US20030201842A1 - Coaxial probe for high temperature and high pressure applications - Google Patents
Coaxial probe for high temperature and high pressure applications Download PDFInfo
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- US20030201842A1 US20030201842A1 US10/134,885 US13488502A US2003201842A1 US 20030201842 A1 US20030201842 A1 US 20030201842A1 US 13488502 A US13488502 A US 13488502A US 2003201842 A1 US2003201842 A1 US 2003201842A1
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- Prior art keywords
- seal element
- probe
- outer sleeve
- cylindrical
- center conductor
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Classifications
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S13/00—Systems using the reflection or reradiation of radio waves, e.g. radar systems; Analogous systems using reflection or reradiation of waves whose nature or wavelength is irrelevant or unspecified
- G01S13/88—Radar or analogous systems specially adapted for specific applications
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01F—MEASURING VOLUME, VOLUME FLOW, MASS FLOW OR LIQUID LEVEL; METERING BY VOLUME
- G01F23/00—Indicating or measuring liquid level or level of fluent solid material, e.g. indicating in terms of volume or indicating by means of an alarm
- G01F23/22—Indicating or measuring liquid level or level of fluent solid material, e.g. indicating in terms of volume or indicating by means of an alarm by measuring physical variables, other than linear dimensions, pressure or weight, dependent on the level to be measured, e.g. by difference of heat transfer of steam or water
- G01F23/26—Indicating or measuring liquid level or level of fluent solid material, e.g. indicating in terms of volume or indicating by means of an alarm by measuring physical variables, other than linear dimensions, pressure or weight, dependent on the level to be measured, e.g. by difference of heat transfer of steam or water by measuring variations of capacity or inductance of capacitors or inductors arising from the presence of liquid or fluent solid material in the electric or electromagnetic fields
- G01F23/263—Indicating or measuring liquid level or level of fluent solid material, e.g. indicating in terms of volume or indicating by means of an alarm by measuring physical variables, other than linear dimensions, pressure or weight, dependent on the level to be measured, e.g. by difference of heat transfer of steam or water by measuring variations of capacity or inductance of capacitors or inductors arising from the presence of liquid or fluent solid material in the electric or electromagnetic fields by measuring variations in capacitance of capacitors
- G01F23/268—Indicating or measuring liquid level or level of fluent solid material, e.g. indicating in terms of volume or indicating by means of an alarm by measuring physical variables, other than linear dimensions, pressure or weight, dependent on the level to be measured, e.g. by difference of heat transfer of steam or water by measuring variations of capacity or inductance of capacitors or inductors arising from the presence of liquid or fluent solid material in the electric or electromagnetic fields by measuring variations in capacitance of capacitors mounting arrangements of probes
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01F—MEASURING VOLUME, VOLUME FLOW, MASS FLOW OR LIQUID LEVEL; METERING BY VOLUME
- G01F23/00—Indicating or measuring liquid level or level of fluent solid material, e.g. indicating in terms of volume or indicating by means of an alarm
- G01F23/22—Indicating or measuring liquid level or level of fluent solid material, e.g. indicating in terms of volume or indicating by means of an alarm by measuring physical variables, other than linear dimensions, pressure or weight, dependent on the level to be measured, e.g. by difference of heat transfer of steam or water
- G01F23/28—Indicating or measuring liquid level or level of fluent solid material, e.g. indicating in terms of volume or indicating by means of an alarm by measuring physical variables, other than linear dimensions, pressure or weight, dependent on the level to be measured, e.g. by difference of heat transfer of steam or water by measuring the variations of parameters of electromagnetic or acoustic waves applied directly to the liquid or fluent solid material
- G01F23/284—Electromagnetic waves
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
- G01S7/02—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S13/00
- G01S7/03—Details of HF subsystems specially adapted therefor, e.g. common to transmitter and receiver
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01Q—ANTENNAS, i.e. RADIO AERIALS
- H01Q13/00—Waveguide horns or mouths; Slot antennas; Leaky-waveguide antennas; Equivalent structures causing radiation along the transmission path of a guided wave
- H01Q13/08—Radiating ends of two-conductor microwave transmission lines, e.g. of coaxial lines, of microstrip lines
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01Q—ANTENNAS, i.e. RADIO AERIALS
- H01Q9/00—Electrically-short antennas having dimensions not more than twice the operating wavelength and consisting of conductive active radiating elements
- H01Q9/005—Electrically-short antennas having dimensions not more than twice the operating wavelength and consisting of conductive active radiating elements for radiating non-sinusoidal waves
Definitions
- This invention relates to guided wave radar measurement instruments and, more particularly, to a probe for a guided wave radar measurement instrument.
