DE10314024B4 - Apparatus and method for determining a fluid flow - Google Patents

Abstract

Device for determining a flow mass of a fluid flow through a pipeline with an inlet pipe, with a downstream volume meter, with at least one pressure tapping point arranged at the inlet pipe and with at least one further pressure tapping point for measuring a fluid pressure by means of a differential pressure transducer, wherein the measuring signals of the differential pressure transducer together with a measuring signal of Volumen counter for determining the flow mass are evaluated, characterized in that the at least one further pressure tapping point (10 ') on the volume meter (3) at the location (X _p ,) of the lowest static pressure (p _stat ) is arranged.

Description

The The invention relates to a device according to the preamble of the claim 1 and a method according to the preamble of claim 7.

wobei ρ die Fluiddichte, p den Fluiddruck, T die Fluidtemperatur und χ die Kompressibilität bezeichnen. Die Indices B und N stehen für die Werte der entsprechenden Messgröße bei Betriebs- bzw. Normalbedingungen. In diesem Zusammenhang erfolgt eine direkte Dichtemessung, beispielsweise mit einem Schwinggabeldichtemesser, was jedoch besonders bei aggressiven oder stark verschmutzten Messstoffen, wie Abgasen, nicht ohne Probleme möglich ist. Weitere Probleme ergeben sich, wenn sich die Normdichte aufgrund einer variablen Gaszusammensetzung ändert. In diesem Fall reicht eine einfache Zustandsmengenumwertung über Druck und Temperatur nach Gleichung 1 nicht aus.The determination of mass flow plays an important role in many areas of everyday industrial practice. In the conventional mass flow measurement of fluids, ie, liquids or gases, the operating flow volume V detected by a volumetric meter such as an oval gear meter or the like becomes. _{B of} the fluid after separate measurement of pressure and temperature, and possibly taking into account the compressibility and a standard density for determining the standard mass or the standard volume using an equation of state. This results in the mass flow or the mass flow (mass per unit time) to:

where ρ is the fluid density, p the fluid pressure, T the fluid temperature and χ the compressibility. The indices B and N stand for the values of the corresponding measurand under operating or normal conditions. In this context, there is a direct density measurement, for example, with a vibrating fork densitometer, but this is not without problems especially in aggressive or heavily polluted media such as exhaust gases. Further problems arise when the standard density changes due to a variable gas composition. In this case, a simple state quantity conversion over pressure and temperature according to equation 1 is not sufficient.

The mentioned above Disadvantages apply in particular to the functioning according to the vibration principle known Coriolis mass meter.

The generic US 3,785,204 shows a flow meter for a pipeline in which a flow through the pipe is determined by means of a designed as a vortex meter volumetric meter, which has a introduced into the pipeline obstruction to generate fluid vortices. In order to determine the fluid density which is likewise required for indicating the mass flow, a fluid pressure in the pipeline is furthermore determined in the subject matter of the cited publication at symmetrically upstream and downstream of the disruptive body in the pipeline, and evaluated by means of a differential pressure transducer. The disadvantage is that due to the special arrangement of the pressure tapping points only relatively small pressure difference signals can be recorded, which also have only a limited reproducibility due to the vortex formation on the bluff body. In addition, the known flow meter has a predetermined by the distance of the pressure tapping points relatively large structural length.

The DE 30 03 599 A1 shows a measuring system for flowing through a pipe section flowable material, wherein in the pipe section a perforated plate and on both sides of the perforated plate in each case a pressure tapping point is provided, which are designed to determine a differential pressure between the fluid vorpress and behind the perforated plate. Furthermore, a paddle wheel flow meter is arranged in a bypass line surrounding the perforated plate, so that after a suitable mathematical combination of the determination values of the differential pressure meter and the flow meter, a mass flow through the tube section can be determined. The differential pressure measurement takes place symmetrically in front of and behind the perforated plate.

The DE 29 33 824 A1 shows a sensor for measuring the flow rate of fluid in which in a pipeline a turbine-like trained impeller is arranged in a complex manner to determine a measure of the mass and flow rate of a flowing medium via a determination of forces and angular momenta or their change. In conjunction with pressure and temperature measurements should thus be determined by the subject of said document a mass flow.

