Apparatus to control fluid flow

Apparatus to control a fluid flow are disclosed. An example fluid flow control apparatus described herein includes a signal stage comprising a signal stage relay having a supply plug being operatively connected to a valve seat at a first end and an exhaust seat at a second end and a seal operatively coupled to the supply plug such that the seal provides a feedback area to apply a fluid pressure feedback force to the exhaust seat.

FIELD OF THE DISCLOSURE

This disclosure relates generally to fluid flow control devices and, more particularly, to apparatus to control fluid flow.

BACKGROUND

Industrial processing plants use control devices in a wide variety of applications. For example, a level controller may be used to manage a final control mechanism (i.e. valve and actuator assembly) to control the level of a fluid in a storage tank. Many process plants use a compressed gas, such as compressed air, as a power source to operate such control devices. In certain hydrocarbon production facilities, compressed air is generally not readily available to operate the control devices. Natural gas is often used as the supply gas to operate these control devices. However, many control devices may bleed natural gas to the atmosphere, which is costly due to the value of the natural gas and the environmental controls and regulations associated with such exhaust gases. Thus, minimizing or eliminating the bleed of natural gas to the atmosphere by the control devices is an important concern.

It is generally understood that typical level controllers used in the hydrocarbon production industry may be single stage, low-bleed pneumatic devices operated by natural gas. To minimize the consumption of natural gas during operation, such level controllers are designed to include a dead band to reduce amounts of bleed gas. However, such designs generally have low operational sensitivity or gain resulting in large vessel spans or oversized sensors.

It is also common to improve the gain of such single stage devices by fashioning a dual-stage pneumatic control device to produce the desired response characteristic with higher output sensitivity. The first stage, often called the signal stage, converts a mechanical or fluid pressure input signal to a pressure output. The signal stage has a low volume flow rate and a low-pressure output that provides the response and control characteristics for the desired process control application. A second stage, often called the amplifier stage, provides high pneumatic capacity and responds to the output of the signal stage to achieve the desired response characteristics while providing a higher output flow rate and/or pressure necessary to operate the final control mechanism. Many of these devices do not provide control action proportional to an input signal and/or suffer from excessive loss of supply gas, such as natural gas, during operation.

FIG. 1andFIG. 2illustrate a known direct-acting, dual-stage pneumatic control device1that includes a reverse-acting signal stage A comprising a signal stage valve110coupled to a reverse-acting amplifier stage B having an amplifier stage relay10(as explained in greater detail below). In operation, an input signal (such as a motion or displacement) from a mechanical device, such as a linkage connected to a displacer in a fluid tank (not shown), may be applied to a valve stem tip135of the signal stage valve110to initiate a pneumatic control signal to the amplifier stage relay10. However, it should be appreciated by those of ordinary skill in the art that the input signal might also be derived from any number of well-known inputs including pressure signals and other direct mechanical forces.

The amplifier stage relay10of the amplifier stage B is the four-mode pneumatic relay disclosed in U.S. Pat. No. 4,974,625, which is hereby incorporated by reference herein in its entirety. Those desiring more detail should refer to U.S. Pat. No. 4,974,625. This relay provides user selectable direct or reverse and proportional or snap-acting operational modes. One of ordinary skill in the art appreciates that a direct or reverse acting mode refers to the relationship of the output signal with respect to an input signal such that, for example, direct mode means the output signal increases with an increasing input signal. Whereas a proportional or snap-acting mode refers to the response of the output signal such that, for example, proportional means changes in the output signal are substantially linear with respect to an input signal change and snap-acting means changes in the output signal are bi-stable and non-linear with respect to an input signal change.

Although the pneumatic relay disclosed in U.S. Pat. No. 4,974,625 may provide four modes, the dual-stage pneumatic control device1illustrated inFIG. 1andFIG. 2may disadvantageously utilize only two modes of operation—direct and reverse/snap-acting modes. This is because the dual-stage pneumatic control device1provides very little feedback or proportioning force between the amplifier stage relay10and the signal stage valve110. That is, there is no specific mechanism to feedback output pressure from a signal diaphragm90of the amplifier stage relay10to offset the applied input force at the valve stem tip135of the signal stage valve110.

In general, the amplifier stage relay10of the control device1includes a series of input and output ports that communicate with respective chambers formed within the amplifier stage relay10. By selectively controlling the fluid communication between various input and output ports through the user selectable switches, the single amplifier stage relay10may provide the multiple operational modes previously described to interface with various control elements.

