Abstract: A system for providing fuel to a combustor of a gas turbine includes an annular fuel distribution manifold that at least partially defines a fuel plenum. The fuel distribution manifold includes a forward end axially separated from an aft end, a flange that extends radially outward and circumferentially around the forward end and an annular support ring that extends downstream from the flange. A LLI assembly extends downstream from the fuel distribution manifold. The LLI assembly includes a unibody liner that at least partially defines a primary combustion zone and a secondary combustion zone within the combustor. A LLI injector extends substantially radially through the unibody liner and provides for fluid communication through the unibody liner into the secondary combustion zone. A fluid conduit in fluid communication with the fuel plenum extends between the LLI injector and the fuel distribution manifold.

Abstract: An example gas turbine engine hose arrangement includes a hose configured to communicate fluid and a thermal blanket directly adjacent the hose. A section of the hose is radially closer to a centerline of the gas turbine engine than the thermal blanket. An example method of limiting hose exposure to thermal energy includes routing a hose near a combustor housing of a gas turbine engine and covering the hose with thermal blanket to limit thermal energy communicated radially inwardly toward an axial center of the combustor housing.

Abstract: An endcover assembly for a turbine combustor adapted to support one or more combustor nozzles includes a substantially flat plate having one side which in use, faces a combustion chamber and an opposite side which, in use, faces away from the combustion chamber. At least one fuel passage extends through the substantially flat plate. A fuel manifold porting block is secured to the opposite side of the flat plate with at least one port aligned with the at least one passage. A fuel restrictor insert formed with multiple flow orifices is located between the flat plate and the fuel manifold porting block in alignment with the at least one fuel passage and the at least one port.

Abstract: A method and system for controlling delivery of fuel to a dual stage nozzle in the combustor of a gas turbine. A liquid fuel is conveyed from a single stage fuel supply through a plurality of primary fuel supply lines to a first nozzle stage including a plurality of primary nozzles. A predetermined operating condition of the gas turbine is identified and a signal is produced in response to the identified operating condition. The signal effects actuation of valves located on secondary fuel supply lines extending from each of the primary fuel supply lines to supply fuel to respective secondary nozzles.

Abstract: A fuel distribution system for a gas turbine with multistage burner arrangement is provided having at least one distribution line that branches off from a common fuel feed line and leads to a burner stage, and at least one element is provided in each distribution line for the specific influencing of the fuel mass flow. In such a fuel distribution system simplified controlling is made possible by a control valve being arranged in each case in one section of the distribution lines as an element for the specific influencing of the fuel mass flow, and in that a restrictor, which is preset in a fixed manner, is arranged in each case in the remaining distribution lines as an element for the specific influencing of the fuel mass flow.

Abstract: A fluid manifold apparatus includes: (a) an array of spaced-apart manifold fittings, each manifold fitting aligned in a predetermined angular orientation. Each manifold fitting includes: (i) a tubular neck; (ii) a pair of spaced-apart tubular arms extending away from a first end of the neck; and (iii) a coupling connected to a second end of the neck; and (b) a plurality of curved tubes, each tube being coupled to one arm of each of two adjacent manifold fittings.

Abstract: A combustor system includes a pylon fuel injection system coupled to a combustion chamber and configured to inject fuel to the combustion chamber. The pylon fuel injection system includes a plurality of radial elements, each radial element having a plurality of first Coanda type fuel injection slots. A plurality of transverse elements are provided to each radial element. Each transverse element includes a plurality of second Coanda type fuel injection slots.

Abstract: Fuel control arrangements for gas turbine engines generally comprise an injector and a fuel control valve. Typically the fuel control valve is controlled in terms of fuel demand through fuel pressure presented to the valve. Fuel demand may vary and in such circumstances stagnation of fuel adjacent to the valve may cause degradation of the fuel and therefore spurious operational performance. By providing a dedicated working fluid, and typically hydraulic, pressure to the valve a variable aperture port can be displaced to alter the fuel flow configuration within the valve. In such circumstances different fuel valve conditions can be generated by altering the available area of aperture in the port to divert or present fuel to the injector between and across first or primary fuel paths and second or pilot fuel paths as required. Thus more flexibility with regard to fuel presentation to the injector is achieved as well as consistency with respect to avoiding fuel degradation as fuel demand varies.