The US 3,307,396 shows a fluid flow measuring device with a turbine-like, helical device for determining a volume flow through a pipeline, in which also a reference pressure determination is provided for determining a mass flow. The pressure measurement takes place on the one hand on the pipe wall, on the other hand within the scar of the helical device, where nothing is carried out at the prevailing pressure conditions.

Of the Invention is based on the object, an apparatus and a method of the generic type in that the determination of mass flows by a pipeline with improved accuracy and increased measuring reliability allows is, wherein the device should be made compact.

The This object is achieved with a device of the aforementioned Sort of solved, having the characterizing features of claim 1. Farther The object is achieved by a method of the type mentioned solved with the characterizing features of claim 7.

The Invention thus provides an integrated structural arrangement of volume meter and differential pressure measuring device, which on the one hand the measurement as making them safer and more accurate and, on the other hand, a compact one Embodiment of the device according to the invention allows by placing the pressure point in place of the lowest static Pressure is arranged or, there is the further pressure measurement. By means of the simple pressure measurement is a second independent direct measurement available, by the measurement signal of the volumetric counter is largely accurate Convert way to determine the mass flow.

There is a continuity equation for source-free or mass-free mass flows of fluids m. = ρ · u · A = const. (2)

In equation 2, u denotes the flow rate of the fluid and A the cross-sectional area of the pipeline. Consideration of the potential and kinetic energy of the flowing fluid leads to the Bernoulli equation, which states that the sum of static pressure (operating pressure + geodetic pressure) and dynamic pressure (dynamic pressure) is a constant. The dynamic pressure is generally calculated according to the formula:

und ist für den statischen Druck negativ (Druckabnahme), für den dynamischen Druck dagegen positiv (Druckzunahme). Den Faktor c_p bezeichnet man als Druckwiderstandsbeiwert, der von der Geometrie des umströmten Körpers und der Art der Fluidströmung (laminar bzw. turbulent) abhängt. Die vorstehende Gleichung 4 lässt sich nach der Fluiddichte umformen:

In a flow around the bodies arranged in the fluid flow, a constriction of the flow occurs at the side of the flowed body, so that at these points according to the equation of continuity - Equation 2 - the fluid flow gains in velocity, the static pressure according to the Bernoulli equation together with equation 3 takes a minimum. If, under suitable conditions, the liquid layers adjacent to the body flowing around become detached, vortex formation occurs. With suitable flow parameters, a Kármán vortex street is formed, with temporally periodically mutually alternating from the body around which the fluid flows in opposite directions. For example, vortexes form on the back of a streamlined body in which the fluid particles move very rapidly, which, according to the Bernoulli equation, results in an increased back pressure (dynamic pressure) and a reduced static pressure. This creates a pressure difference Δp in front of and behind the body that flows around. This is calculated according to

and is negative for the static pressure (pressure decrease), positive for the dynamic pressure (pressure increase). The factor c _p is referred to as the pressure resistance coefficient, which depends on the geometry of the body flowing around and the type of fluid flow (laminar or turbulent). The above equation 4 can be reformed according to the fluid density:

The pressure resistance coefficient c _p is a device constant, the flow velocity u results according to u = V ./A from the continuity equation (equation 1).

Due to the general formula for mass flow, m. = V · · ρ is determined by the mass flow after insertion into Equation 5:

According to the invention is preferred a determination unit for determining the flow mass from the Differential pressure measuring signals and the measurement signal of the volumeter provided.

to Determination of for the flow mass relevant Pressure difference is provided according to a development that the Pressure measurement on the volume counter takes place at the location of the lowest static pressure. This is after a highly preferred Further development of the flow device according to the invention provided, that the pressure tapping point on the volumetric counter is at the location of the lowest static pressure is arranged.

According to a further embodiment of the flowmeter according to the invention it is provided that the volume meter is a vortex meter, which is preferably designed for measuring a vortex frequency of fluid vortices. A vortex meter is used to determine a volume flow V. by a pipeline, by the Ablösefrequenz of eddies (vortex frequency) is determined when flowing around a arranged in the volume to be measured body, for example, on the occurring during the alternate detachment of the Karman vortex pivotal movement of the body (vortex body). The volume flow V. _B is linked via a counter factor K with the measured vortex frequency f.