- MIR micropower impulse radar
- UWB ultra-wideband
- ETS equivalent time sampling
- a very fast pulse with a rise time of 500 picoseconds, or less is propagated down a probe, that serves as a transmission line, in a vessel.
- the pulse is reflected by a discontinuity caused by a transition between two media.
- that transition is typically where the air and the material to be measured meet.
- GWR guided wave radar
- Plastic materials such as PTFE, are often used as process seals for lower temperature and lower pressure probes.
- Plastic has a low dielectric constant. This permits the design to achieve the needed electrical impedance within dimensional constraints imposed by conventional 3 ⁇ 4′′ inch NPT process connections while maintaining adequate structural integrity. Sealing of the plastic process seal is usually performed by o-rings. However, plastic material and o-rings are not capable of withstanding the high temperatures encountered in steam service, and the like.
- Ceramic materials offer high temperature capability and resistance to temperature and steam. However, sealing ceramics to metals is difficult and the ceramic materials are susceptible to cracking under thermal shock. Typically, ceramic seals are brazed to the metal. However, the coefficients of thermal expansion for ceramics are much less than for high temperature and pressure steels. The large differences in the amount that the ceramic and steel expand and contract between the braising, steam service and room temperature results in large stresses. These stresses tend to damage the ceramic and/or the braised joint. Even a pin hole leak in the ceramic or braising will cause the probe to fail. Compensating for the thermal expansion differences results in complex and costly seal designs.
- the present invention is directed to overcoming one or more of the problems discussed above, in a novel and simple manner.
- a probe is reliably sealed without susceptibility to thermal shock while providing transparency for electrical signals.
- a probe defines a transmission line for use with a measurement instrument including a pulse circuit connected to the probe for generating pulses on the transmission line and receiving reflected pulses on the transmission line.
- the probe includes a center conductor for conducting the pulses.
- a conductive outer sleeve is coaxial with the center conductor.
- the conductive outer sleeve has a process end and a connector end. The process end is exposed, in use, to a process environment being measured.
- a first cylindrical seal element between the center conductor and the outer sleeve is at the sleeve process end.
- the first cylindrical seal element is of a first material adapted to withstand a relatively high temperature.
- a second cylindrical seal element is between the center conductor and the outer sleeve disposed between the first cylindrical seal element and the connector end.
- the second cylindrical seal element is of a second material adapted to withstand a lower temperature than the first cylindrical seal element.
- FIG. 1 is an elevation view of a measurement instrument including a probe in accordance with the invention
- FIG. 2 is a sectional view of the probe of FIG. 1 mounted to a process vessel
- FIG. 3 is a generalized block diagram illustrating operation of the measurement instrument of FIG. 1.
- a level measuring process instrument 10 uses micropower impulse radar (MIR) in conjunction with equivalent time sampling (ETS) and ultra-wide band (UWB) transceivers for measuring level.
- MIR micropower impulse radar
- ETS equivalent time sampling
- UWB ultra-wide band
- the instrument 20 uses guided wave radar for sensing level. While the embodiments described herein relates to a MIR level sensing apparatus, various aspects of the invention may be used with other types of process instruments for measuring various process parameters.
- the process instrument 10 includes a control housing 12 , a probe 14 and a connector 16 for connecting the probe 14 to the control housing 12 .
- the probe 14 is typically mounted to a process vessel 18 , see FIG. 2, using a threaded fitting 20 .
- the housing 12 is then secured to the probe 14 as by threading the connector 16 to the probe 14 and to the housing 12 .