Abstract: A fuel control module for a gas turbine engine and method of installation. The fuel control module includes a first frame unit, a second frame unit, a fuel path, and a fuel controller components. The first frame unit and the second frame unit include features for installation in multiple installations and/or multiple configurations.

Abstract: A late lean injection (LLI) manifold is provided and includes a central nozzle and first and second side nozzles positioned at circumferential locations defined around a vessel, a connector, a first leg, to which the connector is connected, formed to define a tube extending from the central nozzle to the first side nozzle such that fuel is communicable between the connector, the central nozzle and the first side nozzle and a second leg. The second leg is formed to define a tube extending from the central nozzle to the second side nozzle such that the fuel is communicable between the central nozzle and the second side nozzle.

Abstract: A fuel nozzle for a gas turbine generally includes a main body having an upstream end axially separated from a downstream end. The main body at least partially defines a fuel supply passage that extends through the upstream end and at least partially through the main body. A fuel distribution manifold is disposed at the downstream end of the main body. The fuel distribution manifold includes a plurality of axially extending passages that extend through the fuel distribution manifold. A plurality of fuel injection ports defines a flow path between the fuel supply passage and each of the plurality of axially extending passages.

Abstract: A method for operating a burner including a burner outlet opening with at least two sectors, each sector is assigned at least one fuel nozzle, is provided. The method is characterized in that fuel is supplied separately to the fuel nozzles of different sectors. Also described is a burner which includes at least two sectors wherein each sector is assigned at least one fuel nozzle. The burner includes at least two separate fuel supply lines and a device for adjusting the fuel mass flow which flows through the respective fuel supply line. The fuel supply lines supply fuel to the fuel nozzles of different sectors. Also described is a gas turbine which is fitted with at least one burner.

Abstract: A system and method control a gas turbine subject to fuel composition variation. The method includes operating a first effector to control the gas turbine based on fuel composition. The method also includes operating a second effector to maintain operation of the first effector within a first boundary limit, the second effector operation being initiated when the operating the first effector reaches a second boundary limit within the first boundary limit.

Abstract: An integrated plate is provided for use with a combustor including a casing, a fuel plenum extending circumferentially about the casing, and a fuel nozzle extending axially through the casing. The integrated plate includes a plurality of fuel injection pegs that extend radially between the fuel plenum and the fuel nozzle.

Abstract: A fuel supply system supplies fuel to a plurality of combustion sectors of a combustion section of a gas turbine. Each of the combustion sectors includes at least two combustors. The fuel supply system generally includes a fuel distribution manifold that is in fluid communication with a fuel supply. A first flow path is defined between the fuel distribution manifold and a first combustion sector. A second flow path is defined between the fuel distribution manifold and a second combustion sector.

Abstract: A premixer is provided for injecting premixed fuel-air mixture into the inlet of a combustion apparatus. In one embodiment, the premixer assembly comprises a plurality of concentric, aerodynamic injector rings, with radially-directed injection holes. The injection holes have a plurality of different diameters, facilitating good mixing over a broad power range. Due to configuration and hole sizes, the assembly is gas or liquid compatible. The radial, concentric injection formation allows for a short injection path.

Abstract: A fuel manifold for distributing fuel to a gas turbine engine includes a plurality of interconnected manifold segments. Each manifold segment extends between a pair of fuel injector inlet fittings. Each manifold segment includes a fuel liner defining an internal fuel passage therethrough fluidly connecting a pair of fuel injector inlet fittings. A wire braid layer surrounds the fuel liner, a fire sleeve surrounds the wire braid layer, and a flexible thermal shield surrounds the fire sleeve. An insulation space is defined between the fire sleeve and the thermal shield to thermally isolate the fire sleeve from conditions external to the thermal shield.

Abstract: A combustor for a gas turbine generally includes a radial flow fuel nozzle having a fuel distribution manifold, and a fuel injection manifold axially separated from the fuel distribution manifold. The fuel injection manifold generally includes an inner side portion, an outer side portion, and a plurality of circumferentially spaced fuel ports that extend through the outer side portion. A plurality of tubes provides axial separation between the fuel distribution manifold and the fuel injection manifold. Each tube defines a fluid communication path between the fuel distribution manifold and the fuel injection manifold.

