Combustion staging system

Abstract

A combustion staging system includes a splitting unit receiving a metered fuel flow and controllably splitting the received flow into pilot and mains flows. Pilot and mains fuel manifolds distribute fuel. A cooling flow recirculation line provides a cooling flow to the mains manifold during pilot-only operation, and a return section to collect mains manifold cooling flow. The cooling flow enters a delivery section and exits the return section. A fuel recirculating control valve on the delivery section has an open position so that the cooling flow enters the delivery section during pilot-only operation; a shut off position prevents the cooling flow entering the delivery section through the cooling flow orifice during pilot and mains operation. A supplementary valve bleeds or feeds cooling flow. The mains manifold cooling flow pressure is determined by the cooling flow and pressure raising orifices flow numbers, and a control setting of the supplementary valve.

Claims

1. A combustion staging system for fuel injectors of a multi-stage combustor of a gas turbine engine, the system including: a splitting unit which receives a metered fuel flow and controllably splits the metered fuel flow into a pilot fuel flow and a mains fuel flow for injecting respectively at a pilot fuel stage and a mains fuel stage of the injectors to perform staging control of the combustor; and a pilot fuel manifold and a mains fuel manifold respectively distributing fuel from the splitting unit to the pilot fuel stage and the mains fuel stage; wherein the splitting unit (1) is operable to select the pilot fuel manifold and deselect the mains fuel manifold for a pilot-only operation in which the pilot fuel flow from the splitting unit is nonzero and the mains fuel flow from the splitting unit is zero, and (2) is operable to select both the pilot fuel manifold and the mains fuel manifold for pilot-and-mains operation in which the pilot fuel flow and the mains fuel flow from the splitting unit are both nonzero; wherein the system further includes a cooling flow recirculation line having (1) a delivery section arranged to provide a cooling fuel flow to the mains fuel manifold when it is deselected during the pilot-only operation so that the mains fuel manifold remains primed with relatively cool fuel, and (2) a return section arranged to collect the cooling fuel flow from the mains fuel manifold; wherein the cooling fuel flow enters the delivery section from a high pressure fuel zone of the engine at a cooling flow orifice and exits the return section to a low pressure fuel zone of the engine at a pressure raising orifice; wherein the system further includes a fuel recirculating control valve on the delivery section adjacent the cooling flow orifice, the fuel recirculating control valve having an open position so that the cooling fuel flow enters the delivery section at the cooling flow orifice during the pilot-only operation, and a shut off position which prevents the cooling fuel flow from entering the delivery section through the cooling flow orifice during the pilot-and-mains operation; wherein the system further includes a supplementary valve which is controllable to bleed the cooling fuel flow from the recirculation line or feed the cooling fuel flow into the recirculation line; and wherein the pressure of the cooling fuel flow in the mains fuel manifold is determined by respective flow numbers of the cooling flow orifice and the pressure raising orifice and a control setting of the supplementary valve.

2. The combustion staging system according to claim 1, further including a controller which controls the control setting of the supplementary valve to meet a target fuel pressure in the mains fuel manifold.

3. A-The combustion staging system according to claim 2, wherein the controller selects the target fuel pressure which ensures that a fuel pressure of the mains fuel manifold remains above a gas pressure in the combustor.

4. The combustion staging system according to claim 2, further including a pressure sensor which measures a fuel pressure of the mains fuel manifold, the controller performing feedback control based on the fuel pressure of the mains fuel manifold to meet the target fuel pressure.

5. A-The combustion staging system according to claim 1, wherein the supplementary valve is a servo-valve which is configured to bleed or to feed continuously variable amounts of the cooling fuel flow.

6. A-The combustion staging system according to claim 1, wherein the supplementary valve bleeds or feeds the cooling fuel flow from or into the recirculation line adjacent the pressure raising orifice.

7. The combustion staging system according to claim 1, wherein the supplementary valve is integrated with the splitting unit.

8. The combustion staging system according to claim 1, further including a recirculating flow return valve on the return section adjacent the pressure raising orifice, the recirculating flow return valve having an open position so that the cooling fuel flow exits the return section at the pressure raising orifice during the pilot-only operation, and a shut off position which prevents the cooling fuel flow from exiting the return section through the pressure raising orifice during the pilot-and-mains operation.

