Contact-type conductivity sensor with corrosion diagnosis

Abstract

A contact-type conductivity sensor is provided. The sensor includes a first electrode configured to contact the liquid, and a second electrode configured to contact the liquid. The second electrode has a first end and a second end. A first conductor is coupled to the first electrode, a second conductor is coupled to a first end of the second electrode, and a third conductor is coupled to a second end of the second electrode. The contact-type conductivity sensor is configured to provide a conductivity measurement of the liquid using the first conductor and the second conductor, and is configured to provide a corrosion diagnosis using the second conductor and the third conductor. A conductivity measurement system using the sensor, and a method of using the sensor, are provided.

Description

Background Art

[0001] Liquid conductivity measurement systems are used to measure the conductivity of water and aqueous solutions, or non-aqueous solutions, in environments, medical, industrial, and other applications that require indication of the ionic content of a liquid.

[0002] Liquid conductivity is measured in a variety of situations to provide a relatively inexpensive parameter that may sometimes be related to the bulk ion concentration. In the presence of a single type of ion, the conductivity may actually be related to the specific ion concentration. Even in the presence of multiple different ionic compounds, the measurement of the bulk liquid conductivity can still provide very useful information. Accordingly, conductivity measurements have been widely adopted and utilized industrially for a variety of different purposes.

[0003] Typically, contact-based conductivity measurement systems include a conductivity sensor or cell, and an associated conductivity analyzer or conductometer. The conductometer generates an AC current through the electrodes of the conductivity cell. The conductometer then senses the voltage obtained between the electrodes of the cell. This voltage is typically a function of the conductivity of the liquid to which the cell is exposed. Summary of the Invention

[0004] A contact-type conductivity sensor is provided. The sensor includes a first electrode configured to contact a liquid, and a second electrode configured to contact the liquid. The second electrode has a first end and a second end. A first conductor is coupled to the first electrode, a second conductor is coupled to the first end of the second electrode, and a third conductor is coupled to the second end of the second electrode. The contact-type conductivity sensor is configured to provide a conductivity measurement of the liquid using the first conductor and the second conductor, and is configured to provide corrosion diagnosis using the second conductor and the third conductor. A conductivity measurement system using the sensor, and a method using the sensor are provided. Brief Description of the Drawings

[0005] Figure 1 Shows a typical contact-type conductivity sensor and an associated conductivity analyzer or conductometer.

[0006] Figure 2A Is a schematic perspective view of a typical contact-type conductivity sensor.

[0007] Figure 2B Is Figure 2A An enlarged schematic view of a portion of the contact-type conductivity sensor shown.

[0008] Figure 3 Is a schematic diagram of a simple cooling water system.

[0009] Figure 4A And Figure 4BSchematic top view and perspective view of a contact-type conductivity sensor according to an embodiment of the present invention.

[0010] Figure 5 Circuit diagram showing the measurement of electrode corrosion of a contact-type conductivity sensor according to an embodiment of the present invention.

[0011] Figure 6 Flowchart of a method for measuring corrosion of a contact-type conductivity sensor according to an embodiment of the present invention. Detailed Description

[0012] Contact-type conductivity sensors are widely used in industry to determine the ion concentration in solutions. Some ions, such as hydrogen and chlorine, are highly corrosive to metal components (e.g., pipes, heat exchangers, and other devices). Although pH probes are used to measure the hydrogen ion concentration in solutions, conductivity probes do not provide information about the type of ions in the mixture. It is a measurement of the total ion concentration. Additionally, even if the pH value of the solution is between 6 and 9, processes such as seawater treatment may still be corrosive due to the high concentration of other ions. The total ion concentration in a solution is referred to as conductivity.

[0013] Figure 1 Shows a typical contact-type conductivity sensor and an associated conductivity analyzer or conductometer. The conductometer generates an AC voltage between the electrodes of the conductivity cell. Then the conductometer senses the current obtained between the electrodes of the cell. This current is typically a function of the conductivity of the liquid to which the cell is exposed. The current between the electrodes depends not only on the solution conductivity but also on the length, surface area, and geometry of the sensor electrodes. Due to the size and geometry of the sensor, the probe constant (also known as the sensor constant or cell constant) is a measure of the sensor's response to a conductive solution. Its unit is cm -1 (length divided by area), and the probe constant required for a given conductivity range is based on the measurement circuit of a specific conductivity analyzer. The probe constant can vary in the range from 0.01 cm -1 to 50 cm -1 and generally, the higher the conductivity, the larger the probe constant required.

