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

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:

$Q = K\sqrt{\frac{\Delta {P}}{ρ}}$

k = a constant
ΔP = differential pressure across device
ρ = density of the fluid.

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 ρ 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 ρ 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.

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
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

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.

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 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 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. 

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