Calibration of combined oxygen and combustibles analyses.

This invention relates to the calibration of combined oxygen and combustibles analysers.

Many known volatile atmosphere processes, such as those carried out in boiler furnaces and coal pulverisers, require constant monitoring to detect potentially flammable or explosive conditions and alert an operator to such conditions.

Presently, several systems are being used to monitor potentially dangerous coal pulveriser atmospheres. Thermocouples and infra-red gas analysers are two systems used to monitor pulverisers. A shield is needed to protect these systems from the corrosive coal particles in the coal pulveriser. The shield reduces heat conduction to the thermocouple system, thereby reducing response time. Infra-red gas analysers need to condition the sample gas, also decreasing response time and sensitivity.

A more reliable method used to monitor volatile atmospheres in coal pulverisers is to provide signals indicative of either oxygen, CO, or both. A certain quantity of each of these elements in a volatile atmosphere becomes indicative of the presence of a hazardous condition. Combination oxygen/combustibles analysers such as the Model OL230 supplied by Bailey Controls Company, a division of The Babcock & Wilcox Company, are used to provide such signals or combinations of them.

These analysers must be calibrated frequently to make sure that they are providing an accurate measurement of the aforementioned elements in the volatile atmosphere. Presently, the calibration procedures for such analysers are not automated with the entire analyser control system.

The present method for calibrating these analysers is manual. An operator manually introduces a test gas into the analyser, maintaining the air pressure as if the analyser was in operation. While maintaining the test gas pressure, calibration potentiometers are manually adjusted and test voltage outputs are monitored by hand-held voltmeters until desired calibrated outputs are attained.

A known method of calibrating a combustibles signal consists of introducing a test gas into the analyser and maintaining the sample air pressure as in the manual calibration of the oxygen signal. Zero and span (maximum scale value) values from the test gas are adjusted by an operator. The incoming sample signal is then calibrated according to these adjusted values.

The known calibration systems for combination oxygen/combustibles analysers are also manual. This inhibits the calibration of a plurality of analysers by one operator. The oxygen calibration operation requires at least fifteen minutes of operator time. This manual operation introduces the possibility of operator error and does not allow for repeatability of zero and span values. Further, the calibration of both combustibles and oxygen signals is not coordinated and thus hinders a plurality of analysers from being calibrated concurrently.

United States Patent US-A-3 924 442 describes a system for continuously measuring volumetric concentrations of gas which is automatically self calibrating and will correct volumetric measurements to a predetermined reference level. The system generates signals in response to zero and span gas phases which signals are used to generate motor drive signals for resetting potentiometers controlling the gain of an analyser in order to adjust the output thereof.

Since, however, the oxygen signal can drift significantly out of calibration before an operator becomes aware of the need for recalibration, a significant error with the oxygen signal is generated by the drift before it is detected and corrected.

According to a first aspect of the invention there is provided an automatic, periodically calibrating system for continuous output of calibrated signals from a combined oxygen and combustibles analyser comprising:    a combined oxygen and combustibles analyser for sensing a level of oxygen and a level of combustibles in a volatile atmosphere and for producing a first sample signal indicative of the oxygen level and a second sample signal indicative of the combustibles level;    means for introducing zero and span calibration test gases into the analyser; and    means for periodically calibrating the analyser including: <ul id="ul0001" list-style="none"><li>(a) a data control unit for automatically controlling the calibration of the analyser, for presetting optimum zero and span values for oxygen and combustibles, and for presetting alarm limits for oxygen and combustibles;</li><li>(b) a timer unit connected to the data control unit for setting discrete time intervals to activate the means for introducing the zero and span calibration test gases into the analyser;</li><li>(c) a mechanical unit connected to the timer unit and to the analyser, for introducing zero and span calibration test gases into the analyser;</li><li>(d) means for calculating zero and span values for oxygen and combustibles while the calibration test gases are introduced into the analyser;</li><li>(e) means for comparing said calculated zero and span values for oxygen and combustibles to the preset alarm limits for oxygen and combustibles;</li><li>(f) means for activating an operator alarm if the zero and span values exceed the preset alarm limits for oxygen and combustibles;</li><li>(g) means for calculating oxygen and combustibles drift adjustments based upon the zero and span values for oxygen and combustibles obtained from the calibration test gases, and upon the preset zero and span values for oxygen and combustibles; and</li><li>(h) a memory unit, connected to the data control unit, and to the means set forth in (d), (e), (f) and (g) above, for storing the preset optimum zero and span values, and the alarm limits; and    means for applying the oxygen and combustibles drift adjustments concurrently to the first and second sample signals, according to a predetermined mathematical relationship, to obtain calibrated output signals indicative of the oxygen and combustibles levels in the volatile atmosphere.</li></ul>

