Mining Science and Technology, 11 (1990) 191-197 191

Elsevier Science Publishers B.V., Amsterdam

Flow determination in underground airways from small differential pressure measurements

E. Jacques and J. Patigny Department of Mechanical Engineering, Division of Thermodynamics, Catholic University of Louvain, Place du Levant 2,

B-1348 Louvain-la-Neuve (Belgium)

(Received December 8, 1989; accepted February 21, 1990)

ABSTRACT

Jacques, E. and Patigny, J., 1990. Flow determination in underground airways from small differential pressure measurements. Min. Sci. Technol., 11: 191-197.

A new technique for measuring the flowrate in underground airways has been developed. It takes advantage of a very sensitive differential pressure measuring transducer, equipped with a microcomputer. After a brief description of the sensor and its control unit, some of the technical characteristics investigated by tests carried out in the laboratory and in the field are discussed. Special attention is paid to the appropriate set-up of the equipment.

Introduction

When optimizing the control of the ventila- tion system for an entire mine, the need for an accurate measurement system is undenia- ble. Common techniques, especially the one used to measure air flow, may well be accept- able for monitoring the ventilation, but its shortcomings introduce many difficulties [1] if these flow measurements are used in con- trol loops.

With a few exceptions [2] the flow rate q is indirectly obtained from parameters such as the velocity c and the cross-sectional area of the flow A [3,4,5], according to the equation:

q = k c A

where k denotes the ratio of the mean cross- sectional velocity to a representative point velocity measurement. As far as steady, or near steady flows are concerned, k may be considered to be a constant and can be ex-

perimentally obtained for any particular type of cross-section. Hence, careful calibration of air flow measurements by means of, for ex- ample, a velocity integration technique [6], periodically repeated, could provide very rea- sonable results.

Unfortunately, in the case of either the cross-sectional area being partially obstructed or the velocity profile being regularly dis- turbed by slow-moving objects, this method rapidly becomes unsuitable for control or even monitoring purposes: errors exceeding 20% are not unusual.

Measurement method

The method proposed here is based upon the measurement of the pressure loss Ap be- tween two cross-sections (L metres apart) caused by the friction of the fluid flowing through a duct. Provided the Reynolds num-

0167-9031/90/$03.50 © 1990 - Elsevier Science Publishers B.V.

192 E. JACQUES AND J. PATIGNY

bet is large, which is always the case for main airways, the flow rate q can be calculated from the equation:

A p = Rq 2

where R denotes the resistance of the part of the duct under consideration. The value of R depends upon the energy loss coefficient X, the geometrical characteristics of the duct, such as its length and hydraulic diameter, and the mean value of the air density p.

Since this method has been developed for continuous monitoring of the flow rate, one can assume that these geometrical character- istics and the energy loss coefficient of the duct are known before applying this measure- ment method. If not, the resistance value or, more precisely, the quanti ty R / p may be

obtained with sufficient accuracy by using an accurate calibration method.

The only real difficulty encountered whilst applying this method in a mining environ- ment is the measurement of a differential pressure of less than 100 Pa. Indeed, in most cases, the differential pressure will hardly ex- ceed 20 Pa, which should be measured with an accuracy of at least 0.2 Pa. Until recently instruments with that sensitivity were only being used in laboratories.

Advantages of this method

(1) The method does not in any way ob- struct the flow and the entire cross-section is kept flee. The latter feature is particularly

I Surface I i Underground 9

I O-10V

I J ~ = = @ 6 V ' K - - ] - -

4 5

[ I

\ \ \ \ \ \ \ \ \ \ \ \ ~ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ w , ~ \ \ \ \ \ \ \

F i g . 1. Schematic equipment set-up. 1 = Airway or passageway; 2, 3 = Measurement cross-sections; 4, 5 = Long hoses; 6 = 3-way electrovalve; 7 = Electromanometer (transducer); 8 = Control unit and A/D converter; 9 = Digital trans- mission; 10 = Microcomputer and peripherals.

F L O W D E T E R M I N A T I O N IN U N D E R G R O U N D A I R W A Y S 193

important in airways along which objects or persons have to move.

(2) Taking a square root is always an er- ror-reducing operation.

(3) A stray object in a measurement sec- tion usually leads to error in the flow rate calculations. When using a point velocity method, this error is due to a partial reduc- tion in the cross-section and the resulting distortion of the velocity profile. The method described here is based on the measurement of a differential pressure and the calculations should take into account the additional fric- tion losses caused by the object. If this is not done, there will also be an error but in most cases it will be smaller.

Flow meter

A schematic drawing of the measurement system designed for underground applications is shown in Fig. 1. Only the lower part of it refers to the flow meter described in this paper. Figure 2 shows pictures of the instru- ment installed in an underground airway.

