| 1. | An overpressure protection system for use with a differential pressure sensor having a high side and a low side, said system comprising: a high side volume adjustable cavity having at least one high side movable wall portion engageable with a first pressure source; a low side volume adjustable cavity having at least one low side movable wall portion engageable with a second pressure source; one of said high and low side cavities being in fluid communication with one side of said sensor and with said movable wall portion of the other of said cavities; the other of said high and low side cavities being in fluid communication with an other side of the sensor; wherein during operation of the sensor to detect the pressure difference between the first and second pressure sources, the other of said high and low side cavities functions as both a normal process volume for the other side and as an overpressure process volume for the one cavity. |
| 2. | The overpressure protection system of claim 1, wherein the other of said high and low side cavities is disposed in fluid communication with said movable wall portion of the one of said cavities, so that during operation of the sensor to detect the pressure difference between the first and second pressure sources, the one of said high and low side cavities functions as both a normal process volume for the one side and as an overpressure process volume for the other cavity. |
| 3. | The system of claim 1 wherein said moveable wall portion of said one cavity comprises a plurality of diaphragms. |
| 4. | The system of claim 3 wherein at least one of said plurality of diaphragms is compressible into surface to surface engagement with an other of said plurality of diaphragms. |
| 5. | The system of claim 3 wherein said plurality of diaphragms comprise two diaphragms attached proximate their edges and one of said diaphragms is engaged with a disklike body. |
| 6. | The system of claim 3, comprising a plurality of interconnected pathways, which connect said high and low side cavities to the sensor. |
| 7. | The system of claim 6 wherein said high side and low side cavities are filled with a substantially incompressible fluid. |
| 8. | The system of claim 3 wherein one of said plurality of diaphragms is an overrange diaphragm releasably biased toward surfacetosurface engagement with a mating surface of a substantially rigid body. |
| 9. | The system of claim 8, wherein both said overrange diaphragm and said mating surface are substantially concave, said overrange diaphragm having a deeper cavity than said mating surface and a periphery of said overrange diaphragm is rigidly fastened to said mating surface, wherein a central portion of said overrange diaphragm is releasably biased into engagement with said mating surface. |
| 10. | The system of claim 9, wherein said other cavity is disposed in fluid communication with said overrange diaphragm of said one cavity, so that pressure within said other cavity is applied against the bias of said overrange diaphragm of said one cavity. |
| 11. | The system of claim 2 wherein said high side moveable wall portion and said low side moveable wall portion both comprise a plurality of diaphragms. |
| 12. | The system of claim 11 wherein at least one of said plurality of diaphragms is compressible into surface to surface engagement with an other of said plurality of diaphragms. |
| 13. | The system of claim 11 wherein at least one of said plurality of diaphragms is made of steel. |
| 14. | The system of claim 13 wherein said at least one of said plurality of diaphragms is made of 316 stainless steel. |
| 15. | The system of claim 11 wherein said plurality of diaphragms comprise two diaphragms attached proximate their edges and one of said diaphragms is engaged with a disklike body. |
| 16. | The system of claim 11, comprising a plurality of interconnected pathways, which connect said high and low side cavities to the sensor. |
| 17. | The system of claim 16 wherein said high side and low side cavities are filled with a substantially incompressible fluid. |
| 18. | The system of claim 11 wherein one of said plurality of diaphragms is an overrange diaphragm releasably biased toward surfacetosurface engagement with a mating surface of a substantially rigid body. |
| 19. | The system of claim 18, wherein both said overrange diaphragm and said mating surface are substantially concave, said overrange diaphragm having a deeper cavity than said mating surface and a periphery of said overrange diaphragm is rigidly fastened to said mating surface, wherein a central portion of said overrange diaphragm is releasably biased into engagement with said mating surface. |
| 20. | The system of claim 19, wherein said low side cavity is disposed in fluid communication with said overrange diaphragm of said high side cavity, so that pressure within said low side cavity is applied against the bias of said overrange diaphragm of said high side cavity. |
| 21. | The system of claim 19, wherein said high side cavity is disposed in fluid communication with said overrange diaphragm of said low side cavity, so that pressure within said high side cavity is applied against the bias of said overrange diaphragm of said low side cavity. |