According to a second aspect of the invention there is also provided a method of automatically and periodically calibrating output signals from a combined combustibles and oxygen analyser comprising the steps of:    providing a source of predetermined time sequenced control signals;    purging the analyser by backflushing atmospheric air through the analyser for a predetermined period in response to the control signals;    establishing a signal indicative of the end of the purge;    introducing a zero calibration test gas to the analyser in response to the signal indicative of the end of the purge and providing a signal indicative of the end of the zero calibration test gas introduction;    introducing a maximum scale value calibration test gas to the analyser in response to the signal indicative of the end of the zero calibration test gas introduction and providing a signal indicative of the end of the maximum scale value calibration test gas introduction; and    adjusting an output of the analyser to read zero and maximum scale upon the signal indicative of the end of the span gas introduction.

A preferred embodiment of the present invention described hereinbelow overcomes or at least alleviates the problems associated with the non-automated control systems for oxygen/combustibles sensors by providing an improved safety monitoring system for oxygen and combustibles analysers using automatic calibration. The preferred system coordinates an automatic periodic calibration system with a signal sensing and safety alarm system to provide an entire monitoring/alarm/calibration system for oxygen and combustibles analysers. The system calibrates both the oxygen and combustibles signals in a fully automated fashion. A calibration procedure is initiated automatically on a predetermined interval such as every twenty-four hours. The calibration is performed concurrently on the oxygen signal and the combustible signal. Further, an alarm function is set into operation whenever calibrated zero and span values go beyond a preset limit. The preferred system then ensures that an accurate, hands-off calibration will be periodically performed. The preferred system is an accurate and reliable automatic calibration system which periodically calibrates the oxygen signal of the oxygen/combustibles analyser and coordinates it into a total control system for monitor/alarm/calibration of an entire safety monitoring unit. Further, the preferred system automatically adjusts both the span and zero values of both the oxygen and combustibles signals to minimise the effect of drift on the current analyser signal.

The invention will now be further described, by way of illustrative and non-limiting example, with reference to the accompanying drawings, in which: <ul id="ul0002" list-style="none"><li>Figure 1 is a schematic drawing of an automated calibration control system for oxygen and combustibles analysers according to a preferred embodiment of the present invention;</li><li>Figure 2 is a schematic view of control logic for a data control unit and a mechanical unit shown in Figure 1;</li><li>Figure 3 is a logic diagram for a logic unit shown in Figure 1;</li><li>Figure 4 is a logic diagram for an oxygen signal unit shown in Figure 1;</li><li>Figure 5 is an oxygen calibration graph used by the oxygen signal unit of Figure 4;</li><li>Figure 6 is a logic diagram for a combustibles (CO_e) signal unit shown in Figure 1; and</li><li>Figure 7 is a combustibles calibration graph used by the combustibles signal unit of Figure 6.</li></ul>

Referring now to the drawings generally, with particular reference to Figure 1, an automatic calibration system or calibration control system 10 includes a known process control computer 12, such as a Bailey Controls Company Model Network 90, and a known oxygen/combustibles analyser 14, such as a Bailey Controls Company Model OL230 gas analyser. All operations and calculations performed by the computer 12 use known hardware/firmward algorithms. These algorithms are called function block units or function blocks because each algorithm performs a specific function. The hardware/firmware used by the function blocks includes combinations of microprocessors with internal, unchangeable operation commands to perform the desired function and will be referred to as units.

More particularly, the system 10 includes a data control unit 16 which energises a mechanical unit 18, by way of a timer unit 20, allowing a manual input of data to be microprocessor memory unit 22. The mechanical unit 18 allows calibration gases to be introduced into the analyser 14. The memory unit 22 provides storage for and access to all necessary values used by the entire system 10. The timer unit 20 is an internal digital clock circuit which controls all timing functions needed by the system 10. A logic unit 24 receives inputs from the memory unit 22 and alerts an operator if the inputs go beyond preset limits for zero and maximum scale (hereafter referred to as span) and performs calculations needed by a combustibles (hereafter referred to as CO_e) signal unit 26 and an oxygen (hereafter referred to as 0₂) signal unit 28. The signal units 26 and 28 respectively perform the actual calibration on a CO_e indicating signal 30 and an 0₂ indicating signal 32, originating from the oxygen/combustibles analyser 14.