Abstract: A combustor of the present invention includes: a plurality of nozzles which is arranged around the central axis of a combustor and of which each base end is connected to an oil supply tube; a nozzle tube base which supports the plurality of nozzles and includes an oil chamber supplying fuel to the nozzles via an oil inflow portion provided in each oil supply tube; a first guide portion which faces the center of the combustor inside the oil chamber and guides the oil remaining inside the oil chamber to the oil inflow portion; and a second guide portion which faces the base end of the combustor inside the oil chamber and guides the oil remaining inside the oil chamber to the oil inflow portion.

Abstract: A duct burner assembly for a HRSG having a casing that defines a combustion chamber with a liner for communicating an exhaust gas. A firing runner attaches to the liner and extends through the combustion chamber. The firing runner defines a plurality of orifices for emitting combustible gas and sustaining a flame. A flame stabilizer attaches to the firing runner and is configured to at least partially shield the plurality of orifices from the exhaust gas. A guide plate attaches to the firing runner and is configured to define a slot between the liner and the guide plate. The guide plate has an upstream end and a downstream end wherein the downstream end is closer to the lining than the upstream end to reduce turbulent flow of the exhaust gas through the slot and cool the liner.

Abstract: In one embodiment, alternative combustion cavity layouts are provided for practicing fuel staging in a trapped vortex (TVC) combustion apparatus comprising an inlet premixer, for injecting fuel-air mixture into the inlet of the combustion apparatus and one or more vortex premixers, for injecting fuel-air mixture into the recirculating vortex within each of one or more trapped vortex cavities. A plurality of TVC cavities, may, for example, be laid out axially, radially, peripherally, internally, or in combinations of such arrangements. These layouts may be used in conjunction with a fuel staging method whereby the relative proportion of mixture introduced through the inlet and the respective vortex premixers can be varied as a function of operating conditions.

Abstract: A combustor 500 is composed of a plurality of premixed combustion burners 100 each comprising a fuel nozzle 110 provided in a burner tube 120, the fuel nozzle 110 having a plurality of swirl vanes 130 on an outer peripheral surface thereof. Injection holes 133a, 133b are formed in each swirl vane 130. Staging control is exercised such that when a gas turbine is in a full load state, a fuel is injected through the injection holes 133a, 133b of all the swirl vanes 130, and when the gas turbine is under a partial load, the fuel is injected only through the injection holes 133a, 133b of a specific number of the swirl vanes 130 adjacent in a circumferential direction, and fuel injection through the injection holes 133a, 133b of the remaining swirl vanes 130 is stopped.

Abstract: A unitary fuel injection manifold for a secondary fuel nozzle improves fuel-air mixing and offers flexibility to alter the mixing profile through adaptability to a variety of number, types, and orientation of discharge outlets to the combustion air mixing space around the secondary fuel nozzle. An aerodynamic surface with reduced extension into the mixing space reduces pressures drop and interference with design airflow. Manifold integrity is enhanced by elimination of fillet welds to mount external pegs.

Abstract: In one embodiment, a system includes a variable geometry resonator configured to couple to a fluid path upstream from a combustor of a turbine engine. The variable geometry resonator is configured to dampen pressure oscillations in the fluid path and the combustor.

Abstract: A swirler for fuel injection in a gas turbine engine includes a frustoconical swirler body. A first and a second air flow path direct air in generally opposed circumferential directions into the swirler. These air paths intermix and create turbulence. As this turbulence encounters fuel droplets, the fuel is atomized, and uniformly distributed within the air flow. A shear layer is created adjacent an inner surface of the swirler body. In a separate feature, a third air flow path is directed into the air.

Abstract: A combustor includes a chamber mixing and burning fuel and air and an air hole plate disposed on a wall surface of the chamber. The air hole plate includes a plurality of rows disposed concentrically of a plurality of air holes jetting coaxial jets of fuel and air into the chamber. A first fuel nozzle and a second fuel nozzle are disposed near a fuel hole for jetting fuel into an air hole row on an inner peripheral side. The first fuel nozzle is structured to suppress turbulence of surrounding air flow and the second fuel nozzle is structured to promote turbulence of a surrounding air flow. In accordance with the aspect of the present invention, good flame stability can be maintained while further reducing NOx in a combustor using coaxial jets.

