Patent Application
20060096862
Process analytic sensors are generally configured to couple to a given process, such as an oil refining process or a pharmaceutical manufacturing process, and provide an analytical output relative to the process. Examples of such analytical outputs include, but are not limited to: measurement to pH; measurement of oxidation reduction potential; selective ion measurement; measurement of dissolved gases, such as dissolved oxygen; and measurement of pressure. These analytical measurements can then be provided to a control system such that process control can be effected and/or adjusted based upon the analytic measurement. Such sensors are generally continuously, or substantially continuously, exposed to the process medium.
One particular type of process analytic sensor is known as an insertion-type process analytic sensor. An insertion-type process analytic sensor is generally configured to couple to a given process by passing through a vessel wall and mounting thereto. Accordingly, an insertion-type process analytic sensor has a distal end that is configured for exposure to the process and a proximal end that is disposed on an opposite side of the vessel wall from the distal end. Generally, insertion-type process analytic sensors are mounted directly to the vessel itself. The vessel may be a pipe, a container, or any other structure that contains a process fluid of interest.
Process analysis is very demanding. On the one hand, industry requires higher and higher accuracy and precision with respect to process analytical measurements. On the other hand, the processes to which such sensors are exposed are becoming more demanding in terms of pressure and temperature. A failure mode that is becoming increasingly common to process analytic sensor, as both the temperature and pressure of industrial requirements rise, is the loss of signal integrity due to decreased signal isolation. Once signal integrity is lost, it is necessary to replace or repair the process analytic sensor, which can potentially require that the entire process be taken offline. Accordingly, providing process analytic sensors, particularly of the insertion-type, that are more robust and better able to withstand the increased temperatures and pressures of the processes to which they are exposed, will benefit the process analytic industries.
An insertion-type process analytic sensor has a distal end that is configured to couple to a process and a proximal end that is configured to couple to the ambient environment. The sensor is constructed from materials that reduce the occurrence of vapor within the sensor as well as facilitate its venting through the proximal end. Additionally, aspects of the invention include selecting materials for internal components of the insertion-type process analytic sensor that provide little or no extractables even when vapor is present within the sensor.
Many embodiments of the present invention arise based upon the unique understanding of the manner in which degradation of process analytic sensors occurs in demanding environments. Accordingly, before describing various process analytic sensor constructions in accordance with embodiments of the present invention, it is first useful to address a failure mode currently experienced in process analytic sensors.
A principal mechanism in the failure process of analytic sensors is the accumulation of electrolytes within the sensor construction. These electrolytes both reduce signal strength through decreased resistivity between the signal and instrument common, and inject spurious voltages and currents through galvanic activity. In a sensor of uncompromised mechanical integrity, these electrolytes are generally comprised of vapor from the high temperature and/or high volatility process liquid being analyzed. These vapors permeate the mechanical components of the sensor housing. As the interior of the sensor housing is usually cooler than the sample, the vapor condenses within the sensor. Sensor construction polymers can sometimes generate amines, phenols, aldehydes, alcohols and other organics that are soluble by the vapor and/or condensate resulting therefrom. The internal wiring within the sensor can sometimes also generate soluble metallic ions. The electrolyte can permeate the polymers within a sensor with high moisture equilibrium, such as epoxies, commonly used to fill the voids within the sensor. Vapor also accumulates and migrates in and through the interfaces formed at unadhered component and filler material surfaces.
In order to better describe the problems observed above, a specific example of a pH sensor monitoring an aqueous solution will be provided. For example, consider a plastic housing pH sensor monitoring an aqueous solution at 105° Centigrade. Water vapor moves through the polyphenylene ether (PPE) or polyphenylene oxide (PPO) housing, the ethylene propylene o-rings sealing the reference, pH, and solution ground electrodes to the housing, and the epoxy filling the voids within the housing. The vapor scavenges sulfides from the plastic, peroxides from the o-rings, and amines from the epoxy filler. This condenses within the interfaces between the epoxy filler and the glass electrodes, the shrink sleeve insulation covering the wiring solder connections, the heavily plastisized vinyl wiring insulation, and the interior wall of the plastic housing. The condensate dissolves lead and tin ions from the solder connections at the electrodes, further increasing the conductivity of the condensate and creating galvanic potentials between the electrode connections within the sensor. This electrolyte even permeates the typical epoxy filling, at a rate increasing with temperature, until the epoxy becomes a significant conductor and voltage potential source. As the electrolyte becomes more concentrated and migrates further over time, the signal isolation between the pH electrode and the other electrodes and the temperature sensor will progressively decrease. Because the pH electrode signal typically has a source impedance of 100 megohms, a reduction of internal isolation from 106 to 105 megohms will noticeably reduce sensor performance. Further, a reduction to 104 megohms causes the sensor to fail. Traditional materials and methods of construction are clearly not adequate for use as industrial process temperatures rise from 85° Celsius towards 135 and 150° Celsius.