9. The combustion staging system according to claim 1, wherein the splitting unit is configured to divert a portion of the mains fuel flow into the delivery section during the pilot-and-mains operation, the portion re-joining a remainder of the mains fuel flow in the mains fuel stage of the injectors.

10. The combustion staging system according to claim 9, wherein the splitting unit has a slidable spool, a position of the spool determining a flow split between an outlet of the splitting unit to the pilot fuel manifold, an outlet of the splitting unit to the mains fuel manifold, and an outlet of the splitting unit to the delivery section of the cooling flow recirculation line.

11. The combustion staging system according to claim 1, wherein the delivery section includes a delivery manifold which distributes the cooling fuel flow to the injectors en route to the mains fuel manifold.

12. The combustion staging system according to claim 1, wherein the fuel injectors have integrated mains check valves which are arranged to open when a fuel pressure within the mains fuel manifold exceeds a predetermined fuel pressure relative to a gas pressure in the combustor.

13. The combustion staging system according to claim 1, wherein the pilot fuel manifold includes a segment restrictable by a lean blow out protection valve to decrease a proportion of the pilot fuel flow delivered to the injectors fed by the segment relative to a total pilot fuel flow delivered to all the injectors of the combustor.

14. A gas turbine engine having the combustion staging system according to claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which:

(2) FIG. 1 shows schematically a combustion staging system for a gas turbine engine in pilot and mains operation mode;

(3) FIG. 2 shows a longitudinal cross-section through a ducted fan gas turbine engine;

(4) FIG. 3 shows schematically a combustion staging system for a gas turbine engine in pilot-only operation mode;

(5) FIG. 4 shows schematically the staging system of FIG. 3 in pilot and mains operation mode;

(6) FIG. 5 shows schematically the staging system of FIGS. 3 and 4 in shut down mode;

(7) FIG. 6 shows a graph of mains manifold pressure relative to combustor gas pressure (P30) against metered fuel flow from the HMU; and

(8) FIG. 7 shows two modes of operation, (a) and (b), of a supplementary valve which provides fine control of the mains manifold pressure.

DETAILED DESCRIPTION AND FURTHER OPTIONAL FEATURES

(9) With reference to FIG. 2, a ducted fan gas turbine engine incorporating the invention is generally indicated at 10 and has a principal and rotational axis X-X. The engine comprises, in axial flow series, an air intake 11, a propulsive fan 12, an intermediate pressure compressor 13, a high-pressure compressor 14, combustion equipment 15, a high-pressure turbine 16, an intermediate pressure turbine 17, a low-pressure turbine 18 and a core engine exhaust nozzle 19. A nacelle 21 generally surrounds the engine 10 and defines the intake 11, a bypass duct 22 and a bypass exhaust nozzle 23.

(10) During operation, air entering the intake 11 is accelerated by the fan 12 to produce two air flows: a first air flow A into the intermediate-pressure compressor 13 and a second air flow B which passes through the bypass duct 22 to provide propulsive thrust. The intermediate-pressure compressor 13 compresses the air flow A directed into it before delivering that air to the high-pressure compressor 14 where further compression takes place.

(11) The compressed air exhausted from the high-pressure compressor 14 is directed into the combustion equipment 15 where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive the high, intermediate and low-pressure turbines 16, 17, 18 before being exhausted through the nozzle 19 to provide additional propulsive thrust. The high, intermediate and low-pressure turbines respectively drive the high and intermediate-pressure compressors 14, 13 and the fan 12 by suitable interconnecting shafts.

(12) The engine has a pumping unit comprising a low pressure (LP) pumping stage which draws fuel from a fuel tank of the aircraft and supplies the fuel at boosted pressure to the inlet of a high pressure (HP) pumping stage. The LP stage typically comprises a centrifugal impeller pump while the HP pumping stage may comprise one or more positive displacement pumps, e.g. in the form of twin pinion gear pumps. The LP and HP stages are typically connected to a common drive input, which is driven by the engine HP or IP shaft via an engine accessory gearbox.