Referring toFIG. 2, to accommodate the operational modes in the amplifier stage relay10, an input port11communicates with a chamber15and an output port12. A pressure outlet17communicates with a chamber16; an input port13communicates with a chamber18, an output port14communicates with chamber20and the pressure outlet17may be connected to a final control mechanism such as a valve and actuator assembly (not shown).

FIG. 1shows a cut-away illustration of the port switches of the amplifier stage B of the control device1used to select the various operational modes. First and second generally triangular-shaped port switches70and72are pivotally mounted on the amplifier stage relay10by pins71and73, respectively. The port switches70and72are sectioned to reveal serpentine channels74and76, respectively, which pneumatically couple the various input and output ports of the amplifier stage relay10from a pressure inlet78and the pressure outlet17to provide alternate modes of operation. As illustrated inFIG. 1, the first port switch70is positioned such that the input port13is in communication with the pressure inlet port78, and the input port11is vented to atmosphere. The second port switch72is shown to vent the output port14. It should be appreciated from U.S. Pat. No. 4,974,625 that this switch configuration places the amplifier relay stage10in a reverse/snap-acting mode, which when combined with the reverse-acting signal stage valve110provides a direct/snap-acting pneumatic control device1.

That is, a decrease in pressure in a chamber88results in movement of a cage assembly59to the left with respect toFIG. 2, which provides an increasing output pressure at the pressure outlet17. Thus, in operation when an increasing input signal moves the stem tip135of the signal stage valve110, the reverse-acting mode of the signal stage valve110provides a decrease in its output pressure in passageway82and consequently a decrease in pressure in the chamber88to provide a direct-acting pneumatic control device1. The alternate switch configuration for the control device1couples the input port11to the pressure inlet port78and the input port13is vented to atmosphere with the second port switch72configured to couple port14to the output port12. This alternate configuration places the amplifier stage relay10in a direct/snap-acting mode and, therefore, the pneumatic control device1operates in a reverse/snap-acting mode. The remaining possible switch configurations for the amplifier stage relay10render the relay inoperable because there is no feedback mechanism present in the described embodiment of control device1.

As shown inFIG. 2, the signal stage valve110includes a single plug130, a first valve seat120and a second valve seat122. In a first state, a first plug end132does not engage the first valve seat120and a second plug end134engages the second valve seat122. In a second state, the first plug end132engages the first valve seat120and the second plug end134does not engage the second valve seat122. In an intermediate state, neither plug end132and134engages either of the respective valve seats120and122.

In operation, a linkage may apply a force to the valve stem tip135to move it toward the amplifier relay10or to the right (with reference toFIG. 1andFIG. 2). The rightward movement of the valve stem tip135causes movement of the stem130of the signal stage valve110that results in the first plug end132and the second plug end134being simultaneously separated from their respective first and second valve seats120and122in the intermediate state. During this separation, the supply gas, such as natural gas, from a supply port85is vented or bled through the second valve seat122to the atmosphere past valve stem tip135. This venting to atmosphere of the supply gas is often called transition bleed, which may cause excessive loss of supply gas, such as natural gas, to the atmosphere. When the rightward movement of the stem130continues, the stem130ultimately engages the first plug end132with the first valve seat120and the transition bleed ceases, and the fluid pressure within a through feedback passage114of the signal stage valve110and the chamber88of the amplifier stage relay10is at atmospheric pressure.

The change from supply gas pressure to atmospheric pressure within the chamber88results in the diaphragm cage assembly59being moved toward the left inFIG. 2by a spring48in the chamber16. The cage assembly59includes a valve seat30and valve plug40. The leftward movement of the valve seat30and the valve plug40causes a valve plug38to engage a valve seat42and terminate the transmission of supply gas to the output port12. The valve seat30is then moved away from the valve plug40as the diaphragm cage assembly59moves to the left so that fluid pressure in the chamber16flows through the T-shaped opening to the chamber18to vent the fluid pressures from the chambers16and18.

While the use of the signal stage valve110with the amplifier stage relay10provides sensitivity to the input signal from the linkage, it also provides a significant transition bleed of natural gas during the operation of the dual-stage pneumatic control device1. It should also be appreciated that one way to reduce the transition bleed and maintain most of the gain of the dual-stage pneumatic control device1is to couple together two amplifier stage relays10for serial operation. However, coupling the two amplifier stage relays10together to create a tandem device increases the cost and results in a relatively larger, dual-stage pneumatic control device1.

