Patent Description:
The subject matter of this disclosure relates to improvements to pressure regulators that address these challenges. Of particular interest are embodiments with a design that integrates two-path control with a balanced trim assembly on a single device. The embodiments may have a spring-operated valve that opens and closes in response to variations in downstream demand. Nominally, the valve has an "equilibrium" position that maintains pressure equally on both upstream and downstream sides of the device. The valve opens from this position to allow fluid to flow through the device to maintain downstream pressure at a relatively constant level (typically in response to increases in downstream demand). The valve moves to reduce flow as demand decreases, often eventually reaching its equilibrium position again. In some applications, the valve may have a fully "closed" position that prevents fluid flow altogether. This proposed design allows for better, more accurate downstream pressure in response to demand. It also increases the operating pressure of the pressure regulator to accommodate applications with an inlet pressure of at least <NUM> PSI.

Reference is now made briefly to the accompanying drawings, in which:.

Where applicable, like reference characters designate identical or corresponding components and units throughout the several views, which are not to scale unless otherwise indicated. The embodiments disclosed herein may include elements that appear in one or more of the several views or in combinations of the several views. Moreover, methods are exemplary only and may be modified by, for example, reordering, adding, removing, and/or altering the individual stages.

Manufacturers often take opportunities to improve construction of industrial equipment. These opportunities may lead to better, more reliable devices or provide new functions or features on the same. In many cases, the improvements may also lead to innovative solutions that drive savings in the form of lower costs of parts, labor and assembly, or maintenance and repair.

As noted above, pressure regulators play an important role in fluid delivery systems. These devices accurately maintain flowing fluids at specific desired pressures. For many industrial applications, pressure regulators must adopt particularly robust or sturdy designs to withstand high pressures, caustic environments, or simply to provide reliable, long-lasting operation. The designs may require construction (e.g., materials, fastening techniques, etc.) that are particularly costly or time-consuming to make or build to specification.

The discussion below describes various embodiments of a pressure regulator. In contrast to known devices, the pressure regulators herein incorporate a loading system that provides two-path control of a main valve along with a balanced plug in the main valve. This construction allows the device to deploy in applications with high inlet pressures, but with the benefits of highly accurate control and response to changes in downstream demand. As an added benefit, the proposed design arranges some construction as modular parts or sub-assemblies. This modular approach simplifies repair and maintenance. It also allows operators to tailor the device for a particular control scheme. Parts of this modular design are also compatible with other types of flow control devices, like control valves. Other embodiments and configuration are within the scope of the subject matter herein.

<FIG> depicts a schematic diagram of an exemplary embodiment of a pressure regulator <NUM>. This embodiment is shown as part of a gas distribution system, identified generally by the numeral <NUM>. The system <NUM> may include pipe <NUM> that carries material <NUM>. As also shown, the pressure regulator <NUM> may include a flow control <NUM> with a balanced trim <NUM> and an actuator <NUM>. A control system <NUM> may couple with the actuator <NUM>. The control system <NUM> may include a fluid circuit <NUM> with a pilot unit <NUM> that controls a limited-flow feed of material <NUM> to the actuator <NUM>.

Broadly, the pressure regulator <NUM> may be configured for use in applications that have high inlet pressures. These configurations may integrate pilot valves to cause "sense pressure" (or downstream pressure) to register at both the actuator and the pilot valves. This arrangement may prove useful for much better and highly accurate pressure control downstream of the device. In addition, the device incorporates a balanced-pressure plug that provides bubble-tight shut off across a wide range of pressure differentials.

The gas distribution system <NUM> may be configured for material to transit industrial sites or parts of larger networks. These configurations may find use at facilities that process or distribute hydrocarbons, like natural gas or "fuel gas. " Municipalities and utilities may deploy intricate networks to deliver resources to consumers, including residential and commercial fuel gas customers. All of these networks may include myriad devices to regulate flow, including pressure regulator <NUM>. These devices may install in-line with the pipes <NUM>, which may embody high capacity pipelines that can carry fluids at high-pressure. However, in addition to fluids (e.g., liquids and gasses), material <NUM> may also embody solids and solid/fluid mixes as well.

