FORM - 2

THE PATENTS ACT, 1970
(39 OF 1970)
Complete Specification
(See Section 10)
TITLE OF INVENTION
"IMPROVED FLOWMETER CALIBRATION RIG"
(a) KROHNE MARSHALL PRIVATE LIMITED
(b) having administrative office at
A-34/35, MIDC Estate, 'H' Block, Pimpri, Pune 411 018, State of Maharashtra, India
The following specification particularly describes the nature of the invention and the manner in which it is to be performed.

FIELD OF INVENTION
The present invention relates to an improved calibration rig and a method thereof for calibration of Flow metering device used for incompressible fluids; in particular, Electromagnetic, Ultrasonic, Vortex and Turbine Flow meter; with current / voltage or pulse output.
BACKGROUND OF THE INVENTION
Flow metering device or Flow meter is used for measurement of (incompressible / compressible) fluid flow in any closed conduit. Calibration of the flow meter is of paramount importance because precision of measurement of flow depends upon the accuracy with which any flow meter is calibrated.
Diameter of flow meter is one of the indicators of its size. In general, size of flow meters range from 10 mm to 3000 mm.
With the increase in size (diameter) of flow meter and its fluid handling capacity, facilities required to calibrate these meters, specially civil set up of the calibration rig, physical size of fluid reservoirs, pumps size, circulation pipes, pumping power & operating cost, etc. become enormously large.
Various systems and methods for calibration of flow meters reported in literature and standards are as follow:
• International Standard ISO : 8316
Measurement of liquid flow in closed conduits - Method by collection of the liquid in a volumetric tank.
• International Standard ISO : 4185
Measurement of liquid flow in closed conduits - Weighing method.
• International Standard ISO : 9104
Measurement of liquid flow, in closed conduits - Method of evaluating the
performance of Electromagnetic flow meter for liquids.
ASMEMFC-9M -1988
Measurement of Liquid Flow in closed Conduits by Weighing Method.
• Calibration and Standards in Flow Measurement by Richard Paton, National Engineering Laboratory, Scotland, UK.
• The concept of a New Primary Standard for Liquid Flow Measurement at PTB Braunschweig.
The basic principle of calibration in the afore mentioned methods and corresponding calibration rigs is to pump fluid through the flow meter to be calibrated followed by weighing or measuring the volume- of the pumped fluid in particular time interval to measure volume/mass flow rate of fluid. As the size of the flowmeter to be calibrated increases, pumping power increases enormously. For example, to calibrate flow meter of 2000 mm diameter, at fluid velocity of 1m/sec (that is minimum recommended), using aforementioned methods and calibration rigs, pumping power of around 900 KW is essentially required. Further, high capacity Weigh Bridge to weigh several tones of fluid or volume vessels of several cubic meter capacity, is necessary which adversely affects accuracy and techno-economics of the calibration rig.
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Apart from the aforesaid methods, the calibration/ testing devices for flow meters reported in prior art are primarily based on the following principles:
• calibration of a flow meter under test using a pre-calibrated Flow measurement device; a comparison method.
• measurement of displacement of a piston in a precise cylindrical bore, relating to volume of fluid displaced in a known time interval.
Prior art related to use of Pre-calibrated device ( comparison method)
United States Patent no. 4.566.307 discloses pipeline flow measurement proving system. A pipeline flow measurement proving system has a custody transfer insertion turbine meter mounted at a first location on a fluid pipeline. A proving insertion turbine flow meter is mounted at a second location on the pipeline upstream from the custody meter. Counting the pulses produced by the two meters during a selected time period carries out the calibration.
United States Patent no. 5,548.990 discloses methods and systems for calibrating flow meters. A Method for in situ calibration of a flow meter under test installed in a pipe involves positioning a pre calibrated insertion meter with in the bore of pipe in series with meter and using the output from the insertion meter to calibrate the meter.
Method and devices used in flow measurement is disclosed in United States Patent no. 5.932.812. To check whether the flow meter measures correctly or not, the noise in the measurement signal is analysed. One or more distribution values for the measurement signal are determined and compared with the predetermined reference values that have been determined for operating conditions for which the flow transducer is intended.
These systems, however, suffer from the drawback that for calibration of large size flow meters (generally above 500 mm diameter), enormous pumping power is necessary to circulate the fluid with minimum recommended fluid velocity of 1 m/s. Also, the inaccuracy of measurement of Pre calibrated device adds on to the overall inaccuracy of the calibration set up and in turn to the inaccuracy level of the device to be calibrated.
