Master Meter Testing at Water Plants and Pump Stations Jeff Cruickshank, PE NC AWWA-WEA Spring Conference Asheville, NC April 11, 2015
Presentation Outline What types of meters are used at WTPs and pump stations? Why test master meters? How are master meters tested? Examples of master meter tests
Types of Large Flowmeters Turbine & Propeller Orifice Plate Venturi Annubar Variable Area Mag Meters Ultrasonic Vortex
Propeller Meters Rotational velocity directly proportional to flow velocity Straightening vanes upstream Accurate within 2% 10:1 range Linear output No external power required Requires calibration factor Moving parts - maintenance
Venturi Meters No moving parts Long life Accurate within 0.75% 10:1 range Large, heavy, expensive Non linear output square root Tubing can leak or plug Inaccurate at low flow
Mag Meters No moving or wetted parts Accurate within 0.5% 30:1 range Linear output Short run of straight pipe Require calibration Moderately expensive
Ultrasonic Meters No moving or wetted parts Accurate within 1% 20:1 range Linear output Accurate to 0.1 fps Sensitive to air and noise Measure mean velocity, not weighted by area
All Types of Master Meters Need Symmetric Velocity Profiles Pumps, bends, and fittings distort velocity profiles Skewed asymmetric profiles produce inaccurate metering Swirl, crossflow and flow separation cause errors Straight pipe upstream and downstream is critical
Typical Velocity Profiles in Water Mains Theoretical velocity factor 0.5 for laminar flow Profile shape for turbulent flow depends on pipe size, roughness, and velocity Velocity factor, ratio of average to center velocity, quantifies profile shape
Velocity Factors for Turbulent Flow VF= 1 1+1.33*sqrt(f) Equation from hydraulics text VF= 1 1+22.8*V^-0.075*D^-0.085*C^-0.925 Equation solved for more familiar terms D= 30 V= 2.0 C VF 40 0.652 60 0.731 80 0.780 100 0.814 120 0.838 140 0.856 Large variation with roughness V= 2.0 C = 140 D VF 6 0.839 12 0.847 24 0.854 30 0.856 48 0.861 54 0.862 Higher in big pipes (flatter profile) D= 12 C = 140 V VF 0.5 0.833 2% 1 0.840 2 0.847 0% 5 0.855 10 0.862-2% Increases with velocity but within 2% of 2 fps
Why Calibrate Large Flowmeters? In-place conditions different than laboratory Distorted profiles and cavitation create errors Mechanical meters wear and calibration changes DP meters inaccurate if leaking or at low flows Mag & ultrasonic meters need periodic calibration Pump Station Meter
Need Meter Tests For Water Audits
Testing Meters at Booster Stations and PRVs Allows Audits within Each Pressure Zone Corrected meter registration gives total flow into each zone Billing and GIS provide water sold in each zone Calculate non-revenue water by pressure zone Track flow into each zone at night to detect leakage
Meter Testing not the Same as Calibration Calibration usually checks electronic or secondary meter components Apply reference signal Check indicated flow Does not check primary element signal producer Testing checks both primary and secondary meter elements
How to Test Master Meters Clearwell drawdown test Second meter in series Clamp-on meters Portable probes Mag meter probes Pitot tubes
Pitot Tube for Testing Meters Invented in 1730 No moving parts Directly measures velocity head as differential pressure between upstream & downstream tips Accuracy 1-2% for stable profiles at velocities > 0.5 fps Reversible for foolproof check
Pitot Tubes Require Taps for Insertion 1-inch minimum tap size 2-inch maximum tap size Air valve tap works if not offset 10-20 diameters straight pipe upstream and 5 downstream Vertical clearance depends on pipe size Pipe at pitot tap must carry same flow as meter