[0014] Figure 2A Is a schematic perspective view of a typical contact-type conductivity sensor, and Figure 2B Is Figure 2A An enlarged schematic view of a part of the contact-type conductivity sensor shown in Figure 2AAs shown, the contacting conductivity sensor 10 generally includes a distal portion 12 configured to be exposed to the process liquid. Typically, the sensor 10 is installed into the process penetration by extending the distal portion 12 through the process penetration and engaging the external threads 14 with the internal threads of the process penetration. A hexagonal flat 16 is provided to allow the sensor 10 to be tightened into the process penetration using a wrench. The sensor body 18 may include one or more electrical components, such as circuitry, and a suitable electrical connection between the sensor 10 and a multi-core cable 18 that is connected to an analyzer ( Figure 2A not shown in the figure).

[0015] The distal portion 12 of the sensor 10 includes an end having an annular space 20 between an inner electrode 22 and an outer electrode 24. The annular space 20 between the electrodes 22, 24 is filled with a process solution that flows out through a leakage hole 26 on one side of the distal portion 12. An analyzer (such as Figure 5 the analyzer 120 shown) applies an AC voltage between the inner electrode 22 and the outer electrode 24, and as a result, ions in the solution move back and forth between the two electrodes 22, 24, forming an ionic current. Then, the sensor or the analyzer measures the ionic current. According to Ohm's law (V = IR), the analyzer calculates the resistance of the solution and obtains the reciprocal of the resistance.

[0016] As described above, a contact conductivity sensor measures the resistance of a solution, which is converted to conductance and multiplied by a cell constant to yield conductivity. The cell constant is determined by calibrating the sensor against a standard solution or a solution with a known conductivity. The theoretical definition of the cell constant is the distance between the inner and outer electrodes divided by the inner surface area of the electrodes. Commercially available contact conductivity sensors are available with various cell constants: a 0.01 / cm cell constant sensor, made for high purity water applications (such as boiler feed water) with conductivities below 10 μS / cm; a 0.1 / cm cell constant, made for clean water applications such as cooling water and drinking water; and a 1.0 / cm cell constant, made for dirtier processes such as wastewater treatment. Sensors with these different constants are available from Emerson Electric Co. of St. Louis, MO. In some cases, these sensors can be factory calibrated to obtain a more accurate cell constant than the nominal value. Then, the end user can simply input the calibrated cell constant into the user's analyzer and begin making conductivity measurements. However, over time, the cell constant may shift due to debris buildup, coating on the electrodes, or due to the gradual corrosion of the electrodes by the process. The embodiments described below generally provide a contact conductivity sensor with real-time or near real-time corrosion diagnosis. It is believed that the embodiments described herein will be particularly beneficial for applications such as cooling water and boiler blowdown systems. An additional benefit of such diagnosis is corrosion monitoring of pipes and equipment to ensure asset integrity.

[0017] Figure 3 is a schematic diagram of a simple cooling water system. System 50 is an example of a cooling water system that is widely used in industries ranging from power plants to refineries to reject large amounts of process heat. The cooling tower 52 stores a large amount of cooling water or chilled water, which is ultimately delivered to one or more heat exchangers 54 throughout the facility. The cooling water absorbs excess heat from the process before leaving the heat exchanger 54 and returning to the cooling tower 52. As Figure 3 shown, when hot water (i.e., water that has passed through the heat exchanger 54) enters the cooling tower 52, the hot water interacts with the air 56 to cool. To facilitate cooling, the air 56 is moved through the cooling tower 52 by a fan 58. Makeup water 60 from a city or local reservoir is used to replenish water lost during blowdown. As indicated by reference numeral 62, blowdown is the process of discharging cooling water from the system to reduce conductivity. Corrosion diagnosis is important in a cooling water system. A heat exchanger fouled due to corrosion may impede optimal heat transfer between the process and the cooling water, affecting process quality. In addition, equipment failures within the cooling tower can be costly to repair and result in unplanned downtime.

[0018] Several substances can cause corrosion in a cooling water system. Leakage through the pipes or the housing of a heat exchanger can cause a significant change in the pH of the cooling water. As Figure 3 shown, a pH probe 64 is typically installed downstream of the heat exchanger 54 to monitor for leakage. Next, as makeup water is continuously supplied from city water or other sources, microbiologically induced corrosion can occur due to bacteria and algae. Chemical additives 66 (e.g., free chlorine) and / or biocide additives 68 are injected into the water to control biological fouling. These additives 66, 68 are monitored with suitable sensors (e.g., an oxidation-reduction potential (ORP) sensor 70 and / or a chlorine sensor 72). However, some corrosion can still occur due to leakage in the housing and / or pipes of the heat exchanger 54, or the presence of high concentrations of salts (e.g., sodium chloride) in a failed ion exchange resin. Installing a contact conductivity sensor (e.g., conductivity sensor 74) in such applications is important for determining changes in the conductivity of the cooling water. Additionally, in a boiler feedwater system, an oxygen probe is installed after the deaerator to control the concentration of oxygen that may leak in through gaskets in fans and / or pumps.

