GB2128754A

GB2128754A - Method and apparatus for testing a fluid

Info

Publication number: GB2128754A
Application number: GB8228435A
Authority: GB
Inventors: Jaime Arturo Donoso Parra
Original assignee: Sarasota Automation Ltd
Current assignee: Sarasota Automation Ltd
Priority date: 1982-10-05
Filing date: 1982-10-05
Publication date: 1984-05-02
Also published as: GB2128754B

Abstract

Apparatus for testing a fluid comprises a flow conduit 10, a probe 15 having an inlet opening 32 which is disposed in a circular cross-section portion 16 of the probe 15 and which communicates with an outlet opening 35 which is disposed in a non-circular cross-section portion 17 of the probe 15, the latter portion 17 being such that the points at which boundary layer separation occurs are not substantially affected by variations in the fluid flow rate turbulent intensity or by the characteristics of the fluid, the probe 15 being provided, between the said circular and non- circular cross-section portions 16, 17 with a flange or plate 18 and measuring means 41, 42. Flow rate is measured using electrically heated thermisters or Pt resistance thermometers. Other parameters such as density, temperature, pH, conductivity and oxygen content may be measured. <IMAGE>

Description

SPECIFICATION Method and apparatus for testing a fluid This invention concerns a method and an apparatus for testing a fluid, e.g. to determine a parameter or parameters thereof such as its mass flow, volumetric flow, density, temperature, pH or conductivity, or to effect a chemical and/or physical analysis as its percentage content of oxygen. The invention is also applicable to the simultaneous determination of a plurality of parameters e.g. the simultaneous determination of density and temperature.

In US Patent No. 4,215,565 a flow or density meter is disclosed having a probe which passes through an opening in the wall of a conduit so as to extend into a main flow of a fluid passing through said conduit. The probe has an inlet opening and an outlet opening therein such that the passage of the main flow of fluid through the conduit causes a sampling flow of the fluid to be drawn from the conduit and to be passed through the probe from the inlet opening to the outlet opening and so back to the conduit, testing means being provided for testing said sampling flow. The probe is of constant crosssection substantially thoughout its length, the preferred cross-section being circular so that the probe may be easily introduced into and sealed in a circular drilled hole in the conduit.

Such a probe of circular cross-section throughout its length, however, suffers from the fact that the points at which boundary layer separation occurs vary in dependence upon factors such as the fluid flow rate and the characteristics of the fluid. If, on the other hand, the cross-section of the probe thoughout its length is square, rectangular or triangular, as is also suggested in US Patent No. 4,215,565, then it is difficult to seal the probe in a drilled hole in the conduit.

According, therefore, to the present invention, there is provided apparatus for testing a fluid comprising a conduit through which a main flow of a fluid may be passed; a probe inserted into said conduit, said probe having an inlet opening which is disposed in a circular cross-section portion of the probe and which communicates with an outlet opening which is disposed in a non-circular crosssection portion of the probe, the latter portion being such that the points at which boundary layer separation occurs from the said noncircular cross-section portion are not substantially affected by variations in the fluid flow rate turbulent intensity or by the characteristics of the fluid, the probe being provided, between the said circular and non-circular cross-section portions, with a flange or plate which extends outwardly of said portions, the said main flow, in passing in operation through the conduit, causing a sampling flow of the fluid to be drawn from the conduit and to be passed through the probe from the inlet opening to the outlet opening and so back to the conduit; and testing means for testing said sampling flow.

The flange or plate separates the flow around said portions from each other, and thus makes it possible to provide the probe with said portions. It also prevents any flow around the circular portion (where the point of separation is varying) from influencing the flow around the non-circular portion. The inlet opening may face either upstream or downstream and is preferably disposed adjacent to the wall of the conduit. The outlet opening may face'downstream or may be disposed in an end surface of the probe. In the latter case, the axis of the outlet opening is preferably separated from the end surface by a distance of between once and twice the greatest crosssection dimension of the said non-circular portion.

The arrangement may be such that t < W/4 where t is the thickness of the said noncircular portion, and W is its greatest crosssectional dimension.