Abstract: A fuel injection assembly for use in a turbine engine is provided. The fuel injection assembly includes an end cover, an endcap assembly, a fluid supply chamber, and a plurality of tube assemblies positioned at the endcap assembly. Each of the tube assemblies includes a housing having a fuel plenum and a cooling fluid plenum defined therein, the cooling fluid plenum downstream from the fuel plenum and separated therefrom by an intermediate wall, a plurality of tubes extending through the housing, each of the plurality of tubes in flow communication with the fluid supply chamber and a combustion chamber downstream from the tube assembly, and an aft plate at a downstream end of the cooling fluid plenum, the aft plate including at least one aperture defined therethrough. The fuel injection assembly further includes at least one fuel delivery pipe coupled to at least one of the plurality of tube assemblies.

Abstract: A fuel manifold for turbine engines, such as the large turbines used for generating electrical power, has a housing with straight internal porting for routing fuel and purge media streams to the combustion area of the turbine. A pressure balanced fuel valve regulates fuel using a poppet actuator. A piston section of the actuator dynamically seals off actuation pressure from the fuel. A valve section of the actuator dynamically seals off a vented area from the fuel and regulates fuel flow through the valve. The piston section has internal porting to a collector tube within the vented area for directing media leaked past the piston seal(s) to a vent port, and thereby prevent mixing of the fuel and actuation media.

Abstract: The present combustor of gas turbine engine includes: a fuel injection unit including a fuel spray part to spray a fuel so that a diffusion combustion region is formed in a combustion chamber, and a pre-mixture supply part to supply a pre-mixture of a fuel and an air so that a pre-mixture combustion region is formed in the combustion chamber; and a fuel supply unit to supply the fuel to the fuel spray part and pre-mixture supply part. The fuel supply unit includes: a pilot fuel passage and a main fuel passage to supply the fuel to the fuel spray part and the pre-mixture supply part, respectively; an assembled fuel passage to supply the fuel to the pilot and main fuel passages; and a fuel distributor disposed at a branch point where the assembled fuel passage is connected to both the pilot and main fuel passages. The fuel distributor is configured to automatically control amounts of the fuel to be distributed to the pilot fuel passage and to the main fuel passage in accordance with the fuel pressure.

Abstract: A fuel manifold assembly for a gas turbine engine fuel system comprises a fuel manifold at least partly enclosed by a heat shield, the internal manifold being made of a first material having a first coefficient of thermal expansion, and the heat shield being made of a second material having a second coefficient of thermal expansion that is lower than the first coefficient of thermal expansion.

Abstract: A segment of a fluid manifold includes a flexible thermal shield with an insulation space defined inboard of the thermal shield to provide thermal isolation from conditions external to the thermal shield. The thermal shield includes a helically corrugated metal tube having opposed first and second ends. An outer collar is threaded to an end of the helically corrugated metal tube. The outer collar includes a crimped portion and an uncrimped portion. The uncrimped portion is spaced apart from the corrugated helical metal tube more than the crimped portion to define a vent between the outer collar and the corrugated helical metal tube in venting communication with the insulation space to vent hot gases.

Abstract: A fuel system for a gas turbine engine includes a plurality of duplex nozzles arranged on each side of top dead center and a plurality of simplex nozzles. A primary manifold is operable to communicate fuel to a primary flow jet in each of the plurality of duplex nozzles and a secondary manifold is operable to communicate fuel to a secondary flow jet in each of the plurality of duplex nozzles and a secondary flow jet in each of the plurality of simplex nozzles. An equalizer valve that is in communication with both the primary manifold and the secondary manifold distributes fuel at various pressures to both the primary and secondary manifolds.

Abstract: A gas turbine combustor has a chamber supplied with fuel and air and a multi-burner having a plurality of burners provided with an air hole plate having a plurality of air holes, and fuel nozzles for supplying fuel to the air holes in the air hole plate; the multi-burner is made up of a center burner disposed in the center and a plurality of outer burners around the center burner, the outer burners are divided into inner fuel nozzles and outer fuel nozzles to separately supply fuel through fuel systems, and the fuel is supplied to the fuel nozzles in the center burner or to the fuel nozzles in the center burner and the inner fuel nozzles in the outer burners disposed around the center burner in a partial load condition in which the load is lower than that of when all the fuel systems are used to supply fuel.

Abstract: A fuel shroud assembly (100) into which fuel (118) is injected for mixing with an air stream (120) in a fuel manifold. The shroud assembly (100) comprises a plurality of parallel fuel scoops (102) each receiving the injected fuel (118). The fuel stream (118) flows through each scoop (102), exiting at an open scoop end (114A). The air stream (120) flows between scoops, creating a shear region proximate each scoop end (114A) where the fuel exits. The shear causes mixing of the air (120) and the fuel (118) streams, wherein the degree of mixing is not dependent on the momentum ratio of the air (120) or fuel (118) streams.