(13) A fuel supply system then accepts fuel from the HP pumping stage for feeds to the combustor 15 of the engine 10. This system typically has a hydro-mechanical unit (HMU) comprising a fuel metering valve operable to control the rate at which fuel is allowed to flow to the combustor. The HMU further typically comprises: a pressure drop control arrangement (such as a spill valve and a pressure drop control valve) which is operable to maintain a substantially constant pressure drop across the metering valve, and a pressure raising and shut-off valve at the fuel exit of the HMU which ensures that a predetermined minimum pressure level is maintained upstream thereof for correct operation of any fuel pressure operated auxiliary devices (such as variable inlet guide vane or variable stator vane actuators) that receive fuel under pressure from the HMU. Further details of such an HMU are described in EP 2339147 A.

(14) An engine electronic controller (EEC) commands the HMU fuel metering valve to supply fuel to the combustor at a given flow rate. The metered fuel flow leaves the HMU and arrives at a staging system 30, shown schematically in FIG. 3 in pilot-only operation mode and in FIG. 4 in pilot and mains operation mode, at a pressure P.sub.fmu.

(15) The staging system 30 splits the fuel into two flows: one at a pressure P.sub.p for first 31a and second 31b segments of a pilot manifold and the other at a pressure P.sub.m for a mains manifold 32. Fuel injectors 33 of a combustor of the engine are split into two groups. The injectors of one group are connected to the first pilot manifold segment 31a, while the injectors of the other group are connected to the second pilot manifold segment 31b. The mains manifold feeds secondary nozzles of the fuel injectors. Pilot weight distributor valves (WDVsdiscussed in more detail below) 39 at the injectors improve injector-to-injector fuel distribution by compensating for the pilot manifold pressure head, while mains flow scheduling valve (FSVs) 40 at the injectors retain a primed mains manifold when de-staged and at shut-down.

(16) A fuel flow splitting valve (FFSV) 34 receives the metered fuel flow from the HMU at pressure P.sub.fmu. A spool is slidable within the FFSV under the control of a servo-valve 35, the position of the spool determining the outgoing flow split between outlets to, respectively, a pilot connection pipe 36 which delivers fuel to the pilot manifold segments 31a, b, a mains connection pipe 37 which delivers fuel to the mains manifold 32, and a delivery pipe 41 of a recirculation line (discussed below). The spool can be positioned (as shown in FIG. 3) so that the mains stage is deselected, with the entire metered flow going to the pilot stage. An LVDT 38 provides feedback on the position of the spool to an engine electronic controller (EEC), which in turn controls staging by control of the servo-valve.

(17) Between the FFSV 34 and the second pilot manifold segment 31b, the pilot connection pipe 36 communicates with a lean blow out protection valve 50 which controls communication between the pilot connection pipe 36 and the second pilot manifold segment 31b. The lean blow out protection valve is spring biased towards an open position. A solenoid operated control valve 52 is operable to apply a control pressure to the valve member of the lean blow out protection valve to move it against the action of the spring biasing to a closed position, restricting the communication between the pilot connection pipe 36 and the second pilot manifold segment 31b, when required.

(18) The recirculation line provides the mains manifold 32 with a cooling flow of fuel when the mains manifold is deselected in pilot-only operation mode (as shown in FIG. 3). The recirculation line has a delivery section including the delivery pipe 41 which receives the cooling flow from a cooling flow orifice (CFO) 46 and a fuel recirculating control valve (FRCV) 42, and a recirculation manifold 43 into which the delivery pipe feeds the cooling flow. The recirculation manifold has feeds which introduce the cooling flow from the recirculation manifold to the mains manifold via connections to the feeds from the mains manifold to the mains FSVs 40.

(19) In addition, the recirculation line has a return section which collects the returning cooling flow from the mains manifold 32. The return section is formed by a portion of the mains connection pipe 37 and a branch pipe 44 from the mains connection pipe, the branch pipe extending to a recirculating flow return valve (RFRV) 45 from whence the cooling flow exits the recirculation line through a pressure raising orifice (PRO) 47.

(20) At entry to the CFO 46, the cooling flow for the recirculation line (obtained from the HMU) is at a high pressure HP.sub.f, and after exiting from the PRO 47 is returned to the pumping unit at a lower pressure LP for re-pressurisation by the HP pumping stage. Between the CFO and the PRO, the cooling flow in the mains manifold 32 is at an intermediate pressure, measured by a pressure sensor 58. Fine control of this pressure is performed by a supplementary valve 57 termed a mains manifold pressure control valve (MMPCV), the control of which is discussed in more detail below. A check valve 48 accommodates expansion of fuel trapped in the mains system during shutdown. The FRCV 42 and the RFRV 45 are operated under the control of the EEC. The HMU also supplies fuel at pressure HP.sub.f for operation of the servo-valve 35 and the RFRV 45.