- the housing 12 and connector 16 may be as generally described in Mulrooney, et al., U.S. Pat. No. 6,062,095, issued May 16, 2000, the specification of which is hereby incorporated by reference herein.
- a probe comprises a high frequency transmission line which, when placed in a fluid, can be used to measure level of the fluid.
- the probe is controlled by electronics in the housing 12 for determining level in the vessel 18 .
- the electronics mounted in the housing 12 may be, for example, as described in Carsella, et al., U.S. application Ser. No. 09/336,194, filed Jun. 18, 1999, the specification of which is hereby incorporated by reference herein, or as described in Benway, et al., U.S. application Ser. No. 09/878,895, filed Jun. 11, 2001, the specification of which is also incorporated by reference herein.
- electronics in the control housing 12 includes a pulse circuit 22 , see FIG. 3, connected to the probe 14 .
- the pulse circuit 22 generates pulses, represented by an arrow 24 , on the transmission line and receives reflected pulses, represented by an arrow 26 , from the transmission line.
- the reflected pulses represent impedance changes on the transmission line.
- the probe 14 is illustrated in greater detail.
- the probe 14 includes a seal assembly, as described, which reliably seals the probe 14 without susceptibility to thermal shock while providing transparency for electrical signals.
- the probe 14 includes the adapter fitting 20 having a nut 30 at an upper end and an NPT thread 32 at a lower end for securing the probe 14 to the process vessel 18 .
- the adapter fitting 20 includes a cylindrical through opening 36 and a counter bore 38 at the upper end defining a shoulder 40 .
- the adapter fitting 20 is constructed of metal.
- a metal, tubular outer sleeve 42 has a process end 44 and a connector end 46 .
- the outer sleeve process end 40 is connected to the adapter fitting 20 .
- the outer sleeve 42 is received in the counter bore 38 abutting the shoulder 40 and is secured thereto as by a weld joint 48 .
- the inner wall of the outer sleeve 42 includes threads 56 at the connector end 46 to receive an electrical connector 48 .
- the electrical connector 48 includes a female connector pin 50 .
- the electrical connector 48 defines a quick connect using the pin 50 for connection to the pulse circuit 22 , see FIG. 3.
- An insulating bushing 52 centers the center pin 50 and is held in place with a lock nut 54 threaded into the outer sleeve inner threads 56 .
- the outer sleeve 42 also includes outer threads 58 at the connector end 46 for threading the probe 14 to the connector 16 , see FIG. 1.
- a metal center shaft 60 defining a center conductor, is received in the female connector pin 50 and is secured thereto in a conventional manner.
- the center conductor 60 is coaxial with the outer sleeve 42 and also the adapter fitting 20 .
- the center conductor 60 is of a length so that it extends downwardly through the adapter fitting 20 .
- the probe 14 further comprises an extender shaft 62 connected to the center conductor 60 and extending downwardly into the vessel 18 .
- An extender outer sleeve 64 is coaxial with the extender shaft 62 and is connected to a lower end of the adapter fitting 20 .
- the length of the extender shaft 62 and extender outer sleeve 64 depend on the particular application.
- the pulse circuit 22 is connected to the probe 14 via the electrical connector 48 .
- the pulse circuit 22 generates pulses on the transmission line, defined by the center conductor 60 and extender shaft 62 , and also receives reflected pulses on the transmission line.
- the outer sleeve 42 , adapter fitting 20 , and extender outer sleeve 64 act as a ground.
- the probe 14 is provided with an improved seal assembly adapted to be impervious to process fluid and to provide a temperature drop to ensure that all seal elements are capable of withstanding temperatures they are exposed to.
- the seal assembly comprises a ceramic seal element 70 , first and second plastic seal elements 72 and 74 , respectively, a set of outer o-rings 76 and a set of inner o-rings 78 .
- the seal elements 70 , 72 and 74 are generally cylindrical having an outer diameter slightly less than an inner diameter of the outer sleeve 42 to be received therein.
- the seal elements 70 , 72 and 74 also have inner diameters slightly greater than an outer diameter of the center conductor 60 for receiving the same.
- the ceramic seal element 70 is positioned at the outer sleeve process end 44 resting on the adapter fitting shoulder 40 .