Transducer

The pressure transducer is of the di- aphragm capacitance type. Two cavities are separated by a taut metal diaphragm with an electrode fixed close to it on either side, so constituting two air dielectric capacitors. The capacitors form two tuned circuits which are coupled to a local transistor oscillator. A pressure difference between the two cavities deflects the diaphragm, changing the capaci- tance in each of the tuned circuits and conse- quently unbalances the voltage across them. This imbalance, properly rectified and ampli- fied, provides a low impedance output signal ranging from 0 to 10 V, proportional to the applied differential pressure.

The following technical specifications were given by the manufacturer:

(1) Measuring ranges: any range between 10 and 10,000 Pa.

(2) Zero drift: 0 .05%/° C from 20 ° C. (3) Long-term drift: 1%. (4) Hysteresis: 0.25%. (5) Linearity: < 0.5%. (6) Overload: 100 times. (7) Zero position effect: 50% of the range. (8) Span adjustment: from 100% to 20%.

Control unit

When the transducer is used as a flow meter, it may be fitted with a flow lineariser circuit, e.g. an integrated circuit connected in feedback mode to provide an electrical output signal which is the square root of the applied input signal. However, since the information measured has to be processed, it is worthwhile replacing the flow lineariser circuit with a data processing unit such as a microcom- puter. Besides enabling a square root to be extracted, this solution has many advantages:

(1) The output signal of the transducer is digitized into an eight-bit set and stored for a certain period of time. This is particularly useful when time-related damping or smooth- ing operations are to be carried out.

(2) Integrated, self-calibrating procedures are possible;

(3) A calibration curve may be fed in in- stead of a factor

(4) If necessary, automatic temperature and pressure compensation may be applied in order to correct the air density value.

(5) For long-distance transmission of re- sults measured, digital transmission is without doubt the most reliable mode. Appropriate communication protocols may be programm- ed to carry this out.

(6) Continuous display in units familiar to the user, optional print out, alarm-levels set- ting, etc., are also benefits which should not be overlooked.

Moreover, this powerful tool can meet other specific demands: for instance, the square

194 E. JACQUES AND J, PATIGNY

Fig. 2. Flow meter installed in a stonedrift .

F L O W D E T E R M I N A T I O N I N U N D E R G R O U N D A I R W A Y S 195

root could be replaced by a more general equation of the type q = KAp x, with x > 0.5 for those airways where the Reynolds num- bers are small.

Testing and results

In the laboratory

The transducer has been subjected to many different tests. Most of them confirm the technical data previously mentioned; on some points the results were even better. However, where the zero and gain drifts are concerned, the specifications of the manufacturer turned out to be too optimistic. Whereas a calibra- tion of the gain factor from time to time (twice a year seems to be satisfactory) may enable the drift to be maintained within rea- sonable limits, the error due to the zero drift requires a more elaborate solution. This has been achieved by mounting a three-way elec- trovalve between the two pressure ports of the transducer, as shown in Fig. 1. The applied differential pressure is thus zero. The re- corded residual output voltage is then used in the next stage to correct the measured values.

In the field

Some of these instruments have been work- ing in underground mine airways for several years. The environment is, as far as humidity and dust are concerned, rather severe, and thus forms one of the most stringent tests the meter can be put through. The scale range choosen depends on the magnitude of the flows and on the average energy loss coeffi- cient of the duct; in the application described here it was from 0 to 50 Pa.

When installed, the instrument has one sampling point at the up-stream cross-section and the other at the down-stream cross-sec- tion which are connected by two long hoses. Their length is at least L/2, where L is the

total length of the measurement section of the duct, i.e., the distance separating the two cross-sections between which the differential pressure is being measured.

From experience gained from the tests over a long period of time the following practical observations can be made.

(1) Since the "velocity head" term might be considerable compared to the static dif- ferential pressure, special care should be taken when installing the free ends of the hoses. They should be fixed in such a way that the velocity head has no effect upon the measure- ment.

(2) Due to all kinds of instabilities in the flow, the complete input signal is not useful. Depending on the frequencies to be cut off, damping should be achieved in an electrical, a mechanical or a numerical way. In practice, however, these three are often combined, since their effects are not entirely the same.

Electrical damping: The transducer is fitted with a variable time constant on the output signal, which can be set by a potentiometer at up to 30 s. For this application a time of 15 s seems to be adequate.

Numerical damping: Using a microcom- puter, any type of filter can be implemented. However, experience showed that the averag- ing of N consecutive values recorded, after having removed the highest and the lowest values, turned out to be very satisfactory. Depending on the programmed scan rate ( . . . I Hz . . . ) and the expected response time ( . . . 15 s . . . ) values around ten were allocated to N.