(21) During pilot-only operation (FIG. 3), the FRCV 42 adopts an open position to allow fuel to pass to the delivery pipe 41 from the CFO 46. The RFRV 45, which is operatively connected to the FRCV via connecting line 59, likewise moves to an open position to allow fuel to exit the branch pipe 44 through the PRO 47. When the mains is staged in (FIG. 4), the FRCV 42 moves to a shut off position which prevents the cooling flow entering the recirculation line therethrough, and similarly the RFRV moves to a shut off position. However, the spool of the FFSV 34 also moves to a position in which the outgoing flow from the FFSV is split between its outlets to the pilot connection pipe 36, the mains connection pipe 37, and the delivery pipe 41. In this way a portion of the mains flow is diverted into a relatively cool flow of fuel that is directed through the recirculation manifold 43 before going on to rejoin the rest of the mains flow at the FSVs 40. The flow keeps the recirculation manifold 43 filled with relatively cool fuel to avoid coking therein. Moreover, as the flow avoids the FRCV 42, the pressure drop through the FRCV can be eliminated. Advantageously, the FFSV 34 can be configured such that the minimum mains flow during pilot and mains operation is 20% or less, and preferably 10% or less of the total pilot+mains flow.

(22) If one of the mains FSVs 40 fails open, the FRCV 42 can be closed so that no fuel is directed through the recirculation manifold 43. Thus there is no leakage through the failed valve into the combustion chamber during pilot-only operation, although the cooling effect of the recirculation manifold is therefore sacrificed.

(23) In a windmill relight situation the FRCV 42 can also be closed so that no fuel is directed through the recirculation manifold 43 during pilot-only operation. Instead all the flow is directed through the segments 31a, b of the pilot manifold, which increases the available fuel for relight.

(24) A failure mode associated with the system of FIG. 1 is that a pilot FSV 139 may fail partially staged in (i.e. stuck partially open) during a relatively low flow operating condition (i.e. pilot-only idle descent). A consequence of this failure scenario is that fuel flows constantly into the combustion chamber through the failed open pilot FSV, resulting in a reduction in fuel flow through the other pilot FSVs 139 in that manifold segment and possibly their closure due to the reduced pressure drop across them. A concern is that there will be hot streaks in the pilot stage of the combustor and resultant downstream turbine damage. Equivalent failure of a mains FSV 140 in the system of FIG. 1 is not believed to be as significant as mains flow enters the combustor through an annulus area of each injector 133 rather than the centred, and hence more concentrated, fuel flow from the pilot stage.

(25) Advantageously, in the system of FIGS. 3 and 4, the pilot FSVs 139 are replaced with pilot weight distributor valves (WDVs) 39. In order to reduce or eliminate the above-mentioned hazard. WDVs are described in more detail in U.S. 2010/0263755. Around the circumference of the combustion chamber the fuel manifold pressure head changes as a result of gravity. The WDVs operate to correct for the effects of such differential fuel manifold pressure head on the fuel distribution to the injectors 33. For injectors located in the lower sector of the combustion chamber the WDVs operate with the fuel pressure acting against a spring force (+valve piston mass), whereas in the upper sector of the combustion chamber they operate with the fuel pressure acting against the spring force (valve piston mass). Trigonometry dictates the resultant forces acting on the WDVs for injectors located in the intermediate sectors of the combustion chamber. This arrangement results in a slight variation in fuel flow restriction at each injector position, which provides an improved injector-to-injector fuel flow distribution and associated engine operating characteristics.

(26) The pilot WDVs 39 have a relatively low crack pressure and open fully at a low fuel flow, so that the difference between a failed open WDV and a correctly-functioning WDV is small. If one fails open it still works the same as an FSV in that as one is taking most of the flow, the flow through the others is reduced. However, as the WDVs are fully open at a much lower flow rate than FSVs it is possible to re-open the other WDVs more quickly, thereby avoiding hot streaks.