| 22. | A method of measuring differential pressures while providing overpressure protection, said method comprising the steps of: a) providing a'sensor having a high side and a low side; b) providing a high side volume adjustable cavity having at least one high side movable wall portion; c) engaging one of the at least one high side movable wall portion with a first pressure source; d) providing a low side volume adjustable cavity having at least one low side movable wall portion; e) engaging the at least one low side movable wall portion with a second pressure source; f) disposing one of the high and low side cavities in fluid communication with one side of the sensor and with the movable wall portion of the other of said cavities; g) disposing the other of the high and low side cavities in fluid communication with the other side of the sensor; and h) using the sensor to detect the pressure difference between the first and second pressure sources, wherein the other of the high and low side cavities functions as both a normal process volume for the other side and as an overpressure process volume for the one cavity. |
| 23. | The method of claim 22, further comprising the step of: i) disposing the other of said high and low side cavities in fluid communication with the movable wall portion of the one of said cavities, so that during operation of the sensor to detect the pressure difference between the first and second pressure sources, the one of said high and low side cavities functions as both a normal process volume for the one side and as an overpressure process volume for the other cavity. |
| 24. | An overpressure protection system for use with a differential pressure sensor having a high side and a low side, said system comprising: a high side volume adjustable cavity having at least one high side movable wall portion engagable with a first pressure source; a low side volume adjustable cavity having at least one low side movable wall portion engagable with a second pressure source; said high side cavity being in fluid communication with said high side of the sensor and with said at least one low side movable wall portion; said low side cavity being in fluid communication with said low side of the sensor and with said at least one high side movable wall portion; wherein during operation of the sensor to detect the pressure difference between the first and second pressure sources, said high side cavity functions as both a high side normal process volume and as a low side overpressure process volume, and said low side cavity functions as both a low side normal process volume and as a high side overpressure process volume. |
Often these differential pressure sensor devices are designed to measure pressure differences as small as 1/100 of a psi or less and often they are used in high pressure fluid systems. Fluid pressures may reach as high as 3,500 psi or more.
Optimal accuracy of the device is obtained by designing the device to function in specific differential and absolute pressure range conditions.
However the device may become damaged if it is used outside these normal pressure ranges.
Thus, overpressure protection for differential pressure sensors is needed to prevent differential pressure sensors from being exposed to high differential conditions.
Previous overpressure protection systems for conventional differential pressure sensor devices have used large volumes of fill fluids in many different chambers, diaphragms and pathways and therefore are relatively large in size. One drawback associated with this arrangement is that these larger systems tend to be adversely affected by temperature changes to the internal fill fluid.
Changes to fill fluid volume, caused by temperature changes, can miscalibrate the device. This problem is called thermal error.
Another drawback associated with large overpressure protection systems is cost. Large systems require extensive raw materials to fabricate. In addition, larger complex systems with many pathways and chambers generally require complicated and expensive manufacturing methods.
If a sensor i. s exposed to high differential pressure, it may become permanently damaged.
Therefore, there is a need for overpressure protection systems that quickly eliminate, reduce, shunt or otherwise block these pressures. However, large overpressure systems have large volumes of fluid and therefore tend to have large fluid movement. Unfortunately, the larger the fluid movement, the slower the response to pressure changes.
Thus, a need exists for an overpressure system and method for use with a differential pressure sensor element, to enable accurate measurement of differential pressure using a minimal amount of internal fill fluids and chambers. There is also a need for this system to be lightweight, small and inexpensive to fabricate. A need also exists for an overpressure system capable of having a relatively quick response time to changing pressures.
SUMMARY The present invention is directed to an overpressure protection system and method for use
with a differential pressure sensor element, which satisfies a need for accurately measuring pressure using a minimal amount of fluid. The system also satisfies a need for a quick response time to pressure changes.
Accordingly, the invention provides in one aspect an overpressure protection system for use with a differential pressure sensor having a high side and a low side. The system includes a high side volume adjustable cavity with a high side movable wall portion engageable with a first pressure source.
