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  • ߣMüt ԴWַ: lڣ2012-4-25 11:44:57
  • Flow Measurement

    Instrument Training Material
    Aim
    Understand the basic principle of flow measurement.
    Knowing different type of flow measuring device available in YYCG.
    Flow!
    What?
    Movement of a liquid or a gas from one point to another.
    Requirement for flow to occur?
    Upstream pressure MUST be higher then downstream.
    Flow Profile Types
    Flow Measurement
    Through a closed piping is one of the most important aspects of process control.
    Flow is generally measured by measuring velocity V through a known area A.
    Flow Measurement
    Volumetric flow:
         Q v   =   A V
    Where:
     A = the cross sectional area of the flow carrier (pipe)
     V = the fluids velocity
    Units:
    m3/day,
    m3/hr,
    liters/min,
    gal/min,
    ft3/sec,
    Flow Measurement
    Mass flow,
           Q m =  Q v r
    Where:
    Qv is the Volumetric Flow Rate.
    r is the density of the liquid.
    Units:
    kg/hr,
    ton/day,
    lb/hr,
    lb/sec.
    Flow Measurement
    Flow rate.
    Measure of flow at specific point of measurement.
    Reynolds Number
    Basic equations of flow assume the velocity of flow is uniform across a given cross section.
    Re = vDr/m
    Where n = the flow velocity
    D the inside diameter of the pipe
    r is the fluid density
    m is the fluid velocity
    Differential Pressure Flow meter
    Contains two elements
    Primary element
    Some type of restriction that is place inside a pipe to cause a differential pressure
    Secondary element
    Normally a differential pressure transmitter
    DP Flow Meters
    When liquid flow through the restriction
    Velocity increases
    Pressure decreases
    Pressure upstream is higher than the pressure downstream
    DP Flow Meters
    Flow rate through the restriction is proportional to the square root of the DP across the restriction
    V = k Sq (DP)
    The DP TX measures the different in pressure or delta P and converts that value to an output that can be used by signal conditioning equipment to indicate flow rate

    Head Meters or DP Flow Meters
    Generally cheaper, simple, reliable and offer more flexibility than other flow measurement methods.
    Consists of two components: Primary device - placed in the pipe to restrict the flow and develop a differential pressure.
    Secondary device - measures the differential pressure and provides a readout or signal for transmission to a control system.
    Types of Primary Elements
    Orifice plate
    Widely used in industrial applications
    It is simple and least expensive
    Produces a greater overall pressure loss
    Effectively utilized for clean fluid where line pressure losses or pumping costs are not critical
    Orifice plate
    Operating principle:
    Plate inserted in process line.
    Differential pressure developed across the orifice plate is measured to determine the flow rate.
    Maintain steady flow through the orifice plate, the velocity must increase as it passes through the orifice.
    Lowest pressure occurs where the velocity  is the highest.
    Greater the flow larger the dp across the orifice plate.
    Types of Primary Elements
    Type of orifice Plate:

    Types of Primary Elements
    Venturi Tubes.
    exhibit a very low pressure loss.
    the largest and most costly.
    operate by gradually narrowing the diameter of the pipe.
    Their applications are generally restricted to those requiring a low pressure drop and high accuracy reading.
    Types of Primary Elements
    Flow nozzles may be thought of as a variation on the Venturi tube. The nozzle opening is an elliptical restriction in the flow but with no outlet area for pressure recovery. The flow nozzle is a high velocity Flowmeter used where turbulence is high such as in steam flow at high temperatures. The pressure drop of a flow nozzle falls between that of the Venturi tube and the orifice plate.
    Types of Primary Elements
    Pitot Tubes consists of two hollow tubes that sense the pressure at different places within the pipe. These tubes can be mounted separately in the pipe or installed together in one casing as a single device. One tube measures the stagnation or impact pressure (velocity head plus potential head) at a point in the flow. The other tube measures only the static pressure (potential head), usually at the wall of the pipe.

    Advantages of DP flowmeters
    Universally suitable for applications involving liquids, gas and steam.
    Excellent method, even under extreme process conditions, high temp and high pressure.
    Dp transmitters can be replaced during operation with process shutdown.
    Robust primary elements, no entirely mechanical and no moving parts.
    Disadvantages of Dp flowmeters
    Suitable for low-viscosity fluids
    Metering gas flow require steady process conditions, pressure, temperature must be constant
    Measuring system required additional valves and impulse lines
    Variable area flowmeters
    Simple and Cost Effective for measuring Liquid or Gases.
    Consist of upright, tapered tube in which a float made of glass or metal hovers on the fluid flow entering the bottom of the tube.
    The force exerted on the float by the fluid depends on its density, viscosity and rate of flow.
    Variable area flowmeters
    Higher the flow rate and, consequently, the force of the flow, the higher the float will rise in the tapered, graduated tube.
    The annular gap between the float and the wall of the tube widens as the float moves up, until the force acting on the float are balanced and the float hovers at a steady height.
    Variable area flowmeters
    Advantages:
    Usable for liquid, gas or steam application
    Low cost method of flow metering due to the simple design
    No power supply needed
    Devices with sight glasses allow for an easy and dependable on-site process and flow monitoring
    Low pressure losses

    Variable area flowmeters
    Disadvantages.
    Measuring accuracy depends on the process conditions and the fluid properties.
    Fluid specific calibration necessary.
    Limited turndown (Max. 10:1).
    Affected be entrained matter.
    Can be used only for low-viscosity fluids.
    No totalizing function.
    Meter has to be installed in a vertical pipe.
    Vortex Meters
    The operating principle.
    based on the phenomenon of vortex shedding known as the von Karman effect.
    As fluid passes a bluff body, it separates and generates small eddies or vortices that are shed alternately along and behind each side of the bluff body.
    These vortices cause areas of fluctuating pressure that are detected by a sensor.
     The output depends on the calibration constant, or K-factor.

    Vortex Equation
    Vortex Meter
    In this example, the passage of a vortex causes a slight bow of a wing placed downstream of the bluff body.
    The bend is measured by a piezoelectric crystal sensor in contact with the top of the wing.
    Magnetic Flow meter
    Based on Faradays Law of Magnetic Induction, which states that a voltage is induced into a conductor moving through a magnetic field.
    With electromagnetic measuring principle, the flowing fluid is the moving conductor.
    Magnetic Flow meter
    The induced voltage is proportionally related to the flow velocity.
    Using the pipe cross-sectional area, the flow volume is calculated.
    The DC magnetic field is generated by a switched direct current of alternating polarity.
    Turbine Flow Meter
    Uses a multi-bladed rotor that is supported by bearings within a pipe section perpendicular to the flow.
    Fluid drives the rotor at a velocity that is proportional to the fluid velocity and, consequently, to the overall volume flow rate.
    A magnetic coil outside the meter produces an alternating voltage as each blade cuts the coils magnetic lines of flux.
    Turbine Flow Meter
    Typical schematic of pick-up assembly and generated waveform are shown.
    When the blade at point A passes the cone point B , the magnetic field produced by the permanent magnet is deflected by its presence, generating a voltage in the coil (positive pulse in the waveform).
    Turbine Flow Meter
    A negative pulse is produced when the blade travel from B to C.
    One pulse represents a unit volume, hence, turbine output can be rated in pulses per gallon or any other units desired.
    Thank You
    Next.. Control Valve


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