GB2276943A

GB2276943A - Conductivity measuring system incorporating probe cleaning.

Info

Publication number: GB2276943A
Application number: GB9306684A
Authority: GB
Inventors: James Ronald Taylor
Original assignee: Spirax Sarco Ltd
Current assignee: Spirax Sarco Ltd
Priority date: 1993-03-31
Filing date: 1993-03-31
Publication date: 1994-10-12
Anticipated expiration: 2013-03-31
Also published as: DE4410953A1; GB2276943B; GB9306684D0

Description

2276943 CONDUCTIVITY PROBE This invention relates to conductivity probes.

Conductivity probes are commonly used to monitor the condition of water in a boiler of a steam generating plant or other processes. In such a plant, the concentration of impurities in the water is of critical importance. Such impurities take the form of dissolved gases or solids and suspended solids. The measurement of the conductivity of the water provides one means of ascertaining the concentration of total dissolved solids (TDS). If the TDS concentration is allowed to get too high then process efficiency can be reduced and serious damage can result to steam and condensate systems in the plant resulting from corrosion and deposition on heat transfer surfaces.

It is known to monitor the level of total dissolved solids (TDS) in the boiler and, when the concentration is unacceptably high, to discharge (or "blow down") a proportion of the contaminated water and to introduce fresh water into the system. However, blowdown loses energy from the system and it is therefore important that there should be accurate control of blowdown, which therefore requires accurate monitoring of the level of impurities in the water. Known conductivity probes operate by measuring the conductivity of the boiler water by monitoring the current flow between the probe tip and an earthed component such as the boiler wall. For this purpose, it is common to apply a constant a.c. voltage to the probe. The use of an a.c. voltage avoids polarity effects which can lead to inaccuracy. Such probes can suffer from the problem that scale builds up at the tip of the probe. This can distort the measurements obtained and consequently it is necessary periodically to drain the boiler so that the probe can be removed for cleaning.

1 According to the present invention there is provided a conductivity sensor comprising a probe and operating circuitry which is adapted to apply an a.c. voltage to the probe during normal operation of the sensor, the operating circuitry including means for applying a d.c. voltage to the probe during a probe cleaning operation.

It has been found, surprisingly, that a relatively short probe cleaning operation (of, for example, 30 to 300 seconds at a d.c. current of 100 to 500 mA) restores the probe to a clean condition, in the sense that the conductivity readings provided by the probe correspond to those of a probe having no build-up of scale. In fact, the application of the d.c. voltage does not necessarily remove the scale, but gas bubbles formed by electrolytic action render the scale microporous so that the boiler water is able to penetrate to the surface of the probe itself. Thus, any electrical properties of the scale have no effect on the conductivity readings provided by the sensor.

The means for applying the d.c. voltage may be a permanent part of the operating circuitry connected to the probe, in which case the operating circuitry also includes timing means which determines the interval between, and duration of, the probe cleaning operation.

Alternatively the means for applying the d.c. voltage may be incorporated in a portable unit, so that it can be releasably connected to the rest of the operating circuitry, for example by a plug-in connector. The portable unit may then be connected in turn by a service engineer to a series of probes to perform a cleaning operation.

According to a second aspect of the present invention there is provided a method of cleaning a conductivity probe to which, in normal operation, an a.c. voltage is applied, the method comprising 4 interrupting the application of the a.c. voltage and applying a d.c. voltage to the probe.

The d.c. voltage may be applied for a duration of between 30 seconds and 300 seconds, and is preferably applied for a duration of between 60 seconds and 180 seconds.

The interval between each application of the d.c. voltage may be not less than 1 hour and not more than 10 hours, and preferably the interval is not less than 3 hours and not more than 5 hours.

For a better understanding of the present invention and to show how it may be carried into effect reference will now be made by way of example to the accompanying drawings, in which:

Figure 1 shows, in schematic form, a conductivity probe and associated control circuitry; Figure 2 shows, in perspective view, the conductivity probe of the apparatus of Figure 1; Figure 3 shows, in cross-section, the probe tip of the apparatus of Figure 1.

As shown in Figure 1, a probe assembly 2 is attached to a boiler 4 by means of a boss 5. A lead 6 feeds the probe assembly 2. The probe assembly 2 comprises a threaded housing 8 to which a probe shaft 10 is attached. At the end of the probe shaft 10, which extends into the boiler 4, there is a probe tip 12. The lead 6 is electrically connected to the probe tip 12 by the probe shaft 10. The probe shaft 10 has an insulating sleeve 7. 30 The lead 6 is connected to operating circuitry 13 for the probe 2. The operating circuitry comprises control circuitry 14 and cleaning circuitry 18 which are alternatively connected to the probe 2 by means of a switch 16 controlled by a timer 20. The control circuitry 14 is for applying an a.c. voltage to the probe tip and for monitoring the -4 resulting a.c. current flow between the probe tip 12 and the wall of the boiler 4. Both the control circuitry 14 and the cleaning circuitry 18 are connected to the boiler 4, which acts as an electrical 5 earth.