- the first plastic seal element 72 is abutting the ceramic seal element 70 .
- the second plastic seal element 74 is abutting the first plastic seal element 72 having its upper end 76 proximate the outer sleeve connector end 46 .
- the seal elements 70 , 72 and 74 are loosely received in the outer sleeve 42 and are captured between the adapter fitting 20 and the electrical connector 48 .
- the outer o-rings 76 are between the second plastic seal element 74 and the outer sleeve 42 .
- the inner o-rings 78 are between the center conductor 60 and the second plastic seal element 74 .
- the o-rings 76 and 78 comprise elastomeric o-rings.
- the sets of o-rings 76 and 78 are positioned longitudinally above a center position of the second plastic seal element 74 .
- seal elements 70 , 72 and 74 , the center conductor 60 and outer sleeve 42 form a coaxial transmission line with an integral high temperature, high pressure seal.
- the seal elements 70 , 72 and 74 are designed to be impervious to process fluids and to provide a temperature drop to ensure that the individual seal elements 70 , 72 and 74 are capable of withstanding the temperatures they are exposed to.
- the ceramic seal element 70 being directly exposed to the process material, is adapted to withstand a relatively high temperature compared to the plastic seal elements 72 and 74 .
- the center conductor 60 carries and guides the probe pulse in a conventional manner.
- the outer sleeve 42 provides the process connection, pressure boundary and ground path for the electrical signal.
- the ceramic seal element 70 and plastic seal elements 72 and 74 are placed end to end between the center conductor 60 and outer sleeve 42 and perform several functions. Particularly, they serve to position the center conductor 60 along the center line axis of the outer sleeve 42 , create a fiducial signal, control impedance and provide a thermal barrier between the process fluid and the o-ring seals 76 and 78 .
- the ceramic seal element 70 is positioned at the process interface at the outer sleeve process end 44 .
- the ceramic seal element 70 provides high temperature and steam resistance.
- the ceramic material blocks the steam from freely traveling up the length of the probe. Heat is lost to the ambient atmosphere through convection and radiation from the outer sleeve 42 , resulting in a temperature drop going up the probe.
- the length of the ceramic seal element 70 provides a sufficient temperature drop so that the temperature at the interface with the first plastic seal element 72 is below the maximum of service temperature of the plastic material.
- the ceramic seal element 70 is loose, but captured within the outer sleeve 42 , as described, the ceramic material is subjected to only pressure and thermal shock stresses. The ceramic material is unaffected by the pressures encountered in steam service. Since no seal is made between the metal parts and the ceramic, if the ceramic material cracks due to thermal stress, then the seal integrity is not compromised. The electrical performance is also unaffected by cracks.
- the plastic seal elements 72 and 74 act similarly to the ceramic seal element 70 . They are of sufficient length to result in a temperature drop such that the temperature at the o-rings 76 and 78 is within the allowable rating for the o-ring elastomeric material. Plastic materials are inherently resistant to cracking from thermal shock, so that the overall design is highly resistance to rapid changes in temperature as might be encountered if the probe 14 is installed into a hot chamber.
- the geometry and dielectric constant of the plastic seal elements 72 and 74 creates a drop in impendence with respect to the nominal impedance of the probe and coaxial cable system.
- the impendence drop at the top of the probe 14 produces a fiducial signal from which level measurements are referenced. Since the electrical signal travels slower through the plastic seal elements 72 and 74 , and the ceramic seal element 70 , due to the higher dielectric constant of those materials, the fiducial signal is separated in time from the level pulse in all cases, permitting flooded cage operation. This design also reduces the complexity of the center conductor 60 since no undercut of the shaft is required to produce the fiducial signal.
- composition of the plastic seal elements 72 and 74 and the ceramic seal element 70 may be varied to account for different process conditions such as temperature, pressure and process fluid.
- the length of the ceramic seal element 70 could be increased relative to the plastic seal element 72 and 74 to allow operation at a higher temperature.
- the plastic seal element 72 and 74 could also be replaced entirely to permit usage of the seal assembly at temperatures or pressures too high for plastic elements.
