system wars: episode i the phantom tank levels · system wars: episode i . the phantom tank levels....
TRANSCRIPT
NC AWWA / WEA2018 Spring SymposiumMarch 2018 - Grove Park Inn, Asheville
System Wars: Episode I The Phantom Tank LevelsJohn Collett PE, Hazen and Sawyer
A long time ago, in a water system far, far away…
SYSTEM WARS
Episode I
The Phantom Tank Levels
Overview
• Why is tank water level important to know?• How is the water level measured in tanks?• 3 case studies to demonstrate problems encountered• Tips on how to evaluate the tanks in your water utility
The Purpose of TanksEqualize Daily Demands Buffer Changes in Pressure
Provide Fire Protection Maintain Pressure During Emergency
How Water Level Is Measured In Tanks
Floats provide visual indication of tank level• Not typically integrated with SCADA• Can get stuck or frozen
How Water Level Is Measured In Tanks
Height?
Pressure transducerPressure at base = 52 psi1 psi = 2.31 ftHeight to water = 120.1 feet
Tank Water Level Higher Than Measured
Tank Overflow• Water loss• Erosion
Force Altitude ValveTo Close• Over pressurize
system
Tank Water Level Lower Than Measured
Reduced Customer Pressure• 10 ft difference = 4.3 psi
Fire Protection• Tank only has ½ the
indicated water volume
Could result in tank going empty without warning
30’
SCADA = 20’
ACTUAL = 10’
Case Study 1
Field Test Results Show Discrepancy
• Calibrated a hydraulic model as part of a water system master plan
• Performed hydraulic gradient and fire flow tests throughout system
• Unable to match static pressure measurements with model within a particular pressure zone
Sources of Error Checked
• Tests at two separate locations showed discrepancies with model results
• Checked SCADA historian to confirm tank level at time of tests
• Checked elevation of hydrants using 2nd source of data
• Reviewed hydraulic model for errors (tank geometry, open pipe, etc.)
• Asked Utility to investigate • Model predicted SCADA reading 14.5 ft high
1st Attempt To Calibrate Tank Water Level Reading
• 1.0 MG elevated tank• Constructed in 2009• Utility first attempted to measure
water level by climbing tank and lowering a tape measure
• Difficult to compare reading to reference elevation then compare to SCADA readout
• Results inconclusive
2nd Attempt to Calibrate Tank Water Level Reading
• Recommended overflowing tank then turning off pump
• See if SCADA readout shows “full tank”
• Had to bypass altitude valve controls
Results:• Tank water level was 15 feet
lower than indicated by SCADA
Operation Due to Measurement ErrorUtility reduced high water level due to low demand and water quality concerns
0 ft
20 ft
40 ft
0 ft
20 ft
40 ft
9 ft Operating Range
Planned Operation Actual Operation
9 ft Operating Range
Impact to Available Fire Storage
Nearby a large distribution facility relies on the water utility for fire protection (no on-site tanks or pumps)• Intended emergency storage during operation: 0.53 MG
3,500 gpm for 2.5 hours• Actual emergency storage during operation: 0.11 MG
3,500 gpm for 30 minutes
Case Study 2
Project to Test Level Measurements For Each Tank in System
• Use elevations from drawings where possible
• Digital pressure gauges calibrated to within 1 ft(0.4 psi) using dead weight tester
• Tank level recorded by SCADA for comparison
Example of Test Results
Project Findings
Tank
NominalVolume
(MG)
Overflow Elevation
(ft)
Low Water Line
Elevation(ft)
Full Level
(ft)
Reference
Elevation(ft)
Reference Location
SCADA Level
(ft)
Measured Level
(ft)
Level Diff.+/-(ft)
Tank 1 1.0 1005.0 965.0 40.0 858.5 Finish Grade 16.5 17.0 +0.5
Tank 2 2.0 1005.9 942.4 63.5 943.6Top of altitude valve
25.3 27.7 +2.4
Tank 3 1.5 1005.5 964.2 41.3 856.4Top of altitude valve
25.6 27.7 +2.1
Tank 4 2.0 1005.0 960.0 45.0 873.0 Top of Foundation 21.0 27.5 +6.5
Tank 5 1.5 1005.5 970.5 35.0 889.5 Top of Foundation 24.0 24.7 +0.7
Tank 6 2.0 1010.0 975.0 35.0 880.0 Tank footing 22.2 15.5 -6.7
Tank 7 1.0 920.0 880.0 40.0 710.0 Finish Grade 25.9 25.7 -0.2
Tank 8 0.5 1070.0 1040.0 30.0 951.0 Tank footing 7.2 4.5 -2.7
Tank 9 1.5 1065.0 1025.0 40.0 885.0 Top of Foundation 19.3 22.3 +3.0
Tank 10 1.5 1120.0 1080.0 40.0 950.0 Finish Grade 24.2 23.3 -0.9
+0.5+2.4 +2.1
+6.5
+0.7
-6.7
-0.2
-2.7
+3.0
-0.9
-8
-6
-4
-2
0
2
4
6
8
Leve
l (ft)
Difference: Measured - SCADA
Operational Impact Prior to Calibrating Tank Level Measurements
• Pressure zone has multiple tanks that float and multiple pump stations that supply water
• Inaccurate tank level indication caused operators to believe tanks were imbalanced and would then operate pump stations at higher flow rates to compensate
Case Study 3
Model Calibration For Master Plan Identifies Problem
• Unable to match static pressure measurements with model from fire flow test
• Utility overflowed tank and confirmed that SCADA measurement is accurate
Additional Testing
• Static pressure measured at multiple locations in pressure zone, discrepancy with hydraulic model still present
• Turned attention back to tank, measured HGL of tank using nearby hydrant
Utility Had Been Using Wrong Overflow Elevation For Years
• Made calibrating hydraulic model difficult• Recent design of new pump station used incorrect elevation
• Pump sizing
Tank Grd Elev.(ft)
Tank Ht. (ft)
Overflow (ft)
Tank Capc. (gal)
Tank 1 216 114 330 500,000Tank 2 201 139 340 300,000Tank 3 290 129 419 1,000,000Tank 4 444 146 590 500,000
Master Tank Spreadsheet Paper Drawings
How Do Sensors Get Out of Adjustment?
• Accidental zeroing of pressure transducer during maintenance
• Incorrect SCADA calculation• Aging sensors
Method 1 - Overflow Tank
1. Fill tank until overflow2. Stop filling then immediately
compare to SCADA
+Eliminates measurement & calculation error-Possible erosion-May require defeatingaltitude valve
Method 2 – Calculate From Independent Measurement
1. Accurately measure pressureat known elevation near tank
2. Convert to HGL3. Compare to SCADA
+Does not require unusualsystem operation+Measures HGL-Requires accurate pressure gauge-Opportunity for calculation errors
