WO1996011390A1

WO1996011390A1 - Transmitter sensor

Info

Publication number: WO1996011390A1
Application number: PCT/IB1995/000896
Authority: WO
Inventors: Mark Kroll
Original assignee: Honeywell S.A.
Priority date: 1994-10-08
Filing date: 1995-10-09
Publication date: 1996-04-18
Also published as: EP0784786A1; AU3617995A; AU3617695A; WO1996011389A1

Abstract

A temperature transmitter in an oil refinery has a thermowell located in a pipe within the petroleum processing section; the thermowell has two thermocouples (1 and 2), for sensing and measuring the petroleum flow. The rate of change of resistance of the thermocouples is regularly checked to estimate any malfunction e.g. failure or deterioration.

Description

TRANSMITTER SENSOR

The present invention relates to a transmitter sensor.

A transmitter sensor for use in manufacturing processes has a sensing probe in the medium to be measured and linked to the main unit of the transmitter for initial processing and/or analysis of the sensed data. The unit and probe are often placed a number of meters apart due to the hazardous or unpleasant nature of the medium or inaccessibility of the location in which the sensing has to be done.

The sensing probe has a thermowell to contain and protect a thermocouple to measure characteristics of the medium. Frequently a thermocouple breaks, causes the manufacturing operation to be halted.

The present invention provides a transmitter sensor comprising thermocouple sensor means for the sensing of a parameter for measurement in the locality of the sensor, characterised in that the sensor comprises means to monitor the thermocouple sensor means to predict failure or deterioration of the thermocouple sensor means.

In this way, the invention can ensure that normal transmitter sensor operation is maintained regardless of any deterioration or failure of an individual thermocouple.

The present invention may also provide any one or more of the following features: means to estimate failure of thermocouple sensor means comprising means to determine whether the rate of change of one or more ch racteristics of the thermocouple sensor means has exceeded a threshold value; means to estimate failure of thermocouple sensor means based on measured values of resistance with respect to time of thermocouple sensor means; the means to monitor and/or the estimation means comprises means to measure the change in resistance of thermocouple sensor means with respect to time; the means to monitor and/or the estimation means comprises means to note a combination and/or sequence of values of resistance, and of change in resistance with respect to time, of thermocouple sensor means; the means to monitor and/or the estimation means includes means to apply a current signal to thermocouple sensor means; means to generate a warning signal in response to an output of the monitor means and/or the estimation means.

Accordingly, the present invention can provide a simple and low cost arrangement to predict and/or estimate thermocouple failure and/or deterioration and so can reduce or eliminate the inconvenience and expense of unplanned stoppages.

The present invention is applicable to all types of transmitter sensors including flow transmitter sensors, temperature transmitter sensors and pressure transmitter sensors.

In order that the invention may more readily be understood, a description is now given, by way of example only, reference being made to the accompanying drawings in which:- Figure 1 is a schematic diagram of a transmitter sensor embodying the present invention;

Figure 2 is a block diagram of operation of the sensor of Figure 1; and

Figure 3 is a flow chart showing operation of the sensor of Figure 1.

A temperature transmitter sensor for use in an oil refinery has a thermowell located in a pipe within the petroleum processing section such that the thermowell is in the stream of petroleum as it passes along the pipe. The thermowell is linked to a main unit of the transmitter situated on the exterior of the pipe. The thermowell contains two high temperature thermocouples 1 and 2 each of which has a platinum/6% rhodium alloy wire and a platinum/30% rhodium alloy wire.

The main unit has an electronic controller 3 whose port A is linked to one side of both thermocouples while port B is linked to the other side of thermocouple 1 and port C is linked to the other side of thermocouple 2. Ports D and E are used as outputs.

In normal operation, ports A and B (see Figure 1) are enabled so that thermocouple 1 is used for sensing and measurement of the petroleum flow by measurement of the e.m.f. from the thermocouple with processing including amplification with configurable gain at amplifier 4, digital conversion at A D converter 5 and treatment by the microprocessor 6. Every 0.5 seconds, constant current generator 7 in microprocessor unit 3 injects via multiplexer 8 a current signal ij of 170 micro Amperes and duration 80 milli seconds out through port A and into thermocouple 1. The resultant voltage drop Vi across ports A and B, which is dependent on the resistance B.\ of thermocouple 1, is measured and if it exceeds a threshold voltage of 20 millivolts (the actual size of voltage can vary according to application) microprocessor 6 disables port B and enables port C while outputting a non- critical alarm to the operator of the oil refinery. Current generator 7 injects a current signal, ij, in similar manner as for thermocouple 1 through port A and the voltage drop V2 between ports A and C is measured; provided that V2 is less than the same threshold voltage, the microprocessor unit 6 utilizes the e.m.f. of thermocouple 2 in all subsequent measurements of the parameter of the petroleum flow. Failed thermocouple 1 is replaced by an engineer at the next convenient opportunity for interrupting the processing in the relevant part of the oil refinery.