Prior art related to method based on use of piston displacement
United States Patent no. 4,619,134 discloses testing device for flowmeter. The device includes a calibration container connected to the pipeline across a switchover valve. A measuring piston is guided for reciprocating movement in the container between a starting and a terminal position. The measuring piston supports two signal releasing rings which are spaced apart from the other about a distance which when multiplied by the inner clearance of the calibration container determines a calibration volume of he measuring path of the piston.
Apparatus and method for determining the flow characteristics of a volumetric flowmeter is disclosed in United States Patent no. 5.111.682. In this apparatus, the displacement of the measuring piston is sensed as it travels through the measuring
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cylinder during a test run, while the pulses produced by the flow meter are counted during the time interval in which the piston displaces a given volume.
Pipeline flowmeter prover control system and method is disclosed in United States Patent no. 5.170.656. A reciprocable piston and cylinder type flow meter prover is calibrated by providing a liquid conducting manifold for conducting liquid to one chamber formed by the cylinder and piston while displacing fluid from the other chamber to a measuring tank and vice versa.
United States Patent no. 6,629.447 discloses machine and process for the calibration and proving of flow sensors. The system comprises of a magnetically coupled, small volume positive displacement multi axial isokinetic flow sensor calibrator or prover that has one or more serially interconnected flow tubes through which a cylindrical convoluted displacer, with embedded omni directional magnet(s) and anti compression device(s), that sequentially isokinetically dispenses calibration fluid through test flow sensor(s).
These systems suffer from several deficiencies such as:
• necessity of higher capacity pump for circulation of fluid through the flow meter to be calibrated;
• enormous increase in pumping power with increasing size of flow meter to be calibrated;
• limited capacity of the system due to the size of the calibration container i.e. prover housing;
Thus the study of the related prior art reveals several technological gaps such as:
• increase in pumping power with the increase in size (diameter) and fluid flow rate required for flow meter to be calibrated;
• adverse effect on best measurement capability of calibration rig with increased flow rate required for large diameter flow meters;
• limitations to provide calibration at a desired high velocity of fluid flow;
• high capital investment and operating cost;
• requirement / necessity of high capacity ( 100 tons plus for DN 2000) Weigh bridge to weigh several tones of fluid;
Necessity has constantly been felt to develop a simple, accurate and economical calibration rig and a method of calibration to substantially reduce capital investment, pumping power and yet enhance accuracy specially for calibration of higher size flow meters having diameter of the order of 500 mm and above.
The present invention addresses drawbacks of the prior art.
SUMMARY OF THE INVENTION
The main object of the invention is to provide a calibration rig and a calibration method thereto, with substantially reduced rig size, capital investment, energy consumption (pumping power) and enhanced accuracy for calibration of flow metering device of different diameters used for incompressible fluids; in particular whose output is in electrical form i.e. pulses, current/ voltage.
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Another object of the invention is to facilitate calibration of the flow metering device at different velocities without sacrifice of accuracy.
Thus, in accordance with the present invention, the calibration rig comprises of:
• vertical fluid containment tower with plurality of fluid level measuring means;
• flow straightening means;
• variable area conduit for fluid flow with a provision for mounting flow measuring device/s of different size under calibration;
• Transfer standard, a high accuracy flow measuring device, duly calibrated with traceability to National/ International standards.
• . Reference standard (volume vessel) of precise known capacity, duly
calibrated with traceability to National Standards.
• control means for maintaining stable and near constant fluid flow rate;
• isolating and control valves;
• pumping means;
• data acquisition and processing means;
wherein control means comprises of:
• high performance, electrically actuated, control valve;
• feed back flow measuring device, output of which is used to correct the opening of control valve for maintaining stable and near constant fluid flow with falling fluid level in vertical containment;
wherein data acquisition and processing means operate in steps of:
• receiving / compiling / processing signal from the flow metering devjce under calibration;
• receiving / compiling / processing signal from the level measuring means of the tower;
• comparing the two said signals, duly processed to the same units of measurement;
• reporting the error and suggesting the new Flow meter constant for performance with in desired accuracy;
wherein fluid passes from fluid containment tower, which provides necessary head for the fluid flow via flow straightening means, to the flow metering device under calibration which is fitted in a variable area conduit of the said calibration rig to the fluid reservoir via control valve;
wherein calibration process is carried out in steps of:
• comparing signals generated by the flow meter under calibration against known volume of fluid, indicated by signals from the level measuring means fitted on the tower, that has passed through the meter;
• measuring fluid volume between any two successive level switches in the tower using Transfer Standard i.e. a flow meter of high accuracy;
• verifying accuracy of Transfer Standard periodically using a Reference Standard;
• verifying accuracy of the volume of Reference standard periodically at National Standards laboratory.