(27) The staging system 30 has a balancing pressure check valve (BPCV) 54 on the pilot connection pipe 36. The BPCV maintains a pressure balance relative to the pressure in the mains manifold 32 for improved split control of the received fuel flow by the FFSV 34. More particularly, adopting the pilot WDVs 39 changes the fuel flow restriction to the injectors, potentially affecting the fuel flow split control. However, the BPCV cooperates with the mains FSVs 40 to maintain the necessary pressure balance, for example, during the pilot and mains operating mode illustrated in FIG. 4. The BPCV effectively replaces the pressure control of the pilot FSVs 139 of the system shown in FIG. 1, providing a variable restriction in parallel with the mains FSVs 40 so that the required pilot/mains split is achieved over the desired fuel flow range.

(28) Locating the BPCV 54 upstream of the lean blow out protection valve 50 provides a flatter over-fuelling ratio to the injectors of the first pilot manifold segment 31a in the event of a lean bow out, making the engine more robust to engine flame-out during slam decelerations.

(29) The BPCV 54 can also accommodate back-purge of the pilot manifold segments 31a, b via a back purge non-return valve 56 (although, alternatively, a direct line to a dump valve of the HMU and thence to a drains tank may be used). FIG. 5 shows schematically the staging system of FIGS. 3 and 4 in shut down mode. Boost pressure can be prevented from entering the manifolds 31a, 31b, 32 on shut down, for example by providing a robust spring-closed drip-tight seal in the RFRV 45, feeding the control valve 52 of the lean blow out protection valve 50 from P.sub.fmu, making the BPCV 54 drip tight, and making the solenoid of the FRCV 42 drip tight. As a further precaution, P.sub.fmu, rather than HP.sub.f can be used as a servo source pressure for the RFRV 45.

(30) Returning to the control of the fuel pressure in the mains manifold 32 during pilot-only operation, FIG. 6 shows a graph of mains manifold pressure relative to combustor gas pressure (P30) against metered fuel flow from the HMU. Plotted on the graph are data points (squares) each indicating a mains manifold pressure (i.e. the cracking pressure of the mains FSVs 40 less a percentage of the pressure to account for possible ripple) for a given flow rate above which fuel flows into the injector nozzle. Accordingly, up to about a flow rate of 13000 pph, the square data points represent an upper specification limit (USL) for the mains manifold fuel pressure during pilot-only operation. Also plotted on the graph are further data points (triangles) each indicating a mains manifold pressure for a given flow rate below which P30 gas can be ingested into the manifold. Thus, again up to about a flow rate of 13000 pph, the triangular data points represent a lower specification limit (LSL) for the mains manifold fuel pressure during pilot-only operation. Both sets of data points are determined from numerical modelling of the staging system 30 over a range of engine operational conditions.

(31) Between these two limits, in the flow rate range from zero to about 13000 pph (5900 kg/h), a preferred design space (corresponding approximately to the shaded grey triangle in FIG. 6) can be defined in which the pressure difference across the mains FSVs 40 may be situated in order to avoid fuel leakage through the FSVs and to avoid P30 gas ingestion.

(32) FIG. 7 shows two modes of operation, (a) and (b), of the supplementary valve 57 (mains manifold pressure control valveMMPCV) which provides fine control of this pressure P. The MMPCV is controllable to either (a) bleed cooling flow from the recirculation line to low pressure (LPtypically between the LP and HP pumping stages of the pumping unit of the engine), or (b) feed additional cooling flow into the recirculation line from high pressure (HPtypically downstream of the HP pumping stage) in continuously variable amounts immediately upstream of the PRO 47. More particularly, in mode (a) the cooling flow rate Q from the mains manifold is reduced by bleeding an amount Q.sub.sv from the recirculation line, resulting in a reduced flow Q.sub.pro through the PRO and hence reducing the differential pressure (P-P30) across the mains FSVs. In contrast, in mode (b) the cooling flow rate Q from the mains manifold is increased by feeding an additional amount Q.sub.sv into the recirculation line, resulting in an increased flow Q.sub.pro through the PRO and hence increasing the differential pressure (PP30) across the mains FSVs.