In addition, while certain designs may provide a feedback force to the above-described device, it may be less desirable. One approach is to provide a diaphragm between the stem130and the valve body112in the signal stage valve110. However, the diaphragm has to be clamped or retained at its inner and outer diameters, which results in a larger signal stage that subsequently requires undesirable changes in the linkage and the displacer.

SUMMARY

An example fluid flow control apparatus described herein includes a signal stage comprising a signal stage relay having a supply plug being operatively connected to a valve seat at a first end and an exhaust seat at a second end and a seal operatively coupled to the supply plug such that the seal provides a feedback area to apply a fluid pressure feedback force to the exhaust seat.

In yet another example, a dual-stage fluid flow control apparatus described herein includes a signal stage having a proportional output, the signal stage comprising a signal stage relay including a supply plug having a first end adjacent a valve seat and a second end adjacent an exhaust seat, a signal stage input post is adapted to couple the signal stage to a control device, and means for urging a seat load across the supply plug toward either the valve seat or the exhaust seat. An amplifier stage comprising an amplifier stage relay is operatively connected to the signal stage via a signal passage, the amplifier stage having a fluid supply responsive member adapted to move a relay member to provide an amplified fluid supply output such that a shift in the seat load across the valve seat and the exhaust seat provides a predetermined engagement of either the valve seat to the first end of the supply plug or the exhaust seat to the second end of the supply plug to provide either a proportional or snap-acting and a direct or reverse acting output of the amplifier stage relative to an input signal at the signal stage input post.

In yet another example, a fluid flow control apparatus described herein includes a signal stage having a proportional output. The signal stage comprises a signal stage relay including a supply port, a supply plug having a first end adjacent a valve seat and a second end adjacent an exhaust seat, a signal stage input post adapted to couple the signal stage to a control device and means for urging a seat load across the supply plug toward either the valve seat or the exhaust seat. An amplifier stage comprising an amplifier stage relay is operatively connected to the signal stage via a signal passage. The amplifier stage relay having a fluid supply responsive member adapted to move a relay member to provide an amplified fluid supply output such that a shift in the seat load across supply plug of the signal stage closes the exhaust seat of the signal stage prior to opening the valve seat of the signal stage to substantially eliminate a transition bleed in the signal stage.

DETAILED DESCRIPTION

In general, the example apparatus and methods described herein may be utilized for controlling fluid flow in various types of fluid flow processes. An example fluid flow control apparatus includes a dual-stage fluid control device having a compact, low bleed signal stage with proportional output to improve the control of fluid flow. Additionally, while the examples described herein are described in connection with the control of product flow for the industrial processing industry, the examples described herein may be more generally applicable to a variety of process control operations for different purposes.

FIG. 3is a cut-away illustration of an example direct-acting dual-stage pneumatic control device200comprising a signal stage having signal stage relay300and an amplifier stage further comprising an amplifier stage relay210. The direct-acting signal stage relay300provides a signal stage C and the direct-acting amplifier stage relay210provides an amplifier stage D of the example dual-stage pneumatic control device200. The amplifier stage relay210of the amplifier stage D is similar to the four-mode pneumatic relay valve disclosed in the U.S. Pat. No. 4,974,625 and the amplifier stage relay10disclosed inFIG. 2, including the port switches70and72illustrated inFIG. 1. Those components in the amplifier stage relay210ofFIG. 3that are the same as or similar to the components in the amplifier stage relay10ofFIG. 2have the same reference numerals increased by 200.

As described in detail below, it should be appreciated by those of ordinary skill in the art that the signal stage relay300improves the operation of the previously described dual-stage relay illustrated inFIG. 1andFIG. 2by providing a throttling or proportioning action, thereby permitting utilization of the four modes available in the amplifier stage relay210while substantially reducing the transition bleed associated with signal stage valve110. A throttling or proportional/direct mode of operation is described below as an example operation of the control device200. Those desiring more detail or description should refer to U.S. Pat. No. 4,974,625, which describes therein the other three modes of operation of a four-mode pneumatic relay valve similar to the amplifier210ofFIG. 3.

Referring toFIG. 3, the signal stage relay300of the signal stage C includes a relay body312having a through feedback passage314, a transverse port316, an inlet318, a first valve seat320, and a second valve seat322. The second valve seat322is located on an exhaust seat325having a seal or an o-ring326engaging and sealing against an inner surface317of the through feedback passage314. As described in greater detail below, the o-ring326provides an effective area on which fluid pressure in the feedback passage314of the signal stage relay300may act to create a feedback force to provide the throttling or proportioning action in the control device200.

