CH684657A5 - Method and apparatus for detecting the flow in a pipe. - Google Patents

Description

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description

The invention relates to a method and a device for detecting the flow in a conduit according to the preambles of claims 1 and 6.

Such methods are advantageously used to record the flow rate of a liquid or gaseous flow medium, the flow rate recorded being used, for example, for energy billing.

A method for determining the flow of a medium flowing through a throttle point is known from DIN 1952 (flow measurement with orifices, nozzles and Venturi tubes in fully flowed tubes with a circular cross section, German Institute for Standardization, July 1982). In the process, the temperature of the medium and the difference between the static pressures above the throttle point are measured and the flow rate is then calculated using a standard formula.

The standard formula for calculating the volume flow q is therefore in a first equation (G1):

q = a • E • (d2Jt / 4) • (2 • 8p / r) i'2 (Q1)

where a is the flow number,

E the expansion number,

d the diameter of the opening of the throttle point,

8p is the pressure difference across the throttle point and r is the temperature-dependent density of the medium.

As the pressure difference becomes smaller, the standard formula becomes increasingly imprecise; the influences of the Reynolds number and the temperature of the medium become relatively large.

The invention has for its object to provide a method for detecting the flow, which is accurate even with small volume flows, and to provide a device for accurate detection of the flow, which can be compared with little effort.

According to the invention, the stated object is achieved by the features of claims 1 and 6. Advantageous configurations result from the dependent claims.

Exemplary embodiments of the invention are explained in more detail below with reference to the drawing.

Show it:

Fig. 1 shows the basic structure of a device for detecting the flow and

2 shows a data flow diagram for comparing the device.

In FIG. 1, 1 means a conduit pipe through which a liquid or gaseous flow medium flows and which has a throttle point 2. A device for detecting the flow q in the conduit 1 has a pressure difference measuring device 3 arranged above the throttle point 2, a temperature measuring device 4 and a computing unit 5 shown in broken lines.

The temperature T of the flow medium can be measured with the temperature measuring device 4, while the pressure difference 5p in the flow medium above the throttle point 2, which is referred to as differential pressure in DIN 1952, can be detected by the pressure difference measuring device 3.

The basic structure of the computing unit 5 is illustrated using a data flow diagram. In the chosen type of representation known from the literature (DJ Hatley, IA Pirbhai: Stratégies for Real-Time System Specification, Dorset House, NY 1988), a circle means an activity, a rectangle an adjacent system (terminator) and an arrow a communication channel (channel) for the transmission of data and / or events, the arrowhead pointing in the essential data flow direction. A data store (pool), which is generally available for several activities, is represented by two parallel lines of the same length. Furthermore, for example, an arrangement of two activities connected by a communication channel is equivalent to a single activity which fulfills all the tasks of the two activities.

Activities are implemented as an electronic circuit and / or as a process, program piece or routine.

The computing unit 5 has a first input 6 for a first communication channel 7 assigned to the temperature T, a second input 8 for a second communication channel 9 assigned to the pressure difference 8p and an output 10 for a third communication channel 11 assigned to the flow q. Furthermore, the computing unit 5 has a first activity 12, a second activity 13 and a data memory 14 connected to the second activity 13. The first activity 12 is connected to the second activity 13 via a fourth communication channel 15 assigned to the density r of the flow medium .

The first communication channel 7 connects the temperature measuring device 4 with the first activity 12 and also with the second activity 13, while the second communication channel 9 from the pressure dif2

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Reference measuring device 3 leads to the second activity 13. An application unit 16 is connected to the second activity 13 via the third communication channel 11.

The device for detecting the flow in the conduit 1 is connected in FIG. 2 to carry out a calibration method with a calibration device 17 shown in dashed lines, which has a third activity 18. The conduit 1 is guided through a reference flow meter 19 such that the flow of the flow medium in the conduit 1 as a reference flow qR can be detected with high precision by the reference flow meter 19.

The reference flow meter 19 is, for example, a known magnetic-inductive flow meter.

The flow q can be transmitted from the second activity 13 to the third activity 18 via the third communication channel 11. Furthermore, the temperature measuring device 4 are connected to the third activity 18 via the first communication channel 7, the pressure difference measuring device 3 via the communication channel 9 and the reference flow meter 19 via a fifth communication channel 20 assigned to the reference flow qR.

Furthermore, the third activity 18 is advantageously connected to the data memory 14 via a sixth communication channel 21.

If required, only a single bidirectional channel is physically realized between the computing unit 5 and the matching device 17, via which all the data to be exchanged between the computing unit 5 and the matching device 17 can be conducted.

1 shows the data flow of the device in an operating phase, in which the computing unit 5, from the temperature T measured by the temperature measuring device 4 and the pressure difference Sp measured by the pressure difference measuring device 3, shows the flow q q usable for the application unit 16, for example in the direction of an arrow 22 through the throttle 2 flowing fluid.

The second activity 13 advantageously calculates the flow q using a new formula using the temperature T, the density r of the flow medium calculated by the first activity 12, the pressure difference 8p and at least one correction constant, an adjustment value K and an adjustment temperature Ta.