Referring now to Figure 2, the data control unit 16 has a manual data input station 34. The manual data input station 34 may be a known computerised data input keyboard or any other known manual data input apparatus. The manual data input station 34 allows an operator to selectively set the control unit 16 in an automatic mode 36, or to start a calibration sequence manually through a manual start button 38, to preset optimum zero and span values for CO_e (40, 44) and 0₂ (42, 48), respectively, and to preset alarm limits 50 for the aforementioned optimum zero and span values. The preset values 40, 42, 44, 48, 50 may be tuned or adjusted by the operator at any time through the manual data input station 34.

Usually, the control unit 16 will be set in the automatic mode 36. When in the automatic mode 36, the analyser 14 will be calibrated on a twenty-four hour interval 52. The interval will be maintained by the timer unit 20. The timer unit 20 provides that a calibration sequence will be peformed on the analyser 14 every day to ensure the accuracy of the oxygen and combustibles signals 30, 32 originating from the analyser 14. The manual start button 38 may also initiate a calibration sequency and will override the timer unit 20 to enable a calibration sequence to be performed at any time desired by the operator.

When a calibration sequency is initiated either by actuating the manual start button 38 or by the timer unit 20, an energise signal 54 is sent to the mechanical unit 18 along a line 56 from the process control computer 12. The mechanical unit 18 is physically attached by any known method to the analyser 14.

The mechanical unit 18 begins the calibration sequence by performing a cleaning operation 58 on the analyser 14. The cleaning operation 58 is called a "puffback". In this operation, the mechanical unit 18 forces air back through the analyser 14 to clean the analyser 14. The cleaning operation 58 is controlled by the timer unit 20 to continue for a period of ten seconds.

When the cleaning operation 58 is complete, known solenoid valves 60 and 62 were energised to open by a control signal 64 allowing a zero calibration gas 66 to flow through a flowmeter 68 into the analyser 14. The gas 66 flows through the analyser 14 for a seven minute interval 70 as controlled by the timer unit 20. The timer unit 20 sends a control signal 67 to deenergise the valve 60 when the seven minute interval 70 is over. The gas 66, used for zero calibration, usually consists of 1.0% 0₂ and 0 ppm CO_e.

During the seven minute interval 70, zero value signals for CO_e and 0₂ as sensed by the analyser 14 are sent along a line 72 to the computer 12. When the seven minute interval 70 is over, signals 74, 76 (Figure 3) indicative of a one minute moving average of zero values for both CO_e (74) and 0₂ (76) are calculated by an internal averaging function block unit 78 of the computer 12. The signals 74 and 76 are then sent to the memory unit 22 along a line 80.

At the end of the seven minute interval 70, a third solenoid valve 82 is energised by a control signal 84 originating from the timer unit 20, allowing a span gas 86 to flow through the flowmeter 68 to the analyser 14. The span gas 86 usually contains 20.9% 0₂ and 250 ppm CO_e. The span gas 86 flows through the analyser 14 for a five minute interval 88. At the end of the five minute interval 88 the timer unit 20 sends a control signal 90 to deenergise the valves 68 and 82.

During the five minute interval 88, span values for CO_e and 0₂ as sensed by the analyser 14 are sent to the computer 12 along the line 72. When the five minute interval 88 lapses, one minute moving averages of span values 92, 94 (Figure 3) for CO_e (92) and 0₂ (94) as sensed by the analyser 14 are calculated by the averaging function block unit 78 of the computer 12. The averages 92 and 94 are then sent to the memory unit 22 of the computer 12 along the line 80. The analyser 14 then returns to analysing a sample gas 96 from a coal pulveriser 98 and sending the signals indicative of CO_e and 0₂ content, 30 and 32 respectively, to the CO_e signal unit 26 and the 0₂ signal unit 28, respectively.

Referring now to Figure 3, when the data control unit 16 energises the mechanical unit 18, it also energises the memory unit 22. This allows the CO_e and 0₂ zero values, 74 and 76 respectively, and the CO_e and 0₂ span values, 92 and 94 respectively, to be stored in the memory unit 22.

When the memory unit 22 receives the zero values 74, 76 and the span values 92, 94, the logic unit 24 accesses them. The logic unit 24 first compares the values 74, 76, 92, 94 to the preset alarm limits 50 in a compare function block unit 100. If any of the zero or span values 74, 76, 92, 94 for CO_e and 0₂ are above their respective alarm limits 50, an alarm 102 will alert the operator to an inferior calibration condition and he will take steps necessary to correct it, including but not limited to a manual adjustment of the analyser. These instances are rare due to daily calibration and the daily cleaning operation 58.