[0019] Typically, metal corrosion coupons are used in a cooling water system to monitor the corrosion rate. A corrosion coupon is a pre-weighed metal sheet that is installed on a bypass rack. The coupon is removed from the bypass rack and weighed periodically (e.g., every 3 or 4 months). The difference in weight from the previously examined weight is the corrosion rate, which is typically expressed in mils per year. Corrosion coupons on a bypass rack are generally the least expensive corrosion monitoring technique. However, such methods do not provide the operator with continuous, immediate measurements. Additionally, many sites may conduct corrosion inhibitor experiments and use corrosion probes to experiment with corrosion inhibitor types and dosages. Again, immediate measurements are more accurate and convenient than using corrosion coupons.

[0020] In the above applications, over time, electrode corrosion will also occur in a contact conductivity sensor. Corrosion of the electrodes reduces the surface area of the annular space between the inner and outer electrodes. This will result in an incorrect conductivity reading. Calibration can be performed to adjust the cell constant, and the reading can be brought back within tolerance. However, since panel mounting will discharge valuable cooling water into the atmosphere, users sometimes choose to install the contact conductivity sensor directly in the pipe. Additionally, unplanned shutdowns are more difficult to manage, and users may not have a spare sensor. Accordingly, providing a contact conductivity sensor with immediate, continuous, or substantially real-time corrosion diagnostics will benefit users of such sensors and will be of particular value for cooling water applications. Since the corrosion measured by the conductivity sensor can indicate overall corrosion in the cooling system, sensor diagnostics can also be used as a predictive tool for maintenance and planned shutdowns.

[0021] Figure 4A and Figure 4B are a schematic top view and a perspective view of a contact-type conductivity sensor according to an embodiment of the present invention. The sensor 100 includes a sensor body 102 ( Figure 4A and Figure 4B a portion of which is shown in), a pair of electrodes 104, 106 extending from the sensor body 102 and having a space 108 therebetween. A process liquid 108 flows through or is present in the space 108, and when an AC voltage is applied to the electrodes 104, 106, contact-type conductivity measurement is performed. In the example shown, the electrode 104 generally has a strip-like, bar-like, or ring-like form extending around the inner electrode 106, and the inner electrode 106 is generally shown as a rectangular block. In some embodiments, one or both of the electrodes 104, 106 may be replaceable. The shape of the electrode 104 facilitates the generation of corrosion diagnosis, and the resistance of the electrode 104 can be measured by an analyzer. The conductor resistance is a property of a conductor at a specific temperature and is defined as the resistance encountered when current flows through a conductive medium. The resistance of a conductor depends on the cross-sectional area of the conductor, the length of the conductor, and its resistivity. The electrode 104 does not change its resistivity or length. Therefore, a change in the resistance of the electrode 104 will indicate a change in the cross-sectional area of the electrode 104, and thus indicate corrosion. Additionally, although the embodiments described herein only show one of the electrodes 104, 106 having a strip-like or bar-like shape, it is expressly contemplated that both electrodes can be formed in a strip-like, bar-like, or ring-like shape such that the resistance of both electrodes can be measured.

[0022] Figure 5 is a circuit diagram showing the electrode corrosion measurement of a contact-type conductivity sensor according to an embodiment of the present invention. For illustrative purposes, Figure 5 the inner electrode 106 is not shown. The contact-type conductivity sensor 100 includes a sensor body 102 coupled to an analytical instrument 120 via a plurality of conductors 122, 124, 126, 128, and 130. Figure 5 shows an outer electrode 104 having a ring-like shape with a first end 132 close to the sensor body 102 and a second end 134 close to the sensor body 102. As shown, the conductor 122 is coupled to the proximal end 132 of the electrode 104, while the conductor 126 is coupled to the proximal end 134 of the electrode 104. This configuration allows the analytical instrument 120 to measure the resistance of the electrode 104.