In a first advantageous variant of the computing unit 5, the correction constant / correction constant, the adjustment value K and the adjustment temperature Ta are stored in the data memory 14 and are available for the second activity 13.

In a second advantageous variant of the computing unit 5, only the correction constant / correction constant and the adjustment value K are stored in the data memory 14, while the adjustment temperature Ta is realized as a predetermined value of the second activity 13.

With a single constant Z, the standard formula given in the first equation (G1) has the structure according to a second equation (G2):

q = Z ■ (8p / r) 1/2 (G2)

The only constant Z of the second equation (G2) is advantageously replaced by the product formed from the comparison value K and a first correction constant Di, which results in the structure according to a third equation (G3) for the standard formula:

q = K • Di - (Sp / r) i / 2 (G3)

The new formula with which the corrected flow qb can be calculated is advantageously developed from the third equation (G3) by supplementing the third equation (G3) with a first term 01 and a second term 02 to a fourth equation (G4) becomes:

qb = K • Di • (5p / r) 1/2 + 61 + 02 (G4)

With a second correction constant D2, the first term © i is advantageously defined according to a fifth equation (G5):

01 = K2 • Da • (Sp / r) (G5)

and the second term 02 is advantageously determined with a third correction constant D3 according to a sixth equation (G6):

02 = K • Di • D3 • (T-Ta) • (5p / r) 1/2 (G6)

By inserting the fifth equation (G5) and also the sixth equation (G6) in the fourth equation (G4), a seventh equation (G7) results in a first advantageous variant of the new formula with which the flow qb can be calculated as a corrected volume flow:

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qb = K • Di • (8p / r) 1/2 + K2 • D2 • (8p / r) + K • Di • D3 • (T-Ta) • (8p / r) i'2 (G7)

It goes without saying that instead of the volume flow, the mass flow can be corrected in the manner shown if the corresponding standard formula for the mass flow is expanded instead of the first equation (G1). The alignment of the process to volume flow is therefore to be understood as an example.

A second advantageous variant of the new formula is formed by omitting the second term 62 in the fourth equation (G4). The second variant is therefore an eighth equation (G8):

qb = K • Di • (8p / r) i / 2 + K2 - D2 • (Sp / r) (G8)

The two variants of the new formula are advantageously constructed in such a way that they contain the only adjustment value K and the correction constants Di, D2 and D3 which can be determined with little effort in one phase of the adjustment method.

A correction method is described below by means of which the adjustment value K and the correction constants Di, D2 and D3 can be determined.

For the sake of clarity, the seventh equation (G7) is rewritten as a ninth equation (G9):

qb = {1 + D3 • (T-Ta)} • K • Di • (5p / r) iß + K2 - D2 • (5p / r) (G9)

If the value of the first correction constant Di is set to one and the values of the second correction constant D2 and the third correction constant D3 are set to zero in the ninth equation (G9), an incorrect flow q * can be calculated according to a tenth equation (G10):

q * = K - (8p / r) i / 2 (G10)

An eleventh equation is derived from the tenth equation (G10), with which the adjustment value K can be determined at an adjustment point at the adjustment temperature Ta:

K = q0 • (ro / 5po) 1/2 (G11)

where the quantities indicated with zero at the selected adjustment point q0 can be measured or calculated at the adjustment temperature Ta.

A measurement deviation e related to the reference flow qR - the true flow - is defined in a twelfth equation (G12):

e = (q * / qR) - 1 (G12)

With a correction function b compensating the measurement deviation e, a thirteenth equation (G13) is established for the corrected flow qb:

qb = (1-b) • q * (G13)

whereupon for the correction function b a fourteenth equation (G14) is formed from the three equations (G13), (G9) and (G10):

b = [1- {1 + D3 • (T-Ta)} • Di] - D2 • q * (G14)

A total measurement deviation f of the corrected flow qb related to the reference flow qR is defined in a fifteenth equation (G15):

f = (qb / qR) -1 (G15)

The two equations (G12) and (G13) are used in equation (G15) and a sixteenth equation (G16) is formed by transformation:

f = e- b- e- b (G16)

Assuming that the measurement deviation e is small, the mixed term e • b is omitted and a seventeenth equation (G17) is defined for the total measurement deviation f:

f <= e - b (G17)

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To determine the three correction constants Di, D2 and D3, zero is used for the total measurement error f in the seventeenth equation (G17) and an eighteenth equation (G18) is formed from it:

e = b (G18)

By inserting the fourteenth equation (G14) in the eighteenth equation (G18), a nineteenth equation (G19) is formed for the measurement deviation e:

e = [1 - {1 + D3 - (T-Ta)} ■ Di] - D2 • q * (G19)

With three advantageously defined determination points, of which a first determination point is, for example, the maximum usable reference flow value qR2 with the occurring measurement deviation e2 and a second determination point the minimally usable reference flow value qm with the occurring measurement deviation ei, the three correction constants Di, D2 and D3 determines:

With Di = 1 and T = Ta and the two defined determination points, a twentieth equation (G20) is formed from the nineteenth equation (G19):

D2 = (ei-e2) / (qR2-qm) (G20)

The second correction constant D2 can advantageously be determined according to the twentieth equation (G20).

