Abstrict Method and apparatus for determining (i.e., proving) the accuracy
of a gas flow meter includes the use of a flow determining valve
to derive flow data representative of actual gas flow rate simultaneously
with similar flow data obtained from the meter. Real time quantitative
analysis of the gas stream constituents is employed during meter
proving (as may be accomplished with an automated gas chromatograph)
so as to condition the flow data obtained from the flow determining
valve and hence provide a more accurate determination of actual
gas flow rate. This conditioned flow data is then compared to the
flow data obtained from the meter to thereby derive a variance factor
for the meter at given sensed flow rates. Therefore, flow rate data
obtained from the meter may then be adjusted as a function of the
derived variance factor so that a more accurate determination of
actual flow rate through the meter is possible.
Claims What is claimed is:
1. A method for determining the accuracy of a gas flow meter comprising
the steps of:
allowing gas to flow simultaneously through the meter and a flow
determining valve;
deriving flow data from the meter and said flow determining valve;
comparing said derived data to obtain a variance factor for said
meter, and
determining the relative quantities of gas constituents in said
gas flow simultaneously with said step of deriving flow data from
the meter and the flow determining valve, and then conditioning
said derived flow data based upon said determined gas constituent
quantities.
2. A method for determining the accuracy of gas flow measurements
of a meter installed in a gas flow line comprising the steps of:
obtaining a first signal from said meter which is expected to be
indicative of gas flow measurement by said meter corresponding to
an actual gas flow rate through said line;
obtaining a second signal from a gas flow calibration valve which
is also expected to be indicative of said actual gas flow rate;
determining the quantity of at least one constituent of the gas
flowing through the line in real time with said step of obtaining
the first signal from the meter, and then conditioning in real time
said second signal on the basis of said determined quantity of said
at least one gas constituent so as to obtain a conditioned gas flow
signal; and
deriving an error factor for said meter at said expected actual
gas flow rate by comparing said first signal and said conditioned
second signal.
3. A method as in claim 2 wherein said step of determining the
quantitative presence of said at least one gas constituent includes
continually withdrawing a sample of said gas flowing through the
gas flow line, and passing the withdrawn sample to a gas chromatograph
means which determines said quantitative presence.
4. A method as in claim 2 wherein the gas flowing through the gas
flow line is natural gas.
5. A method for determining the accuracy of gas flow measurements
of a meter comprising the steps of:
obtaining a first signal indicative of meter flow measurement at
an actual gas flow rate;
obtaining a second signal indicative of said actual gas flow rate;
and
comparing said first and second signals so as to derive a correction
factor for said meter flow measurement, wherein
said step of obtaining said second signal includes determining
a quantitative presence of at least one constituent of the gas at
said actual flow rate in real time with said step of obtaining the
first signal, and then conditioning, in real time, said second signal
prior to said signal comparing step, whereby said correction factor
is adjusted on the basis of said determined quantitative presence.
6. A method for determining the accuracy of a gas flow meter installed
in a gas line through which a gas mixture flows, said method comprising
the steps of:
generating a flow measurement signal from said meter at a predetermined
actual flow rate;
passing said gas through a sonic nozzle valve and generating a
flow determining signal in response thereto;
determining the quantitative presence of at least one gas constituent
of said gas mixture substantially simultaneously with said step
of generating a flow measurement signal from the meter, and deriving
a gas constant based upon said substantially simultaneously determined
quantitative presence;
adjusting said flow determinating signal on the basis of said derived
gas constant; and
comparing said flow measurement signal and said flow determinating
signal to obtain a flow correction factor for said meter at said
predetermined actual flow rate.
7. A method as in claim 6 wherein said step of determining the
quantitative presence of said at least one gas constituent includes
continually withdrawing a sample of said gas flowing through the
gas flow line downstream of said sonic nozzle, and passing the withdrawn
sample to a gas chromatograph which determines said quantitative
presence.
8. A method as in claim 6 wherein the gas flowing through the gas
flow line is natural gas.
9. Apparatus for determining the accuracy of gas flow measurements
of a meter which outputs a measurement signal indicative of meter
flow measurement at an actual gas flow rate therethrough, said apparatus
comprising:
valve means for obtaining a calibration signal indicative of said
actual gas flow rate; and
means for comparing said measurement and calibration signals so
as to derive a correction factor for said meter flow measurement
at said actual gas flow rate, wherein
said means for obtaining said calibration signal includes;
(i) gas constituent determining means for determining a quantitative
presence of at least one constituent of the gas at said actual flow
rate in real time with the measurement signal; and
(ii) means for conditioning said calibration signal on the basis
of said determined quantitative presence so that said comparing
means compares said measurement signal and said conditioned calibration
signal, whereby said correction factor is adjusted on the basis
of said determined quantitative presence.