The system also includes a low side volume adjustable cavity having a low side movable wall portion engageable with a second pressure source.
One of the cavities is also in fluid communication with one side of the sensor and with the moveable wall portion of the other of the cavities. The other of the cavities is also in fluid communication with an other side of the sensor. The other cavity functions as both a normal process volume for the other side of the sensor and as an overpressure process volume for the one cavity.
The present invention provides in another aspect, a method of measuring differential pressures while providing overpressure protection. This method includes the steps of providing a sensor having a high side and a low side, and providing a high side volume adjustable cavity having a high side movable wall portion. The high side movable wall portion is engaged with a first pressure source. A low side volume adjustable cavity having a low side movable wall portion is also provided.
The low side movable wall portion is then engaged with a second pressure source. One of the high and low side cavities is disposed in fluid communication with one side of the sensor and with the movable wall portion of the other cavity. The other cavity is disposed in fluid communication with the other side of the sensor. The sensor is then used to detect the pressure difference between the first and second pressure sources, so that the other cavity functions as both a normal process volume for the other side, and as an overpressure process volume for the one cavity.
In another aspect, the invention includes an overpressure protection system for use with a differential pressure sensor having a high side and a low side. The system includes a high side volume adjustable cavity and has a high side movable wall portion engagable with a first pressure source. The high side cavity is also in fluid communication with the high side of the sensor and with a low side movable wall portion. The high side cavity functions as both a high side normal process volume and as a low side overpressure process volume.
The system also includes a low side volume adjustable cavity having a low side movable wall portion engagable with a second pressure source.
The low side cavity is also in fluid communication with the low side of the sensor and with at least a high side moveable wall portion. The low side cavity functions as both a low side normal process volume and as a high side overpressure process volume.
BRIEF DESCRIPTION OF THE DRAWINGS The above and other features and advantages of this invention will be more readily apparent from a reading of the following detailed description of various aspects of the invention taken in conjunction with the accompanying drawing in which: Fig. 1 is a side sectional view of an overpressure protection system of the present invention, coupled with a fluid-flow conduit and including a differential pressure sensor.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the figure set forth in the accompanying Drawing, the illustrative embodiments of the present invention will be described in detail hereinbelow.
The present invention includes an overpressure system 10 for use with a differential pressure sensor 60, to protect the sensor while enabling accurate measurement of differential pressure using a minimal amount of internal fill fluids and
chambers. This system is lightweight, small and inexpensive to fabricate. The system 10 is also quick to respond to pressure changes.
Referring now to Fig. 1, the system 10 of the present invention will be more thoroughly described.
A body 12 has a disk like shape and also has a high side 18 and a low side 20. In one embodiment, both sides of the body are machined to have a concave shape. In this embodiment the body 12 is preferably formed of 316 or 400 stainless steel, or alternatively from other materials such as aluminum or non-metals.
The overpressure protection system 10 also includes two separate fluid filled chambers, a high side chamber 14 and a low side chamber 16. The high side chamber 14 includes a high side overrange diaphragm 22, which is attached to the high side of the body 18 with welds 24. The low side chamber 16 also includes a low side overrange diaphragm 28, which is attached to the low side of the body 20 with welds 24. Other fastening devices, such as bolts or screws may also be used to attach the overrange diaphragm to the body 12. In a preferred
embodiment the diaphragms and the body 12 are made of the same material so they will have the same coefficients of thermal expansion. This will allow the diaphragms and body to expand and contract at nominally the same rate during changing ambient temperature environments. This will also reduce excessive stresses in the device and associated metal fatigue and cracking problems.
In a preferred embodiment, the diaphragms and body 12 are formed of 316 stainless steel, or alternatively from 400 stainless steel or other materials such as aluminum or non-metals.
Typically the overrange diaphragms 22 and 28 are circular in shape and designed to flex under specific pressures. In preferred embodiments, the overrange diaphragms 22 and 28 will range from. 001 to. 010 inches thick depending on the structural requirements of a particular application and may be approximately 0.75 inches or more in diameter.
