Basics of Flow Measurement with the Orifice Flow Meter I ~ Learning Instrumentation And Control...

5/7/14 Basics of Flow Measurement with the Orifice Flow Meter I ~ Learning Instrumentation And Control Engineering

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Basics of Flow Measurement with the Orifice Flow Meter I

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The differential pressure measurement method is a universally utilized

measuring principle for flow measurement. The orifice flow meter is a

type of differential pressure flow meter that can be used for measuring

gases and liquids.

As shown in Flow Instrumentation: principles and Formulas, we know that

the relationship between flow and differential pressure in a flow

restriction device like the orifice meter is given by:

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Where

k = a constant

P = differential pressure across device

= density of the fluid.

Flow Measurement Engineering Handbook

In the above formula, fluid density is a key factor in flow measurement computation in both liquids and gases. If fluid

density is subject to change over time, we will need some means to continually calculate so that our inferred flow

measurement will remain accurate. Variable fluid density is typically experienced in gas flow measurement, since all gases

are compressible by definition. A simple change in static gas pressure within the pipe is all that is needed to make

change, which in turn affects the relationship between flow rate and differential pressure drop. Therefore in gas flow

measurement, change in fluid density with static pressure is compensated for.

In liquid flow measurement, the scenario is different. Liquids by definition are considered to be incompressible for all

practical purposes since a change in pressure has little or no effect on the density hence they are neglected in flow

measurement applications.

Both liquid and gas density change with temperature as a result, they are compensated for in a differential pressure flow

measurement system like the orifice plate flow meter.

ISO 5167-2:2003, Measurement of fluid flow by means of pressure differential devices:Orifice plates

Flow Measurement Setup

The complete flow measurement installation of an orifice flow meter system consists of the following elements:

1. The Orifice plate (the differential pressure source)

2. Orifice plate fittings(more convenient for large pipe size)

3. Pressure piping (impulse lines)

4. Isolation and Equalizing Valves Manifold for Impulse lines and Transmitter

5. Differential pressure transmitter

6. Pressure transmitter

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7. Temperature sensor/Transmitter

8. A flow computer

9. A DCS, PLC/HMI System etc.

The Orifice plate

The orifice plate is the primary element in the measurement system and it is the source of the differential pressure being

used to infer flow measurement. You can learn more about orifice plates from:

Introduction to orifice plates

Basics of the orifice flow meter

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Orifice Plate Fittings

Orifice plates for flow measurement could be installed between flanges, typically in an orifice flange union arrangement

for small pipe sizes used for low flow rates.

For relatively low flow rates, an alternative arrangement is the integral orifice plate. This is where a small orifice plate

directly attaches to the differential pressure-sensing element, eliminating the need for impulse lines.

Conditioning orifice plate for wet gas flow measurement: An article from: Pipeline & Gas Journal

For large pipe sizes and higher flow rates, it is more convenient to use an orifice plate within an orifice fitting. The

fitting makes easy removal, inspection and replacement of the orifice plate possible during inspection and maintenance

Orifice Plate Flow Meter Gas Flow Measurement Orifice Fittings

( 17 ) Control Valves ( 16 ) InstrumentationDiagrams ( 14 ) Calibration ( 12 ) Flow meters ( 10 ) DPTransmitters ( 8 ) LevelMeasurement ( 8 ) PressureSwitch ( 8 ) Orifice Plates ( 7 ) pressure sensors ( 7 ) Process Control ( 6 ) RTD ( 5 ) Thermocouples ( 5 ) Basics of

Instrumentation ( 4 ) Electrical Noise

( 4 ) Pressure Gauges ( 4 ) Transmitters

( 3 ) 4 - 20mA Signals ( 2 ) Digital or

Analog Multimeters ( 2 ) Fluke pressure

calibrator ( 2 ) Measurement terminology

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operation.