The flow control <NUM> may be configured to regulate flow of fuel gas <NUM> through the pressure regulator <NUM>. These configurations may embody a valve that operates in response to differential pressure across its inlet and outlet. This feature matches flow of fuel gas <NUM> to demand, e.g., on the network <NUM>. As noted above, the valve may have various operating positions or conditions to manage flow. One position may maintain pressure in equilibrium across the upstream or "supply" side and the downstream or "demand" side of the device. This position may change in response to variations in downstream demand. An increase in downstream demand, for example, may reduce downstream pressure and cause the valve to move to a position that allows more fluid to flow. The resulting flow meets the downstream demand. As pressure equalizes, the valve may move back to its prior "equilibrium" position. In one implementation, the valve may employ a fail-to-close design that causes the valve to default to a "fully-closed" position in response to control pressure loss, mechanical failure, or other problems on the device or in the network <NUM>. This position prevents flow of gas through the device altogether.

The balanced trim <NUM> may be configured to accommodate higher inlet pressures on the upstream side of the valve. These configurations may include a plug (or closure member) that moves relative to a seat (to instantiate the variable positions of the valve noted above). For "balanced" trim designs, fluid forces balance on either side of the plug at equilibrium. The plug may incorporate openings or like design features for this purpose. The openings may vent fluid from the upstream side of the plug into a chamber in the pressure regulator <NUM> "above" or on the opposite side of the plug.

The actuator <NUM> may be configured to regulate the position of the plug relative to the seat. These configurations may embody devices (or mechanical elements) that can apply a load on the plug. Examples of mechanical devices may include a diaphragm that is sensitive to changes in pressure. A spring may find use to provide a spring force that supplements the diaphragm. For fail-to-close devices, the spring force will direct the plug into its fully-closed position.

The control system <NUM> may be configured to apply gain to the system. These configurations can embody devices that multiply a small change in downstream pressure into a larger change at, e.g., the diaphragm. These devices improve response time and provide stable, accurate control of the position of valve in response to changes in downstream demand.

The fluid circuit <NUM> may be configured to direct fuel gas <NUM> among the parts of the control system <NUM>. These configurations may leverage a local network of conduit (or piping or tubing). The conduit may extend from locations or taps found on the pipe <NUM> on both the upstream and downstream sides of the pressure regulator <NUM>. These taps allow fuel gas <NUM> into the conduit. The fuel gas <NUM> transits the local network to the actuator <NUM> and through control system <NUM>.

The pilot unit <NUM> may be configured to regulate pressure to the actuator <NUM>. These configurations may embody devices (or "pilot") with a manifold that houses valves (or "pilot valves"). For two-path control, conduit may couple the pilot valves with the actuator <NUM> so as to effect gain that enhances response of the plug (or movement of the plug relative to the seat in response) to the changes in downstream demand. The manifold may have internal flow pathways that place multiple pilot valves in flow connection with one another to allow fuel gas <NUM> to flow between them. The pilot valves may employ designs for fixed differential pressure or a variable differential pressure, as desired. This design can allow for any number of pilot valves (and other devices, like check valves or orifices) to incorporate into a single unit on the pressure regulator <NUM>. In one implementation, the manifold has a modular design with various parts that fit or attach together. This arrangement may accommodate combinations of fixed and variable pilot valves to match any proposed application of the pressure regulator <NUM> or to permit an end user to effectively tune performance of the pressure regulator <NUM> as desired.

<FIG> depicts a perspective view of one example of the pressure regulator <NUM> of <FIG>. Parts including parts that instantiate the control system <NUM> are not shown in this example for clarity. The device may have a housing <NUM> of robust design, typically made of cast or machined metals to make the device compatible with high pressures and caustic, harsh, or corrosive materials (like fuel gas <NUM>). The housing <NUM> may have several parts or members, shown here to include a pair of cartridge members (e.g., an upper cartridge member <NUM> and a lower cartridge member <NUM>) and an adapter member <NUM>. The housing members <NUM>, <NUM>, <NUM> may mate with one another at a peripheral, outer flange <NUM>. Apertures <NUM> may populate the outside of one or more of these parts. Some of the apertures <NUM> may operate as ports <NUM> with threaded openings to accommodate fluid fittings, as noted more below. Others may operate as threaded holes <NUM> in a mounting area <NUM> that receives, for example, parts of the pilot unit <NUM>. In one implementation, the housing <NUM> may include a valve body <NUM> that houses the balanced trim <NUM> discussed above in <FIG>. The valve body <NUM> may have openings <NUM> at either end of an internal passage <NUM>. Flanges <NUM> (or butt-weld ends) may allow the valve body <NUM> to mount in-line with sections of pipe <NUM>. In one example, the valve body <NUM> may include ports <NUM>, which may also have threaded openings to receive fluid fittings as well.