[0023] As described above, the change in the resistance of electrode 104 over time indicates corrosion. The resistance of electrode 104 is typically low, and the resistance difference caused by corrosion (at least initially) may be very small. Thus, in some examples, the embodiments described herein include a reference element 136 that preferably has substantially the same length, width, and composition as electrode 104. However, reference element 136 is disposed within sensor body 102 and is not exposed to the process liquid. As shown, reference element 136 has a first end 138 coupled to conductor 128 and a second end 140 coupled to conductor 126. Analyzer 120 includes a voltage source 142 shown as an AC voltage source. Although voltage source 142 may be the same AC voltage source used to obtain conductivity measurements, it is expressly contemplated that voltage source 142 may be different from the voltage source used for conductivity measurements and need not even be an AC voltage source. Analyzer 120 includes a potentiometer of voltage measurement device 144 coupled to conductor 128 via resistor 146 and coupled to conductor 124 via resistor 148. Additionally, voltage measurement device is coupled to conductor 126. In some examples, resistors 146, 148 have the same resistance such that the circuit is completely balanced under initial conditions without corrosion. However, one or both of resistors 146, 148 may be variable resistors to allow the end user to adjust the circuit for initial balance when the sensor is first installed.

[0024] When corrosion begins to occur, the metal electrode 104 (shown as the “exposed element”) will become smaller. As electrode 104 becomes smaller due to corrosion, the resistance of the electrode will increase. As the resistance of electrode 104 increases, the resistance of reference electrode 136 will remain the same. Thus, the circuit will become unbalanced and the voltage will be detectable by voltage measurement device 144. Analyzer 120 uses the detected voltage from the voltage measurement device to provide a corrosion diagnosis that can be immediate, real-time, or substantially real-time. Additionally, even in embodiments where the corrosion diagnosis is not provided as a continuous real-time value, the corrosion diagnosis can still be readily provided at a relatively high frequency (e.g., every 10 minutes, every hour, every day) as opposed to the 3 or 4 months specified for corrosion coupons. Additionally, because the corrosion diagnosis is provided so frequently, the corrosion diagnosis can also be used to monitor asset integrity in essence.

[0025] Figure 6A flowchart of a method for measuring corrosion of a contact-type conductivity sensor according to an embodiment of the present invention. Method 200 begins at block 202, where an analyzer measures the conductivity between a pair of electrodes of a contact-type conductivity sensor exposed to a process liquid (e.g., cooling water). In some examples, the pair of electrodes may include an electrode with an annular shape disposed around an inner electrode. However, embodiments cover any suitable physical arrangement where at least one of the electrodes has a configuration that allows measurement of its resistance while still providing a suitable surface area for its function as an electrode of a contact-type conductivity sensor. Next, at block 204, the analyzer provides a conductivity output based on the measured conductivity. Then, method 200 proceeds to optional block 206 to determine whether corrosion diagnosis is needed. Since embodiments can be practiced where corrosion diagnosis is performed with each conductivity measurement, block 206 is optional. However, since corrosion typically occurs more slowly than changes in the conductivity of the process liquid, corrosion diagnosis can be performed periodically (e.g., every minute, every 5 conductivity measurements, once a day). Block 206 is typically performed by a controller within analyzer 120. The controller can be a microprocessor that may include or be coupled to a real-time clock to support the programmed determination provided by block 206. If corrosion diagnosis is not needed, method 200 repeats by returning to block 202, as indicated by line 207.

[0026] If it is determined at block 206 that corrosion diagnosis is needed, or in embodiments where block 206 is not provided, method 200 proceeds to block 208 to measure the corrosion of at least one electrode. As described above, this corrosion measurement is done by measuring the resistance of at least one electrode of the contact-type conductivity sensor. In embodiments where the resistance of two electrodes is measured, the measurements can be combined to provide an average corrosion. Additionally or alternatively, the highest resistance of the two measurements can be used such that the reported corrosion is the corrosion of the most severely corroded electrode. Next, at block 210, the result of the corrosion diagnosis is provided. This output can be in the form of an alarm 214 generated when the corrosion reaches a preselected threshold. The alarm can be a local audio and / or visual alarm. Additionally or alternatively, the alarm can be a message (e.g., SMS, email, process communication, etc.) generated to a responsible party or maintenance technician indicating the current corrosion level and / or the estimated time when the sensor will need to be recalibrated or replaced.

[0027] As Figure 6 shown, the corrosion output can be an adjustment to the calibration interval 216 of the sensor. Thus, as the sensor begins to corrode, it can be calibrated more frequently. Figure 6It is also shown that the corrosion diagnosis output can be a conductivity output that simply compensates for the corrosion determined during corrosion diagnosis. As described above, as the electrodes corrode, the cell constant will change. Other suitable logic or components of the controller or analyzer can include a look-up table that correlates the measured corrosion with the cell constant change. Thus, the analyzer 120 can use the determined corrosion value to look up the look-up table to obtain the corresponding cell constant change and apply the changed cell constant to the conductivity measurement output. Additionally, in the case where the conductivity measurement compensates for corrosion, the analyzer 120 can provide such an indication.