The cross-section of the said non-circular portion preferably has at least two corners at which the boundary layer separation occurs.

Thus the said non-circular portion may be substantially triangular, rectangular, trapezoidal or sector-shaped.

The flange or plate may be solid or may be perforated with one or more apertures.

Preferably, the diameter of the flange or plate is at least 1 + times the diameter of the circular portion.

The conduit preferably has a recess or pocket within which the inlet opening is disposed, the said flange or plate being substantially aligned with the wall of the main portion of the conduit, the said flange or plate being such that the fluid may enter the recess or pocket from the main portion of the conduit.

A part of the testing means is preferably provided within the probe.

The testing means may comprise two electrically heated devices one of which is disposed in a passage through which said sampling flow passes, and the other of which is disposed in a chamber which communicates with but does not form part of said passage.

Alternatively, as described in our co-pending British Patent application No. 8228036 the testing means may comprise at least one electrically heated device, means for supplying electrical power to the or each heated device, means for varying the amount of electrical power to the or each said heated device so as to vary its temperature differential with respect to that of the fluid while ensuring that the temperature of the heated device or at least one of the heated devices is always above that of the fluid, means for sensing the temperature of the or each heated device, and means for determining the power lost by the latter when it is respectively at at least two different temperatures and for determining therefrom the power loss of the or each heated device per unit temperature, and means for employing the said power loss per unit temperature to calculate the mass flow.

In this case there may be two electrically heated devices which are disposed in a common passage through which said sampling flow passes.

The electrically heated device may be thermistors or platinum resistance thermometers.

The invention also comprises a method of testing a fluid comprising passing a main flow of a fluid through a conduit, inserting into said conduit a probe having an inlet opening which is disposed in a circular cross-section portion of the probe and which communicates with an outlet opening which is disposed in a non-circular cross-section portion of the probe, the latter portion being such that the points at which boundary layer separation from the said latter portion occur are not substantially affected by variations in the fluid flow rate turbulent intensity or by the characteristics of the fluid, the probe being provided, between the said circular, and non-circular cross-section portions, with a flange or plate which extends outwardly of said portions, the said main flow, in passing through the conduit, causing a sampling flow of the fluid to be drawn from the conduit and to be passed through the probe from the inlet opening to the outlet opening and so back to the conduit; and testing said sampling flow. Thus the sampling flow may be tested to determine the mass flow or volumetric flow of the fluid passing through the conduit.

The method of the invention is particularly applicable in cases in which the main flow of the fluid is dirty.

the invention is illustrated, merely by way of example, in the accompanying drawings, in which: Figure 1 is a diagrammatic cross-sectional view of a first embodiment of an apparatus for testing a fluid according to the present invention, Figures 2 and 3 are cross-sections taken on the lines 2-2 and 3-3 respectively of Fig. 1, Figure 4 is a graph illustrating the relationship between the flow coefficient of a fluid passing through a conduit and the Reynolds Number at the tip of a probe extending into said conduit, Figure 5 shows various cross-sections which may be used for a non-circular cross-section portion of a probe shown in Fig. 1, Figure 6 is a view similar to Fig. 1 but illustrating a second embodiment of the present invention, Figure 7 is a diagrammatic cross-sectional view of a third embodiment of an apparatus for testing a fluid according to the present invention, and Figures 8 and 9 are cross-sections taken on the lines 8-8 and 9-9 respectively.

In Figs. 1-3 there is shown an apparatus according to the present invention for testing a fluid, e.g. for determining the mass flow of a main flow of fluid passing through a conduit 10. The conduit 10 is drilled to provide a circular opening 11 in which is located and to which is sealed an inverted cup-shaped or "top-hat", member 1 2. The conduit 10 is thus provided with a recess, or "pocket", 1 3 which is displaced within the inverted cupshaped member 1 2 and which communicates with the main portion of the conduit 10.The end surface of the inverted cup-shaped member 1 2 is drilled to form a circular crosssection hole 14 therein, through which extends a probe 1 5. Although, for purposes of simplification, this is not shown in Fig 1, the inverted cup-shaped member 1 2 may in practice comprise a pipe stub, a valve, and a short length of pipe provided with a sealing gland to allow withdrawal of the probe 1 5 from the conduit 10. Thus the construction may be as shown in British Patent Specification No.

2,058,247 the description of which is incorporated herein by reference.

The probe 1 5 has a circular cross-section portion 1 6 which is mounted and sealed in the hole 14, a non-circular cross-section portion 17, and a flange or plate 1 8 which is disposed between and extends radially outwardly of the portions 16, 1 7. The flange or plate 18 may be solid as shown or may be perforated with one or more apertures (not shown). The flange or plate 1 8 may, moreover, as shown, be integral with the probe 1 5 or may be a separate member secured thereto, the flange or plate 18 serving to separate the flow around the portions 16, 1 7 from each other This separation of the flows is important in that without it the accuracy of the apparatus will be impaired.To ensure adequate separation, the diameter of the flange or plate 18 should be at least 1 > times either the diameter of the circular portion 1 6 or the maximum cross-sectional width of the non-circular portion 1 7.

As shown in Fig. 2, the portion 1 7 is trapezoidal having parallel surfaces 21, 22 which extend substantially at right angles to the flow through the conduit 10, and nonparallel surfaces 23, 24 which extend in the general direction of said flow. The portion 1 7 thus has corners 30, 31 at which boundary layer separation occurs and, by reason of the said trapezoidal shape, these corners 30, 31 constitute points at which the boundary layer separation always occurs irrespective of variations in the fluid flow rate turbulent intensity and irrespective of the characteristics of the fluid. This construction provides both a large and very stable differential pressure between inlet and outlet openings 32, 35 of the probe 1 5 and is very sensitive at low velocity flows.

As will be appreciated, therefore, the portion 1 6 constitutes a region in which the flow separation varies, whereas the portion 1 7 constitutes a region in which the flow separation is fixed. The inlet opening 32 is disposed in the portion 16 and communicates by way of internal passages 33, 34, with the outlet opening 35 which is disposed in the portion 1 7. The inlet opening 32 is disposed within the recess or pocket 1 3 and is thus disposed adjacent to the wall of the conduit 10.The outlet opening 35 is disposed in an end surface or tip 36 of the probe 1 5. Due to the relative pressures prevailing at the inlet and outlet openings 32, 35, the passage of the fluid through the conduit 10 causes a sampling flow of the fluid to be drawn from the conduit 10 and to be passed through the probe 1 5 from the inlet opening 32 to the outlet opening 35 and so back to the conduit 10.

It will be noted that the inlet opening 32 faces downstream and if this inlet opening is disposed in a main portion of the conduit 10, as opposed to being disposed in the recess or pocket 13, it is very desirable that it should so face downstream so as to reduce the amount of dirt which will pass into it, especially if the fluid is a dirty one. If, however, the inlet opening 32 is disposed in the recess or pocket 1 3 as shown, it does not greatly matter whether the inlet opening 32 faces downstream or upstream, although the downstream facing disposition shown is still preferred.

As will be seen from Fig. 1, the flange or plate 1 8 is substantially aligned with the wall of the main portion of the conduit 10, the flange or plate 1 8 being however such that the fluid may enter the recess or pocket 1 3 from the main portion of the conduit 10.This construction assists in maintaining the flow through the conduit 10 parallel to the axis of the latter and ensures that the flow through the main portion of the conduit 10 will not cause edge tones to be generated at the shoulder 40 provided at the radially inner end of the inverted cup-shaped member 1 2. The flange or plate 18 also acts as a damper for damping vibrations of the fluid in the recess or pocket 13, since the inverted cup-shaped member 1 2 can act like an organ pipe if excited by edge tones. Moreover, the flange or plate 1 8 produces a very thin boundary layer which does not distort the vortex shedding process.

Mounted so as to extend into the passage 33 is a "flow" thermistor or platinum resistance thermometer 41. A "reference" thermistor or platinum resistance thermometer 42 is mounted so as to extend into a chamber 43 which communicates with the passage 33- but does not form part of the latter so that the fluid in the chamber 43 is relatively stagnant.

The thermistors or platinum resistance thermometers 41, 42 constitute electrically heated devices which form part of a testing means of a flow meter of the kind disclosed in British Patent No. 1,463,507, the description of which is incorporated herein by reference.

Thus the thermistors or platinum resistance thermometers 41, 42 are provided within the probe 15.

Fig. 4 is a graph whose ordinate represents the flow coefficient K of a fluid passing through the conduit 10 and whose abscissa represents the Reynolds Number of the tip 36 of the probe calculated at the mean velocity of the conduit 10.

The curve 44 is that of a probe (not shown) which is of circular cylindrical shape throughout its length. As will be seen, in the case of the curve 44 the value of the flow coefficient K varies considerably and non-linearly with the Reynolds Number. If, moreover, the fluid flow is turbulent in this case, the curve 44 becomes even more non-linear, as indicated by the dotted line 45. If, however, it were possible to give the probe the shape of an ideal flat plate, it would have a substantially perfectly linear characteristic 46 which would be substantially independent of the Reynolds Number and which, as indicated by the dotted line 47, would hardly be affected by turbu lence.It is not in practice possible to give the probe such a flat plate shape, but the curve of a non-circular cross-section probe which can be provided, and which is constituted by a plate whose thickness is 1/4 of its width, is shown at 48. As will be seen, the curve 48 is substantially linear and its flow coefficient K does not vary substantially with the Reynolds Number. Moreover, as indicated by the dotted line 49, such a non-circular cross-section probe is not greatly affected by turbulence.

Fig. 5 illustrates various cross-sections which the portion 1 7 may have in addition to that shown in Fig. 2. Fig. 5(a) shows a platelike rectangular cross-section; Figs. 5(b) and 5(c) show sector-shaped cross-sections; Fig.

5(d) shows a diamond-shaped cross-section; Figs. 5(e) and 5(f) show triangular cross-sections; and Fig. 5(g) shows a substantially diamond-shaped cross-section having surfaces 25, 26 which extend parallel to the flow.

Fig. 6 shows a second embodiment of the present invention which is identical to that shown in Fig. 1 except that the passage 34 does not extend to the end surface or tip 36 but instead communicates with a downstream facing outlet opening 50 which is disposed adjacent to the end surface or tip 36. The axis of the opening 50 is preferably separated from the end surface or tip 36 by a distance of between once and twice the greatest crosssection dimension of the portion 1 7. This construction helps to prevent the deposition of dirt in the outlet opening 50 and its subsequent partial blocking and also improve the accuracy of the apparatus since the outlet opening 50 is removed from the vortices which occur at the end surface or tip 36 and which cause substantial fluctuations in' the readings produced by the apparatus.Thus such fluctuations may amount to 25% of the readings.

A third embodiment of the present invention is shown in Figs. 7-9 which will not be described in detail since it is generally similar to that of Figs. 1-3, like reference numerals indicating like parts.

In the construction of Figs. 7-9, however, an inverted cup-shaped member 1 2 is not employed, and the portion 1 6 is disposed in the main portion of the conduit 10 instead of being disposed in the recess or pocket 1 3.

The flange or plate 1 8 is thus disposed within the conduit 10 and is well spaced from the wall of the latter.

As will be seen from Fig. 7, the flange or plate 1 8 has a "smooth" or radiused edge 52, although it could, if desired, have a "sharp" edge 53, as in the construction of Fig. 1.

The portion 1 7 has a width, or greatest cross-sectional dimension, W which may be the same as the diameter d of the portion 1 6.

The portion 1 7 has a thickness t such that W t < -.

Claims

1. Apparatus for testing a fluid comprising a conduit through which a main flow of a fluid may be passed; a probe inserted into said conduit, said probe having an inlet opening which is disposed in a circular crosssection portion of the probe and which communicates with an outlet opening which is disposed in a non-circular cross-section portion of the probe, the latter portion being such that the points at which boundary layer separation occurs from the said non-circular crosssection portion are not substantially affected by variations in the fluid flow rate turbulent intensity or by the characteristics of the fluid, the probe being provided, between the said circular and non-circular cross-section portions, with a flange or plate which extends outwardly of said portions, the said main flow, in passing in operation through the conduit, causing a sampling flow of the fluid to be drawn from the conduit and to be passed through the probe from the inlet opening to the outlet opening and so back to the conduit; and testing means for testing said sampling flow.