Various embodiments of the present invention make use of this understanding of the mechanism by which process analytic sensors are susceptible to degradation and adjust or otherwise modify sensor construction materials in order to reduce such degradation.
Sensor 10 includes a tubular plastic outer housing 28 that is formed of PPE (polyphenylene ether) or PPO (polyphenylene oxide) and is generally configured for exposure to the process fluids. Reference fill solution 30 is disposed within sensor housing 28 between sensor housing 28 and inner housing 22. Reference fill solution 30 constitutes an electrical half cell with silver/silver chloride reference wire 32 and peripheral reference junction 26. Silver/silver chloride reference wire 32 is actually a silver wire with a silver chloride coating disposed thereon. Sensor 10 also includes internal temperature sensor 34 that is disposed within inner housing 22. Temperature sensor 34 is coupled to cabling 36 by temperature sensor lead to cable wire solder connections 38. Electrode 20 is coupled to insulated copper wire 40 by lead tin solder joint 42. A rubber seal 44 with a relatively higher vapor permeability than other materials inside sensor 10 generally seals fill solution 30 of sensor 10 from proximal end 18. Silver/silver chloride reference wire 32 generally includes vinyl sleeving insulation 46 that abuts rubber seal 44. The interior of sensor 10 beyond rubber seal 44 and pH electrode 20, excepting the space occupied by fill solution 30, is generally filled with a cast-in-place epoxy insulation and mechanical back fill 48. Finally, a cable exit/dress cap 50 is provided at proximal end 18 to cover proximal end 18. Dress cap 50 includes an aperture allowing signal cable 52 to exit such that sensor 10 can be coupled to a suitable analyzer such as analyzer 14 (illustrated in
Various embodiments of the present invention generally include insertion-type process analytic sensors having components formed of various materials that facilitate venting, or otherwise resist degradation. Various embodiments of process analytic sensor construction provide materials and configurations such that the distal, sampling end of the sensor has very low vapor permeability while the interior of the sensor has a relatively high vapor permeability, with a similarly high or greater vapor permeability out through the proximal, ambient environment end of the sensor. Additionally, only relatively low extractable materials are used in the interior construction of the process analytic sensor, thereby providing very low unreacted material residue. Further, such low, unreacted material residue is of a non-conductive and non-corrosive nature. The interior materials are preferably selected for mutual adhesion, ensuring the absence of unbonded interfaces. The resulting process analytic sensors in accordance with various embodiments of the present invention, suffer from relatively little or no accumulation of reactive electrolytes, as the process sample vapor, such as water vapor, entering the sensor through the distal, sensing, end, is quickly dissipated out the back of the sensor by the high vapor permeability materials before condensing into a solvent, and while the use of low extractable polymers obviates residual reactive solutes. The absence of unbonded interfaces eliminates spaces in which condensate and unreacted components can potentially accumulate, reside or migrate to the detriment of the insulation properties of the sensor internal construction. With little condensed vapor solvent, no or few unreacted material solutes, and relatively no interface spaces for accumulation or migration thereof, relatively no electrolyte will form, thereby preventing galvanic potentials and shorting between the various internal electrodes and wire connections as the sensor ages. This is believed to provide a process analytic sensor with relatively longer high-temperature life as the signal is not dissipated by deteriorating insulation properties of the internal construction.