- the probe 14 includes a ceramic seal element 70 that is approximately one half inch long.
- the first plastic seal element 72 is two inches long and the second plastic seal element 74 is 2.5 inches long.
- This configuration provides temperature capability to approximately 600° F.
- all three seal elements 70 , 72 and 74 could be made from high temperature plastic with the same overall length as just described.
- the overall length of the seal elements can be increased by adding additional ceramic elements 70 of one half inch length. Each additional one half inch length ceramic seal element provides approximately a 50° F. increase in maximum allowable temperature.
- the seal elements 72 and 74 may comprise ceramic seal elements.
- Alumina ceramic for example, can be machined to a sufficiently low surface finish to permit sealing by the O rings 76 and 78 .
- the electrical characteristics of alumina ceramic are such that the fiducial signal would be generated in the same manner as with the use of plastic seal elements.
- Ceramic seal elements have higher allowable compressive stress levels and service temperatures compared to plastics. Hence, an all ceramic design is capable of increased temperature and pressure limits.
- the illustrative example described above uses the minimum amount of ceramic necessary for temperature compatability while minimizing manufacturing costs as ceramic is more expensive than an equivalent amount of plastic.
- a probe having an improved seal assembly that is not susceptible to thermal shock while providing transparency for electrical signals.
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Abstract
Description
- This invention relates to guided wave radar measurement instruments and, more particularly, to a probe for a guided wave radar measurement instrument.
- Knowledge of level in industrial process tanks or vessels has long been required for safe and cost-effective operation of plants. Many technologies exist for making level measurements. These include buoyancy, capacitance, ultrasonic and microwave radar, to name a few. Recent advances in micropower impulse radar (MIR), also known as ultra-wideband (UWB) radar, in conjunction with advances in equivalent time sampling (ETS), permit development of low power and lost cost time domain reflectometry (TDR) instruments.
- In a TDR instrument, a very fast pulse with a rise time of 500 picoseconds, or less, is propagated down a probe, that serves as a transmission line, in a vessel. The pulse is reflected by a discontinuity caused by a transition between two media. For level measurement, that transition is typically where the air and the material to be measured meet. These instruments are also known as guided wave radar (GWR) measurement instruments.
- The design of a GWR liquid level measurement probe for steam and other high temperature/pressure applications requires a seal assembly that satisfies both mechanical and electrical requirements. Mechanically, the seal must simultaneously be resistant to the corrosive effect of steam, withstand process temperatures of 600° F. and above, pressures in excess of 2000 psi, and carry the tensile and bending loads induced by a center conductor extending into the process vessel. Electrically, the seal assembly must be electrically transparent to the radar signal to allow proper functioning of the system. In addition, the mechanical design affects the electrical characteristics so that the two are interrelated.
- Plastic materials, such as PTFE, are often used as process seals for lower temperature and lower pressure probes. Plastic has a low dielectric constant. This permits the design to achieve the needed electrical impedance within dimensional constraints imposed by conventional ¾″ inch NPT process connections while maintaining adequate structural integrity. Sealing of the plastic process seal is usually performed by o-rings. However, plastic material and o-rings are not capable of withstanding the high temperatures encountered in steam service, and the like.
- Ceramic materials offer high temperature capability and resistance to temperature and steam. However, sealing ceramics to metals is difficult and the ceramic materials are susceptible to cracking under thermal shock. Typically, ceramic seals are brazed to the metal. However, the coefficients of thermal expansion for ceramics are much less than for high temperature and pressure steels. The large differences in the amount that the ceramic and steel expand and contract between the braising, steam service and room temperature results in large stresses. These stresses tend to damage the ceramic and/or the braised joint. Even a pin hole leak in the ceramic or braising will cause the probe to fail. Compensating for the thermal expansion differences results in complex and costly seal designs.
- The present invention is directed to overcoming one or more of the problems discussed above, in a novel and simple manner.
- In accordance with the invention, a probe is reliably sealed without susceptibility to thermal shock while providing transparency for electrical signals.