Abstract: A system for supplying a working fluid to a combustor includes a fuel nozzle, a combustion chamber, and fuel injectors circumferentially arranged around the combustion chamber. A combustor casing surrounds the combustion chamber. A distribution manifold encloses the fuel injectors, and a plenum passes through the combustor casing. A method for supplying a working fluid to a combustor includes flowing a working fluid from a compressor through a combustion chamber, diverting a portion of the working fluid into a plenum, and flowing the diverted portion of the working fluid outside of the compressor and the combustor. The method further includes flowing the diverted portion of the working fluid through a combustor casing and through a distribution manifold that encloses fuel injectors circumferentially arranged around the combustion chamber.

Abstract: A system for supplying a working fluid to a combustor includes a fuel nozzle and a combustion chamber downstream from the fuel nozzle. A flow sleeve circumferentially surrounds the combustion chamber, and fuel injectors provide fluid communication to the combustion chamber. A distribution manifold circumferentially surrounds the fuel injectors and defines an annular plenum. A fluid passage through the distribution manifold provides fluid communication through the distribution manifold. A radial cross-sectional area of the annular plenum varies around the flow sleeve. A method for supplying a working fluid to a combustor includes flowing a working fluid through a combustion chamber, diverting a portion of the working fluid through a distribution manifold that circumferentially surrounds fuel injectors circumferentially arranged around the combustion chamber, and changing at least one of a pressure or flow rate of the diverted portion of the working fluid.

Abstract: A late lean injection system transition piece includes a flowsleeve interface extending circumferentially around a transition interior. Also included is an impingement sleeve mounting surface extending circumferentially around the transition interior. Further included is a plurality of fuel injectors integrated substantially within the transition piece and disposed between the flowsleeve interface and the impingement sleeve mounting surface. Yet further included is an inner surface extending circumferentially around the transition interior, wherein the inner surface is spaced radially inward of the flowsleeve interface and the impingement sleeve mounting surface.

Abstract: Embodiments of this invention provide a premixed pilot assembly for use with a fuel nozzle for a turbine. The level of nitrogen oxide (NOx) emitted by the pilot is reduced by mixing the fuel and air fast, at the end of the pilot nozzle, thereby avoiding significant zones of rich fuel and air mixtures. The air is mixed with the fuel through the use of openings in a first cylinder and a second cylinder, one of which carries air and one of which carries pilot fuel. The openings can be configured as necessary to obtain a desired effect on the pilot.

Abstract: A gas turbine engine fuel delivery system which includes an external fuel manifold fastened to the outer surface of a casing surrounding the combustor of the engine and which includes a plurality of inlet manifold tubes circumferentially disposed in serial flow communication. A plurality of fuel nozzles are mounted to the casing and are axially spaced apart from the fuel manifold on the outer surface of the casing. A plurality of jumper tubes are disposed outside the casing and feed fuel flow from the fuel manifold to the fuel nozzles. The jumper tubes have a rigidity that is less than that of the inlet manifold tubes of the fuel manifold such that the jumper tubes elastically deflect under load.

Abstract: A flow divider assembly includes a dividing valve and a balancing valve. The dividing valve includes a dividing cavity configured to receive a flow of fuel and a piston device arranged within the dividing cavity and selectively positioned to divide the flow of fuel in the dividing cavity into the first dividing valve port and the second dividing valve port. The balancing valve is fluidly coupled to the dividing valve and configured to receive the first portion and the second portion of the flow. The balancing valve includes a balancing cavity and a balancing device arranged within the balancing cavity and selectively positioned to direct the first portion of the flow into the first balancing cavity port at a third pressure and the second portion of the flow into the second balancing cavity port at a fourth pressure such that the first pressure is approximately equal to the second pressure.

Abstract: A fuel delivery system for a gas turbine engine includes one or more subsets of combustors, and at least two fuel manifolds including a fuel manifold coupled to each combustor subset and configured to deliver fuel to only the subset of combustors during a predetermined gas turbine engine operating mode. A gas turbine engine assembly including a fuel delivery system and a method of operating a gas turbine engine is also described herein.

Abstract: A method of noise control from a combustor of a gas turbine engine includes selectively forming a plurality of local circumferential zones with different fuel-air ratios within the combustor.