<FIG> depicts an elevation view of the cross-section of the pressure regulator <NUM> taken at line <NUM>-<NUM> of <FIG>. The cartridge members <NUM>, <NUM> may form an internal chamber <NUM> that encloses parts of the actuator <NUM>. These parts may include a diaphragm <NUM>, preferably an annular disc of flexible material including metals, rubbers, or composites. The annular disc <NUM> may be arranged with its outer, periphery portion "sandwiched" between the peripheral outer flange <NUM> of the cartridge members <NUM>, <NUM>. This arrangement separates the internal chamber <NUM> into two chambers (e.g., a first chamber <NUM> and a second chamber <NUM>). Ports <NUM> in the housing members <NUM>, <NUM> may form flow passages that extend to each of the chambers <NUM>, <NUM>. Additional parts of the actuator <NUM> may include support plates <NUM>, <NUM> that reside on either side of the annular disc <NUM>. A compression spring <NUM> may reside in the first or "upper" chamber <NUM> to apply force to the upper support plate <NUM>. In the second or "lower" chamber <NUM>, a seal pack <NUM> may insert into an opening <NUM> in the cartridge member <NUM>. The seal pack <NUM> may have an annular body <NUM> with an outer seal <NUM>, like an o-ring that resides in a groove. The annular body <NUM> may also have a centrally-located through bore to receive inner seals (e.g., a first inner seal <NUM> and a second inner seal <NUM>) and a bushing <NUM>. A plate <NUM> may reside on top of the annular body <NUM>. Fasteners F<NUM> may penetrate through the annular body <NUM> and the plate <NUM> into the cartridge member <NUM>. Additional seals <NUM>, <NUM> may find use to seal any interstitial gap between the cartridge member <NUM> and the adapter <NUM> and the interface between the adapter <NUM> and the valve body <NUM>. As also shown, the device may include a valve stem <NUM> that can move axially through seals <NUM>, <NUM> and bushing <NUM>. This movement may result from changes in demand downstream of the pressure regulator <NUM>. One end of the valve stem <NUM> may couple with the diaphragm <NUM>. An indicator <NUM> may couple with this end, for example, using magnets (although other fastening techniques are readily acceptable as well). The indicator <NUM> may penetrate through the upper cartridge member <NUM> into an indicator housing <NUM> that forms a seal with the same.

The other end of the valve stem <NUM> may reside in the valve body <NUM> along with other parts of the balanced trim <NUM>. These parts may reside in a chamber <NUM>. In one implementation, the balanced trim may include a cage <NUM>, shown here as a hollow cylinder <NUM> with openings <NUM> disposed circumferentially in its peripheral wall. A plug <NUM> may reside in the cage <NUM>. As noted herein, the plug <NUM> may be configured for inlet pressure to balance on either side. These configurations may leverage a bi-furcated design, for example, with a first plug member <NUM> that has an elongate portion that extends into a second plug member <NUM>. The elongate portion may receive the end of the valve stem <NUM>. Openings <NUM> in the plug members <NUM>, <NUM> may allow pressure to balance across the bi-furcated plug <NUM>. An annular seal <NUM> may reside in a peripheral groove that circumscribes the outside of the second plug member <NUM>. Examples of the annular seal <NUM> may utilize a rubber ring (with plastic backup ring, if necessary). The rubber ring <NUM> may contact the inner surface of the peripheral wall on the cylinder <NUM>. This arrangement creates a circumferential seal that circumscribes this inner surface (and the outer surface of the second plug member <NUM>). In one implementation, the plug <NUM> my incorporate an insert <NUM>, like a Teflon® or nitrile ring that resides between the plug members <NUM>, <NUM>. A portion of the insert <NUM> may engage with a seat <NUM> to effect the fully "closed" position of the plug <NUM>.