10. Apparatus as in claim 9 wherein said gas constituent determining
means includes a gas chromatograph.
11. Apparatus as in claim 10 wherein said gas constituent determining
means includes a sample port for withdrawing continually a sample
of said gas downstream of said flow determining valve and for passing
said withdrawn sample to said gas chromatograph.
12. Apparatus as in claim 9 also comprising connection means adapted
to being connected operatively with a gas flow line, said connection
means including flexible conduits adapted to being removably connected
to stationary supply and discharge nipples associated with the gas
flow line.
13. Apparatus for determining the accuracy of a gas flow meter
comprising:
a flow determining valve;
means which allow gas to flow simultaneously through the meter
and said flow determining valve;
means for deriving real time flow data from said meter and said
flow determining valve; and
means for comparing said derived data to obtain a variance factor
for said gas flow meter, wherein
said means for deriving flow data from said determining valve includes
means for determining in real time the relative quantitative of
gas constituents in said gas flow and then deriving said real time
flow data based upon said real time determined gas constituent quantities.
14. A mobile gas meter proving system comprising:
a vehicle; and
means housed within said vehicle for proving accuracy of a gas
meter, said means for proving accuracy including,
(i) a flow determining valve;
(ii) means adapted to being connected operatively with a gas flow
line in which the gas flow meter is located so as to allow the gas
to flow simultaneously through the meter and said flow determining
valve;
(iii) means for deriving flow data from said meter and said flow
determining valve; and
(iv) means for comparing said derived data to obtain a variance
factor for said gas flow meter, wherein
(v) said means for deriving flow data from said determining valve
includes means for determining the relative quantities of gas constituents
in said gas flow and then deriving said flow data based upon said
gas constituents quantities.
15. A mobile gas meter proving system as in claim 14 wherein said
gas constituent determining means includes a gas chromatograph.
16. A mobile gas meter proving system as in claim 14 wherein said
gas constituent determining means includes a sample port for withdrawing
continually a sample of said gas downstream of said flow determining
valve and for passing said withdrawn sample to said gas chromatograph.
17. A mobile gas meter proving system comprising:
a vehicle; and
means housed within said vehicle for proving accuracy of a gas
flow meter, said means for proving accuracy including,
(i) a flow determining valve;
(ii) means adapted to being connected operatively with a gas flow
line in which the gas meter is located so as to allow the gas to
flow simultaneously through the gas flow meter and said flow determining
valve;
(iii) means for deriving flow data from the gas flow meter and
said flow determining valve;
(iv) means for comparing said derived data to obtain a variance
factor for the gas flow meter, and
(v) means for quantitatively determining at least one constituent
of the gas and for conditioning said derived flow data based upon
said determined quantity of said at least one gas constituent.
18. A mobile gas meter proving system as in claim 16 or 17 wherein
said gas line includes inlet and outlet conduits and valve means
associated with said inlet and outlet conduits, and wherein said
system further includes flexible conduit means adapted to being
interconnected to said inlet and outlet conduits so as to continuously
supply gas from the gas line to said flow determining valve from
the gas line, and return gas to the gas line from the flow determining
valve.
19. A mobile gas meter proving system as in claim 17 wherein said
flow determining valve is a sonic nozzle valve.
Description FIELD OF THE INVENTION
This invention relates to method and apparatus for determining
the accuracy of gas flow meters (i.e., sometimes called "proving"
a meter in art parlance). In a specific embodiment, the invention
is in the form of a mobile meter proving unit which can be dispatched
to the site of a meter for field-proving. The invention (whether
embodied in mobile or stationary units) is capable of determining
the quantity of at least one chemical constituent of the gas flowing
through the meter, and then conditioning a proving signal (i.e.,
which is indicative of the actual gas flow through the meter) on
the basis of the detected chemical constituent quantity. This, in
turn, provides for a more precise generation of an error factor
for the meter at this actual gas flow (i.e., to determine the amount
by which the indicated signal generated by the meter deviates from
a true signal representative of the actual flow).
BACKGROUND AND SUMMARY OF THE INVENTION
Proving the accuracy of flow meters is very important for suppliers
of natural gas and/or any other gas which is purchased by a supplier's
customers. That is, since the cost of the supplied gas is computed
by the amount of gas flowing through the meter, then any meter deviation
from the actual amount of gas which is supplied to the customer
will result in an undercharge/overcharge for that particular customer.
As may be appreciated, over time, the underchanging/overcharging
could result in significant/customer, respectively.
In order to reduce such potential economic losses, it has been
the industry practice for some time to periodically check the accuracy
of flow meters in the field (i.e., provide stationary on-site proving
apparatus at the meter in the gas pipeline). There are, however
some disadvantages to these conventional meter proving systems.
