DE3732583A1 - Flow-measuring (metering) probe - Google Patents

Abstract

Flow-measuring probe, quantity measurement technology, pressure take-off apertures, aperture ratios, total pressure, negative pressure, modifiable differential pressure, free selection of probe diameter, capability to influence the output signal, adaptation to available measuring technology. The flow-measuring probe is used in quantity measurement technology for flowing liquids and gases. The probe, designed as a cylindrical body, is inserted radially into the pipework over its whole diameter, has pressure take-off apertures directed upstream and towards the pipe wall and has, according to the invention, a diameter fixed as a function of a preset aperture ratio m = F2/F1, the aperture ratio being between 0.3 and 0.95. On the apertures directed upstream a total pressure is measured and on the apertures directed towards the pipe wall a negative pressure, resulting from the constriction by the probe body, with the static pressure superimposed, is measured. The pressure difference existing at the output of the probe can be modified by the choice of the probe diameter. In this way, a capability to influence the output signal and adaptation to available measurement technology are achieved. <IMAGE>

Description

The invention is used in quantity measurement technology for flowing liquids and gases.

There are differential pressure flow probes for use in flowable Medium-carrying pipelines known (DE 19 35 989), in which a congestion probe designed as a hollow tube over extends the entire diameter of the pipeline and with is provided with several upstream openings, the pairs and symmetrical to a downstream one Opening for the detection of the static pressure in the Center of the flow cross-section outside the center are arranged.  

The shortcomings inherent in this probe are that

• - The output signal exclusively on the measurement of the dynamic pressure according to the relationship based, where ρ is the density of the flowing substance and c is the flow velocity.
  This dynamic pressure is very low compared to the differential pressures that can be generated with the conventional measuring methods, ie devices of this type are not a real alternative to the conventional throttle measuring methods, regardless of their advantages;
• - The output signal is subject to a deviation, which is based on the Reynold number between Re = 1 × 10⁵ and Re = 2.5 × 10⁵ from -15% to 0%, in order then to drop again;
• - For the output signal, due to the fact that the pressure is removed for the static pressure in the outflow area of the probe body lies, an overlay of the same by the flow phenomena forming on the downstream side occur, making the output signal square over the Flow velocity is not linear.

A pitot tube pressure meter is also known (DE 28 42 676), where the above defect under painting 3, d. H. the negative influence of free turbulence on the static Pressure in the outflow area of the probe body, thereby eliminated is that two surfaces with a sharp contour are provided and so causes a fixed boundary layer detachment and preventing the boundary layer from resuming. It However, the disadvantages under paint 1 and 2 remain.

Finally, a differential pressure probe is known (DE 28 42 414), in which the holes for absorbing the static pressure in Flow direction behind the hole to record the dynamic Pressure portion are arranged and in pairs to both Sides in a probe body in front of the area of decreasing probe body width lie.  

The task of this measure was to check the linearity of the output signal, especially at low flow speeds, to improve to the deficiency mentioned under painting 3 counteract.

The shortcomings listed under painting 1 and 2 can however, this measure cannot be used either. The described differential pressure probe is clearly based on the Principle of the Pitot tube according to Prandtl, which consists of an in Direction of flow arranged thin tube exists and at which the static pressure is detected in an annular gap, the downstream of the upstream pressure tapping port for the detection of the total pressure.

Ultimately, pitot tubes are also known, in contrast to the above-mentioned multiple tapping points for the detection of the static pressure on the downstream Are located in the direction of the pipe wall a lower dependence of the measurement result on the Reynoldsche Number to achieve (automation technology practice, Verlag Oldenburg Munich, 28th vol., Issue 3, p. 124 (1986). The disadvantage here is that only the dynamic pressure is measured becomes.

It is therefore not possible for all of the listed facilities for a given flow and a certain Differential pressure meter to adjust the output signal. There is only the possibility from which To infer dynamic pressure to the flow velocity in the pipe, what in industrial practice due to the relative low signal level poses significant problems.

The known flow meters using the differential pressure method (Method at constant local altitude) are based on the one hand the dynamic pressure process and on the other hand on the throttle process. Both processes are derived from the energetic Relationships of pipe flow.  

In the dynamic pressure process, the speed of the pipe flow with a flow that is negligibly constricting Sensor determined and the flowing amount from the product of Speed and pipe cross section determined.

In the throttling process, the entire flow is forced to flow faster due to a narrowing of the pipe cross-section, and the flowing amount is derived from the pressure drop resulting from the narrowing of the pipe cross-section F 1 to a pipe cross-section F 2 .

The dynamic pressure process is compared to the throttle process significantly disadvantaged by the low dynamic pressure but the advantage of a far smaller mass of the sensor, the Possibility of removal and installation without the pipeline must be emptied when liquid flows through it becomes. Furthermore, this measuring method requires only a small one Calming section in front of the sensor.