(33) The following considerations are used to determine the position of the MMPCV servo-valve 57. If 165 psid (1.14 MPa) is the minimum pressure at which the mains FSVs 40 will seal, then a maximum pressure (P.sub.manifold_high) for the mains manifold fuel pressure can be set at:
P.sub.manifold_high=P30+165 psid10%P.sub.manifold_high
whereby P.sub.manifold_high=(P30+165 psid)/110%
and a minimum pressure (P.sub.manifold_low) for the mains manifold fuel pressure can be set at:
P.sub.manifold_low=P30+15%P.sub.manifold_low
whereby P.sub.manifold_low=P30/85%
the 10% and 15% values being respectively the expected high and low system pressure ripple amplitude as a percentage of the pressure in the manifold. Accordingly, the exact percentages may vary between implementations. Similarly, different minimum pressures than 165 psid may be applicable for different FSVs. On FIG. 6 the lines for P.sub.manifold_high and P.sub.manifold_low are respectively indicated by dashed and solid thick lines.

(34) The MMPCV 57 can be controlled by the EEC, which selects the higher of P.sub.manifold_high and P.sub.manifold_low to ensure it meets the above requirements. Thus the EEC controls the MMPCV to operate such that the mains manifold fuel pressure follows the thick dashed line (P.sub.manifold_high) until the lines cross at around 11000 pph (5000 kg/h), and thereafter follows the solid thick line (P.sub.manifold_low). Below the LSL (triangular data points) there is a potential for combustion chamber gases to be ingested, which is a safety hazard. Above the USL (square data points) unmetered fuel can dribble into the combustion chamber, which increases coking and decreases component life but is not a safety hazard. Thus this control strategy avoids gas ingestion, even if at the expense of some dribble (e.g. at flow rates above about 13000 pph outside the preferred design space), but prevents dribble where possible.

(35) Thus the EEC can calculate a respective flow number for the MMPCV 57 required for each operating condition based on the target pressure, the estimated HP and LP pressures, and the flow numbers of the CFO 46 and the PRO 47. The EEC then sets the servo-valve current of the MMPCV 57 based on its nominal characteristic to achieve that flow number. Moreover, the EEC can use feedback from the pressure sensor to compensate for variation in servo-valve characteristics, leakage through the FFSV 34, and variation in CFO and PRO characteristics by setting a trimming current. When there is a switch to pilots and mains operation (e.g. when there is no longer a need to control pressure in the mains manifold 32), the trim current can be stored for use when the system reverts to pilot-only operation. In FIG. 3 (pilot-only operation mode), the MMPCV 57 is shown bleeding cooling flow from the recirculation line. This is likely to occur at the extremes of burnt flow, e.g. both low and high flows in the pilot-only range. In FIG. 4 (pilot and mains operation mode) and FIG. 5 (shut down mode), the MMPCV 57 is shown with its servo-valve in a zero current position, with no power consumption.

(36) Advantageously, the variable orifices 60 of the MMPCV servo-valve 57 provide for precise control of the MMPCV flow number, and hence control of the pressure drop across the PRO 47. This tight control of the mains manifold pressure makes the selection of the CFO 46 and PRO and the manufacture of the FFSV 34 less critical. An option is to replace the servo-valve with e.g. a solenoid (switching) type valve. Such a device would not provide the variable pressure control of the servo-valve, but rather two-position fixed pressure switching. Another option is to replace the servo-valve with a modification to the FFSV 34 in which the FFSV spool is lengthened and provided with additional ports to the HP feed and the LP return that can be used to pressurise/depressurise the mains manifold 32. Thus according to this option, the supplementary valve which performs fine control of mains manifold pressure can be integrated into the FFSV.

(37) An additional advantage of the MMPCV servo-valve 57 is that the servo-valve port can be selected so that, in the event of loss of power to the servo-valve, the valve tends to supply HP fuel into the mains manifold 32, preventing P30 gas ingestion. The PRO 47 can then be sized accounting for the minimum zero current flow (the minimum flow which can be delivered when there is zero control current supplied to the servo valve) to ensure the manifold pressure never falls below the LSL preventing P30 ingestion even in a failure case. Sizing the PRO in this way helps to maximise the used capacity of the MMPCV servo-valve, while ensuring the ingestion requirement is always met. Making the PRO any smaller, while raising the manifold pressure further, tends not to be beneficial as the MMPCV then has to be capable of bleeding more flow from the manifold at the maximum pressure conditions.

(38) While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.

(39) All references referred to above are hereby incorporated by reference.