Pitot Tube Measures Point Velocity v = c 2gd / 12 c is coefficient (0.9) v is velocity in fps g is 32.17 feet/sec^2 d is differential in inches Differential recorded as a 4-20 ma signal 12 ma is zero dp Zero dp Check
Pitot Tube Positioned at Centers of 10 Rings of Equal Area to Determine Average Velocity
Flow Calculated from Continuity Equation Q = V * A V is average velocity determined from profile A is area of pipe calculated from inside diameter Inside diameter measured with pipe caliper
Meter Test Compares Pitot Tube Measurement With Registration on Meter Totalizer 30 Best Fit Average Maximum Minimum 25 Pipe Location - inches 20 15 10 5 0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Velocity - fps Q = V * A Q = V / T
Procedure for Testing Master Meters 1. Measure inside diameter 2. Plot velocity profile 3. Read meter to start test 4. Record velocity 5. Read meter at end of test Pipe Location - inches Best Fit Average Maximum Minimum 16 14 12 10 8 6 4 2 0 0.0 1.0 2.0 3.0 4.0 5.0 Velocity - fps City Greensboro, NC Type of Test Meter Tap Location Stoney Creek PS Tap Number 1 0.959 Velocity F Nominal Size 16 inches 0.8903 Table E Co Caliper Size 16.625 inches 1.080 Area Corre PCR Range -4 @4mA 4 @20mA Zero Head 12:02 12.10 ma Forward: 10:20 8.88 ma -1.56 inches Reverse: 10:35 15.24 ma 1.62 inches Test Head 11:57 16.16 ma DP Calibration 1.75 inches vs 1.88 Actual Test Date/Time 5/2/2013 10:15 to 10:45 Rod being reversed from 10:29 to 10:30 TEST RESULTS 1.579 inches 2.554 fps Time ma inches fps Remarks ---------- --------- --------- --------- --------- 1 10:15:00 8.91-1.547 2.528 OK 2 10:16:30 8.93-1.537 2.520 OK 3 10:18:00 8.90-1.550 2.531 OK 4 10:19:30 8.87-1.563 2.541 OK 5 10:21:00 8.87-1.563 2.541 OK 6 10:22:30 8.87-1.567 2.544 OK 7 10:24:00 8.89-1.553 2.533 OK 8 10:25:30 8.83-1.587 2.560 OK 9 10:27:00 8.80-1.600 2.570 OK 10 10:28:30 8.90-1.550 2.531 OK 11 10:29:00 RevRod 12 10:30:00 15.22 1.610 2.578 OK 13 10:31:30 15.23 1.617 2.584 OK 14 10:33:00 15.28 1.640 2.602 OK 15 10:34:30 15.29 1.643 2.605 OK 16 10:36:00 15.06 1.530 2.514 OK 17 10:37:30 15.21 1.607 2.576 OK 18 10:39:00 15.15 1.573 2.549 OK 19 10:40:30 15.19 1.593 2.565 OK 20 10:42:00 15.15 1.573 2.549 OK 21 10:43:30 15.17 1.583 2.557 OK 22 10:45:00 23 24 25 26 27 28 29 30 ---------- --------- --------- --------- Average 1.579 2.554 OK
Example Showing Typical Test Results Velocity Factor = 0.887 30 Best Fit Average Maximum Minimum 25 Pipe Location - inches 20 15 10 5 0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 Velocity - fps Test Date - 2013 20-May 20-May Totalizer Registration - kgal 61.0 72.0 Elapsed Time - minutes 20.52 20.16 Pitometer Flow - mgd 4.40 5.26 Totalizer Flow - mgd 4.28 5.14 Totalizer Error -2.8% -2.2%
Example Showing Consistent Under-Registration Over Wide Flow Range
Example Showing Close Instantaneous Rate But 28% Over-Registration for Totalizer Test Data Test Date - 2013 29-Apr WTP Totalizer Totalizer Registration - kgal 724 Elapsed Time - minutes 30.35 Pitometer Flow - mgd 26.79 Totalizer Flow - mgd 34.35 Instantaneous Meter Rate - mgd 27.65 Totalizer Error 28.2% Instantaneous Meter Rate Error 3.2%
Example Showing Consistent Meter Registration Regardless of Pump in Operation Velocity Factor = 0.916 Pipe Location - inches Best Fit Average Maximum Minimum 16 14 12 10 8 6 4 2 0 0.0 1.0 2.0 3.0 4.0 5.0 Velocity - fps Test Date - 2013 2-May 2-May 2-May Pump 1 Pump 2 Pump 3 Totalizer Registration - kgal 8.0 4.5 4.5 Elapsed Time - minutes 20.75 29.51 29.39 Pitometer Flow - mgd 5.69 2.25 2.26 Totalizer Flow - mgd 5.55 2.20 2.20 Totalizer Error -2.4% -2.3% -2.5%
Example Showing Venturi Meters Accurate Except at Low Flow Rates
Questions