Abstract: A gas-turbine burner for a gas turbine includes a fuel nozzle 1 having several fuel exit holes 23, which are each connected to a fuel line 5, 7, 29, 30, through which fuel can be passed selectively Between individual fuel exit holes 23, different static pressures of the airflow are provided between fuel lines flown by fuel and fuel lines not flown by fuel.

Abstract: The present application provides a dual gas fuel delivery system and a method of delivering two gas fuels. The dual gas fuel delivery system may include (a) a low energy gas delivery system comprising a low energy gas inlet, a gas split, a low energy gas primary manifold outlet, and a low energy gas secondary manifold outlet; (b) a high energy gas delivery system comprising a high energy gas inlet and a high energy gas primary manifold outlet; (c) a primary manifold; and (d) a secondary manifold, wherein the low energy gas primary manifold outlet and the high energy gas primary manifold outlet are coupled to the primary manifold, and wherein the low energy gas secondary manifold outlet is coupled to the secondary manifold.

Abstract: A combustor section is provided and includes one or more annular quaternary fuel manifolds mounted within an annular passage defined between a casing and a cap assembly of a combustor through which air and/or a fuel/air mixture flows upstream from a fuel nozzle support, the manifold including a body to accommodate quaternary fuel therein, the body defining injection holes through which the quaternary fuel is injected into a section of the passage at a location upstream from the fuel nozzle support.

Abstract: A combustor section is provided and includes a segmented annular manifold mounted upstream from a fuel nozzle support in a section of a passage through which an oxidizer flows, each segment of the manifold being substantially axially aligned and including a body to accommodate fuel internally that is formed to define injection holes through which the fuel is injected into the passage through which the oxidizer flows upstream of the fuel nozzle support.

Abstract: An apparatus and process for depositing an overlay weld on a substrate in a manner that reduces dilution of the substrate material. A consumable electrode is positioned in proximity to the surface of the substrate, and an electrical potential is applied between the electrode and substrate to generate an electrical arc therebetween. The arc melts the electrode and forms a molten spray that deposits on the substrate surface. Energy of the electric arc is absorbed to reduce the temperature at the substrate surface by feeding an additional filler material into the electric arc toward its center axis. The filler material continuously melts prior to reaching the center axis of the electric arc, and the electrode and filler materials are simultaneously deposited to form the overlay weld on the substrate. Sufficient energy is absorbed by the filler material to reduce intermixing between the overlay weld and the substrate.

Abstract: A fuel injector for a gas turbine engine is disclosed. The fuel injector includes an injector housing extending from a first end to a second end along a longitudinal axis. The second end of the housing is fluidly coupled to a combustor of the turbine engine and the housing includes a liquid fuel gallery annularly disposed about the longitudinal axis. The fuel injector also includes a stem extending longitudinally from the first end of the housing to a third end. The stem includes a liquid tube configured to deliver liquid fuel to the fuel injector. The fuel injector also includes an annular shell extending along the longitudinal axis from the first end to the third end and circumferentially disposed about the stem. The fuel injector further includes an insulating air shroud formed inside the shell. The air shroud includes a layer of air between the shell and the stem.

Abstract: A gas turbine engine fuel supply assembly which includes a fuel manifold mounted to a casing surrounding a combustor of the engine, and a plurality of fuel nozzles mounted to the fuel manifold. A mating interface between the fuel nozzles and the fuel manifold has at least one sealing element, and at least one aperture defined in the fuel nozzle proximate to the mating interface. The aperture is a non fuel conveying passage, and reduces conductive heat transfer to the sealing element by preventing a direct conductive heat transfer path between a hot region of the fuel nozzle and the sealing element.

Abstract: A variable performance valve for use in a fuel nozzle is presented. The valve includes a spring, an inner spool having a port, an outer sleeve, and an orifice, which in a fuel nozzle application may be a calibration orifice. In a low flow condition, the inlet to downstream pressure is the spring force divided by the area on which the pressure acts. At low flow the orifice does not cause any appreciable pressure drop. As the flow increases, a pressure drop develops across the orifice. Since the pressure drop across the valve cannot be greater than the spring force divided by valve area, the valve is forced to open to compensate. As flow is increased, the valve will stroke completely open and the pressure drop at the port becomes negligible and the pressure drop across the orifice is nearly 100% of the pressure drop across the valve.