The logic unit 24 then performs a calculation using the values 74, 76, 92, 94; the preset optimum zero values for CO_e and 0₂, 40 and 42 respectively; and the preset span values for CO_e and 0₂, 44 and 48 respectively. The preset values 40, 42, 44, 48 are accessed from the memory unit 22. The calculations are performed in difference function block units 104 and 106 and a dividing function block unit 108. The same calculation is performed on both the CO_e values 74, 92, 40, 44 and the 0₂ values 76, 94, 42, 48 according to the following equation:<maths id="math0001" num=""><math display="block"><mrow><mtext>Da= </mtext><mfrac><mrow><msub><mrow><mtext>P</mtext></mrow><mrow><mtext>s</mtext></mrow></msub><msub><mrow><mtext> - P</mtext></mrow><mrow><mtext>z</mtext></mrow></msub></mrow><mrow><msub><mrow><mtext>A</mtext></mrow><mrow><mtext>s</mtext></mrow></msub><msub><mrow><mtext> - A</mtext></mrow><mrow><mtext>z</mtext></mrow></msub></mrow></mfrac></mrow></math><img file="EP0242946B1_D0001.tif" /></maths>    where:       Da = drift adjustment;       P_s = preset span value (CO_e-44 0₂-48);       P_z = preset zero value (CO_e-40 0₂-42);       A_s = actual span value (CO_e-92 0₂-94); and       A_z = actual zero value (CO_e-74 0₂-76).

The logic unit 24 sends calculated drift adjustments DaCO_e 110 and Da0₂ 112 to the CO_e signal unit 26 and the 0₂ signal unit 28, respectively. The drift adjustments DaCO_e 110 and Da0₂ 112 are drift adjustments to be applied to the CO_e and 0₂ sample signals 30 and 32, respectively. The drift adjustments 110 and 112 minimise the drift of the zero values 74, 76 and span values 92, 94 from their preset values 40, 42 and 44, 48, respectively.

The 0₂ signal unit 28, which may be embodied as shown in Figure 4, takes the Da0₂ calculation 112 from the logic unit 24, the 0₂P_z value 42 and the 0₂A_z value 76 from the memory unit 22, and the 0₂ uncalibrated signal 32 from the analyser 14, and performs the following calculation on them:<maths id="math0002" num=""><math display="block"><mrow><msub><mrow><mtext>0₂Cal = Da(0₂s) + P</mtext></mrow><mrow><mtext>z</mtext></mrow></msub><msub><mrow><mtext> - Da(A</mtext></mrow><mrow><mtext>z</mtext></mrow></msub><mtext>)</mtext></mrow></math><img file="EP0242946B1_D0002.tif" /></maths>    where:       0₂Cal = calibrated 0₂ output 114;       Da = drift adjustment 112;       O₂s = 0₂ sample signal 32;       P_z = preset 0₂ value 42; and       A_z = actual 0₂ zero value 76

The calculation to arrive at the calibrated 0₂ output 114 is achieved by proportional function block units 116 and 118, a summation function block unit 120, and a difference function block unit 122.

The output 114 is continously monitored in a compare function block unit 124 to ensure that the coal pulveriser 98 being monitored does not go into a potentially dangerous situation. When the output 114 goes above its preset alarm value 50, the operator is alerted of the dangerous situation by an alarm 126 and can take necessary steps to stabilise the coal pulveriser 98.

The output 114 is sent to a known chart recorder 128 and also may be displayed on a known cathode ray tube monitor 130.

The 0₂ signal unit 28 creates a calibration line graph 132, depicted in Figure 5, through its calculation. The Da0₂ value 112 is actually a proportion of error between the preset optimum scale and the scale the analyser 14 is outputting. The Da0₂ value 112 is then multiplied by the analyser 0₂ output 32 to arrive at the calibrated 0₂ output 114 with an offset added in for the preset zero value 42. The calibration graph 132 depicts the calibrated output 114, equal to 17.625% 0₂ content, for the given uncalibrated analyser output 32, equal to 19% 0₂. This calculation is performed continuously on the uncalibrated output 32 to give the continuous calibrated 0₂ signal 114.

The CO_e signal unit 26, which may be embodied as shown in Figure 6, operates in the same fashion as the 0₂ signal unit 28. The CO_e signal unit 26 uses the same calculation used in the 0₂ signal unit 28, but performs this calculation using the CO_e values 30, 74, 110. This calculation has no zero offset added in, however, because the preset zero value 28 is always zero. This calculation is performed in a difference function block unit 134 and a proportional function block unit 136. The CO_e signal unit also uses a compare function block unit 138 to monitor a calibrated CO_e signal 140 and alert the operator to a dangerous condition by an alarm 142. The calibrated CO_e signal 140 may also be displayed on the cathode ray tube monitor 130 or the recorder 128.

A CO_e calibration graph 144 is shown in Figure 7.