The overrange diaphragms 22 and 28 may be formed using a die that has a deeper dish than the concave machined shaped sides of the body 12. This deeper dish allows the overrange diaphragms 22 and
28 to be preloaded or pretensioned when placed into surface-to-surface engagement with the shaped sides of the body 12. After the overrange diaphragms 22 and 28 are preloaded into their respective sides of the body 12, they are welded to the body 12.
The high side chamber 14 also includes a high side process diaphragm 30, which is attached to the high side overrange diaphragm 22 with diaphragm welds 26. The low side chamber 16 also includes a low side process diaphragm 32, which is attached to the low side overrange diaphragm 28 with diaphragm welds 26. Other fastening devices, such as bolts or screws may also be used to attach the process diaphragms 30 and 32 to their respective overrange diaphragms 22 and 28.
The process diaphragms 30 and 32 are usually the same size and shape as the overrange diaphragms 22 and 28. However, the process diaphragms 30 and 32 are more flexible than the overrange diaphragms 22 and 28. In preferred embodiments, the process diaphragms 30 and 32 may be approximately. 001 to . 010 inches thick depending on specific application, and may be approximately one inch or more in
diameter or to the same diameter of the overrange diaphragms 22 and 28.
The high side chamber 14 also includes a high side diaphragm cavity 34. The high side process diaphragm 30 and high side overrange diaphragm 22 are joined near their edges to form the high side diaphragm cavity 34. The low side chamber 16 also includes a low side diaphragm cavity 36. The low side process diaphragm 32 and low side overrange diaphragm 28 are joined near their edges to form the low side diaphragm cavity 36.
The high side chamber 14 also includes high side pathways 38 that are interconnected. The high side pathways 38 have three ends. The high side first end 40 is in fluid communication with the high side diaphragm cavity 34 through an opening in the high side overrange diaphragm. The high side second end 42 is connected to the high side of the sensor 44 and sensor housing 46. The high side third end 48 is connected to the body side of the low side overrange diaphragm 50.
The high side chamber 14 is a closed system and is filled with a high side fill fluid. The low side
chamber 16 is a separate closed system and is filled with a low side fill fluid. Typically, these fill fluids are either silicone oil such as available from Dow Corning, or other inert fluids such as FluorinertT^, FC-43 available from 3M Company of St.
Paul, Minnesota.
The low side chamber 16 also includes low side pathways 52 that are interconnected with one another. The low side pathways 52 have three ends.
The low side first end 54 is in fluid communication with the low side diaphragm cavity 36 through an opening in the low side overrange diaphragm. The low side second end 56 is connected to the low side of the sensor 58 and sensor housing 46. The low side third end 61 is connected to the body side of the high side overrange diaphragm 62.
A sensor 60, having a high side of the sensor 44 and a low side of the sensor 58, is located in sensor housing 46. The sensor 60 may be a piezo- resistive sensor 60 or similar type device. In a preferred embodiment, the sensor 60 is made of silicon or polysilicon and can therefore be quite small and highly sensitive. The sensor housing 46
can be made of metal, plastic or other suitable materials.
Also shown in Fig. 1 is a restriction 63 inside pipe 64. This restriction 63 may be an orifice plate as shown, or a filter, pressure reducer, valve or other device. Upstream of the restriction (on the high side) is a first opening 66 in the pipe 64 that is coupled to the high side of the body 18 by a high side process cover 67. The pipe process fluid applies pressure (high side process pressure 68) to the high side process diaphragm 30 through the first opening 66 and within this high side process cover 67.
The process fluid pressure in the pipe 70 is lowered by going through the restriction 63.
Downstream of the restriction 63 (on the low side) is a second opening 72 in the pipe 64 that is coupled to the low side of the body 20 by a low side process cover 74. This lower pressure process fluid 70, downstream of the restriction 63, applies pressure (low side process pressure 76) to the low side process diaphragm 32 through this second opening 72 and within this low side process cover
74. The process covers are typically steel but may also be plastic or any other durable material. The process covers 67 and 74 can be welded, bolted or mechanically fastened to the body 12.