It should be appreciated that at a quiescent point in the throttling or proportional mode, valve plugs330,240and238are in a “closed” position. That is, closed position means the valve is “substantially in contact with” the valve seat. However, one skilled in the art appreciates that for such a valve seating surface, for example, a metal-to-metal valve seat arrangement, in a closed position with the limited seat loads available such valve-seat arrangements are known to leak small quantities of fluid (i.e. not bubble tight). This leakage at the seats yields a fluid flow to provide throttling action of the pneumatic control device in operation. That is, unlike a snap-acting operation wherein the valves are substantially moving into and out of contact with the valve seats, a throttling or proportional mode is, in part, defined by shifts in corresponding seat load to modify a pressure balance across the relay components. The shifting seat loads provide a modification in seat leakage during quiescent operation to shift the pressure balance across the signal stage C and the amplifier stage D in proportion to supply input and sensor feedback. It should also be appreciated that other materials of construction having sufficient hardness will yield similar leakage flows during operation.

As shown inFIG. 3, the exhaust seat325has an input post327and is retained within the through feedback passage314by an end cap329. The supply valve plug330is located in the through feedback passage314and includes a first plug end332adjacent (e.g., situated immediately adjacent) the first valve seat320and a second plug end334adjacent (e.g., situated immediately adjacent) the second valve seat322. The exhaust seat325includes a shoulder336that receives a spring340. The spring340also engages a shoulder313to urge the exhaust seat325into engagement with the end cap329and away from the second plug end334. A second spring344engages a valve body shoulder315and the first plug end332to urge the first plug end332into engagement with the first valve seat320.

The signal stage relay300is positioned within an opening280in an end cover236of the amplifier stage relay210. An end cover236includes a signal passage282that fluidly couples the transverse port316of the signal stage relay300with a signal chamber288defined partially by a signal diaphragm290located between the end cover236and an intermediate piece239. The end cover236also includes a supply port285that provides supply gas to the inlet318of the signal stage relay300.

In a quiescent operational mode, the first plug end332is in contact with the first valve seat320and the second plug end334is in contact with the second valve seat322. A supply gas is provided to the signal stage relay300via the supply port285and the inlet318. The first plug332is seated at the first valve seat320with sufficient seat load so that the supply gas is substantially prohibited from passing the first valve seat320and the seat load of the second plug end334is seated at the second valve seat322of the exhaust seat325so supply gas is substantially prohibited from exhausting from the exhaust seat325. However, as previously explained, in throttling or proportional mode, at a quiescent operating point, both first and second valve plug ends332and334, when engaged with the respective valve seats320and322, substantially prohibit fluid flow, with only a leakage flow present. The slight leakage creates a proportional, shifting pressure balance across the signal and amplifier stages C and D to modify the respective seat loads in proportion to the supply fluid wherein a feedback force is coupled through a linkage connected to a displacer in a fluid tank (not shown). The input signal may be derived from any number of well-known inputs including pressure signals and direct mechanical forces.

For example, the supply plug330is shown in its left most position, with respect toFIG. 3, in contact with the first valve seat320. In operation, such as a level control application, a buoyant force is applied to a displacer by a fluid in the fluid tank, an input or mechanical linkage provides an input force to the input post327of the exhaust seat325. The input force or signal increases the leakage flow across the first valve seat320. This action also causes the seat load of the second valve seat322to sealingly engage the second plug end334and decrease leakage flow through feedback passage314to the atmosphere, and then the first plug end332to increase leakage flow from the first valve seat320to enable a limited quantity of supply gas to enter the feedback passage314.

Subsequently, the supply gas from the supply port285passes through the inlet318, the first valve seat320, through the feedback passage314to the transverse port316, the signal passage282and the signal chamber288to act upon the signal diaphragm290. The pressure of the supply gas increases a force supplied by the signal diaphragm290and a diaphragm cage assembly259, thereby increasing a seat load upon a valve seat230from the valve plug240to decrease a leakage flow therebetween. This pressure also acts upon the inner surface317of the o-ring326to apply a negative feedback force on the linkage to provide a proportional output from the control device200. That is, a force equal to the product of the pressure within the signal passage282and the effective sealing area of the o-ring326(i.e. the cross-sectional area of the o-ring defined by the inner surface317) is applied in opposition to the linkage force.