In addition to the monitoring of resistance, controller 3 uses stored information about measured resistance and calculated change of resistance in order to monitor the change of resistance with time, such that once a threshold limit is exceeded, the switching of use from one thermocouple to another is done in the same way as described above. With both monitoring methods utilised, the switching between thermocouples can be done when one or both (or any specified sequence of) threshold(s) is or are exceeded.

If the voltage V2 exceeds an appropriate threshold, the controller unit outputs a critical alarm to the operator of the oil refinery so that a decision can be made as to whether the petroleum operation is to be stopped for the failed thermocouples to be replaced.

The controller can operate such that, when the failed thermocouple has not yet been replaced so the thermocouple 2 is operating with no back-up, then the appropriate thresholds (e.g. for producing a warning signal) are different, typically lower than when a thermocouple is operating with a back¬ up. The normal voltage drop across the thermocouples 1 or 2 for the injected current would be for example 2 millivolts representing a thermocouple resistance of 10 Ohms, and the threshold value of 20 millivolts (i.e. thermocouple resistance of 118 Ohms) represents the point at which performance of such a thermocouple has significantly deteriorated and a warning signal is to be generated, whereas the further threshold value of 35 millivolts, (i.e. 200 Ohms) represents the point at which there is danger of the thermocouple imminently failing. A resistance change of 10 Ohms/minute is an indication of a problem with the thermocouple, the values when monitoring of resistance change being very application specific. The controller unit is able to measure resistances in the range 0 to 2000 Ohms, and resistance changes in the range 50 Ohms/second to 1 Ohm/day.

Additionally or alternatively to the substitution of thermocouple 1 by thermocouple 2, controller 3 can activate a warning signal indicating failure or deterioration, and/or cause other appropriate action. The controller estimates failure or deterioration of a thermocouple based on the measurement of change of resistance with respect to time (optionally also linked to resistance), and displays or otherwise outputs the results together with the warning signal, or produce a suitably modified warning. The controller 3 can also determine the relationship between the estimated failure time and the next service or maintenance due, and give an appropriate signal. Thus there may be provided means to correlated estimated time of malfunction with expected next service and means to issue an appropriate warning if estimated malfunction is before or just after the next service.

In one modification, the controller 3 provides temperature compensation of the resistance, and/or change of resistance, measurements. In normal use of a thermocouple such as to obtain a temperature reading at the thermocouple, the e.m.f. voltage of the hot junction is measured and compensated with the e.m.f. voltage of the cold junction. Thus, when determining the resistance and/or change of resistance value, the measured voltage after injection of the current is compensated with the previously measured thermocouple e.m.f. to obtain the corrected resistance of the hot junction. Assuming that the resistance of the lead wires and the connection resistance is zero, then vhot-junction ⁼ ( _* measured - Ve.m.f./ / Icont nt.

In applications utilizing long thermocouple wires or compensation cables, the lead wire resistance can be significant in relation to the hot junction resistance (albeit constant in time), such that:-

JΛle«d-wire + connection — (Vmeasured - Ve.m.f.) / loonstant

If controller 3 notes and stores the resistance of the lead wire and of the connection at t = 0 (referred to as Rα>), then the corrected resistance is:- ithot-junction ~ ( Vmeasured - Ve.m.f./ / loom tan t • Jtvto

The microprocessor 6 may effect intermittent checks of thermocouple 2 even when thermocouple 1 is in circuit and being used for measurement of the petroleum, such checks being less frequent than those of thermocouple 1.

The thermowell may have a third (or more) thermocouple^) to be switched into operation in case thermocouple 2 fails before thermocouple 1 is replaced.

The invention ensures that, at minimal complexity and cost, the processing operation of the oil refinery is not interrupted by failure of a thermocouple.

Claims

1. A transmitter sensor comprising thermocouple sensor means for the sensing of a parameter for measurement in the locality of the sensor, characterised in that the sensor comprises means to monitor the thermocouple sensor means to predict failure or deterioration of the thermocouple sensor means.

2. A sensor according to Claim 1 characterised by means to estimate failure of thermocouple comprising means to determine whether the rate of charge of one or more characteristics of the thermocouple sensor means has exceeded a threshold value.