In an alternate embodiment, the system 10 may have process pressure applied to only one side of the system 10. In this scenario, one side has the same system of two diaphragms, pathways, and discrete fluid filled pathways connecting to a sensor, as described hereinabove. The opposite side of the system 10 may simply include an overrange diaphragm for use in overpressure situations.
A preferred embodiment of the invention having been described, the following is a description of the operation thereof.
The system 10 has a high side fill fluid volume and a separate low side fill fluid volume located within high side and low side diaphragm cavities 34 and 36, respectively.
Each of these cavities 34 and 36 (including the fill fluids disposed therein) functions as a normal process volume during the normal operating mode of the sensor 60 and overpressure protection system 10.
These same cavities also function as overpressure process volumes during the overpressure operating mode of the overpressure protection system 10.
The system 10, including sensor 60, is designed to obtain its most accurate pressure readings in normal operating mode. This normal operating mode has a specific differential pressure normal operating range and a specific absolute pressure normal operating range. During normal operating mode, the overpressure protection aspect of the system generally does not operate, and is not needed, because at normal operating pressures the sensor 60 can easily withstand all the pressures.
Rather, during normal operating mode, the fill fluid of system 10 functions as a normal process volume and transmits each respective side's process pressures 68 and 76 to the sensor 60.
The system 10 shifts to its overpressure operating mode in the event the differential pressure rises above the normal operating range.
During overpressure operating mode the overpressure protection system functions to protect the sensor 60 from these damaging excessive pressures.
The fill fluids are an important part of the overpressure protection system. During overpressure operating mode the fill fluid in the cavity on one side functions as an overpressure process volume for the opposite side to help lower the differential pressure back into normal operating range.
Turning to specific operating examples, pipe 64 is filled with process fluid that flows through a pipe restriction 63. The process fluid upstream of the restriction 63 (high side) has a higher pressure than the process fluid downstream (low side) of the restriction 63.
During normal operating mode, the process fluid in the pipe 70 enters inside the high side process cover 67 and applies pressure against the high side process diaphragm 30 which is referred to as high side process pressure 68. The high side process diaphragm 30 flexes which increases the high side fill fluid pressure and this pressure is applied in three important areas. First, the high side fill fluid applies pressure against the high side overrange diaphragm 22 which will not move because it is supported by body 12. Secondly, the high side
fill fluid applies pressure against the high side of the sensor 60. Thirdly, the high side fill fluid applies pressure against the body side of the low side overrange process diaphragm 50.
During normal operating mode the high side fill fluid pressure is less than its opposing force, at the back side of the low side overrange diaphragm 50, comprised of the low side diaphragm preload and low side fill fluid pressure. Therefore the low side overrange diaphragm 28 will not substantially flex.
Also, during normal operating mode, the lower pressure low side process fluid enters inside the low side process cover 74 and applies pressure against the low side process diaphragm 32, which is called the low side process pressure 76. The low side process diaphragm 32 flexes which increases the low side fill fluid pressure and this pressure is applied in three important areas. First, the low side fill fluid applies pressure against the low side overrange diaphragm 28 which will not move because it is supported by rigid body 12. Secondly, the low side fill fluid applies pressure against the
low side of the sensor 60. Thirdly, the low side fill fluid applies pressure against the body side of the high side overrange process diaphragm 62.
During normal operating mode the low side fill fluid pressure is less than its opposing force, comprised of the high side overrange diaphragm preload and high side fill fluid pressure. Therefore the high side overrange diaphragm 22 will not substantially flex.
Finally, the sensor 60 will output the differential pressure.
During overpressure operating mode, the overpressure protection system operates to protect the sensor 60 from excessive differential pressures, which are above the normal operating range.
Hereinbelow are described overpressure protection steps that will actually protect the sensor 60. In this instance, we will assume the high side process pressure 68 is very high relative to the low side process pressure 76 (i. e., due to the low side being vented to atmospheric pressure while pressure 68 remains on the high side) that is causing the very high differential pressure.
The process diaphragms 30 and 32 are designed to increasing flex under increasing amounts of process pressures 68 and 76.