The cleaning circuitry 18 is for applying a d.c. voltage to the probe tip 12.

Both the control circuitry 14 and the cleaning circuitry 18 receive their input power supply from a common source, such as the mains, and process the supply to provide the required a.c. or d.c. output.

The probe assembly 2 is shown in greater detail in Figure 2. The probe assembly comprises an elongate body one end of which is adapted to be inserted into the boiler 4. A threaded portion 22 is provided to enable screwing of the probe into a corresponding threaded bore in the wall of the boiler 4. A gasket 24 provides a seal. The probe tip 12 is mounted on the end of the probe shaft 10 so that the probe tip 10 lies within the boiler 4.

The end region of the probe shaft 10, to which the probe tip 12 is mounted, is shown in cross-section in Figure 3. The probe shaft 10 is made from an electrically conductive material. The probe tip 12 has a projecting threaded portion 26 which is screwed into a threaded bore in the probe shaft 10. An electrically insulating shroud 28 surrounds the end portion of the shaft 10 and the shroud is, for example, made from PTFE. The probe shaft 10 is also covered by an insulating sleeve 30 which also may be of PTFE.

During normal operation of the system the switch 16 is in the position as shown in Figure 1. The control circuitry 14 effects control of the probe assembly 2. An a.c. voltage is supplied by the control circuitry 14 and the conductivity between the probe 12 and the housing of the boiler 4 is calculated from 7 z Z current measurements obtained by the control circuitry 14. This conductivity gives an indication of the level of impurities in the water of the boiler 4. Typically the frequency of the a.c. voltage applied by the control circuitry 14 is around 1000 Hz, and results in a current flow between the probe tip 12 and the wall of the boiler 4. Over a period of operation, scale builds up on the probe tip 12, and this affects the accuracy of the conductivity readings obtained. In order to remedy this, the system shown in Figure 1 periodically connects the probe tip 12 to the cleaning circuitry 18 by means of the switch 16. A d.c. voltage is then applied to the probe assembly 2 resulting in a constant current of, for example, 150 mA, passing through the probe tip 12 to earth (namely to the boiler 4). The cleaning circuitry 18 operates for a predetermined time which is sufficient to enable the probe tip 12 to be returned to a clean condition. The timer 20 controls the switch 16 so as to determine the duration of the cycle during which the cleaning circuitry 18 has control over the probe assembly 2, and the frequency with which the cleaning circuity 18 interrupts the normal control of the system. The operating parameters of the timer 20 may be altered in order to take account of differences in plants which lead to a more rapid or a more gradual build up of scale on the probe tip. Typically, the cleaning cycle may be for 3 minutes every 4 hours.

In an alternative form, the timer 20 may be omitted, and the cleaning circuitry 18 may be incorporated in a portable, mains powered unit. This unit can then be used to service several probes. Since the interval between cleaning operations is likely to be greater when the portable unit is used than when the probe installation has permanently connected operating circuitry 13 as shown in Figure 1, the portable unit will typically provide a higher current flow of, for example, 300 mA.

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Claims

1. A conductivity sensor comprising a probe and operating circuitry which is adapted to apply an a.c. voltage to the probe during normal operation of the sensor, the operating circuitry including means for applying a d.c. voltage to the probe during a probe cleaning operation.

2. A conductivity sensor as claimed in claim 1 in which the operating circuitry includes a switch for switching between normal operation of the sensor and the cleaning operation.

3. A conductivity sensor as claimed in claim 2, in which the operating circuitry includes timing means which controls the operation of the switch.

4. A conductivity sensor as claimed in any one of the preceding claims in which the means for applying a d.c. voltage to the probe comprises a permanent part of the operating circuitry.

5. A conductivity sensor as claimed in any one of claims 1 to 3, in which the means for applying a d.c. voltage to the probe is accommodated in a portable unit for releasable connection to the rest of the operating circuitry.

6. A method of cleaning a conductivity probe to which, in normal operation, an a.c. voltage is applied, the method comprising interrupting the application of the a.c. voltage and applying a d.c. voltage to the probe.

7. A method as claimed in claim 6 in which the d.c. voltage is applied to the probe periodically for a duration of not less than 30 seconds and not more than 300 seconds.

8. A method as claimed in claim 7, in which the duration of each application of the d.c. voltage is not less than 60 seconds and not more than 180 seconds.

9. A method as claimed in any one of claims 6 to C 8, in which the interval between each application of the d.c. voltage is not less than 1 hour and not more than 10 hours.

10. A method as claimed in claim 9, in which the interval between each application of the d.c. voltage is not less than 3 hours and not more than 5 hours.

11. A method as claimed in any one of claims 6 to 10 in which, application of the d.c. voltage to the probe results in a current flow through the probe of between 100 and 500 mA.

12. A conductivity sensor as herein described with reference to and as shown in the accompanying drawings.

13. A method for cleaning a conductivity probe as claimed in claim 6 and substantially as described herein.