As the linkage applies the input signal to the input post327seating forces between the first plug end332and the first valve seat320are diminished or reduced, increasing supply gas pressure to the signal chamber288. The amplifier stage relay200of the amplifier stage D has port switches (not shown) set for proportional/direct operation. Thus, supply gas is applied to an input port211and a chamber215. A chamber216and an output port217are coupled to a final control device. The supply gas is contained within the chamber215as long as a leakage flow across a valve seat242is substantially reduced by the valve plug238to prohibit a pressure increase in the chamber216and the output port217. As pressure increases in the signal chamber288, the force generated by the signal diaphragm290and the diaphragm cage assembly259increases the seat load across the valve seat230. As the seat load increases across the valve seat230and the valve plug240of a plug assembly237, the seat load across the valve seat242and the plug238decreases. The decrease in seat load across the valve seat242and the plug238increases a leakage flow from the chamber215and subsequently into the chamber216. The increase in flow and pressure communicate through the pressure outlet217and into the final control device.

Continuing in operation, as the seat load of the first plug end332and the first valve seat320decreases, the supply gas in the feedback passage314acts upon the exhaust seat325to offset the input signal applied to the input post327by the linkage and provide a proportional amount of supply gas pressure to the signal chamber288. At equilibrium, the valve seat230of the amplifier stage relay210is in contact with the valve plug240and the valve seat242is in contact with the valve plug238with the seat loads in balance so that the output pressure at the pressure outlet217and the final control device is proportional to the input signal at the input post327.

If the input signal at the input post327decreases, the force provided by the diaphragm cage assembly259decreases so that the seat load between the valve plug238and the valve seat242increases and the seat load between the valve seat230and the valve plug240decreases. In this state, the leakage flow between the valve seat230and the valve plug240enable the supply gas in the chamber216to pass through a T-shaped opening232to the chamber218and vent through an input port213, which is exposed to the atmosphere. Changes in the input signal at the input post327results in a new equilibrium state for the amplifier stage relay210with the output pressure at the pressure outlet217being directly proportional to the input signal.

During operation, when the input force at the input post327decreases, the seat load at the second valve seat322decreases and the supply plug330is slightly loaded. That is, the seat load at the first plug end332of the supply plug330and the first valve seat320increases to decrease the leakage flow of supply gas through the first valve seat320. The seat load at the second valve seat322of the exhaust seat325and the second plug end334of the supply plug330decreases. The decrease in seat load permits the supply gas in the signal chamber288, the signal passage282, the transverse port316, and feedback passage314to vent through the second valve seat322to atmosphere.

The signal stage relay300enables the example dual-stage pneumatic control device200to have a high gain, a low transition bleed, and four modes of operation that achieve numerous advantages. For example, the spring340is utilized to overcome a frictional force created by the seal or O-ring326and to keep or maintain the input post327in contact with the input linkage, thereby ensuring that a dead band of operation does not occur during the operation of the linkage. In other words, the input post327is in contact with the input linkage such that a bias force of the spring340substantially maintains contact between the input linkage and the input post327to substantially eliminate a dead band between the input linkage motion and exhaust seat325motion. The high gain, four-modes of operation provided by the example dual-stage pneumatic control device200eliminate the need to use either two-serially aligned amplifier stage relays210to provide a high gain or a diaphragm between the exhaust seat325and the valve body312to provide a feedback force. The use of the seal or O-ring326(i.e., as opposed to the use of a diaphragm) to provide a supply gas pressure feedback force to the exhaust seat325enables the signal stage relay300to have a small diameter and, thus, a small and compact size. This also results in the example dual-stage pneumatic control device200being usable with a smaller displacer and lighter fluids in a fluid vessel, thereby minimizing the cost of the fluid vessel.

The example dual-stage pneumatic control device200utilizes the springs244and248of the amplifier stage relay210and the springs344and340of the signal stage relay300to assist in the control of the flow of the supply gas through or across the respective valve seats242,230and320and322. As a result, the example dual-stage pneumatic control device200may function at any orientation, including horizontal, vertical, and angled without compensating for the affects of gravity.