For example, conventional meter proving systems are usually provided
for each meter in the pipeline for which proving is desired. Thus,
the conventional proving system is stationary at the meter site
thereby requiring a replication of such a system for each meter
for which proving is desired. This, of course, results in greater
costs to the gas supplier associated with proving of the supplier's
meters. Hence, it would especially be desirable if a meter proving
system was provided which was mobile --i.e., a system which could
be dispatched and transported from one meter to be proved to another.
The present practice of proving gas flow meters is to determine
as accurately as possible the actual flow rate of the gas flowing
through the meter. This is usually accomplished with a high precision
sonic valve array downstream of the meter. The sonic valve array
will output a signal indicative of the volumetric flow rate of gas
and this flow rate is then converted to a mass flow rate using well
known algorithms. However, some of the variables used in these known
algorithms depend upon the mass quantities of the various gas constituents
flowing through the pipeline (e.g., for natural gas these constituents
may be CO.sub.2 methane, ethane, propane, and others). The present
practice is to predetermine the quantities of the various gas constituents
in advance of meter proving, (as by submitting a sample of the gas
to a laboratory for quantitative analysis) and then inputting the
results of this quantitative predetermination into the proving system
in the form of "constants". Obviously, the quantitative
gas constituency may change between the time that the analysis is
completed and the time that the meter is actually proved thereby
possibly contributing to a less than accurate error factor for the
meter.
It would therefore particularly be desirable in terms of accuracy
if a meter proving system was provided which measures quantitatively
the various gas constituents in real time (i.e., concurrently with
the meter proving). Thus, if a real time quantitative gas constituent
measurement was possible, then the resulting signal representative
of the actual mass flow of the gas could be conditioned on the basis
of the real time measured quantities thereby providing for more
accurate determination of the actual gas flow rate.
According to the present invention, method and apparatus are provided
which not only permit gas flow meters to be proved on-site via a
mobile gas proving unit, but which also use real time quantitative
measurements of the gas constituents flowing through the meter so
that more accurate meter proving results.
The invention achieves these objectives by providing a self-contained
meter proving system in association with a suitable vehicle. Thus,
all that is needed to prove a meter in the field is to dispatch
the mobile proving unit and to obtain the error factors for particular
meters. These error factors can then be downloaded into the supplier's
main data file, for example, so that subsequent charges for gas
supplied to a customer (based on the meter's output) can be adjusted
by the error factor.
A more accurate determination of the meter's error factor is achieved
by providing a device which is capable of quantitatively deriving
the presence of any preselected number of gas constituents. Preferably,
the device employed with the present invention to accomplish this
function is a gas chromatagraph which outputs a signal representative
of the quantitative presence of each of a predetermined number of
gas constituents in the gas flow. This outputted signal is then
supplied as an input for determining mass flow data for the gas
stream using well known algorithms. In such a manner, real time
quantitative analysis of the gas stream flowing through the meter
is obtained thereby providing a more accurate determination of the
meter's error factor.
These and other advantages of the invention will become more clear
to the reader after consideration is given to the following detailed
description of the preferred exemplary embodiment.
BRIEF DESCRIPTION OF THE DRAWINGS
Reference will hereinafter be made to the accompanying drawings
wherein like reference numerals throughout the various FIGURES denote
like structural elements, and wherein;
FIG. 1 is a schematic view of a mobile meter proving unit of this
invention at the field site of a meter to be proved; and;
FIG. 2 is a schematic block diagram of a preferred system employed
by the present invention to prove a gas flow meter.
DETAILED DESCRIPTION OF THE PREFERRED EXEMPLARY EMBODIMENT
A schematic representation of a mobile meter proving unit 10 which
had been dispatched to a field site 12 of a glass flow meter 14
installed in a gas pipe line 16 is shown in accompanying FIG. 1.
The pipe line 16 includes, downstream of the flow meter 14 a full
port block valve 18 which is closed during meter proving so as to
direct gas flow through bypass outlet 20 and into the mobile meter
proving unit 10 via flexible conduit 22. The gas then returns to
pipe line 16 via flexible conduit 24 and bypass pipe 26. Of course,
when meter proving is finished, the flexible conduits 22 and 24
are housed within the mobile unit 10 after first opening block valve
18 and closing the valves 28 and 30 so that gas again flows directly
through the line 16 in the direction of arrow 32. It should be noted
that customer "starvation" does not occur during meter
proving using the mobile unit 10 of this invention since a constant
flow of gas is provided downstream of block valve 18 due to the
bypassing of a portion of the gas stream through valve 60a in bypass
line 60b. That is, bypass line 60b is provided so that at least
some gas flow is diverted around meter 14 and the block valve 18
(and thus also around the proving valve 46 - see FIG. 2).
The mobile proving unit 10 thus permits field-proving of gas flow
meters since the flexible conduits 22 24 may be carried by the
unit 10 and connected quickly to the bypass conduits 20 and 26
respectively. In this regard, any conventional "quick-disconnect"
coupling system may be employed to join conduits 22 and 24 to conduits
20 and 26.
