected
at its other end to the valve chamber housing 52 by means of nut 29. The valve chamber
52 is in the form of an open basket with openings 42. The valving element 36 includes
a frusto-conical surface 37 and a partial cylindrical skirt 38 defining an opening
39 which corresponds with opening 40 in the bypass fluid flow inner wall 20.
[0014] As described in U.S. Patent 5,477,673, the bleed valve, when in an open position
as shown in Fig. 1, allows bleed air from the downstream portion of the compressor
to pass through openings 42 and through opening 40 to the bypass fluid flow path 12
downstream of the bleed valve 10. The spring 44 normally urges the valve to its open
position, as shown in Fig. 1, and the valve is closed pneumatically as described in
the above United States patent.
[0015] It has been found that the valve, under the pneumatic pressure from a source downstream
of the compressor impeller, as described in the above-mentioned patent, will prematurely
close the valve against the spring 44 while the engine is still vulnerable to a surge
condition.
[0016] Referring now to Figs. 2 and 3, the bleed valve, in accordance with the present invention,
is shown and identified as 110. All of the reference numerals which correspond to
reference numerals in Fig. 1 have been raised by 100.
[0017] The bleed valve 110 of Figs. 2 and 3 is shown in cross-section in a radial plane,
that is, at 90° to the cross-section of Fig. 1.
[0018] The bleed valve 110 includes an upper casing 127 defining a piston chamber 132 communicating
with an inlet 150. Bleed valve 110 is a piston-type bleed valve and includes a piston
126 which includes the sleeve 130 having bushings 148 sliding on rod 128. Rod 128
is fixed at the casing 127 by means of nut 131. At the other end, rod 128 mounts a
valve housing 152 in the form of an open basket which defines a valve seat 154 adjacent
the inner wall 120 of the bypass fluid flow path 112.
[0019] The sleeve 130 mounts a piston head 134 which is adapted to slide in sealing engagement
within the chamber 132. At the other end of the sleeve 130 is an aerodynamic cap 146
to which is connected a valving element 136. The valving element 136 includes a frusto-conical
surface surrounded partially by a skirt 138 which is adapted to slide within the basket
152. The valving element 136 defines an annular spring recess 143 which houses a coil
spring 144. The skirt 138 defines an opening 139 in the downstream side of the valving
element 136 (although the opening 139 is shown to one side in Figs. 2 and 3 for the
purposes of illustration only).
[0020] Referring to Fig. 4, the curve N represents the bleed valve as shown in Fig. 1 of
the spring 44. Thus, it can be seen that curve N, as it is closing, passes through
the so-called surge bucket S. As the valve 36 is being closed, it is difficult to
control the valve opening.
[0021] It has been found, however, that by precompressing the spring 144, as shown in Figs.
2 and 3, the air pressure entering inlet 150 in Fig. 2 required to urge the piston
head 134 and thus the piston 126 to close against the valve seat 154 will need to
be higher since the precompressed spring 144 offers more resistance. Since the necessary
force required to overcome the spring 144 will be greater, the valve will remain open
longer and will naturally be larger since the valving member will not readily close
the opening unless a larger force is applied.
[0022] The curve P shown in Fig. 4 represents the schedule for closing valve 136 using a
precompressed spring 144.
[0023] It has been found that a preferred spring rating will include a precompression of
40 lbs. when the valve is completely opened, although a precompression of 20 lbs.
should be sufficient to clear the surge bucket 144. This compares to zero compression
in terms of spring 44 in Fig. 1 when the valve is completely opened. It is anticipated
that the spring could also be precompressed to 50 lbs. It is noted that when the valve
36 is closed, the spring 144 is compressed to 60 lbs. which is similar to the spring
44 in Fig. 1.
1. In a gas turbine engine including a compressor with an axial fluid flow path, a bypass
fluid flow path concentric with the compressor fluid flow path, a bleed valve in fluid
communication with the compressor fluid flow path and the bypass fluid flow path whereby
the bleed valve comprises a piston extending radially, with a piston head radially
remote from the compressor fluid flow path, a pneumatic chamber surrounding a portion
of the piston head and means for introducing compressed fluid into said chamber, the
piston including a valving means and a rigid member extending between the piston head
and the valving means whereby the piston is effective to open or close the communication
between the compressor fluid flow path, the improvement comprising a precompressed
spring associated with the piston to normally urge the piston radially outwardly relative
to the compressor fluid flow, the arrangement being such that the pneumatic pressure
in the chamber surrounding the piston head must overcome the precompressed spring
in order to close said valving means and the precompression spring is such that the
closing schedule of the bleed valve will avoid the surge conditions of the engine.
2. The bleed valve according to Claim 1, characterised in that the precompressed spring
is a coil spring precompressed to between 20 and 50 lbs.
3. The bleed valve according to Claim 2, characterised in that the spring is precompressed
to 40 lbs.
4. The bleed valve according to any preceding claim, characterised in that the bleed
valve includes a casing defining the pneumatic chamber mounted to the outer shroud
of the bypass fluid flow path and a valving means housing is mounted to the other
end of a rod fixed to the chamber casing whereby the valving means housing is mounted
to the inner wall of the bypass fluid flow path, the piston includes the piston head
and an elongated sleeve connecting the piston head to the valving element such that
the sleeve slides on the rod between a valve opened position and a valve closed position
and the precompressed spring is mounted in the valving element housing between the
valving element and the housing so as to urge the piston and the valving element to
an open position.
5. The bleed valve according to Claim 4, characterised in that the spring is precompressed
to 40 lbs.