Under Use of a vortex meter to determine the volume flow is thus due to the regular presence a flow around vertebra ensured, that the measurement according to the invention a pressure difference Δp, which enters directly into the calculation of the flow mass, in optimal Way feasible is.

direkt aus den Messgrößen ohne Verwendung einer Zustandsgleichung für das Fluid, wobei m . den Massendurchfluss, Δp die gemessene Druckdifferenz, V ._B den gemessenen Volumendurchfluss und k eine geometrieabhängige numerische Konstante bezeichnen, wozu die Bestimmungseinrichtung der erfindungsgemäßen Vorrichtung entsprechend ausgebildet ist. Die Bestimmung des Massendurchflusses berücksichtigt folglich nur direkt gemessene Größen und ist von jeder näherungsweisen Zustandsmengenumwertung über Druck und Temperatur unabhängig.According to a highly preferred embodiment of the invention, therefore, a determination of a mass flow occurs by means of the relationship

directly from the measured quantities without using an equation of state for the fluid, where m. the mass flow, Δp the measured pressure difference, V. _B denote the measured volume flow and k a geometry-dependent numerical constant, for which purpose the determination device of the device according to the invention is designed accordingly. The determination of the mass flow thus takes into account only directly measured quantities and is independent of any approximate state quantity conversion over pressure and temperature.

Around one possible uniform inflow of to be measured fluids in the range of pressure measurement and the counter volume to ensure, is provided according to a development of the invention that the Inlet pipe a flow straightener having. In this way, the first pressure measurement takes place before volume meter in the stationary undisturbed Fluid flow and the physical conditions in the subsequent flow of the volumeter or the flow around of the vertebral body are reproducible.

Further Details of the invention will become apparent from the following description an embodiment based on drawings. It shows:

1 a schematic overall view of a flow meter according to the invention; and

2 a qualitative representation of the pressure profile of the fluid flow within the flowmeter according to the invention.

This in 1 illustrated flow meter 1 has an inlet pipe 2 followed by a vortex counter 3 and this downstream an outlet pipe 4 on. The inlet pipe 2 as well as the outlet pipe 4 have connecting flanges 5 . 6 for connecting the flowmeter according to the invention with a pipe leading the fluid flow F. 7 whose measurand is to be determined.

The volume counter 3 is formed in the embodiment shown as a vortex meter and includes a traversed by the fluid flow meter volume 8th and a counter unit 9 for the determination of the volume flow V. _B through the pipeline 7 , In the range of meter volume 8th as well as in the area of the inlet pipe 2 before the meter volume 8th is in each case a pressure tapping point 10 . 10 ' arranged in the form of a bore, which via pressure lines 11 . 12 and valves 10a . 10a ' with a differential pressure transmitter 13 connected is. Furthermore, there is a pressure line connection 11a between the pressure lines 11 . 12 in which also a valve 11a ' is arranged. The differential pressure transmitter 13 is in the embodiment shown by means of a holding part 14 and a clamp connection 15 at the inlet pipe 2 attached.

From the counter unit 9 of the vortex counter 3 and from the transducer 13 lead electrical lines 16 . 17 to a determination unit 18 , a general purpose calculator, and a display device for displaying the calculated mass flow value 19 having.

Inside the inlet pipe 2 is a flow straightener 20 arranged in the form of a pipe axis A coaxial inner tube.

The aforementioned component of the flowmeter according to the invention can be combined in a compact design by means of a common housing (not shown) to form a uniform measuring device. This concerns in particular the counter volume 8th with the valves 10a . 10a ' . 11a ' , the pressure lines 11 . 11a . 12 and the differential pressure transmitter 13 as well as the counter unit 9 of the vortex counter 3 and the determination unit 18 with the associated lines 16 . 17 ,

The mass flow to be measured fluid flow F emerges from the pipeline 7 coming into the inlet pipe 2 the flowmeter according to the invention 1 one. There he is using the flow straightener 20 In terms of its flow characteristics changed so that a spatially homogeneous as possible stationary fluid flow in the interior of the inlet pipe 2 results. For this purpose, the inlet pipe 2 In addition, preferably, an axial extent l, which is greater by a multiple than the pipe diameter D before the decisive for the drop in static pressure to be measured cross-sectional constriction in the interior of the meter volume 8th , The ratio l to D is typically 10 to 20.