- Broadly, in accordance with one aspect of the invention, a probe defines a transmission line for use with a measurement instrument including a pulse circuit connected to the probe for generating pulses on the transmission line and receiving reflected pulses on the transmission line. The probe includes a center conductor for conducting the pulses. A conductive outer sleeve is coaxial with the center conductor. The conductive outer sleeve has a process end and a connector end. The process end is exposed, in use, to a process environment being measured. A first cylindrical seal element between the center conductor and the outer sleeve is at the sleeve process end. The first cylindrical seal element is of a first material adapted to withstand a relatively high temperature. A second cylindrical seal element is between the center conductor and the outer sleeve disposed between the first cylindrical seal element and the connector end. The second cylindrical seal element is of a second material adapted to withstand a lower temperature than the first cylindrical seal element.
- Further features and advantages of the invention will be readily apparent from the specification and from the drawings.
- FIG. 1 is an elevation view of a measurement instrument including a probe in accordance with the invention;
- FIG. 2 is a sectional view of the probe of FIG. 1 mounted to a process vessel; and
- FIG. 3 is a generalized block diagram illustrating operation of the measurement instrument of FIG. 1.
- Referring to FIG. 1, a level
measuring process instrument 10 according to the invention as illustrated. Theprocess instrument 10 uses micropower impulse radar (MIR) in conjunction with equivalent time sampling (ETS) and ultra-wide band (UWB) transceivers for measuring level. Particularly, theinstrument 20 uses guided wave radar for sensing level. While the embodiments described herein relates to a MIR level sensing apparatus, various aspects of the invention may be used with other types of process instruments for measuring various process parameters. - The
process instrument 10 includes a control housing 12, aprobe 14 and a connector 16 for connecting theprobe 14 to the control housing 12. Theprobe 14 is typically mounted to aprocess vessel 18, see FIG. 2, using a threadedfitting 20. The housing 12 is then secured to theprobe 14 as by threading the connector 16 to theprobe 14 and to the housing 12. The housing 12 and connector 16 may be as generally described in Mulrooney, et al., U.S. Pat. No. 6,062,095, issued May 16, 2000, the specification of which is hereby incorporated by reference herein. As described therein, a probe comprises a high frequency transmission line which, when placed in a fluid, can be used to measure level of the fluid. Particularly, the probe is controlled by electronics in the housing 12 for determining level in thevessel 18. - The electronics mounted in the housing12 may be, for example, as described in Carsella, et al., U.S. application Ser. No. 09/336,194, filed Jun. 18, 1999, the specification of which is hereby incorporated by reference herein, or as described in Benway, et al., U.S. application Ser. No. 09/878,895, filed Jun. 11, 2001, the specification of which is also incorporated by reference herein. As described in each of the two referenced applications, electronics in the control housing 12 includes a
pulse circuit 22, see FIG. 3, connected to theprobe 14. Thepulse circuit 22 generates pulses, represented by anarrow 24, on the transmission line and receives reflected pulses, represented by anarrow 26, from the transmission line. The reflected pulses represent impedance changes on the transmission line. - Referring to FIG. 2, the
probe 14 is illustrated in greater detail. Theprobe 14 includes a seal assembly, as described, which reliably seals theprobe 14 without susceptibility to thermal shock while providing transparency for electrical signals. - The
probe 14 includes the adapter fitting 20 having anut 30 at an upper end and an NPT thread 32 at a lower end for securing theprobe 14 to theprocess vessel 18. Theadapter fitting 20 includes a cylindrical throughopening 36 and a counter bore 38 at the upper end defining ashoulder 40. Theadapter fitting 20 is constructed of metal. - A metal, tubular
outer sleeve 42 has aprocess end 44 and aconnector end 46. The outersleeve process end 40 is connected to theadapter fitting 20. Particularly, theouter sleeve 42 is received in the counter bore 38 abutting theshoulder 40 and is secured thereto as by a weld joint 48. The inner wall of theouter sleeve 42 includesthreads 56 at theconnector end 46 to receive anelectrical connector 48. - The
electrical connector 48 includes a female connector pin 50. Theelectrical connector 48 defines a quick connect using the pin 50 for connection to thepulse circuit 22, see FIG. 3. An insulatingbushing 52 centers the center pin 50 and is held in place with alock nut 54 threaded into the outer sleeveinner threads 56. Theouter sleeve 42 also includesouter threads 58 at theconnector end 46 for threading theprobe 14 to the connector 16, see FIG. 1. - A