<FIG> and <FIG> show the pressure regulator <NUM> of <FIG> in partially-exploded form. Fasteners F<NUM> may secure the adapter member <NUM> onto the valve body <NUM>. Fasteners F<NUM> may secure the cartridge members <NUM>, <NUM> together to create an "actuator cartridge" <NUM>. Fasteners F<NUM> may insert through each of the cartridge members <NUM>, <NUM> to secure the actuator cartridge <NUM> to the adapter member <NUM>. Notably, this arrangement creates a modular structure that allows an end user to perform maintenance and repair on the pressure regulator <NUM> in its installed or "in-line" location on the pipe <NUM>. In <FIG>, an end user can remove fasteners F<NUM> to decouple the actuator cartridge <NUM> from the adapter member <NUM>. The end user can lift the actuator cartridge <NUM> off of the adapter <NUM>. This action also removes parts of the balanced trim <NUM> (namely, the plug <NUM> in whole) out of the valve body <NUM>. As best shown in <FIG>, the end user could remove fasteners F<NUM>, F<NUM> to decouple the upper cartridge member <NUM> from the lower cartridge member <NUM>. The end user can lift the upper cartridge member <NUM> off of the lower cartridge member <NUM> to gain access to the internal chamber <NUM> formed by cartridge members <NUM>, <NUM>. This feature can allow the end user to service the diaphragm <NUM> without disturbing other parts, including the seals <NUM>, <NUM> (<FIG>) or the balanced trim <NUM> (<FIG>). In one implementation, the end user could also remove the diaphragm <NUM> to gain access to (and replace) the seal pack <NUM> (<FIG>).

<FIG> depict a perspective view of an example of the pressure regulator <NUM> of <FIG>. Parts have been added in each to continue the discussion of certain features and functionality for the proposed designs. Conduit <NUM> in the form of, for example, metal tubing, may extend between fluid fittings <NUM> to complete fluid connections among the parts of the pressure regulator <NUM> as well as between these parts and locations on the pipe <NUM> that are upstream and downstream of the pressure regulator <NUM>. In some implementations, a filter FL may install into the fluid circuit <NUM>, as well. The pilot unit <NUM> may include an interface block <NUM> to secure a manifold <NUM> to the adapter member <NUM> (at the mounting area <NUM>). The manifold <NUM> may embody a pair of ported blocks (e.g., a first ported block <NUM> and a second ported block <NUM>). As noted above, construction of the manifold <NUM> may permit changes to the pilot unit <NUM> to expand functions. For example, the blocks <NUM>, <NUM> may separate from one another to add parts to the manifold <NUM> that adapt the pressure regulator <NUM> for particular applications, for example, as part of a working monitor arrangement, among others.

<FIG> depicts a perspective view of an example of the pilot unit <NUM> to illustrate this concept. In this example, the manifold <NUM> incorporates a third ported block <NUM>. Examples of the ported blocks <NUM>, <NUM>, <NUM> may embody separate elements of the manifold <NUM>, for example, machined billets of metal like aluminum, steel, or steel alloys. The billets may include openings <NUM>, which may embody clearance or threaded holes. The holes <NUM> may accommodate fasteners F<NUM> to secure the ported blocks <NUM>, <NUM>, <NUM> together. This arrangement facilitates the modular design for the manifold <NUM>. As also shown, the ported blocks <NUM>, <NUM>, <NUM> may have outer access ports <NUM> that provide threaded openings to receive fittings <NUM> (<FIG>).

<FIG> schematically depicts an elevation view of the cross-section of the pressure regulator <NUM> taken at line <NUM>-<NUM> of <FIG>. Conduit <NUM> may include a supply line SUP<NUM> that couples the manifold <NUM> with the inlet or supply side P<NUM>. Sense lines S<NUM> and S<NUM> may couple the upper chamber <NUM> of the actuator cartridge <NUM> with the outlet or demand side P<NUM> and with the manifold <NUM>, respectively. Load line L<NUM> may couple the manifold <NUM> with the lower chamber <NUM> of the actuator cartridge <NUM>. As also shown, the ported blocks <NUM>, <NUM> may have an internal flow network <NUM>, typically machined bores (or like features) that permits fluid flow among various flow controls that allow a technician to more accurately tune the setpoint of the pressure regulator <NUM>. Examples of the flow controls include an adjustable orifice <NUM> and a check valve <NUM>. The adjustable orifice <NUM> or "restrictor" may have a v-groove on its outer surface. A threaded plug <NUM> may find use to seal one or more of the access ports <NUM>. The flow controls also include a pair of pilot valves (a first pilot valve <NUM> and a second pilot valve <NUM>). The first pilot valve <NUM> has a fixed differential pressure. The second pilot valve <NUM> is configured (with a knob, for example,) to adjust differential pressure across the device. The end user can tune operation of the pilot unit <NUM> by adjusting the variable pilot device <NUM> or rotating the adjustable orifice <NUM> to change radial orientation of the v-groove relative to the internal flow pathway <NUM> in the second ported block <NUM>. Values for the fixed differential pressure of the first pilot valve <NUM> may accommodate design parameters of the second pilot valve <NUM>; exemplary values may be in a range of <NUM> psi to <NUM> psi.