The fluid flow F in the interior of the inlet pipe 2 according to the Bernoulli equation, has a total pressure p which, ignoring geodetic influences such as height differences or the like, results in the sum of a static pressure p _stat and a dynamic pressure p _dyn . The static pressure p _stat is determined by means of the differential pressure transmitter 13 in connection 10a . 11 with the first pressure tapping point 10 in the area of the inlet pipe 2 measured.

Subsequently, the fluid flow F to be measured enters the counter volume 8th of the vortex counter 3 in which, due to the cross-sectional constriction occurring here when flowing around the vertebral body (not shown here), an increase in the flow velocity u occurs, which according to the Bernoulli equation results in a drop in the static pressure to a value p ' _stat . This reduced p ' _stat compared with p _{stat stat} by means of the pressure sensor device 13a in connection 10a ' . 12 with the second pressure tapping point 10 ' measured. The differential pressure transmitter 13 then generates from the measurement signals of the pressure sensor device 13a a differential pressure signal Ap and puts this over the line 16 the determination unit 18 to disposal.

In normal operation of the illustrated flowmeter 1 are the valves 10a . 10a ' open, so that the differential pressure caused by the bluff body of the vortex meter in cooperation with the flow F can be detected. For maintenance work, eg when replacing the differential pressure transmitter 13 , both valves can 10a . 10a ' getting closed. In addition, can be in this way with simultaneous opening of the other valve 11a ' , which is normally closed, set the zero point of the differential pressure transmitter.

The control of the valves 10a . 10a ' . 11a ' can be done electronically in connection with the measurement process. It is also possible to use the valves 10a . 10a ' to switch so that in the normal state, the operating pressure of the fluid flow F not at the transducer 13 is applied. In general, however, a continuous operation for continuous monitoring of the mass flow is preferred.

In the course of an electronic valve control, the Differenzdruckmeßumformer 13 one after the other - according to the control of the valves 10a . 10a ' - determine two different pressure values. The pressure values are accordingly in the differential pressure transmitter 13 before the subtraction (intermediate) fedi chert.

The vortex counter 3 is in addition to generating a pressure drop in the range of the counter volume 8th also for determining the volume flow V. _{B of} the fluid flow F through the pipeline 7 educated. Generally there is a suitable flow around the counter volume 8th a simple relationship between the separation frequency f of Kármán's vortices and the flow velocity u of the fluid flow F, where:

In the above equation, St denotes the Strouhal number, which can be considered as a constant for typical applications. d denotes a characteristic dimension of the vertebral body, eg its diameter transverse to the inflow through the fluid flow F. The volumetric flow rate V. _B results directly from the vortex frequency f in conjunction with a device-dependent meter factor K. The result is an electrical signal over the newspaper 17 also the determination unit 18 provided for evaluation.

zur Bestimmung des Massendurchflusses durch die Rohrleitung 7 allein aus den direkten Messgrößen Differenzdruck Δp und Volumendurchfluss V ._B ausgebildet. In die Konstante k gehen alle konstanten Zahlen und Faktoren der Berechnungsformel, wie der Zählerfaktor K und der Druckwiderstandsbeiwert c_p, ein.The determination unit 18 is according to the relationship

for determining the mass flow through the pipeline 7 solely from the direct measured values differential pressure Δp and volume flow V. _B trained. In the constant k all constant numbers and factors of the calculation formula, such as the counter factor K and the pressure resistance coefficient c _p , are included.

The outlet pipe 4 serves to ensure that the detachment of the vertebrae from the vertebral body not by obstructing the flow of the fluid flow F downstream of the counter volume 8th is compromised, which could distort the result of mass flow determination.

Due to the structurally strong connection of the pressure transducer 13 with the inlet pipe 2 over holding part 14 and clamp connection 15 is for a mechanical relief of the pressure lines 11 . 11a . 12 taken care of.

2 shows qualitatively the behavior of a fluid flow F when flowing around a vertebral body 8a as he is inside the counter volume 8th of the 1 is arranged. The vertebral body 8a is in the illustration after 2 formed as a cylindrical rod, but in practice may have more complex shapes, such as wedge shape.