metal center shaft 60, defining a center conductor, is received in the female connector pin 50 and is secured thereto in a conventional manner. As such, thecenter conductor 60 is coaxial with theouter sleeve 42 and also theadapter fitting 20. Thecenter conductor 60 is of a length so that it extends downwardly through theadapter fitting 20. - The
probe 14 further comprises anextender shaft 62 connected to thecenter conductor 60 and extending downwardly into thevessel 18. An extenderouter sleeve 64 is coaxial with theextender shaft 62 and is connected to a lower end of theadapter fitting 20. The length of theextender shaft 62 and extenderouter sleeve 64 depend on the particular application. - As described, the
pulse circuit 22 is connected to theprobe 14 via theelectrical connector 48. Thepulse circuit 22 generates pulses on the transmission line, defined by thecenter conductor 60 andextender shaft 62, and also receives reflected pulses on the transmission line. Theouter sleeve 42, adapter fitting 20, and extenderouter sleeve 64 act as a ground. - In accordance with the invention, the
probe 14 is provided with an improved seal assembly adapted to be impervious to process fluid and to provide a temperature drop to ensure that all seal elements are capable of withstanding temperatures they are exposed to. - Particularly, the seal assembly comprises a ceramic seal element70, first and second plastic seal elements 72 and 74, respectively, a set of outer o-rings 76 and a set of inner o-rings 78. The seal elements 70, 72 and 74 are generally cylindrical having an outer diameter slightly less than an inner diameter of the
outer sleeve 42 to be received therein. The seal elements 70, 72 and 74 also have inner diameters slightly greater than an outer diameter of thecenter conductor 60 for receiving the same. The ceramic seal element 70 is positioned at the outer sleeve process end 44 resting on theadapter fitting shoulder 40. The first plastic seal element 72 is abutting the ceramic seal element 70. The second plastic seal element 74 is abutting the first plastic seal element 72 having its upper end 76 proximate the outersleeve connector end 46. As described, the seal elements 70, 72 and 74 are loosely received in theouter sleeve 42 and are captured between the adapter fitting 20 and theelectrical connector 48. The outer o-rings 76 are between the second plastic seal element 74 and theouter sleeve 42. The inner o-rings 78 are between thecenter conductor 60 and the second plastic seal element 74. The o-rings 76 and 78 comprise elastomeric o-rings. The sets of o-rings 76 and 78 are positioned longitudinally above a center position of the second plastic seal element 74. - Together the seal elements70, 72 and 74, the
center conductor 60 andouter sleeve 42 form a coaxial transmission line with an integral high temperature, high pressure seal. The seal elements 70,72 and 74 are designed to be impervious to process fluids and to provide a temperature drop to ensure that the individual seal elements 70, 72 and 74 are capable of withstanding the temperatures they are exposed to. Particularly, the ceramic seal element 70, being directly exposed to the process material, is adapted to withstand a relatively high temperature compared to the plastic seal elements 72 and 74. - The
center conductor 60 carries and guides the probe pulse in a conventional manner. Theouter sleeve 42 provides the process connection, pressure boundary and ground path for the electrical signal. The ceramic seal element 70 and plastic seal elements 72 and 74 are placed end to end between thecenter conductor 60 andouter sleeve 42 and perform several functions. Particularly, they serve to position thecenter conductor 60 along the center line axis of theouter sleeve 42, create a fiducial signal, control impedance and provide a thermal barrier between the process fluid and the o-ring seals 76 and 78. - The ceramic seal element70 is positioned at the process interface at the outer
sleeve process end 44. The ceramic seal element 70 provides high temperature and steam resistance. The ceramic material blocks the steam from freely traveling up the length of the probe. Heat is lost to the ambient atmosphere through convection and radiation from theouter sleeve 42, resulting in a temperature drop going up the probe. The length of the ceramic seal element 70 provides a sufficient temperature drop so that the temperature at the interface with the first plastic seal element 72 is below the maximum of service temperature of the plastic material. - Because the ceramic seal element70 is loose, but captured within the