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DESCRIPTION OF THE INVENTION
Features and advantages of this invention will become apparent in the following detailed description and the preferred embodiments with reference to the accompanying drawings.
Figure 1: Schematic of the calibration rig (Sheet 1)
Definitions and Nomenclature
For the purpose of the description contained herein, the definition of the following terms is relevant:
Transfer Standard: A precision flow measuring device, duly calibrated with traceability to National standards, used to periodically calibrate the Tower volume between successive levels.
Reference Standard: A Volume vessel of precisely known volume between high and low levels,.duly calibrated at National Standards laboratory, and used to calibrate Transfer standard.
In one of the embodiments, the calibration rig comprises of a vertical fluid containment tower 1 which is provided with plurality of highly precision level switches (LS) installed at different height. Fluid containment tower 1 provides necessary head for the fluid flow obviating use of higher size pumping means thus reducing pumping power substantially.
A variable area conduit 2, hereinafter called as test line connects near the tower base. Tapering cone sections are provided in 2 to accommodate different size of flow meters which are to be calibrated. Flow straightening means 3, feed back meter 4 and control valve 5 are fitted in 2. Along the path of fluid flow direction 20 from 1, flow meter under calibration 6 is fitted after 3 and before 4. Electrically actuated control valve 5 is provided at down stream after 4. It maintains stable, near constant flow rate in the conduit 2 and the meter to be calibrated 6 irrespective of the falling head in the tower 1 using control signals 21, 22 from the feedback meter 4 via data acquisition and processing means 7.
Signal 23 in the form of pulses from meter to be calibrated 6 is fed to the means 7. Signal 24 in the digital form from level switches of vertical tower 1 are also fed to the means 7.
A flow meter 8 of high accuracy (around 0.03%) hereinafter called as Transfer Standard is provided along with another accurate volume vessel 9 herein after called as Reference Standard. It is a volume vessel with top and bottom level switches. Volume between these two level switches is calibrated at a competent authorized National Standards organization such as National Physical Laboratory (NPL).
Pumping means 11 is provided to pump the fluid from an underground reservoir 10 into the vertical Tower 1. Since pump 11 is used to fill the tower 1 only and not for fluid circulation in the test line 2 and flow meter 6, its fluid handling capacity and pumping power is substantially reduced.
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Calibration of the flow metering device is carried out in following steps:
• Calibration fluid is pumped by pump 11 from an underground reservoir 10 into a vertical Tower 1 through the Test line 2.
• Filled water in the tower is allowed to calm for some time. Calibration is initiated by opening the control valve 5 to the desired level and stabilizing fluid flow rate in the test line.
• Time and pulse count, both at start and finish of the calibration run, is triggered with the change of state 6f the level switch LS when descending water column crosses the level switch in the vertical tower 1.
• Various uncertainties that can affect accuracy of calibration like change in fluid volume in tower due to change of temperature on tower volume and water volume, compressibility effect of the water column, perturbation to the free water surface due to sway of tower under strong breeze etc. are considered while arriving at the Best Measurement Capability (BMC) of the calibration rig.
• Flow meter 6 is calibrated by comparing pulses (23) generated by the meter when known volume of fluid (indicated by triggering signals 24) that has passed through the meter in the same time from the tower 1.
• Fluid Volume between any two successive level switches in the tower 1 is measured using Transfer Standard 8 and recorded in 7. Tower volume between level switches is confirmed periodically and after any physical change that may have occurred to the tower structure.
• Accuracy of Transfer standard is verified periodically using a Reference Standard 9. .•■ ,
• Reference standard is calibrated at National standards laboratory as per their recommendations.
Example
Following example quantitatively establishes that the calibration rig of the present invention results in the following in comparison with the rigs mentioned in the prior art:
• substantially reduced pump capacity, pumping power, sump size, capital investment; and
• enhanced accuracy.