The throttling method is the most precise and therefore the farthest widespread all flow measurement methods. But it has Disadvantages that the mass of the sensors relative to the dynamic pressure probes that is two to five times, the installation and removal in In the case of measurement of liquid quantities, the emptying of the Pipeline presupposes and a calming section both before as well as after the measurement site must exist between the 10 to 20 times the pipe diameter, but higher Demands on the measurement accuracy can be much higher. In addition, the flow resistance is much higher than Pitot tubes.

The aim of the invention is a high material economy through the Reducing the mass of the probe and by reducing the Dimensions of the calming sections, an improvement in Energy economy by reducing the flow resistance, a simplification of maintenance through  Significant facilities for installation and removal, as well as Ability to influence the output signal in analogy to the Throttling devices, adapted to the downstream measuring technology.

There was the task of taking the well-known pitot tubes Maintaining the advantages this has over throttling devices have, such as low mass, low flow resistance, low demands on the upstream and downstream Calming trails, simplifying maintenance, so too shape that in analogy to the throttle devices in the Freely selectable throttle effect and one in size freely selectable output signal and the measuring accuracy the throttling devices is retained.

According to the invention the object is achieved in that the diameter of the cylindrical body is fixed as a function of a predetermined opening ratio m = F ₂ / F ₁, with F ₂ the cross section of the pipe flow restricted by the shadow surfaces of the cylindrical body and F ₁ the cross section of the pipeline into which the cylindrical body is inserted in the radial direction and the opening ratio is between 0.3 and 0.95 and the pressure discharge openings arranged in pairs in the direction of the pipe wall are at the position of the cylindrical body where the cylindrical body is seen in the direction of flow has its greatest width and are located in the channels into which the pressure tapping openings open, dip tubes leading to the outside, the pressure tapping openings lying below the center of the pipeline.

Influencing the size of the freely selectable output signal is thus achieved according to the invention in that the Dynamic pressure principle with the principle of forming a negative pressure is combined at a restriction of the flow.  

That means it won't be like the well-known pitot tube the total pressure and the static pressure measured, but the total pressure and the pressure developing at the constriction point Vacuum. The diameter of the probe is therefore relative determining the pipe diameter for the constriction and thus determining the negative pressure that develops, which in turn is decisive for the line at the exit of the probe Differential pressure. The output signal is accordingly the choice of probe or pipe diameter Object of the invention can be modified according to.

Now the total pressure resulting from the positive dynamic Pressure component and the static pressure composed, and the total pressure resulting from a negative dynamic Pressure component and the static pressure composed, added, in contrast to pitot tubes, this results in a considerable amount Increase in the signal level caused by the negative dynamic Determining pressure component is influenced. Mathematically, this is as follows:

or

wherein the speed c ₂ at the point of the flow restricted by the opening ratio

is determined.

Here are:

c ₁ the speed in the full pipe cross section, F ₂ the free pipe area remaining on both sides of the probe and F ₁ area over the entire pipe cross section.

The flow is calculated in contrast to the Pitot tubes and in analogy to the throttling devices according to the relationship

Q = m · α δ p

where Q is the flowing quantity in m³ / h, α is a flow number and δ p is the differential pressure generated in mm WS.

Embodiments

The invention is based on 2 embodiments and the associated drawings explained in more detail.

Example 1

Fig. 1 shows a flow measuring probe for a pipeline 1 with a nominal width of 100 mm seen in the flow direction. At the front of the probe there are openings 2 and 3 above and below the pipe center. The total pressure, which is composed of a positive dynamic pressure component and the static pressure, takes place at these openings. Furthermore, openings 4 and 5 are provided in the middle of the pipeline, facing the pipe wall, on both sides of the probe. A further total pressure is recorded at these openings, which is composed of a negative dynamic pressure component and the static pressure. The probe is fastened by means of the flange 8, which is firmly connected to the probe body 6 , which is surrounded by a jacket 7 , on a counter flange 9 , which is connected to the pipe 1 by a connecting pipe 10 , in which the medium flows, the quantity of which is detected shall be.

Fig. 2 shows a section through Fig. 1 perpendicular to the viewing plane, from which the structure of the flow measuring probe can be seen inside. In both bores 11 and 12 in the probe body 1 there are dip tubes 13 and 14 which continue in the outlet connections 15 and 16 . Both immersion tubes extend below the middle of the probe or the pipeline. The purpose of the immersion tubes is to exclude air pockets when measuring the amount of liquid in the differential pressure lines connected to the nozzles 15 and 16 .