Pressure Piping (Impulse lines)

Two impulse lines upstream and downstream the orifice plates (installed either between flanges or in an orifice fitting)

are connected to the high and low ports of the differential pressure transmitter to measure the differential pressure

generated by the orifice plate.

-->

Isolation and Equalising Valves Manifold for Impulse lines and Transmitter.

The two impulse lines from the orifice fitting/orifice plate are isolated from the differential pressure transmitter by shut-

off valves. The differential pressure transmitter, protected by a valve combination consisting of three to five valves

(often in a single assembly referred to as a 3-valve or 5-valve manifold) is installed before the transmitter. The valves

shut off the transmitter and allow the pressures in each line to be equalized, enabling the transmitter to be zeroed.

Differential Pressure Transmitter

The differential pressure transmitter measures the differential pressure drop created by the orifice plate. The transmitter

must be capable of withstanding the high static pressure in the installation piping. It must also be very sensitive so that it

can measure low differential pressures at low flow rates as high differential pressures are not desirable because it results

in a high pressure loss.

Additional features that differential pressure transmitters for flow measurement should possess include:

(a) Its material make up should be chemically resistant to corrosive media

(b) It should be able to convert the differential pressure into an analog(4 20mA) or digital output signal.

(c) It should be able to extract the square root in order to achieve a direct linear output proportional to the flow rate.

This is necessary because flow rate is proportional to the square root of differential pressure for an orifice plate meter

and other differential pressure flow meters.

(d) It should be resistant to interference, explosion proof and intrinsically safe.

(e) It should include some self diagnostics features for maintenance purpose and be easy to operate.

(f) It should be capable of modern communication technologies e.g Foundation Fieldbus , Profibus PA etc.

The location of the transmitter in a differential pressure flow measurement installation should be carefully considered in

order not to introduce measurement errors. As a rule, in gas flow measurement with the orifice flow meter, the

transmitter should be installed above the pipe in order to prevent any condensate from entering the pressure lines. For

liquid measurement systems, the transmitter is installed below the pipe to prevent gas bubbles from entering the

measuring system.

-->

Pressure Transmitter.

A pressure transmitter is required to continuously measure static pressure in gas flow measurement setup. This is because

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static pressure variations significantly affect the density of gases and needs to be compensated for. Liquid systems do not

have this problem.

Temperature Sensor/Transmitter

Temperature measurement is required in both liquid and gas flow measurement systems due to the fact that both liquid

and gas density vary with temperature. So a continuous measurement of the temperature of the process is required in

order to compensate for this variation. RTD sensors/transmitter are commonly used for the continuous measurement of

the temperature. The sensor and its Thermowell is usually positioned downstream of the orifice plate so that the

turbulence it generates will have negligible impact on the fluid dynamics at the orifice plate. The American Gas

Association (AGA) allows for upstream placement of the sensor Thermowell, but only if located located 10 diameters

upstream of a flow conditioner.

Flow Computer

The flow computer receives measurement signals from the differential pressure transmitter, the pressure transmitter and

the temperature transmitter. All signals are used to compute the mass and volumetric flow rates according to a set

algorithm programmed into the flow computer. In particular, gas flow measurement applications require an online Gas

Chromatograph and Densitometer that calculates the density of the gas flow stream at flowing conditions in order to

accurately determine the mass and volumetric flow rates.

DCS, PLC/HMI, Controller System

Most times, the signal from the flow computer is required in a central control system located in a centralized control

room. The flow signal could be sent for display in a DCS or PLC/HMI system or a controller for control action.

Go to the concluding part: Basics of Flow Measurement with the Orifice Flow Meter II

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Flow Meter II

Basics of Flow Measurement with the Orifice

Flow Meter I

Basics of The Orifice Plate Flow Meter

Types of Orifice Plates Used in Flow

Measurement

How to Size an Orifice Plate Flow Meter with

Software

A Guide to Sizing Orifice Plate Flow Meters

The Orifice Flow Meter Equation

Learning Instrumentation And Control Engineering

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