One skilled in the art should also appreciate that the feedback area, presented by the effective area of the o-ring326can also be adjusted by changing the internal diameter of the feedback passage314of the signal stage relay300and the external diameter of the seal or o-ring326. That is, the signal stage relay housing312and the seal or o-ring326can be quickly changed or replaced as a replaceable single stage module that provides a predetermined feedback area to accommodate different types of services such as water, condensate or interface, which may provide or exert different linkage forces. For example, a relatively large feedback area (e.g. 0.1080 in2) would be preferable for applications providing a large buoyant force (i.e. corresponding to fluid having an approximate specific gravity of 1.0), such as water. A slightly smaller feedback area (e.g. 0.0625 in2) would accommodate applications providing a moderate buoyant force (i.e. corresponding to a fluid having an approximate specific gravity of 0.8) such as oil and a very small feedback area (e.g. 0.036 in2) would preferably accommodate an oil-to-water interface application with a small buoyant force (i.e. corresponding to fluids having an approximate differential specific gravity of 0.1). Specifically, one of ordinary skill in the art will recognize that this feature provides the user with an improved setup and calibration scenario for level control applications since the lever and the displacer need not be modified or replaced for these different applications.

The example dual-stage pneumatic control device200depicted inFIG. 3may provide very high gain (i.e. increased responsiveness) and very low gas consumption during normal operation. However, in certain applications such high gain or responsiveness may create susceptibility to mechanical vibrations that may lead to instability in control. The source of this instability is generally the rapid application of a feedback force on a controller linkage by the signal stage of the pneumatic controller device. The example pneumatic control device401ofFIG. 4may substantially reduce such susceptibility by: 1) independently controlling the pressure to the signal stage relay; and 2) reducing the feedback area of signal stage relay.

Referring toFIG. 4, a cut-away illustration of an example dual-stage pneumatic control device401having a signal stage E and an amplifier stage F including stabilizing pressure regulators500and510. The stabilizing pressure regulators500and510independently provide supply air to a signal stage relay410and an amplifier stage relay420through a signal supply pressure inlet485and an amplifier supply pressure inlet411. It should be appreciated that such stabilizing pressure regulators500and510could be integrated within the signal stage E and the amplifier stage F, or such regulators could be external to the signal and amplifier stages E and F. Alternatively, it should be appreciated that stabilizing regulator500may be positioned downstream of stabilizing pressure regulator510. The signal stage relay410and the amplifier stage relay420of example device generally function as the previously described example dual-stage pneumatic control device200depicted inFIG. 3except the stabilizing pressure regulators500and510provide independent pressure supply to each stage, signal stage E and amplifier stage F to enhance device stability and to improve overall pneumatic control device performance. For example, the signal stage pressure regulator500may be set to 8 psig, whereas the amplifier stage pressure regulator510may be set to 35 psig. Generally, the signal stage E is set to a lower pressure than the amplifier stage F. That is, the signal stage pressure may be set at a minimal operating point to operate the amplifier stage F. The lower signal stage pressure improves pneumatic control device stability and performance in the following manner: 1) lower signal stage supply pressure directly reduces the feedback force that can be generated by the signal stage relay410(i.e. Force=Pressure×Area); and 2) lower pressure directly reduces the gas consumed by the signal stage relay410.

Additionally,FIG. 5illustrates a signal stage610to further improve pneumatic control device performance. That is, in combination with the low signal stage pressure of the example pneumatic control device ofFIG. 4, the present example signal stage610has a reduced feedback area to further reduce feedback forces on a sensor. The example signal stage relay610includes a relay body612having smaller internal diameter relative to the feedback passage614and/or the previously described relay body312of the example pneumatic control device200depicted inFIG. 3. The corresponding feedback passage614is also reduced in diameter to provide a sealing engagement with a seal or an o-ring626. As previously described, the fluid pressure in the feedback passage614acts upon the inner surface617and the seal or o-ring626to apply a negative feedback force on the linkage to provide a proportional output from a control device. As a result, the reduced feedback area provides a reduced feedback force to a sensor coupled to a pneumatic control device.

The combination of low pressure signal stage and the reduced feedback-area signal stage may improve device stability for feedback sensors with high gain. By controlling the feedback area in a predetermined manner and configuring signal stage pressure independent of amplifier stage pressure, a pneumatic control device can be adapted to stabilize a broad variety of displacement-style level controllers.

In summary it should be appreciated that the example device disclosed herein substantially eliminates the transition bleed of the control device fashioning a dual-stage pneumatic relay that positively closes an exhaust port of the relay before a supply port opens. Additionally, a seal or an o-ring of a signal stage relay provides significant negative feedback area to counteract or offset the lever force on the signal stage relay in a throttling or proportioning manner while providing increased gain to improve overall system performance.