The restrictor <NUM> may be configured to work in conjunction with the second or "main" pilot valve <NUM> to define pressure in the lower chamber <NUM>. In one implementation, the main pilot valve <NUM> may have an internal orifice that increases and decreases in size in response to changes in downstream pressure. This orifice enlarges in response to downstream pressure below the setpoint of the pressure regulator <NUM>. When the orifice becomes larger the orifice of the restrictor <NUM>, more gas flows into the lower chamber <NUM> than passes through the restrictor <NUM> and downstream (through the upper chamber <NUM>). The orifice shrinks in response to downstream pressure above the setpoint so that, when it is smaller than the orifice of the restrictor <NUM>, less gas will flow to the lower chamber <NUM> (than passes downstream). The end user can adjust the size of the orifice of the restrictor <NUM> to manage the relationship between this internal orifice and the orifice of the restrictor <NUM> and, turn, tune accuracy and speed of response of the pressure regulator <NUM>.

The check valve <NUM> may be configured to limit pressure differential across the diaphragm <NUM>. These configurations may prove useful to prevent damage (to the diaphragm <NUM>) that can result from backpressure or related use cases. Backpressure may occur at startup because, if downstream pressure rises quickly, gas can flow into the upper chamber <NUM> faster that it bleeds across the restrictor <NUM>. This imbalance builds pressure in the upper chamber <NUM>. The check valve <NUM> may open in response to downstream pressure above cracking pressure to allow more gas to pass to the lower chamber <NUM>, thus allowing pressure to equalize across the diaphragm <NUM>.

The diagram of <FIG> shows the pressure regulator <NUM> in a first position. This position is consistent with pressure of fuel gas <NUM> on the supply side P<NUM> in equilibrium (or balanced) with pressure of fuel gas <NUM> on the demand side P<NUM>. The diaphragm <NUM> (and spring <NUM>) exert a spring force that maintains the position of the balanced plug <NUM>, <NUM>. As noted here, while the plug <NUM>, <NUM> is shown in contact with the seat <NUM>, this is not always the case. Supply side pressure P<NUM> acts on each side of the balanced plug <NUM>, <NUM> and concomitantly on one side of the pilot valves <NUM>, <NUM>. Demand side pressure P<NUM> acts on the chambers <NUM>, <NUM> through the sense line S<NUM> and on opposite sides of the pilot valves <NUM>, <NUM> through the load line L<NUM> and sense line S<NUM>.

<FIG> also schematically depicts an elevation view of the cross-section of the pressure regulator <NUM> taken at line <NUM>-<NUM> of <FIG>. This diagram show the pressure regulator <NUM> in a second position. This position reflects a change in demand side pressure P<NUM>. The change often corresponds with increased demand, which may rapidly reduce lower side pressure P<NUM> below supply side pressure P<NUM>. In response to the sensed pressure differential (DP<NUM>,<NUM>), the first pilot valve <NUM> operates to reduce or "steps down" supply side pressure P<NUM> at the second pilot valve <NUM> to a lower, intermediate pressure P<NUM>. The second pilot valve <NUM> opens in response to intermediate pressure P<NUM> to increase or "step up" pressure in the lower chamber <NUM> to a loading pressure P<NUM>, which is high enough to overcome the spring force and move the balanced plug <NUM>, <NUM> from its first position (in <FIG>). The new position for the balanced plug <NUM>, <NUM> permits fuel gas <NUM> through openings <NUM> in the cage <NUM> to meet the downstream demand. In one implementation, the balanced plug <NUM>, <NUM> may return to the first position as pressure equalizes between the supply side pressure P<NUM> and the demand side pressure P<NUM>.