In the flow around the vertebral body 8a through the fluid flow F occurs in the region of its flanks 8b to an increase in the flow velocity u, whereby the speed-dependent dynamic pressure of the fluid flow F increases in this range according to equation 3 and the static pressure decreases accordingly. Behind the vertebral body 8a is the page-wise alternate detachment of fluid vortices W recognizable, the separation frequency f (reciprocal of the past between the detachment of two consecutive vortex W time) according to equation 8 is a measure of the flow velocity u and thus to determine the volume flow V. _{B is} usable.

At the bottom of the 2 is the course of the static pressure p _stat in the axial direction x of the pipeline 7 (see 1 ). At x _p the fluid flow F is through the obstruction (the vertebral body) 8a is still substantially unaffected and is at a relatively high static pressure p _stat associated with a correspondingly low dynamic pressure p _dyn . In the course of the flow around the vertebral body 8a Due to the increased flow velocity (the increasing dynamic pressure) and the vortex formation at x _p ', the static pressure drops to a minimum value p' _stat . The pressure difference Δp is detected by the flowmeter according to the invention and used in the course of the method according to the invention for determining the mass flow.

The inventive method is not only with vortex counters, as in the embodiment according to 1 shown, but also with other flowmeters applicable, if the flow can be assigned a pressure difference, by the mass flow according to the invention without inaccurate state quantity conversions is possible.

1

Flowmeter

2

inlet pipe

3

Volume meter / meter body

4

outlet pipe

5

flange

6

flange

7

pipeline

8th

counter volume

8a

vertebra

8b

Flank (from 8a )

9

counter unit

10 10 '

Pressure tapping

10a, 10a ', 11a'

Valve

11

pressure line

11a

pressure line

12

pressure line

13

differential pressure transducer

14

holding part

15

clamp connection

16

management

17

management

18

determining unit

19

display

20

Flow straightener

A

pipe axis

F

fluid flow

W

fluid vortex

p

print

p _dyn

dynamic print

p _stat

static print

Ap

pressure difference

x _p , x _p '

axial Position (longitudinal 7)

Claims (9)

Device for determining a flow mass of a fluid flow through a pipeline with an inlet pipe, with a downstream volume meter, with at least one pressure tapping point arranged at the inlet pipe and with at least one further pressure tapping point for measuring a fluid pressure by means of a differential pressure transducer, wherein the measuring signals of the differential pressure transducer together with a measuring signal of Volumen counter for determining the flow mass are evaluated, characterized in that the at least one further pressure tapping point ( 10 ' ) at the volume counter ( 3 ) is located at the location (X _p ,) of the lowest static pressure (p _stat ). Device according to claim 1, characterized in that that the volume meter as a vortex counter designed to measure a vortex frequency of fluid vortices. Apparatus according to claim 1 or 2, characterized by a determination unit ( 18 ) for determining the flow mass from the differential pressure measuring signals and the measuring signal of the volumetric counter ( 3 ) is trained. Device according to one of claims 1 to 3, characterized by a volume meter ( 3 ) downstream outlet pipe ( 4 ). Device according to one of claims 1 to 4, characterized in that the inlet pipe ( 2 ) a flow rectifier ( 20 ) having. Device according to one of the preceding claims, characterized in that the determination unit ( 18 ) for determining the mass flow rate according to

directly from the measured quantities Δp, V. _{B is formed} without using a state equation for the fluid, wherein m. the mass flow, Δp the measured pressure difference, V. _{B is} the measured volume flow and k is a geometry-dependent numeric constant. Method for determining a flow mass of a Fluid flow through a pipeline, wherein the fluid flow through a Inlet pipe and a volume meter flows and where at the inlet pipe and at another point through a differential pressure transmitter a differential pressure in the pipeline measured and the flow mass due to the measuring signals of the differential pressure transducer and the measuring signal of the volumetric counter is determined, characterized in that the further pressure measurement at the volume counter takes place at the location of the lowest static pressure. A method according to claim 7, characterized in that a mass flow through the relationship

directly from the measured quantities Δp, V. _{B is determined} without using an equation of state for the fluid, where m. the mass flow, Δp the measured pressure difference, V. _{B is} the measured volume flow and k is a geometry-dependent numeric constant. Method according to claim 7 or 8, characterized that the volume flow is measured by means of a vortex counter.