Flow Proportional Sampling Equipment Considerations Presented by: Glenn Hummel, PE
Objective / Agenda Objective: Introduce background and technical information with respect to flow metering and sampling to enable you to begin to navigate this issue and decide how to prepare your response • Sampling • Composite Samples: Time Paced vs. Flow Paced • Flow Measurement • Flow Classifications • Types of Flow Metering Methodologies • Area Velocity • Primary Devices • Flow Metering Technologies • Level Measurement Technology • Interfacing Flow Meters with Sampling Equipment • “The Plan”
Flow Proportional Sampling • The Objective of Flow Proportional Sampling is to collect a Representative Sample relative to a Period of Interest
Composite Sample • A composite sample is a single sample volume constructed of multiple individual samples (aliquots) taken over a specific period of timeor volume of flow (period of interest).
Time Paced Composite • “Time Paced” Composite samples are taken at uniform intervals of time over the period of interest. • Typical: 100 mL every 15 minutes, over a period of 24 hours. This yields 96 samples deposited to a composite bottle = 9,600 mL = 2.5 gallons
Time Paced Composite Challenges: • Building a Representative Sample • Intermittent Flow: could lead to sampling stagnant water or sewage • Overweighting periods of low to no flow • Underweighting (missing) periods of high flow
Flow Paced Composite • “Flow Paced Composite Samples” are taken at fixed intervals of volume metered past a flow measurement point • Challenges: • Variability in discharge volume over period of interest leads to varying sample volume collected.
Flow Pacing a Sampler • Samplers count “Pulses” received as an input signal from a flow meter • “Pulse” is equal to a fixed volume of water passing the meter. • Therefore sampling uniform aliquots into a bottle every X pulses produces a “flow proportional” composite sample.
Flow Pacing a Sampler Expected Volume of Discharge during Period of Interest • Example: 100,000 gallon discharge • Flow Meter Pulse = 100 gallons • Sample every 20 pulses = 2,000 gallons • Yields 50 samples * 100 mL/sample = 5,000 mL = 1.3 gallon composite sample
Flow Pacing a Sampler Minimum Sample Volume Required for Analysis EXAMPLE: Min. Sample Vol. = 3 L (3,000 mL) If Discharge Vol. = 50,000 gal. (50,000 gal.) / (100 gal./pulse) * (100 mL / 20 pulses) Composite Sample Vol. = 2.5 L (2,500 mL)
Flow Pacing a Sampler Maximum Sample Volume (Bottle Size) • Example: 200,000 gallon discharge • Flow Meter Pulse = 100 gallons • Sample every 20 pulses = 2,000 gallons • Yields 100 samples * 100 mL/sample = 10L • Bottle Overfilled (9,400 mL capacity) • Corrective Action: Smaller Aliquots, Larger Sample Interval, Bigger Bottle (2 Samplers)
Flow Pacing a Sampler Minimum Number of Aliquots to Build a Composite • Example: 100,000 gallon discharge • Flow Meter Pulse = 20,000 gallons • Sample every 1 pulses = 20,000 gallons • Yields 5 samples * 1,000 mL/sample = 5,000 mL = 1.3 gallon composite sample • Corrective Action: Finer Flow Meter Output
Flow Pacing a Sampler • Average Discharge Volume over Period of Interest • Max & Min. Discharge Volume (Range) • Minimum Sample Volume Required for Analysis • Minimum Number of Aliquots to Build a Composite • Minimum Volume per Aliquot • Flow Meter Output: Volume Per Pulse
Flow Classifications • Open Channel • Any open channel where liquid flows with a free surface (Non-full pipe. Example: sewer) • Closed Channel • Completely Filled Pressure Conduits (Full Pipe)
Types of Flow Measurement Methods • Timed Gravimetric • Dilution • Slope – Hydraulic Radius (Manning Eq.) • Area*Velocity • Hydraulic Structure (Primary Device)
Types of Flow Measurement Methods • Area*Velocity Q (cfs) = v (ft/s) * A (ft^2) • Primary Measuring Device • This method uses a hydraulic structure that creates a known relationship between flow rate, and a secondary measurement
Magnetic Flow Meter • Area is Fixed (Full Pipe) • Velocity is measured using Faraday’s Law
Magnetic Flow Meter • 1 to 30 ft/s • +/- 0.5% Accuracy • Full Flow Area • Chemical Compatibility • Straight Pipe • No Elbows, Valves, Pumps • Pipe must be full
Venturi Flow Meter • Primary Device (Full Pipe) • creates d/p in relationship to flow rate • Secondary Measurement is Differential Pressure
Flow MeasurementQ = V x A To measure flow rate (Q), we need... 1. The pipe or channel geometry. 3. The average velocity of the flow. 2. The depth of the flow. This is the hard part
Level Measurement (Secondary Devices for Flumes & Weirs)
Level Measurement Technologies • Bubbler • Strength: Good Resolution / Accuracy • Weakness: Contacts flow, compressor hardware
Level Measurement Technologies • Ultrasonic • Strength: Non-contacting (low maintenance) • Weakness: Foam, gas (speed of sound)
Level Measurement Technologies • Submerged Probe / Pressure Transducer • Strength: Good Resolution / Accuracy • Weakness: Contacts flow • NOTE: Doppler Area Velocity Probe uses this technology
Flow Metering Technology Selection Considerations • Hydraulics: • Full (Closed Pipe) • Non-Full (Open Channel) • Pipe Diameter • Pipe Slope • Flow Metering Objective: • Flow Pace Sampling • Billing • Process Control • Maintenance • Installation / Facility Configuration / Constraints
Flow Meter to Sampler Interface • Automatic Samplers utilize “pulses” for Flow Paced Sampling Programs • Pulse is output from Flow Meter every “unit volume” • a.k.a. Pulse Frequency Output • Sampler counts pulses and collects a fixed volume aliquot every X pulses.
Non-Isco Flow Meter to Sampler Interface • A “pulse” must be: Isolated contact closure 5-15 VDC >25 millisecond duration • Not all pulse outputs meet this spec. • Simple 2 wire cable with Military Connector
Analog Flow Signal • Analog Output from Flow Meter can be used to pace sampler • Requires a 4-20mA Sampler Interface Module • Module converts analog signal to pulses. • Generates pulses at a frequency proportional to the analog signal • Full Scale: 20mA = 5 pulses / minute Example: 20mA = 1,000 gpm => 200 gallons / pulse