As can be seen with respect to
Accordingly, various embodiments of the present invention generally employ or provide low-extractable material and/or materials that may also facilitate venting vapor within the sensor to the atmosphere via the proximal end of the insertion-type process analytic sensor. The internal use of exclusively low extractable silicone rubber for electrode seals contacting the housing filling material, insulating all electrode leads, insulating all solder connections, and venting the sensor interior to atmosphere provides not only a high-purity internal environment for the process analytic sensor but also facilitates venting. Moreover, the use of a self priming, adhesive, low extractable solid (such as silicone rubber) filling the sensor and eliminating interface voids dramatically reduces the internal buildup of condensate and greatly restricts the availability of reactive ions. Accordingly, vapor entry from the typically hot and high pressure process near the distal end of sensor 100 is restricted by relatively lower vapor permeability materials. At the same time, venting of vapor from the interior of sensor 100 is facilitated by high vapor permeability materials communicating to the ambient environment at the cable (proximal) end of the sensor 100. Additionally, the high-purity internal components and filler introduce substantially no corrosive or conductive materials, thereby preventing damaging and/or spurious galvanic currents within the sensor. This systemic selection and placement of materials reduces vapor entry, vapor and condensate retention, electrolyte constituents, and electrolyte sites and transport, thereby better isolating electrode signals.
Conventional glass membrane and reference junction impedance sensor diagnostics are becoming less effective with rising process sample temperatures. For example, at 105° Celsius, the glass membrane impedance is only a couple of megohms, but a heat aged membrane may exceed 103 megohms when removed from the process sample for room temperature sensor evaluation. At high temperature the membrane impedance is too low to test and gives no indication of the possible very high room temperature impedance of the pH membrane but will prevent the detection at high temperature of a cracked pH membrane. Diagnostics of the reference electrode are even more variable with temperature and age. Because a very aged, high impedance pH glass membrane may still be effective at high temperature, another diagnostic is needed. In high temperature applications, a common failure mode is the loss of signal isolation, as discussed above, providing reliable isolation at high temperature not only extends sensor life, but the required materials and construction also allow the impedance between the various electrodes and the temperature sensor (not in contact with the process sample) to be an indicator of sensor condition.
Epoxies commonly used for sensor housing void filling insulation lose several orders of impedance magnitude at high temperature, even when new and dry. So, this impedance measurement cannot be used as an indication of detrimental electrolyte buildup within the sensor. Silicone polymer filling of the electrical connection portion of the sensor housing void suffers little impedance reduction with increased temperature. Accordingly, a loss of impedance between the temperature sensor and any electrode indicates failing sensor integrity within the vicinity of that electrode. An instrument or transmitter can thus detect not only sensor failure, but impeding failure, and the importance of the failure. For instance, a 102 megohm reduction between the temperature sensor and the pH electrode would be serious and could be predicted by monitoring the steady decrease in insulation resistance over time. A similar reduction between the temperature sensor and the solution ground would be insignificant to the operation of the sensor, but would be an indicator of decreased sensor integrity. The pH electrode has a typical source impedance of 102 megohms, while the solution ground has a source impedance of ohms. A diagnostic that reliably indicated sensor integrity while in service at high temperature could reduce or eliminate the need for removal of the sensor for room evaluation, which it would very possibly fail due to high pH membrane impedance, even though the sensor was working well at high temperature due to the reduction in membrane impedance with temperature. Significantly reducing the vapor and condensate (solvent) concentration in the sensor, reducing the availability of reactive ions (solutes) and transport thereof via component interfaces, and replacing poor insulation fillers, such as epoxy, with exceptionally good insulation fillers, such as silicone, will lengthen sensor life and allow new, high temperature diagnostics.
Embodiments of the present invention also provide manufacturing benefits. An epoxy filled process analytic sensor cannot be tested until the epoxy has cured. This is because the uncured epoxy acts as a low impedance, electrolyte shorting out the signal. As the epoxy cures the sensor will test better and better. Thus a low signal sensor may be scraped when the only problem is poorly mixed epoxy that cures more slowly than expected and may have yielded a “good” sensor in a few days. This creates both scrap losses and manufacturing delays. A scrap sensor cannot usually be rebuilt due to the hard epoxy. A sensor with two part vulcanizing silicone polymer can be tested immediately after assembly and readily rebuilt if necessary.
Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.
The present application is based on and claims the benefit of U.S. provisional patent application Ser. No. 60/625,933, filed Nov. 8, 2004, the content of which is hereby incorporated by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 60625933 | Nov 2004 | US |