In this instance, excessive pressure on the high side forces some or all the high side fill fluid from inside the high side diaphragm cavity 34 out into the highside pathways 38. At this point the high side fill fluid applies pressure to the low side diaphragm cavity 36 so the fill fluid therein no longer acts as a normal process volume, but rather performs its second function as a overrange process volume for the high side. The high side fill fluid pressure overcomes the low side overrange diaphragm's preload and the opposing low side process pressure 76. As discussed earlier, in this instance, the restriction 63 ensures that the low side process pressure 68 is lower than the high pressure 76 side pressure. Therefore, the low side fill fluid pressure is lower than the high side fill fluid pressure and the low side overrange diaphragm 28 will be flexed away from the low side of the body 20. The low side overrange diaphragm 28 will balloon out away from the low side of the body 20 to
apply pressure to the low side cavity 36 and in turn, balloon the low side process diaphragm 32. In this manner, the low side diaphragm cavity, which serves as the low side 58 normal process volume, also serves as the overpressure process volume for the high side 44.
Many systems need two or more large internal cavities to complete what the present invention accomplishes with this single cavity 36. The low side overrange diaphragm will continue to balloon outwards until the high side process diaphragm 30 is pressed against the high side overrange diaphragm 22. At this point there is no fill fluid left in the high side cavity 34 to transmit higher pressures 68 to the connection 42, thus protecting the sensor 60. The system 10 of the present invention operates similarly but in reverse, in the event an excessively, relatively high pressure is experienced on the low side thereof.
Damage to the sensor 60 thus may be prevented because the differential pressure has been kept safely below the sensor's maximum pressure limits.
One advantage of this overpressure protection system is that all four diaphragms remain relatively motionless as differential pressures change within the normal operating range. This minimizes fluid movement while in this normal operating range, in turn minimizing errors due to changes in differential pressure, and provides relatively quick response to pressure changes.
Another advantage of this system 10 is that it has very low thermal error and therefore the sensor 60 is relatively accurate. Thermal error tends to occur in all fluid systems that encounter temperature variations.
For example, this overpressure protection system 10 is calibrated at a predetermined temperature and at this temperature there is a predetermined volume of fill fluid in the closed fluid chamber. However, this system 10 will see many different ambient and pipe fluid temperatures.
These changes in temperature of the fluid in the pipe and ambient air temperature may then in turn change the temperature of the fill fluid in the sensor 60 and cause it to expand or contract.
Changing the volume of fluid in the pressure sensor 60 may affect the calibration of the system 10.
This is referred to as thermal error.
System 10 minimizes thermal error because it uses less fluid than most systems by virtue of its dual function cavities 34 and 36 and therefore may use smaller diaphragms and provide a smaller system overall. Because it is smaller, the internal fill fluid volume will change less with changes in temperature. This, in turn, will cause the process diaphragms to move less with changes in temperature, minimizing their contribution to thermal error. The internal fill fluid volume will also change less with changes in absolute pressure. This, in turn, minimizes changes in the process diaphragm positions caused by changes in absolute pressure, thus minimizing static pressure errors. Thus, the smaller system of the present invention may have less change in fluid volume and less error, than a system with more fluid volume. Moreover, the effects of any remaining errors due to the internal fill fluid and process diaphragms will tend to cancel due to the symmetry between the high side and
low side systems. Any residual thermal errors that do not cancel may be easily compensated using electronic techniques and/or algorithms well known by those skilled in the art.
The present invention uses only two chambers and minimal pathways. This makes it advantageously small in size and lightweight. This system 10 also uses less materials and labor for manufacturing because of its simple design and therefore tends to be inexpensive to produce.
The present invention requires less material and labor to manufacture and thus tends to be less expensive to produce than larger systems.
The foregoing description is intended primarily for purposes of illustration. Although the invention has been shown and described with respect to an exemplary embodiment thereof, it should be understood by those skilled in the art that the foregoing and various other changes, omissions, and additions in the form and detail thereof may be made therein without departing from the spirit and scope of the invention.
Having thus described the invention, what is claimed is: