Portable wastewater flow meter

Abstract

A portable wastewater flow meter particularly adapted for temporary use at a single location in measuring the rate of liquid flow in a circular entrance conduit of a sewer manhole both under free flow and submerged, open channel conditions and under fill pipe, surcharged conditions, comprising an apparatus having a cylindrical external surface and an inner surface that constricts the flow through the apparatus in such a manner that a relationship exists between (1) the difference between the static pressure head of liquid flowing through the entrance of the apparatus and the static pressure head of liquid flowing through the constriction, and (2) the rate of liquid flow through the apparatus.

Description

This application discloses improvements that were discovered during research funded by the U.S. Department of Energy under the Energy-Related Inventions Program. The invention is described in detail in a report entitled "The Flumeter™: A New Tool for Wastewater Management" prepared for the U.S. Department of Energy by Yellowstone Environmental Science, Bozeman, Mont., May 1988.

This application is a continuation-in-part of U.S. Pat. Application No. 051,325 filed May 19, 1987 now U.S. Pat No. 4,799,388FIG. 12 is another cross-sectional view of the device taken at section 8 shown on FIG. 7.

FIG. 13 is another cross-sectional view of the device taken at section 9 shown on FIG. 7. 912. The body of the device 2' has ports 113 and 115 in the end portions 4", 4"' thereof, at the points where the tubes terminate. The ends 84', 86" of the tubes are inserted in the ports, as seen in FIGS. 8 and 9, and a pair of plugs 117 is inserted in the bottoms of the grooves 66, 68 to close the ports to the outer periphery of the end portions.

Referring now to FIG. 6, compressed gas sources 18 118 is used to pressurize bubbler tubes 92, 94 and 96. The gas may be any nonflammable gas such as nitrogen or air. Compressed gas source 118 may be a cylinder of compressed gas or a compressor. The gas flows through pressure regulators 120, 122 and 124 which lower the pressure to the working pressures of bubbler tubes 92, 94 and 96. These pressure regulators also ensure that changes in pressure in one of the bubbler tubes do not affect the pressures in the other bubbler tubes. The pressure in each bubbler tube is thus determined by the depth of submergence of the open end of the tube (i.e., the static pressure head).

One end of bubbler tube 92 is connected to the pressure port of differential pressure gauge 126. Similarly, one end of bubbler tube 94 96 is connected to the reference port of differential pressure gauge 126. When the entrance section 8, and hence the throat 28, of the device are not filled with liquid, differential pressure gauge 126 senses the liquid level in the entrance section of the device. When the entrance section, and hence the throat, of the device are filled with liquid, differential pressure gauge 126 senses the difference between the pressures in bubbler tube 92 and that in bubbler 94 96. Differential pressure gauge 128 functions in a simmilar manner.

During open channel operation, with the flow direction as shown, the liquid level sensed by differential pressure gauge 126 is compared to the liquid level sensed by differential pressure gauge 128. Since the device is level, the open end of bubbler tubes 92 and 96 94 terminate at the same elevation. In the preferred embodiment, they both terminate at the same elevation as the elevation of the invert of the throat, but any elevation at or below that elevation is acceptable.

In conventional practice, the ratio of the downstream depth of flow to the upstream depth of flow (when expressed as a percentage) is termed the submergence. When the ratio exceeds a certain value, usually in the, range 65-75 percent, a critical flow flume is said to be operating above its maximum submergence or above its modular limit. When such a meter operates below its modular limit, the device is said to be operating in a free flow mode. In a free flow mode, a unique relationship exists between the upstream depth of flow and the flow rate, if the meter is installed in a sewer of low to moderate slope, say up to about 2 percent slope. When such a meter operates above its modular limit, the device is said to be operating in a submerged mode. In a submerged mode, the flow rate predicted by a free flow calibration curve must be corrected by a factor that is a function of the percent submergence. Examples of a free flow calibration curve and a correction curve are presented in FIGS. 10 and 11, respectively. The flow rate obtained from FIG. 10 would be multiplied by the correction factor obtained from FIG. 11 to determine the corrected flow rate.

During open channel operation, with the flow direction opposite that shown, differential pressure gauge 128 is used to sense the "upstream" depth of flow and differential pressure gauge 126 is used to sense the "downstream" depth of flow by means of a signal converter, such as computer 200. Similar calibration and correction curves would be used to relate pressure reading into flow rates. Thus the improved meter is capable of metering flow rates under the following conditions for both forward and reverse flow:

Open channel

Free flow

Submerged flow

Full Pipe

It should be apparent that, at positive sewer slopes appreciably greater than zero, reverse open channel flow will typically occur only momentarily, if at all. This is true because reverse flow is caused by a downstream increase in liquid depth. If the downstream increase in depth occurs slowly, the depth upstream will slowly increase until the increase stops or the sewer fills with liquid, but reverse open channel flow will not occur. If the downstream increase in depth occurs suddenly, then a surge will move upstream as a wave. Only during the passage of the wave might reverse open channel flow occur.

In an alternative embodiment, shown with dashed lines on FIG. 6, bubbler tube 92 is also connected to the pressure port of differential pressure gauge 130 and bubbler tube 96 94 is also connected to the reference port of differential pressure gauge 130. When the device is operating in an open channel mode, differential pressure gauge 130 is used to directly sense the difference between the pressures in the bubbler tubes, and, hence, the difference between the upstream and downstream liquid depths. This difference is compared to the upstream or downstream liquid depth to determine (1) the percent submergence and (2) the correct correction factor, if the meter is operating above its modular limit.

In the embodiment shown in FIGS. 1-5, the bubbler tubes 92 and 96 sense the static pressure head in the annular space between the inside wall of the sewer and the outside wall of the meter. The liquid in the annular space acts as a stilling well to attenuate variations in the sensed pressure. Furthermore, the open ends of the tubes are relatively isolated from the flowing liquid, and thus are less likely to be fouled by gross wastewater solids. Because the end of the annular space is open in the direction of flow, the static pressure head sensed by the tubes includes a very small component of velocity head equal to the head produced by stagnation of that portion of the velocity profile adjacent to the sewer walls as it impinges on the open end of the annular space. Even if the meter is installed in a sewer much larger than the meter outside diameter, the impact of incorporation of a small component of velocity head in the upstream and downstream head measurements does not significantly impact metering accuracy.

In the preferred embodiments of FIGS. 1-5 and 6-9, the entrance section 26, the exit section 30, the entrance transition 22 and the exit transition 32 have circular sections with their centers along the longitudinal axis 18 of the meter. The throat section 28 has a truncated circular section with a center along the same axis. The top 28' of the throat section is flat. In the preferred embodiments, the entrance transition 26 and exit transition 30 converge at a slope of 1:6. This transition slope is best because it causes the least head loss between the throat section and the downstream section and, hence, maximizes the modular limit (maximum submergence of the meter). This design maximizes the amount of submergence (due to tailwater) that can be accomodated by the meter before the modular limit is reached and before two depth measurements are required for metering of open channel flow.

Another improvement in meter design is that the throat section is adapted relative to the entrance section to cause simultaneous filling before the modular limit is reached when the meter is installed in sewers of minimum slope. In conventional practice, a sewer of minimum slope is one which flows full at an average velocity of 2 feet per second. Simultaneous filling occurs earlier (at lower normal depths) in sewers of greater slope providing an additional factor of safety against submerged operation.

In meters of similar design, the modular limit is a function solely of the size (inside diameter) of the meter. The modular limit of meters with nominal diameters of 8 to 12 inches typically ranges from 65 to 75 percent.

Given a particular sewer diameter, the normal depth of flow at a given flow rate can be determined using the well-known Manning formula:

, Q=(1.486/n) AR^2/3 S^1/2

where

Q=flow rate

n=coefficient of roughness (Manning's a)

A=area of flow (which is a function of normal depth of flow)

R=hydraulic radius (which is the area of flow divided by the wetted perimeter, both a function of the normal depth of flow)

S=sewer slope

The above formula is usually solved by trial and error, substituting values for depth of flow into the formula until the sought after flow rate results.

To illustrate the application of the Manning formula, assume the following:

Sewer diameter--8 inches (0.667 ft)

Sewer slope--0.0033 ft/ft

Manning's n--0.013

By trial and error, wastewater flowing at a rate of 0.525 cubic feet per second (cfs) will flow at a normal depth of 0.433 ft (5.2 inches).

As was noted above, both the upstream ad downstream depths of flow are measured by this invention relative to the elevation of the bottom of the throat. The Manning formula, on the other hand, predicts the downstream normal depth of flow relative to the invert elevation of the sewer. With a device with an entrance inside diameter of 6.9 inches and a throat inside diameter of 5.5 inches installed in an 8-inch sewer, the throat invert elevation would be about 1.25 inches (0.104 feet) above the sewer invert, with a relatively low sewer slope. Thus, a downstream normal depth of 0.433 ft would cause a downstream depth reading of 0.433-0.104=0.329 feet=3.95 inches to be registered by the meter.

The equations presented in U.S. Pat. Application No. 051,325 could be used to show that a meter with an entrance section with a 6.9 inch inside diameter and a throat with a centered 5.5 inch inside diameter and a 4.5 inch height would cause simultaneous entrance section and throat section filling at a flow rate of 0.525 cfs. That is, at a flow rate of 0.525 cfs, under free flow conditions, the upstream depth (measured relative to the throat invert elevation) would be 6.9-0.7=6.2 inches, because the throat invert elevation in this design is 0.7 inches above the entrance invert elevation.

With this meter installed in an 8-inch sewer, the ratio of the downstream depth reading (3.95 inches) to the upstream depth reading (6.2 inches) would be 0.64 or 64 percent. With an exit transition of 1:6, the meter would have a modular limit of about 65 percent. Thus, with this design, the throat section and upstream section of the meter would simultaneously fill before the modular limit was reached, if the sewer downstream from the meter were flowing at the normal depth predicted by the Manning formula. This is important because one can be assured that submerged operation will not occur during normal operation of the meter. Metering under open channel conditions in an unsubmerged mode as well as metering under full pipe conditions requires obtaining and manipulating only a single differential pressure reading. On the other hand, metering under open channel conditions in a submerged mode requires obtaining and manipulating two differential pressure readings and, for this reason, is inherently less accurate. Adapting the throat of the meter to cause simultaneous throat and entrance filling at a flow rate below the modular limit is thus a significant improvement in meter design.

A portable wastewater flow metering device has been disclosed for installation in the entrance pipe to a sewer manhole. The device is capable of measuring liquid flow both under free flow, open channel conditions and under full pipe conditions by taking measurements in a sewer adjacent to one sewer manhole.

The invention is not to be construed as limited to the particular forms disclosed herein, since these are to be regarded as illustrative rather then restrictive. It is the intention of this patent to cover all changes and modifications of the example of the invention herein chosen for the purposes of the disclosure, which do not constitute departures from the spirit and scope of the invention.

Claims

1. In the process of metering the flow of liquid which is flowing by gravity in an elongated pipe that is open to atmosphere, wherein:

tubular venturi metering device is installed in the pipe, which has an open-ended bore therethrough having an axis extending end-to-end thereof,

arranging the device in the pipe so that the axis of the bore is disposed substantially parallel to the longitudinal axis of the pipe and the bore thus has an end which is normally oriented upstream of the liquid flow in the pipe and an end which is normally oriented downstream of the liquid flow in the pipe,

the bore having an axially inwardly tapered entrance section adjacent the upstream end thereof which converges toward the axis of the bore in vertical planes paralleling the axis of the bore relatively toward the downstream end of the bore but terminates short of the axis of the bore so that a throat is formed in the bore which opens to the downstream end thereof,

forming a liquid seal between the device and the pipe at the outer periphery of the device so that the liquid in that section of the pipe disposed upstream from the upstream end of the bore of the device, is constrained to flow through the bore of the device, relatively toward the downstream end thereof,

determining the static pressure head in the liquid in the aforesaid upstream section of the pipe when the liquid is flowing in the pipe at a depth less than that adapted to fill the upstream pipe, to meter the flow in the pipe for the less than full condition thereof,

configuring the cross-sectional area of the throat, relative to that of the pipe, transverse the respective axes thereof, so that the throat will fill with liquid substantially simultaneously with the upstream section of the pipe, when the liquid depth rises therein, and

providing means whereby the static pressure head of the liquid in the throat of the device and the upstream section of the pipe can be determined when both the upstream section of the pipe and the throat are filled, so that the difference between the latter two pressure heads can be determined to meter the flow in the pipe for the full condition thereof, and thereby enable the flow in the pipe to be metered for the full condition thereof as well as the less than full condition thereof and the transition therebetween,

the improvement wherein:

arranging the device in the pipe with its axis and the top of its throat horizontal thus leveling the device,

the bore having an axially outwardly tapered exit section adjacent the downstream end thereof which diverges from the axis of the bore in vertical planes paralleling the axis of the bore relatively toward the end of the bore that is normally downstream,

providing means whereby the static pressure head of the liquid in the throat of the device and in both the entrance section and the exit section can be determined, so that the difference between the static pressure heads in the entrance section and in the exit section can be used to determine the direction of flow and, hence, the actual upstream end of the device, and, during the less than full condition, an appropriate correction factor for metering the flow rate, and so that the difference between the throat pressure head and the actual upstream pressure head can be determined to meter the flow in the pipe for the full condition thereof.

2. The process in claim 1 wherein the cross-sectional area of the throat is configured relative to the entrance section of the bore and the exit section of the bore to cause simultaneous filling of the throat and the section of the bore that is actually upstream before the modular limit of the device is reached when the device is installed in a pipe of minimum slope.

3. In the combination wherein there are:

an elongated pipe which is open to atmosphere and adapted for the flow of liquid by gravity therein,

a tubular venturi metering device installed in the pipe and having an open-ended bore therethrough which has an axis extending end-to-end thereof,

the device being arranged in the pipe so that the axis of the bore is disposed substantially parallel to the longitudinal axis of the pipe and the bore thus has an end which is normally oriented upstream of the liquid flow of the pipe and an end which is normally oriented downstream of the liquid flow in the pipe,

the bore having an axially inwardly tapered entrance section adjacent the upstream end thereof, which converges toward the axis of the bore in vertical planes paralleling the axis of the bore and in that axial direction of the bore relatively toward the downstream end of the bore, but terminates short of the axis of the bore so that a throat is formed in the bore which opens to the downstream end thereof,

means for forming a liquid seal between the device and the pipe at the outer periphery of the device so that the liquid in that section of the pipe disposed upstream from the upstream end of the bore of the device, is constrained to flow through the bore of the device, relatively toward the downstream end thereof, and

first means for determining the static pressure head of the liquid in the aforesaid upstream section of the pipe when the liquid is flowing in the pipe at a depth less than that adapted to fill the upstream section of the pipe, to meter the flow in the pipe for the less than full condition thereof,

the cross-sectional area of the throat being configured relative to that of the upstream section of the pipe, transverse the respective axes thereof, so that the throat will fill with liquid substantially simultaneously with the upstream section of the pipe, when the liquid rises therein, and

there being second means for determining static pressure head of the liquid in the throat of the device and in the upstream section of the pipe when both the upstream section of the pipe and the throat are filled, so that the difference between the latter two pressure heads can be determined to meter the flow in the pipe for the full condition thereof, and thereby enable the flow in the pipe to be metered for the full condition thereof, as well as the less than full condition thereof and the transition therebetween,

the improvement comprising

means for leveling the device,

an axially outwardly tapered exit section adjacent the downstream end of the bore which diverges from the axis of the bore in vertical planes paralleling the axis of the bore relatively toward the end of the bore that is normally downstream,

means for determining the static pressure head of the liquid in the throat of the device and both in the entrance section and in the exit section whereby the difference between the static pressure heads in the entrance section and the exit section can be used to determine the direction of flow and, hence, the actual upstream end of the device, and during the less than full condition an appropriate correction factor for metering flow rate and whereby the difference between the throat pressure head and the actual upstream pressure head can be determined to meter the flow in the pipe for the full condition thereof.

4. The combination in claim 3 wherein the cross-sectional area of the throat is configured relative to the entrance section of the bore and the exit section of the bore to cause simultaneous filing of the throat and the section of the bore that is actually upstream before the modular limit of the device is reached when the device is installed in a pipe of minimum slope which at least flows full at an average velocity of about 2 feet/second.

5. The combination in claim 4 wherein means for determining the static pressure head of the liquid in the entrance section and in the exit section comprise tubes that discharge bubbles into the liquid in the annular space between the outside surface of the device and the inside surface of the pipe.

6. The combination in claim 4 wherein means for determining the static pressure head of the liquid in the entrance section and in the exit section comprise tubes that discharge bubbles into the liquid as it flows through the interior of the device.

7. A process of metering the flow of liquid which is flowing in an elongated pipe that is open to the atmosphere, wherein a closed conduit venturi metering device is installed in the pipe, which device has an open-ended bore therethrough extending end-to-end thereof, the bore having an entrance section adjacent a first end thereof, an exit section adjacent the second end thereof, and intermediate the entrance and exit sections, a throat having a top and bottom and a smaller cross-sectional area than the entrance and exit sections, comprising the steps of:

arranging the device in the pipe to accept flow into the entrance from the pipe and otherwise to substantially block the pipe, and

configuring the cross-sectional area of the throat, relative to that of the entrance section, including constricting the throat across the bore at the throat top or bottom, or both so that the throat will fill with liquid substantially simultaneously with the entrance section, when liquid depth rises in the entrance section, and

providing means for determining the head of the liquid in said entrance section, in said throat and, in said exit section, for use at least to determine direction of flow in the device and flow both in less than full and in full flow through the device.

8. The process of claim 7 in which the throat is also configured to impart critical flow depth to liquid flowing through the throat in less than full flow. 9. The process of claim 8 further comprising,

comparing the heads of liquid in said entrance and exit sections of the device flowing in less than full flow or in full flow, and

determining therefrom the direction of flow in said device. 10. The process of claim 9 further comprising,

determining from said head comparisons the percent submergence of the device when the liquid is flowing less than full, and

applying a correction factor to flow rate in less than full flow condition when the percent submergence exceeds the maximum submergence of the device. 11. The process of claim 8 further comprising,

comparing the heads of liquid in said entrance and exit sections of the device flowing in less than full flow,

determining therefrom the percent submergence of the device, and

applying a correction factor to flow rate in less than full condition when the percent submergence exceeds the maximum submergence of the device.

12. The process of claim 8 in which the cross-sectional area of the throat is configured, relative to that of the entrance section, to cause the throat and entrance sections to fill simultaneously below the modular limits of the device, when liquid depth rises in the entrance section. 13. The process in claim 12 wherein the device is arranged in a pipe which at least flows full at an average velocity of about 2 feet/second. 14. A process of metering the flow of liquid which is flowing by gravity in an elongated pipe that is open to the atmosphere, wherein a tubular venturi metering device is installed in the pipe, which device has an open-ended bore therethrough having an axis extending end-to-end thereof, the bore having an entrance section adjacent a first end thereof which converges toward the second end of the bore but terminates short of the axis of the bore so that a throat is formed in the bore which opens to said second end, the bore having an exit section adjacent said second end which diverges from the bore toward said second end, comprising the steps of:

arranging the device in the pipe to accept flow into said entrance and otherwise to substantially block the pipe,

configuring the cross-sectional area of the throat, relative to that of the entrance section, so that the critical flow depth is imparted to liquid flowing through the throat in less than full flow and so that the throat will fill with liquid substantially simultaneously with the entrance section, when the liquid depth rises in the entrance section,

providing means for determining the head of the liquid in said entrance section, in said throat and in said exit section,

comparing the heads of liquid in said entrance and exit sections of the device flowing in less than full flow or in full flow, and

determining the direction of flow. 15. The process of claim 14 further comprising,

determining from said head comparisons the percent submergence of the device when the liquid is flowing less than full, and

applying a correction factor to flow rate in less than full flow condition when the percent submergence exceeds the maximum submergence of the

device. 16. The process of claim 14 in which the cross-sectional area of the throat is configured, relative to that of the entrance section, to cause the throat and entrance sections to fill simultaneously below the modular limit of the device, when liquid depth rises in the entrance section. 17. The process of claim 16 wherein the device is arranged in a pipe which at least flows full at an average velocity of about 2 feet/second. 18. The process of claim 14 wherein said step of configuring includes constricting said throat horizontally across said bore at the throat top or bottom, or both. 19. The process of claim 16 wherein said step of configuring includes constricting said throat horizontally across said

bore at the throat top or bottom, or both. 20. Apparatus for metering flow of liquid which is flowing in an elongated pipe which is open to the atmosphere, comprising:

a closed conduit venturi metering device installed in the pipe and having an open-ended bore therethrough extending end-to-end thereof, said bore having an entrance section adjacent a first end thereof, an exit section adjacent the second end thereof, and intermediate the entrance and exit sections, a throat having a top and bottom and a smaller cross-sectional area than the entrance and exit sections,

said device being arranged in said pipe to accept flow into said entrance from the pipe and otherwise to substantially block the pipe,

the cross-sectional area of the throat, relative to that of the entrance section, being configured, including a throat constriction across the bore at the throat top or bottom, or both, such that the throat will fill with liquid substantially simultaneously with the entrance section, when liquid depth rises in the entrance section, and

means for determining the head of the liquid in said entrance section, in said throat, and in said exit section, for use at least to determine direction of flow in the device and flow both in less than full and in

full flow through the device. 21. The apparatus of claim 20 in which said throat also is configured to impart critical flow depth to liquid flowing through the throat in less than full flow. 22. The apparatus of claim 21, further including means for comparing heads of liquid in said entrance and exit sections in less than full flow or in full flow, and for determining therefrom the direction of flow in said device. 23. The apparatus of claim 21, further comprising means for determining the percent submergence of the device in less than full flow, and for applying a correction factor to flow rate in less than full flow condition when the percent submergence exceeds the maximum submergence of the device. 24. The apparatus of claim 21 in which said cross-sectional area of the throat is configured relative to that of the entrance section to cause the throat and entrance sections to fill simultaneously below the modular limit of the device,

when liquid depth arises in the entrance section. 25. The process of claim 24 in which said device is arranged in a pipe which at least flows full at an average velocity of about 2 feet/second. 26. Apparatus for metering the flow of liquid which is flowing by gravity in an elongated pipe that is open to the atmosphere, comprising:

a tubular venturi metering device arranged in the pipe, which device has an open-ended bore therethrough having an axis end-to-end thereof, said bore having an entrance section adjacent a first end thereof which converges toward the second end of the bore but terminates short of the axis of the bore so that a throat is formed in the bore which opens to said second end, such bore having an exit section adjacent said second end which diverges from the bore toward that second end,

said device being arranged in the pipe to accept flow into said entrance from the pipe and otherwise to substantially block the pipe,

the cross-sectional entrance of a throat being configured relative to that of the entrance section, so that a throat will fill with liquid substantially simultaneously with the entrance section below the modular limit of the device, when the liquid depth rises in the entrance section, and

means for determining the head of liquid in said entrance section and in said throat. 27. The apparatus of claim 26, further comprising means for comparing the heads of liquids in said entrance and exit sections of the device flowing in less than full flow or in full flow, and for determining the direction of flow. 28. The apparatus of claim 27 further comprising means for determining the percent submergence of the device from said head comparisons in less than full flow, and for applying a correction factor to flow rate in less than full flow conditions when the percent submergence exceeds the maximum

submergence of the device. 29. The apparatus of claim 28 wherein said device is arranged in a pipe which at least flows full at an average velocity of about 2 feet/second. 30. The apparatus of claim 26 wherein said configuration of said throat includes a constriction of said throat horizontally across said throat at the throat top or bottom, or both. 31. The apparatus of claim 28 wherein said configuration of said throat includes a constriction of said throat horizontally across said throat at the throat top or bottom, or both.