FIG. 3 shows the section AA indicated in FIG. 1. From this section it can be seen that in the probe body 6 , which is a solid body, there are axial channels in the form of bores 11 and 12 and the openings 4 and 5 , at which the total pressure is released, open into the bore 12 .

As can also be seen, the probe body is covered with the jacket 7 , which is open downstream. This creates the edge that can be seen behind the openings 4 and 5 , as a result of which the flow behind the openings separates. This leads to a stabilization of the measurement. The dip tubes 13 and 14 can also be seen.

FIG. 4 shows the section BB indicated in FIG. 1. It can be seen here that the openings 3 and 2 , from which the second total pressure is taken, open into the bore 11 .

Fig. 5 shows the comparison of the output signals of the prescribed flow measuring probe and a conventional dynamic pressure probe. From this comparison it can be seen that with the flow measuring probe according to the invention an increase in the signal level of about 6 times compared to the dynamic pressure probe occurs when the opening ratio is chosen to be 0.4.

Example 2

Fig. 6 shows a Durchflußmeßsonde for a pipe 1 with a nominal width 1200 mm. As can be seen, the probe is constructed analogously to that described in FIGS. 1 to 4. The channels are not bores in a solid body, but tubes 17 and 18 , which are connected to disks 19 to 21 at openings 2 to 5 , the disks being designed according to FIGS. 7 and 8. The pipes are tightly welded to the plates at the passage points. The probe body is designed as a cladding tube 22 .

The same applies to the attachment of the probe to the pipeline 1 for the pressure tapping points 2 to 5 as for the prescribed probe in example 1.

The probe jacket, which is identical to the cladding tube here, is recessed in the middle to ensure the longitudinal stability of the probe to increase and still the stabilizing effect in the measurement bring about.

The outlet connections 15 and 16 also continue in this probe via the immersion tubes 13 and 14 to below the center of the tube in order to prevent air from entering the differential pressure lines, in particular when measuring quantities of liquid.

FIG. 7 shows the section AA shown in FIG. 6, FIG. 8 the section BB .

The diameter of the flow measuring probe for the nominal size 1200 is 70 mm, the opening ratio = 0.91.

FIG. 9 shows the comparison of the output signals of the flow measuring probe according to the invention and a dynamic pressure probe. From this comparison it can be seen that with the aperture ratio 0.91 the signal level is tripled.

In the design of the flow measuring probe, in the case of the probe for the nominal width 1200, there was a requirement to generate a signal which, at a flow rate of 10,000 m³ / h, enables the use of a transmitter with a measuring range from 0 to 1600 mm WS. Due to the fact that the selection of the cladding tube 22 is tied to standards, the effective pressure δ p = 1160 mm WS shown in FIG. 9 resulted, while only 320 mm WS can be reached with the dynamic pressure probe.

From the examples given and a number of further tests, it follows that with opening ratios of 0.3 less than or equal to m less than or equal to 0.6, the differential pressure that can be achieved at a certain flow is between that which can be achieved with nozzles and orifices.

For m greater than 0.6, the same differential pressure is achieved as with the orifice.

The remaining pressure losses, however, are with the flow measuring probe much less than with the aperture.

The total error achievable with the flow measuring probe The electrode lies, depending on the precision of the production, the installation conditions and the measurement processing, between ± 1% and ± 3%.  

• List of the reference numerals used
• 1 pipe
  2 opening
  3 opening
  4 opening
  5 opening
  6 probe bodies
  7 coat
  8 flange
  9 counter flange
  10 spigot
  11 hole
  12 hole
  13 immersion tube
  14 dip tube
  15 outlet connection
  16 outlet connection
  17 pipe
  18 pipe
  19 disc
  20 disc
  21 disc
  22 cladding tube

Claims (2)

1.flow measuring probe, designed as a cylindrical body, which is introduced in the radical direction into a pipeline through which a medium flows, over the entire pipeline diameter, in which one or more pressure tapping openings are provided upstream and one pressure tapping opening is provided on both sides in the current direction towards the pipe wall, the upstream and the pressure outlet openings directed towards the tube wall each opening into one of two channels located inside the cylindrical body and leading to the outside, characterized in that the diameter of the cylindrical body is fixed as a function of a predetermined opening ratio m = F ₂ / F ₁ is, where F ₂ is the cross section of the tube flow narrowed by the shadow surface of the cylindrical body and F ₁ is the cross section of the pipe into which the cylindrical body is inserted in the radial direction, and the opening ratio is between 0.3 and 0.95 and the pressure tapping openings arranged in pairs in the direction of the pipe wall are located at the location of the cylindrical body where the cylindrical body has its greatest width when viewed in the direction of flow. 2. flow measuring probe according to claim 1, characterized in that that in the channels leading to the outside outside leading immersion tubes are located, the pressure tapping openings arranged below the center of the pipeline are.