<FIG> depicts a perspective view a pair of flow controls of the type discussed with respect to the pressure regulator <NUM> above. These flow controls form a "working monitor" setup with a first or "first stage" regulator A and a second or "second stage" regulator B in series on the pipeline <NUM>. On the first stage regulator A, the manifold <NUM> includes ported blocks <NUM>, <NUM>, <NUM> to accommodate a third pilot valve <NUM>, which preferably is configured to vary differential pressure across the device.

<FIG> schematically depicts an elevation view of the cross-section of pressure regulators A, B taken at line <NUM>-<NUM> of <FIG>. The diagram identifies the pilot valves as Pilot <NUM> and Pilot <NUM> on the first stage regulator A and as Pilot <NUM> on the second stage regulator B. Conduit <NUM> may include a supply line SUP<NUM> that couples the manifold <NUM> on the second stage regulator B to an intermediary section of pipeline <NUM> that extends between pressure regulators A, B. Sense lines S<NUM> and S<NUM> may couple the upper chamber <NUM> on the first stage regulator A with the intermediary section of pipeline <NUM> as well. Load line L<NUM> may couple the manifold <NUM> with the lower chamber <NUM> on the first stage regulator A. As also shown, sense line S<NUM> couples the third ported block <NUM> with the demand side P<NUM> to monitor second stage regulator B.

The diagram of <FIG> shows the pressure regulators A, B in a first position. The diaphragm <NUM> (and spring <NUM>) exert a spring force that maintains the position of the balanced plug <NUM>, <NUM> on both pressure regulators A, B, which may or may not cause the plug <NUM>, <NUM> to contact the seat <NUM>. This position may reflect conditions with pressure of fuel gas <NUM> on the supply side P<NUM> in equilibrium (or balanced) with pressure of fuel gas <NUM> on the demand side P<NUM>. When fully-closed, however, the upstream and downstream pressure may differ, but there will be no flow through one or both of the devices. Moving from left to right in the diagram, supply side pressure P<NUM> acts on each side of the balanced plug <NUM>, <NUM> and concomitantly on one side of the fixed pilot <NUM> and Pilot <NUM> on the first stage regulator A. The loading pressure P<NUM> acts on the chambers <NUM>, <NUM> through sense line S<NUM> and on opposite sides of the fixed pilot <NUM> and Pilot <NUM> and Pilot <NUM> through sense line S<NUM> and load line L<NUM>. Downstream pressure P<NUM> acts on Pilot <NUM> through the sense line S<NUM>. On the second stage regulator B, the loading pressure P<NUM> acts on both sides of the balanced plug <NUM>, <NUM>, and on one side of the fixed pilot <NUM> and Pilot <NUM> through the supply line SUP<NUM>. Demand side pressure P<NUM> acts on the chambers <NUM>, <NUM> though sense line S<NUM> and on opposite sides of the pilot valve <NUM> and Pilot <NUM> through the load line L<NUM> and sense line S<NUM>.

Claim 1:
A pressure regulator (<NUM>), comprising:
a cage (<NUM>) with a peripheral wall forming a hollow cylinder;
a pressure-balanced plug (<NUM>, <NUM>) disposed inside of the hollow cylinder and forming a circumferential seal therewith;
a pneumatic actuator (<NUM>) coupled with the pressure-balanced plug (<NUM>, <NUM>), the pneumatic actuator comprising a diaphragm (<NUM>) creating a pair of separate, airtight chambers (<NUM>, <NUM>);
a valve stem (<NUM>) coupled on one end to the diaphragm (<NUM>) and on the other end to the pressure-balanced plug (<NUM>, <NUM>);
a pilot unit (<NUM>) disposed on the pneumatic actuator (<NUM>); and
conduit coupling the pilot unit (<NUM>) with the pair of airtight chambers (<NUM>, <NUM>);
wherein the pilot unit (<NUM>) comprises:
a manifold (<NUM>) with an internal flow network including part of the conduit and
a fixed differential pressure pilot valve (<NUM>) and a variable differential pressure pilot valve (<NUM>) disposed on the manifold (<NUM>) and in flow connection with one another via the internal flow network;
wherein the pilot valves (<NUM>, <NUM>) are plumbed to one another in series with the variable differential pilot valve (<NUM>) immediately upstream of the actuator (<NUM>).