The physical contents of mobile proving unit 10 and their relationship
to the on-site meter location 12 during a meter proving operation
is shown schematically in block-diagram fashion in accompanying
FIG. 2.
The meter 14 shown schematically in FIG. 2 is of a conventional
turbine variety which generates a pulsed output 14a. The pulsed
output 14a is supplied to a pulse counter 14b which counts the number
of pulses of the gas flow meter 14 and supplies this count to a
flow rate calculator 14c which is part of a microprocessor-based
computer 40 contained within the mobile proving unit 10. The output
of meter flow rate calculator 14c is the actual cubic feet per hour
(ACFH) which is measured by the gas flow meter 14. This signal is
converted in block 14d to standard cubic feet per hour (SCFH) by
inputting pressure and temperature signals from pressure and temperature
transmitters 42 44 respectively, connected operatively to the
gas flow meter 14 and gas pipe line 16 during meter proving.
During a meter proving operation, the output from block 14d is
compared at comparator 44 to a signal indicative of the standard
cubic feet per hour which is measured via a flow sensing valve 46
housing within the mobile unit 10. The flow sensing valve 46 can
be of any conventional, high accuracy type (e.g., sonic nozzle)
gas flow sensing valve. Preferably, it is a valve having a series
of individually actuable, value weighted, digital bistable valve
elements as is described more fully in U.S. Pat. Nos. Re. 29383
issued Sept. 6 1977 (re-issue of U.S. Pat. Nos. 3875955 issued
Apr. 8 1975); 3785389 issued Jan. 15 1974; 3746041 issued
July 17 1973; and 3942553 issued Mar. 9 1976 the entire contents
of each of these prior-issued U.S. Patents being expressly incorporated
hereinto by reference. The preferred valve is a Digital.RTM. valve
which may be commerically obtained from the Digital Valve Company,
Longmont, Colorado.
The output from the flow sensing valve 46 is applied as an input
to a mass flow calculator 47 which calculates mass flow according
to the following equation: ##EQU1## where M is the actual mass flow
rate in pounds, P is the gas pressure in absolute pounds per square
inch (psia), A is the area constant for the nozzle throat of valve
46 in square inches, C* is a critical flow factor, Cd is the discharge
coefficient or actual mass flow rate divided by theoretical mass
flow rate, R is the gas constant which is equal to 48.03 divided
by the molecular weight of the gas stream, and T is the gas temperature
in degrees Rankine.
Important to the present invention, a sample sidestream of gas
is withdrawn from the outlet 24 at sample port 45 (downstream of
flow-sensing valve 46) and is directed to a chromatagraph 48 which
analyzes the gas stream so as to determine the molecular weight
of the gas stream in addition to the quantitative presence of the
gas stream constituents (e.g., CO.sub.2 methane, ethane, propane,
and the like). Each of these constituents is then represented by
an output signal (which may be indicative of the mole percentage
of each individual constituent) which in turn, is supplied as an
input to the mass flow calculator 47.
Any suitable automated gas constituent analyzer may be employed
in the successful practice of this invention. One particularly preferred
analyzer is the Danalyzer.TM. chromatagraph commercially available
from the Electronic Division of Daniel Industries, Inc. of Houston,
Tex.
Mass flow calculator 47 uses known algorithms to generate the value
for C* and the gas constant R based on real time quantitative measurement
of the gas stream constituents. (For a more complete understanding
of the gas flow algorithms employed in mass flow calculator 47 and
elsewhere, the reader's attention is directed to Britton, "Sonic
Nozzles", Appalachian Gas Measurement Short Course, West Virginia
University, Aug. 12-14 1975 the entire content of which is expressly
incorporated hereinto by reference.)
The standard density is then calculated in block 50 as a function
of the pressure and temperature signals from pressure and temperature
transmitters 61 62 respectively, operatively associated with the
proving valve 46. The standard cubic feet per hour (SCFH) of the
proving valve 46 is then calculated in block 52 by dividing the
mass flow rate derived in block 47 (and conditioned by the input
from chromatagraph 48) by the standard density derived in block
50. The prover SCFH and the meter SCFH are then compared in comparator
44 and this comparison is used in block 54 so as to obtain a meter
error factor at a corresponding flow rate which is sensed by the
meter is ultimately obtained. These error factors may then be "downloaded"
into the gas supplier's main data file in the form of an electronic
"look-up" table, for example, so that subsequent charges
to a customer at any given flow rate may be determined more accurately.
While the invention has been described in connection with what
is presently considered to be the most practical and preferred embodiment,
it is to be understood that the invention is not to be limited to
the disclosed embodiment, but on the contrary, is intended to cover
various modifications and equivalent arrangements included within
the spirit and scope of the appended claims. |