Mechanical damping: Although it might not be obvious, the hoses, along which the pres- sure information travels to the input ports of the transducer, have a considerable damping effect, depending on their length and their inner diameter. Putting their behaviour into an equation [7], the resulting mathematical model enables the ratio J AplJ to I Ap0l to be drawn as a function of the fluctuating pres- sure frequency. Apt is the decreasing value of

196 E . J A C Q U E S A N D J . P A T I t 3 N Y

the input differential pressure Ap0, after hav- ing travelled a length l along the hose. Figure 3 depicts this ratio for several values of the internal diameters d. It shows that the damp- ing effect for a given frequency increases with decreasing values of the internal diameter and, more precisely, with the square of i t . How- ever, it should be noted that according to another study [7] the influence of temperature is more important for smaller diameters. This means that the smallest diameter does not necessarily lead to the smoothest and the most accurate input signal at the ports of the meter.

(3) Care should be taken to have both hoses of the same length. If the hoses are not of equal length, a sudden overall pressure rise in the airway, with pressure step Spo applied simultaneously to the end of each hose, will reach the ports with a different time lag. In fact, this pressure step is seen as Sp, by the first sample point of the transducer and, at the same time, as Sp2 by the second sample point. As the flow meter only senses the dif- ferential pressure, it will display, for a certain length of time, a changing flow rate; although in fact it did not change at all.

Figure 4 shows an example of this phenom- enon in the case of a sudden 100 Pa pressure rise. The length of the hoses is respectively 100 m and 200 m, and their internal diameter is 3.4 mm. Figure 4(a) plots both the curves

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10-3 I0 • I0"I IOO

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Fig. 3. Mechanical filtering capacity of a hose with inner diameter d.

'100

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4O

2O

A 0

Sp2

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B 5 l0 15 20 25

[s] Fig. 4. Scale saturation phenomenon due to unbalanced set-up. A. Pressure as a function of time. B. Difference between pressure recorded from the two sampling points with time. (See text for full details).

Sp, and Sp2 as a function of time. The dif- ference between Sp, and Sp2, (Fig. 4b) is in fact the differential pressure sensed by the meter. Since the manomete r is designed for pressures in the range 0-50 Pa, it is remarka- ble to observe that the full scale value is held for about 6 s. The real value of 15 Pa is reached only after 39 s.

Conclusions

When flow rates of 12-15 m3/s and over are to be measured in large airways over a distance of a few hundred meters, the method described here seems to give more reliable results than a point velocity method, particu- larly in the case of passageways. Indeed, if the precautions referred to above are taken,

FLOW D E T E R M I N A T I O N IN U N D E R G R O U N D AIRWAYS 197

the resulting flow rate value should be accu- rate to within _+ 5%. However, at tention is drawn to one stringent limitation: no pressure or heat sources can be permitted in the sec- tion used for measurements.

Several of these instruments have been in operation for m a n y years underground. They have shown good durabil i ty and have withstood the severe conditions within the mine.

Acknowledgements

The work described in this paper was sponsored by the European Economic Com- muni ty and the "Kempense Steenkolenmij- nen N.V.", Belgium, where the equipment is in operation. The laboratory tests were car- ried out a t the Institute for Mining Hygiene, Belgium. The authors must also express their gratitude to Dr. E.A. Spencer O.B.E., who gave useful comments on the text of the first draft.

Notation

A - - cross-sectional area of the duct (m 2) c - - air velocity (m/s ) d - - inner diameter of the hose (m) k - - proportional factor K - - inverse value of R (m8/Ns 2) l - - length (m)

L

N _ _

p - - q - -

R - - A p - - X - - p - -

distance between the cross-sections (In) number of recorded values pressure step (Pa) flow rate (m3/s) resistance of the duct (Ns2 /m 8) differential pressure (Pa) energy loss coefficient air density ( k g / m 3)

References

1 Patigny, J., Cerulus, J. and Jacques, E., Commande centralis6e des ventilateurs et portes regulatrices sous contr61e d'ordinateur. Rev. Inst. Hyg. Mines, Hasselt, Belg., 37 (1) (1982): 5-55.

2 Thimons, E.D. and Kissell, F.N., Using sulphur hexafluoride tracer gas for mine ventilation analysis. Proc. Mine Ventilation Congr. (Johannesburg) (1975), pp. 97-102.

3 Hayward, A.T.J., Flowmeters, a Basic Guide and Source-Book for Users. Macmillan, New York (1979).

4 Rheinhard, A., The Thermistor anemometers, multi- function ATM 689. Doc. Tech. Charbonnages Ft., 4 (1971): 250-252.

5 Berryman, R.J., Measurement of swirling gas flow in ducts and fan inlets. Int. Conf. on Industrial Flow Measurement Onshore and Offshore (London) (1987).

6 Mercenier, P., Patigny, J. and Pirson, J., Int6gration de la carte des vitesses dans une section droite d'une conduite. Extension de la m6thode Log-Tchebycheff pour des sections rectangulaires aux sections d'autre forme. Rev. M Tijdschr., 28 (4) (1982): 293-304.

7 Jacques, E. and Patigny, J., Mechanical and Numeri- cal Filters Used Together with Electromanometers: A Way to Improve the Performance of Air Flow Mea- surement by P.D. Methods. VDI-Verlag, DiSsseldorf (1989), pp. 287-294.