[0028] Once the output of the corrosion diagnosis has been provided, the method 200 repeats by controlling a return to block 202 via line 212.

[0029] Although the invention has been described with reference to preferred embodiments, those skilled in the art will recognize that modifications can be made in form and detail without departing from the spirit and scope of the invention.

Claims

1. A contact conductivity sensor, comprising: a first electrode configured to contact the liquid; a second electrode configured to contact the liquid, the second electrode having a first end and a second end; a first conductor coupled to the first electrode; a second conductor coupled to the first end of the second electrode; a third conductor coupled to the second end of the second electrode; and Wherein, the contacting conductivity sensor is configured to provide conductivity measurement of the liquid using the first conductor and the second conductor, and is configured to provide corrosion diagnosis using the second conductor and the third conductor.

2. The contact type conductivity sensor according to claim 1, wherein: The second electrode is a ring-shaped electrode disposed around the first electrode.

3. The contact type conductivity sensor according to claim 2, wherein: The first electrode is rectangular.

4. The contact type conductivity sensor according to claim 3, wherein: The conductivity measurement is performed on a liquid disposed between the first electrode and the second electrode.

5. The contacting conductivity sensor of claim 1, further comprising a reference element having a first end and a second end, wherein The second end of the reference element is coupled to the third conductor. 6 . The contacting conductivity sensor of claim 5 , further comprising a fourth conductor coupled to the first end of the reference element.

7. The contact type conductivity sensor according to claim 6, wherein: The reference element is configured to have the same length as the second electrode.

8. The contact type conductivity sensor according to claim 7, wherein: The reference element is disposed within the body of the contact-type conductivity sensor and is isolated from the liquid.

9. The contact type conductivity sensor according to claim 1, wherein: Each of the first electrode and the second electrode is in a strip shape.

10. The contact type conductivity sensor according to claim 1, wherein: Each of the first electrode and the second electrode is in a strip shape.

11. The contact type conductivity sensor according to claim 1, wherein: Each of the first electrode and the second electrode is in a ring shape.

12. A conductivity measurement system, comprising: a conductivity analyzer having a voltage source and a voltage measuring device; as well as Contacting conductivity sensor, including: a first electrode configured to contact the liquid; a second electrode configured to contact the liquid, the second electrode having a first end and a second end; a first conductor coupling the first electrode to the conductivity analyzer; a second conductor coupling the conductivity analyzer to the first end of the second electrode; a third conductor coupling the conductivity analyzer to the second end of the second electrode; and The conductivity analyzer is configured to generate a conductivity output based on conductivity measurement of the contacting conductivity sensor using one of the second conductor and the third conductor and the first conductor, and is further configured to generate a corrosion diagnostic output using the second conductor and the third conductor.

13. The conductivity measurement system according to claim 12, wherein: The contacting conductivity sensor includes a reference element having a first end and a second end, wherein the second end of the reference element is coupled to the third conductor.

14. The conductivity measurement system of claim 13, further comprising a fourth conductor coupling the first end of the reference element to the conductivity analyzer.

15. The conductivity measurement system according to claim 14, wherein: The conductivity analyzer includes a voltage measurement device operably coupled to the second electrode and the reference element, the voltage measurement device configured to provide an indication of a change in resistance of the second electrode, the change in resistance indicating corrosion of the second electrode.

16. The contact type conductivity sensor according to claim 15, wherein: The reference element is configured to have the same length as the second electrode.

17. The contact type conductivity sensor according to claim 16, wherein: The reference element is disposed within the body of the contact-type conductivity sensor and is isolated from the liquid.

18. The conductivity measurement system according to claim 12, wherein: The corrosion diagnostic output is an alarm.

19. The conductivity measurement system according to claim 12, wherein: The corrosion diagnostics are regulated at calibration intervals.

20. The conductivity measurement system according to claim 12, wherein: The corrosion diagnostic output is a compensated conductivity output.

21. The conductivity measurement system according to claim 20, wherein: The compensated conductivity output includes an indication that the output was compensated for corrosion.

22. The conductivity measurement system according to claim 12, wherein: The corrosion diagnostic output is provided in real time.

23. A method of operating a contact-type conductivity sensor; the method comprising: measuring conductivity between a first conductivity electrode and a second conductivity electrode, the first conductivity electrode and the second conductivity electrode being in contact with a process liquid; generating a conductivity output based on the measured conductivity; measuring a resistance of at least one of the first electrode and the second electrode; as well as A corrosion diagnostic output is provided based on the measured resistance.

Images