Flow Calibration

FLOW CALIBRATION

Calibration and Standards Flow rate calibration depends on standards of volume(length) and time or mass and time. Classified into two: (a) Primary calibration & (b) Secondary calibration Primary calibration based on the establishment of steady flow through the flowmeter to be calibrated and subsequently measurement of volume or mass of flowing fluid that passes through in a accurately timed interval. Any stable and precise flow meter calibrated by such primary methods becomes a secondary flow rate standard against which less accurate flow meters may be calibrated conveniently.

Calibration and Standards Continued… Significant deviations of the conditions of use from those at calibration will invalidate the calibration Possible sources of error in flow meters include variation in (a) fluid properties - density, viscocity and temperature (b) orientation of meter (c) pressure level (d)flow disturbances-elbows,tees,valves etc,upstream and to a lesser extend down stream etc

Calibration of Flow meters Flow meters are classified into gas flow meters , liquid flow meters and solid flow meters Avoided the solid flow meter calibration in this presentation Focusing on gas flow meter and liquid flow meter Both gas flow meter and liquid flow meter can either be volumetric(volumetric flow) or gravimetric (mass flow)

Calibration of Flow meters Contd... Difference between liquid and gas is that liquid can be collected and measured easily. For eg. A bucket can be used to collect the water and weigh it before and after collecting the certain volume of water which has flown through the system. By collecting and measuring the volume or mass of the liquid which has flown through the flow meter for a certain period of time, we can find volumetric rate or mass flow rate

Gas Flow Calibration Contd We have a meter on test eg. Rotameter and have gas supply which flows through a vertical burette as shown in the figure. At some particular time, allow a soap film/soap bubble to enter the burette.  Gas flow is going to carry film across the burette. Using the photocell along with timing device we can find out the amount of time taken in sweeping a certain volume of burette. By dividing volume sweeped by period of time ,actual volume flow rate can be obtained. By comparing the measured and actual value, we can calibrate the flow meter.

Liquid Flow Meter Calibration Contd Using pump the liquid is circulated across testmeter and either to sump or weighing tank(as per diverter/control valve action) We start measuring process by allowing it to come to weighing tank and after a certain length of time, divert it to sump. Either weigh or measure volume of liquid which is collected. Volume or mass divided by time will give actual volume rate or mass rate. Using the actual value and measured value from meter, we can plot calibration curve.

Flow Calibration(Calibration Curve) contd.

Flow Calibration(Calibration Curve) contd. From Calibration curve we can see the variation between actual value and measured value. We can use the variations as calibration information and can apply correction factor.

Examples of Commercial Flow rate Calibrators Calibrators using dynamic weighing scheme(using liquid) Ballistic flow prover(using liquid) Gas flow calibrator(piston driven by motor)

Liquid Flow rate Calibration using dynamic weighing scheme

Liquid Flow rate Calibration using dynamic weighing scheme Contd… Available in models to cover the range 0.5 to 150,000 lbm/h and have overall accuracy of +/-0.1 % Procedure for carrying out dynamic weighing test are: a) Running operation before test• Fluid contained in the reservoir is pumped through a closed hydraulic circuit • First, it enters the filter and heat exchange equipment,which controls temperature within +/- 1Degree Fahrenheit. • It then passes through control valves, meter undertest,backpressure valves, weigh tank,then back into reservoir

Liquid Flow rate Calibration using dynamic weighing scheme Contd… b)Start of preliminary fill (Tare Time) • When control valves have been adjusted for desired flow,a tare weight is placed on the weigh pan. • Then the cycle start button is pushed, resetting the timer, closing the dumb valve which starts the filling of the weigh tank. c)End of prefill,start of weighing cycle • As the weigh tank fills, the weigh pan rises, tripping the timer actuator, and the electronic timer begins counting in millisecond, starting the actual weighing cycle

Liquid Flow rate Calibration using dynamic weighing scheme Contd… • The preliminary fill, balanced out by the tare weight before actual weighing begins, permits a net measurement of the new fluid added after preliminary fill. • The preliminary fill method permits measurement of only a portion of the cycle,eliminating the mechanical errors in the start and stop portions and allowing dynamic errors to be cancelling. d)Weighing cycle in operation • The weighing cycle is continued as the precision weight is placed on the weigh pan,again deflecting the beam • The cone shape deflector at the inlet of weigh tank permits the even distribution of the metered fluid

Liquid Flow rate Calibration using dynamic weighing scheme Contd… e)End of weighing cycle • As the tank fills, the weigh pan rises, until it again trips the timer actuator, stopping the timer and indicating the time within a thousandth of a second. • By combining the precision test weight with the timed interval, the actual flow rate in pounds/hour is easily accurately determined.From these basic mass units, other flow units can be accurately calculated. f) Emptying for recycling • After the beam movement trips the timer, weigh tank automatically empties in less than 25 seconds,even at maximum flow. • The calibrator is now ready for next flow setting.This cuts total calibrating time as much as 50%.

Ballistic flow prover calibator • Useful for fast-response, high resolution flowmeters such as turbine, positive displacement,vortex shedding etc, types, where steady state can be achieved quickly and the integration of the flow rate to get total flow is accomplished accurately by accumulating the meter pulse rate output in a counter. • The Integration gives accurate total flow even if the flow rate is not perfectly steady. • One such calibrator uses Teflon sealed air driven free piston travelling down a precision honed tube to dispense a precise volume of calibration fluid through the attached flowmeter to be calibrated.

Ballistic flow prover calibrator Contd… • Precise time and displacement measurements on moving piston are utilised in a microprocessor data reduction system to achieve a claimed flow rate accuracy of +/- 0.02% • The small flow volume(above 5 gal for a 700 gal/min full scale unit) involved in checking a single flow rate allows rapid calibration, 20 repeats of a single point typically being achieved in 8 minutes for a 300 gal/min flow rate. • Units are available with maximum flow rates of 100 to 6000 gal/min. • A similar type of gas flow calibrator is shown in next slide (except that the piston is driven by electric motor is available for gas flow)

Gas flow Calibrator (piston driven by electric motor)

Important Points • The calibration of flow meters to be used with gases often can be carried out with liquids as long as the pertinent similarity relations(Reynold no.) are maintained and theoretical density and expansion corrections are applied. • If this procedure is felt to be insufficient accuracy, a direct calibration with actual gas to be employed can be carried out by means of the gasometer system as shown in next slide. • Here the gas flowing through the flowmeter during a time interval is trapped in the gasometer bell and its volume is measured.

Gas-flow Calibration

Important Points Contd.. • Temperature and pressure measurements allow calculation of mass and conversion of volume to any desired standard condition. • By filling the bell with gas, raising it to the top and adding appropriate weights, such a system may be used as a gas supply to drive gas through a flow meter as the bell gradually drops at a measured rate. • By using a precision analytical balance to measure the mass accumulated in a storage essel overtime, accuracies of 0.02% were obtained for flow rates up to 9 Kg /s (20 lbm/s) • The size of the equipment makes it more economical for flow rate; however accuracy is less(+/- 0.08%)

Important Points Contd.. • When the above primary methods cannot be justified,comparison with secondary standard flowmeter connected in series with the meter to be calibrated may be sufficiently accurate. • Turbine flowmeters and their associated digital counting equipment have been found particularly suitable for such secondary standards. • With attention to details of such standards can closely approach the accuracy of primary standards.

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