The comparison is presented between conventional weigh bridge system wherein fluid is pumped through the flow meter to be calibrated followed by weighing the pumped fluid in particular time interval to measure mass flow rate of fluid and the calibration rig of the present invention.
A flow meter of size DN 2000 -is selected and fluid (water) velocity of 1 m/s is considered. To calibrate the flow meter, a set of 5 readings were repeated for three times; that is total 15 readings were considered.
Following table indicates details of the results:
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Particular Conventional Rig Present invention
Pumping capacity 11,300 m3/h 400 m3/h
Installed power 966 kW 44 kW
Electrical consumption for calibration of the said flow meter 2900 kWH 220 kWH
Weight 100 ton , 30 ton
Approx. Sump size 800 m3 360m3
In the conventional weigh bridge system weight of pumped water is measured. This weight is of the order of 100 ton for the calibration of the said flow meter. The accuracy for this weight measurement is of the order of 0.15 to 0.2%.
In the calibration rig of the present invention, tower volume accuracy is of the order of 0.04%, reference standard accuracy is 0.02% and transfer standard accuracy is of the order of 0.03%.
Thus it is evident and quantitatively established from this example that the improved calibration rig of the present invention results in substantial reducing pump capacity, pumping power, sump size, capital investment; yet enhancing accuracy of the measurement.
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claim:
An improved calibration rig comprising:
fluid containment tower with plurality of fluid level measuring means;
a conduit connected to the said tower at the base;
tapering cone section/s mounted in the said conduit for mounting one or plurality of flow meter/s to be calibrated;
a flow straightening means and control means mounted in the said conduit;
wherein the said control means comprises of:
• electrically actuated control valve;
• feed back flow measuring device, output of which is used to correct the opening of control valve for maintaining stable and near constant fluid flow with failling fluid level in vertical containment;
a high accuracy flow measuring device, duly calibrated with traceability to national / International standards connected to the said conduit for measuring fluid volume rate between successive level measuring means of the said tower;
a volume vessel of precise known capacity with top and bottom level switches, duly calibrated with traceability to National Standards; connected to the said high accuracy flow measuring device to verify accuracy of the said flow measuring means;
pumping means, fluid reservoir;
data acquisition and processing means to receive, compile signals from the said fluid level measuring means, the said flow meter to be calibrated and the said control means;
wherein fluid passes from fluid containment tower via flow straightening means, to the flow meter to be calibrated to the fluid reservoir via said control means;
An improved calibration rig as claimed in claim 1 wherein data acquisition and processing means operate in steps of:
• receiving, processing and storing signal from the flow meter under calibration;
• receiving, processing and storing signal from the level measuring means of the tower;
• comparing the two said signals, duly processed to the same units of measurement;
• reporting the error and suggesting the new flow meter constant for performance within desired accuracy.
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3. An improved calibration rig as claimed in claims 1-2 wherein calibration process is carried out in steps of:
• pumping the fluid in the said reservoir to the said tower through the said conduit;
• opening the said electrically actuated control valve of the said control means and stabilizing the fluid flow rate in the said conduit;
• acquiring/ receiving signal from the said level measuring means in the said data acquisition and processing means with respect to descending water level in the said tower;
• acquiring/ receiving signals in the said data acquisition and processing means from the said flow meter under calibration;
• comparing the said signals from the flow meter and the level measuring means generated by the flow meter under calibration;
• measuring fluid volume between any two successive level measuring means in the tower using the said high accuracy flow measuring device;
• verifying accuracy of the said high accuracy flow measuring device; using the said volume vessel of precise known capacity

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Abstract
The present invention relates to an improved calibration rig and a method for calibration of Flow metering device used for incompressible fluids; in particular, Electromagnetic, Ultrasonic, Vortex and Turbine Flow meter; with current / voltage or pulse output. With the increase in size (diameter) of flow meter and its fluid handling capacity, physical size of fluid reservoirs, pumps size, circulation pipes, pumping power & operating cost, etc. become enormously large. The judicious combination of a vertical fluid containment tower to provide head for fluid circulation along with fluid level measuring, feed back flow measuring means, flow measuring devices calibrated to national standard and volume vessel results in substantially reducing pump capacity, pumping power, sump size, capital investment; yet enhancing accuracy of the measurement.