CHAPTER 22-23

Practical Flow Measurement

  • Presented by: Remuel P. Lagrosa

Differential Instrument Connections

  • Differential flowmeters utilize two connections:

    • High-pressure (upstream)

    • Low-pressure (downstream)

  • Purpose: Measure the pressure difference caused by fluid flow.

  • Example: An averaging Pitot tube combines these connections into one tube.

Taps

  • Pressure taps connect at points on flow elements such as:

    • Orifice plates

    • Venturi tubes

  • Common types of taps include:

    • Flange taps

    • Vena contracta taps

    • Pipe taps

  • Positioning is crucial to capture differential pressure accurately.

Impulse Line

  • Connects flow taps to the differential pressure sensor (d/p sensor).

  • For liquids: Install the instrument below to avoid air bubbles.

  • For gases: Position above to allow drainage of condensed liquids.

  • For steam: Also installed below for cooling buffer.

Differential Instrument Location

  • Liquid Flow:

    • Install the instrument below the flow element.

    • Ensure impulse lines are filled with liquid to prevent air bubbles.

    • Length does not affect accuracy if lines start and end at the same level.

  • Gas Flow:

    • Position instrument above the flow element to allow drainage.

  • Steam Flow:

    • Mount instrument below flow element, ensuring condensate fills lines to protect from heat.

    • Manage air removal and condensation by blowing steam or pre-cooling with water.

Flow Transmitter and Orifice Metering

  • d/p cell placement affects flow measurement:

    • Above orifice

    • Below orifice

    • Installation details vary for liquids and gases.

Compact Orifice Flowmeter

  • Simplifies installation with direct connection to d/p cell.

  • Standard form: Requires straight pipe lengths.

  • Conditioning form: Allows shorter installations while maintaining accuracy.

Boiler Airflow Measurement

  • High turndown ratios are essential for accurate combustion airflow measurement.

  • May require custom elements and d/p transmitters for linear output.

Blocking Valves and Manifolds

  • Blocking valves: Isolate instruments for calibration.

  • Manifolds: Attached to d/p cells for streamlined calibration processes.

Accessory Flow Devices

  • Devices such as integrators and flow switches assist in monitoring flow:

    • Integrators: Calculate total flow over time, handling both pulse and analog outputs.

    • Flow switches: Trigger alarms at specific flow rates.

    • Types include differential pressure, blade, thermal, and rotameter switches.

Integrators

  • Function: Calculate and total flow over specified periods.

  • Types: Mechanical, pneumatic, electronic handling differential pressure.

  • Pneumatic versions: Useful in hazardous areas.

Flow Switch Functions

  • Monitor fluid flow and trigger outputs at set flow rates for alarms/shutdowns.

Flow Switch Types

  • Differential Pressure Switches: Two sensors with an adjustable spring detect pressure changes.

  • Blade Switches: Activate when fluid force pushes a flexible blade.

  • Thermal Switches: Detect flow through heated sensors.

  • Rotameter Switches: Use a float with a magnet to trigger external magnetic switches based on flow rate.

Gas Analyzers in Industrial Process Control

  • Importance: Essential for quality, safety, and efficiency of industrial processes.

  • Functions: Measure gas compositions to optimize production and reduce emissions.

Definition and Importance of Analyzers

  • Analyzers measure physical, chemical, or electrical properties to determine composition.

  • Main types: Laboratory analyzers and on-line analyzers.

  • Laboratory analyzers: Highly accurate but introduce delays due to manual sampling.

  • On-line analyzers: Offer continuous or frequent sampling for real-time adjustments.

Types of Analyzers

  1. Thermal Conductivity Analyzers: Measure thermal conductivity for specific gases.

  2. Radiant-Energy Absorption Analyzers: Measure how much radiant energy a gas absorbs.

  3. Oxygen Analyzers: Measure oxygen concentration for efficient combustion.

  4. Opacity Analyzers: Detect particulate matter via opacity measurement.

Analyzer Sampling Systems

  • Composed of pipes, valves, and equipment to transport gas samples to analyzers.

  • May include conditioning elements (filters/dryers) to maintain sample integrity.

  • Critical for accuracy in presence of condensable vapors or particulates.

Overview of Analyzer Sampling Systems

  • In-line analyzers: Connected for continuous sampling, while intermittent systems capture samples for slower processes.

  • Represents a sample faithfully to ensure accurate analysis.

Ensuring Representative Samples

  • Critical to maintain chemical and physical properties of samples.

  • Strategic sampling point placement and sample integrity are crucial.

  • Challenges include changes during transport (e.g., gas condensation).

Challenges in Sample Conditioning and Transport

  1. Temperature and Vapor Content: High temps can lead to condensation, affecting analysis.

  2. Filtration Needs: Necessary to prevent particulates from contaminating the analyzer.

  3. Sample Transportation Lag: Delay between sampling and arrival at the analyzer can impact real-time monitoring.

Stack Gas Sampling Systems

  • Essential for monitoring emissions and ensuring compliance.

  • Components include:

    • Stack gas inlet

    • Filters for particulates

    • Coalescing filters for liquid removal.

Components of a Stack Gas Sampling System

  1. Cartridge Filters: Remove particulates to protect the analyzer.

  2. Sampling Tubes: Transport gas samples to analyzers.

  3. Coalescing Filters: Capture liquid droplets from gas samples.

Importance of Clean Sampling

  • Ensures accurate measurements by removing contaminants.

Thermal Conductivity Analyzers

  • Measure specific gas concentrations using thermal conductivity differences.

  • Two-chamber designs compare sample and reference gases.

Limitations of Thermal Conductivity Analyzers

  • Effective only for binary or pseudo-binary mixtures with significant conductivity differences.

Radiant-Energy Absorption Analyzers

  • Measure gas concentration by analyzing absorption of electromagnetic radiation.

  • Applications in detecting gases like CO2 and SO2.

Applications of Radiant-Energy Absorption Analyzers

  • Ideal for gases with defined absorption characteristics used in environmental monitoring.

Advanced Detection Methods

  • Luff-Type Sensors: Measure pressure changes based on IR absorption.

  • Photoacoustic Detectors: Detect pressure waves created by gas expansion.

Nondispersive Infrared (NDIR) and Ultraviolet (UV) Analyzers

  • Vital for monitoring gas compositions in industrial processes.

  • UI analyzers measure UV absorption, crucial for trace gas detection.

Applications and Advantages

  • Both NDIR and UV analyzers provide rapid, accurate measurements necessary for regulatory compliance.