2. Apparatus as claimed in claim 1 in which the inlet opening faces downstream.

3. Apparatus as claimed in claim 1 or 2 in which the outlet opening faces downstream.

4. Apparatus as claimed in claim 1 or 2 in which the outlet opening is disposed in an end surface of the probe.

5. Apparatus as claimed in claim 4 in which the axis of the outlet opening is separated from the end surface by a distance of between once and twice the greatest crosssection dimension of the said non-circular portion.

6. Apparatus as claimed in any preceding claim in which the inlet opening is disposed adjacent to the wall of the conduit.

7. Apparatus as claimed in any preceding claim in which ttW/4, where t is the thickness of the said non-circular portion, and W is its greatest cross-sectional dimension.

8. Apparatus as claimed in any preceding claim in which the cross-section of the said non-circular portion has at least two corners at which the boundary layer separation occurs.

9. Apparatus as claimed in claim 8 in which the said non-circular portion is substantially triangular, rectangular, trapezoidal, or sector-shaped.

10. Apparatus as claimed in any preceding claim in which the flange or plate has one or more apertures therein.

11. Apparatus as claimed in any preceding claim in which the diameter of the flange or plate is at least 1 + times the diameter of the circular portion.

1 2. Apparatus as claimed in any preceding claim in which the conduit has a recess or pocket within which the inlet opening is disposed, the said flange or plate being substantially aligned with the wall of the main portion of the conduit, the said flange or plate being such that the fluid may enter the recess or pocket from the main portion of the conduit.

1 3. Apparatus as claimed in any preceding claim in which a part of the testing means is provided within the probe.

14. Apparatus as claimed in any preceding claim in which the apparatus is a flowmeter.

15. Apparatus as claimed in claim 14 in which the testing means comprises two electrically heated devices one of which is disposed in a passage through which said sampling flow passes, and the other of which is disposed in a chamber which communicates with but does not form part of said passage.

16. Apparatus as claimed in claim 14 in which the testing means comprises at least one electrically heated device, means for supplying electrical power to the or each heated device, means for varying the amount of electrical power to the or each said heated device so as to vary its temperature differential with respect to that of the fluid while ensuring that the temperature of the heated device or at least one of the heated devices is always above that of the fluid, means for sensing the temperature of the or each heated device, and means for determining the power lost by the latter when it is respectively at at least two different temperatures and for determining therefrom the power loss of the or each heated device per unit temperature, and means for employing the said power loss per unit temperature to calculate the mass flow.

1 7. Apparatus as claimed in claim 1 6 in which there are two electrically heated devices which are disposed in a common passage through which said sampling flow passes.

18. Apparatus as claimed in any of claims 15-17 in which the electrically heated devices are thermistors or platinum resistance thermometers.

1 9. Apparatus for testing a fluid substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

20. A method of testing a fluid comprising passing a main flow of a fluid through a conduit,-inserting into said conduit a probe having an inlet opening which is disposed in a circular cross-section portion of the probe and which communicates with an outlet opening which is disposed in a non-circular crosssection portion of the probe, the latter portion being such that the points at which boundary layer separation from the said latter portion occur are not substantially affected by variations in the fluid flow rate turbulent intensity or by the characteristics of the fluid, the probe being provided, between the said circular and non-circular cross-section portions, with a flange or plate which extends outwardly of said portions, the said main flow, in passing through the conduit, causing a sampling flow of the fluid to be drawn from the conduit and to be passed through the probe from the inlet opening to the outlet opening and so back to the conduit; and testing said sampling flow.

21. A method as claimed in claim 20 in which the sampling flow is tested to determine the mass flow of the fluid passing through the conduit.

22. A method as claimed in claim 20 or 21 in which the main flow of fluid is dirty.

23. A method of testing a fluid substantially as hereinbefore described with reference to the accompanying drawings.