With the second correction constant D2 and T = Ta and a determination point, for example the second, a twenty-first equation (G21) is derived from the nineteenth equation (G19):

Di = 1 - 62 - D2 • qR2 (G21)

The first correction constant Di can advantageously be determined using the twenty-first equation (G21).

Finally, a twenty-second equation (G22) is defined from the nineteenth equation (G19) and with the help of a third determination point, which is determined at a temperature T not equal to the calibration temperature Ta and at a reference flow value qRT and the measurement deviation ei that occurs:

D3 = [1 - eT - D2 • qT - Di] / [Di • (T-Ta)] (G22)

The third correction constant D3 can advantageously be determined according to the twenty-second equation (G22).

The adjustment method according to which the device for detecting the flow in line pipe 1 (FIG. 2) can be adjusted before the operating phase is advantageously defined as follows:

In a first step, the adjustment value K is calculated according to the eleventh equation (G11).

In a second step, the calculated adjustment value K and, if necessary, the adjustment temperature Ta are stored in the data memory 14.

In a third step, which can be carried out before or after the second step, the first correction constant Di is set to the value one in the memory 14, and the two correction constants D2 and D3 are each set to the value zero.

After the three steps, the computing unit supplies the value of the flow q available on the third communication channel 11 or at the output 10 - due to the set values of the three correction constants Di D2 and D3 - according to the third equation (G3), although the second activity does Flow q calculated according to the seventh equation (G7).

After the first three steps, in a fourth step for the three defined determination points qRi, qR2 and qRT, the measurement deviation ei or e2 or eT resulting for the point qRi or qR2 or qRT is determined.

After the fourth step, in a fifth step in the following order

- the second correction constant D2 after the twentieth equation (G20),

the first correction constant Di after the twenty-first equation (G21) and

- the third correction constant D3 is calculated according to the twenty-second equation (G22).

After their calculation, the three correction constants Di, D2 and D3 are stored in the data memory 14 in a sixth step. If necessary, the fifth step and the sixth step can be carried out at least partially in parallel (concurrent).

The adjustment process can advantageously be carried out automatically by the third activity 18.

After the sixth step, the computing unit 5 delivers the corrected flow qt> - available on the third communication channel 11 or at the output 10.

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If the corrected flow qb is calculated according to the second variant of the new formula according to the eighth equation (G8), the adjustment method described is of course simplified accordingly.

Within a homogeneous production lot of the device described, the correction constants Di, Ü2 and D3 are only slightly dependent on a single execution within the production lot. The correction constants Di, D2 and D3 are advantageously determined only once for the production lot. The adjustment value K, however, is determined individually for each facility.

Claims (5)

Claims 1. In a method for detecting the flow of a liquid or gaseous medium in a line pipe (1) having a throttle point (2), the temperature T of the medium and also the pressure difference 8p above the throttle point (2) in the medium are measured and from the measured Pressure difference 8p, the density r of the medium and the measured temperature T the flow is calculated, the method being characterized [a] that a standard formula that can be represented with an expression Z in the form q = Z • (8p / r) 1/2 for calculating the flow q with at least one term having a correction constant Di, D2 or D3 for a new one having a comparison value K Formula is expanded with which the flow q can be calculated, [b] that in a first adjustment phase the adjustment value K is determined at an adjustment point p0 at an adjustment temperature Ta, [c] that in a second adjustment phase the correction constant / correction constants Di or D2 or D3 is / are determined and [d] that in a usage phase following the two adjustment phases, the flow q is calculated using the new formula. 2. The method according to claim 1, characterized in that the new formula has three correction constants Di, D2 and D3 and is constructed as follows q = K • Di • (8p / r) 1/2 + K2 - D2 - (Sp / r ) + K • Di • D3 • (T-Ta) • (<5p / r) 1/2 3. The method according to claim 1, characterized in that the new formula has two correction constants Di and D2 and is structured as follows q = K • Di ■ (8p / r) 1/2 + W ■ D2 • (öp / r) 4. The method according to any one of claims 1, 2 or 3, characterized in that to determine the adjustment value K at the adjustment temperature Ta and the flow rate q0, the pressure difference 8p0 is measured above the throttle point (2) and the adjustment value K according to the formula K = qo • (ro / 8p0) 1/2, where r0 is the density of the medium at the calibration temperature Ta. 5. A device for performing the method according to claim 1, with a pressure difference measuring device (3) with which the pressure difference 8p in the medium above the throttle point (2) can be measured and with a temperature measuring device (4) with which the temperature of the Medium is measurable, characterized in [a] that the device has an arithmetic unit (5) with an output (10) with a data memory (14), the detected flow q being available at the outlet (10) corrected, b] that the arithmetic unit (5) with the Temperature measuring device (4) and the pressure difference measuring device (3) is connected, [c] that the data memory (14) has at least one correction constant Di, D2 or D3, [d] that the device can be adjusted in a calibration phase by a single calibration value K which can be determined in a calibration point p0 at a calibration temperature Ta and [e] that the adjustment value K can be stored in the data memory (14) by an adjustment device (17). 6