3. A sensor according to Claim 1 or 2, characterised by means to estimate failure of thermocouple sensor means based on measured values of change of resistance with respect to time of thermocouple sensor means.

4. A sensor according to any preceding Claim, characterised in that the means to monitor and/or the estimation means comprises means to measure the change in resistance of thermocouple sensor means with respect to time.

5. A sensor according to any preceding Claim, characterised in that the means to monitor and or the estimation means comprises means to note a combination and/or sequence of values of resistance, and of change in resistance with respect to time, of thermocouple sensor means.

6. A sensor according to any preceding Claim characterised in that the means to monitor and/or estimation means includes means to apply a current signal to thermocouple sensor means.

7. A sensor according to any preceding Claim characterised by means to generate a warning signal in response to an output of the monitor means and/or the estimation means.

8. A sensor according to any preceding Claim characterised by means to switch operation from one thermocouple sensor means, for the sending of a parameter for measurement in the locality of the sensor head, to another such thermocouple sensor means which had not been in use, upon malfunction (whether by failure or deterioration), or estimation of such malfunction, of the said one thermocouple sensor means.

9. A sensor according to any preceding Claim characterised by at least two thermocouple sensor means for the sensing of a parameter for measurement in the locality of the sensor head; and means to switch operation from one thermocouple sensor means upon malfunction (whether by failure or deterioration), or estimation of such malfunction, to another which had not been in use.

10. A sensor according to any preceding Claim characterised in that the means to monitor comprises means to test and means to estimate failure of the state of the thermocouple sensor means which is not in use.

11. A sensor according to Claim 11 characterised in that the means to monitor is operable to test and estimate failure of the thermocouple sensor means which is not in use less frequently than the thermocouple sensor means which is in use.

12. A sensor according to any preceding Claim characterised in that the switch means comprises a solid-state device with means to disable a port to one thermocouple sensor means and means to enable a port to another thermocouple sensor means upon malfunction of the one thermocouple sensor means.

13. A sensor according to any of Claims 10 to 12 characterised in that the means to monitor comprises means to determine whether a characteristic of the thermocouple sensor means has exceeded a threshold value.

14. A sensor according to any of Claims 10 to 13 characterised in that the means to monitor comprises means to measure the resistance of thermocouple sensor means and/or means to measure the change in resistance of thermocouple sensor means with respect to time.

15. A sensor according to any preceding Claim characterised by means to correlate estimated time of malfunction with expected next service and emans to issue an appropriate warning of estimated malfunction is before or just after next service.

16. A method of operating a transmitter sensor comprising a thermocouple sensor means for the sensing of a parameter for measurement in the locality of the sensor, the method characterised by monitoring thermocouple sensor means to predict failure or deterioration of thermocouple sensor means.

17. A sensor according to Claim 16 characterised by estimating failure of thermocouple sensor means comprising determining whether the rate of change of one or more characteristics of the thermocouple sensor means has exceeded a threshold value.

18. A method according to Claim 16 or 17 characterised by estimating failure of thermocouple sensor means based on measured values of change of resistance with respect to time of thermocouple sensor means.

19. A method according to any of Claims 16 to 18 characterised in that the monitoring step and/or the estimating step includes applying a current signal to the thermocouple sensor means.

20. A method according to any of Claims 16 to 19 characterised in that the monitoring step and/or the estimating step comprises means to note a combination and/or sequence of values of resistance, and of change in resistance with respect of time, of thermocouple sensor means.

21. A method according to any of Claim 16 to 20 characterised by switching operation from one thermocouple sensor means for the sensing of a parameter for measurement in the locality of the sensor head to another thermocouple sensor means which has not been in use upon malfunction (whether by failure or deterioration), or estimation of such malfunction, of the said one thermocouple sensor means.

22. A method according to any of Claim 16 to 21 characterised by switching operation from one thermocouple sensor means upon malfunction (whether by failure or deterioration), or estimation of such malfunction, to the other which had not been in use.

23. A method according to any of Claims 16 to 23 characterised by testing the state of the thermocouple sensor means which is not in use.

24. A method according to Claim 23 characterised by testing the thermocouple sensor means which is not in use less frequently than the thermocouple sensor means which is in use.

25 A method according to any of Claims 16 to 24 characterised in that the switching step comprises disabling a port of a solid-state device to one thermocouple sensor means and enabling a port to another thermocouple sensor means upon malfunction of the one thermocouple sensor means.

26. A method according to any of Claims 16 to 25 characterised by correlating estimated time of malfunction with expected next service and issuing an appropriate warning if estimated malfunction is before or just after next service.