outer sleeve 42, as described, the ceramic material is subjected to only pressure and thermal shock stresses. The ceramic material is unaffected by the pressures encountered in steam service. Since no seal is made between the metal parts and the ceramic, if the ceramic material cracks due to thermal stress, then the seal integrity is not compromised. The electrical performance is also unaffected by cracks. - The plastic seal elements72 and 74 act similarly to the ceramic seal element 70. They are of sufficient length to result in a temperature drop such that the temperature at the o-rings 76 and 78 is within the allowable rating for the o-ring elastomeric material. Plastic materials are inherently resistant to cracking from thermal shock, so that the overall design is highly resistance to rapid changes in temperature as might be encountered if the
probe 14 is installed into a hot chamber. - Because sealing is accomplished by the o-rings76 and 78 at the upper end of the seal assembly, some process fluid can migrate into the spaces between the seal elements 70, 72 and 74, and the o-rings 76 and 78. Clearances between the
center conductor 60, seal element 70, 72 and 74, andouter sleeve 42 are kept minimal to reduce the amount of fluid permitted in the seal region. A small amount of fluid does not significantly affect the electrical performance of the seal assembly. Additionally, thermal expansion coefficient of the plastic is higher than that of the steelshaft center conductor 60 and theouter sleeve 42. - The geometry and dielectric constant of the plastic seal elements72 and 74 creates a drop in impendence with respect to the nominal impedance of the probe and coaxial cable system. The impendence drop at the top of the
probe 14 produces a fiducial signal from which level measurements are referenced. Since the electrical signal travels slower through the plastic seal elements 72 and 74, and the ceramic seal element 70, due to the higher dielectric constant of those materials, the fiducial signal is separated in time from the level pulse in all cases, permitting flooded cage operation. This design also reduces the complexity of thecenter conductor 60 since no undercut of the shaft is required to produce the fiducial signal. - The composition of the plastic seal elements72 and 74 and the ceramic seal element 70, as well as their dimensions, may be varied to account for different process conditions such as temperature, pressure and process fluid. For example, the length of the ceramic seal element 70 could be increased relative to the plastic seal element 72 and 74 to allow operation at a higher temperature. The plastic seal element 72 and 74 could also be replaced entirely to permit usage of the seal assembly at temperatures or pressures too high for plastic elements.
- In an illustrative example, the
probe 14 includes a ceramic seal element 70 that is approximately one half inch long. The first plastic seal element 72 is two inches long and the second plastic seal element 74 is 2.5 inches long. This configuration provides temperature capability to approximately 600° F. For temperature ranges up to 550° F., all three seal elements 70, 72 and 74 could be made from high temperature plastic with the same overall length as just described. To provide temperature capability above 600° F., the overall length of the seal elements can be increased by adding additional ceramic elements 70 of one half inch length. Each additional one half inch length ceramic seal element provides approximately a 50° F. increase in maximum allowable temperature. - For higher temperature and/or pressure applications, or for process fluids that are not compatible with plastic seal elements, the seal elements72 and 74 may comprise ceramic seal elements. Alumina ceramic, for example, can be machined to a sufficiently low surface finish to permit sealing by the O rings 76 and 78. The electrical characteristics of alumina ceramic are such that the fiducial signal would be generated in the same manner as with the use of plastic seal elements. Ceramic seal elements have higher allowable compressive stress levels and service temperatures compared to plastics. Hence, an all ceramic design is capable of increased temperature and pressure limits.
- The illustrative example described above uses the minimum amount of ceramic necessary for temperature compatability while minimizing manufacturing costs as ceramic is more expensive than an equivalent amount of plastic.
- Thus, in accordance with the invention, there is provided a probe having an improved seal assembly that is not susceptible to thermal shock while providing transparency for electrical signals.
Claims (23)
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US10/134,885 US6642807B1 (en) | 2002-04-29 | 2002-04-29 | Coaxial probe for high temperature and high pressure applications |
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US20030201842A1 true US20030201842A1 (en) | 2003-10-30 |
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