Basics of Flow Measurement With the Orifice Flow Meter
Flow measurement with Orifice meter
Basics of Flow Measurement with the Orifice Flow Meter
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=KΔPρ−−−√ Q=K X Squareroot of change in P/density
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:
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 (Impulselines)
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 Equalizing 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.
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 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.
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.
ACCURACY AND RANGEABILITY OF ORIFICE METERING SYSTEMS
The performance of the orifice meter system just like other differential pressure flow meters depend on the precision of the orifice plate and the accuracy of the differential pressure sensor. Orifice plate accuracy is rated in percentage of actual flow rates whereas the differential pressure transmitters have their accuracy rated in percentage of calibrated span. Due to the fact that flow rate is proportional to
the square root of the differential pressure for an orifice plate, at low flow rates, the errors in differential pressure measurement leads to large errors in the actual flow rates. As a result, orifice metering systems are limited to a turn down ratio or rangeability of 3:1 or 4:1
As shown above, Orifice plate flow meter range-ability can be increased in two ways:
(a) Operating two or more meter runs in parallel. When the total flow rate is great, all meter runs are placed into service and their respective flow rates summed to yield a total flow measurement. When the total flow rate decreases, individual meter runs are shut off, resulting in increased flow rates through the remaining meter runs.
(b) Another option is to stack two or more differential pressure transmitters in parallel onto the same orifice plate element, one for low flow(say 1-30%), the other for high flow (say 30-100% )of full scale differential pressure produced.
A combination of the two approaches above is the common practice today used to improve the rangeability of the orifice plate meter in order to increase its accuracy. However one drawback of these techniques is that they are complex and expensive to implement. To reduce the complexity of these systems and to further improve accuracy, the use of intelligent or smart differential pressure transmitters is becoming the norm. A smart transmitter with the ability to switch between two calibrated spans eliminates the need for two differential pressure transmitters across the orifice plate flow element.
Straight Run Pipe Requirement
Measurement errors can be introduced into an Orifice metering system when there is insufficient straight run pipes upstream and downstream the orifice plate. The pipe runs are required for laminar flow to be established before the flow element is encountered in the flow stream. The amount of straight run pipes required depends on both the beta ratio of the orifice plate installation and on the nature of the upstream components( like elbows, tees, valves etc) in the metering setup.
Straight run requirement can be eliminated in the metering setup by using flow conditioners or straighteners. Flow conditioners adjust the velocity profile, ideally eliminating or greatly reducing the magnitude of the flow distortion caused by the upstream piping configuration.
Basics of The Orifice Plate Flow Meter
As discussed in flow instrumentation: principles and formulas, when there is a flow restriction in a pipe, a differential pressure results which can then be related to the volumetric flow rate through the restriction.
The principle of a flow restriction creating a differential pressure is what is used in the orifice meter to measure the flow rate of liquids, steam and gases.
The orifice flow meter consists basically of:
(a) A primary device, the orifice plate that creates the flow restriction
(b) A secondary device that measures the differential pressure created by the orifice plate.
The Primary Device: Orifice Plate:
The orifice plate is basically a thin metal plate (1.5 to 6 mm in thickness) with a hole bored in the centre. The orifice plate has a tab or printed label on one side where the specifications for the plate are stamped. The upstream side of the orifice plate usually has a sharp, square edge facing into the flow stream. Consider a typical orifice plate shown below:
The ratio of orifice bore diameter (d) to the pipe inside diameter (D) is called
the Beta Ratio (β ).
β = d/D
The orifice plate has a typical bore diameter that ranges from 30% to 75% of the inside diameter of the pipe work in which it is installed. A beta ratio of 0.4 signifies that the orifice bore diameter is 40% of the pipe inside diameter.
How an Orifice Plate Flow Meter Measures Flow
With an orifice plate installed in a flow stream, increase in fluid flow velocity through the reduced area of the orifice develops a differential pressure across the orifice. The differential pressure generated is related to the beta ratio of the orifice plate.The smaller the beta ratio, the higher the differential pressure generated. In practice, the choice of beta ratio is a compromise between the differential pressure desired and the flow rate required.
Shown below is a typical pressure profile of the Orifice plate.
As shown in the pressure profile above, with the orifice plate in the pipe work (in between flanges or an orifice fitting), the static pressure upstream the plate increases slightly due to back pressure effect and then decreases sharply as the flow passes the orifice. Flow downstream the orifice reaches a minimum at a point called the vena contracta where the velocity of the flow is at a maximum. Beyond the vena contracta, static pressure starts to recover but it never gets to the upstream value. In other words, with an orifice meter installation, there is always a permanent pressure loss. In addition to pressure loss, some of the pressure energy is converted into sound and heat at the orifice plate. It can be seen from the pressure profile diagram that the measured differential pressure developed by flow through the orifice plate also depends on the location of the pressure sensing points or pressure taps
Pressure Tap Locations:
For orifice flow meter installations, there are five common locations for differential pressure taps:
· Flange (greater than 2 inches)
· Vena contracta (greater than 6 inches)
· Pipe (full flow)
· Corner taps ( less than 2 inches)
Flange taps are the most common and are generally used for pipe sizes of 2 inches and greater. Flange taps are a convenient alternative to drilling and tapping the main pipe for pressure connections.
Vena Contracta Taps:
Vena contracta taps are limited to pipe sizes greater than 6inches. This limitation is mainly imposed to provide adequate clearance of the tap from the flange. The vena contracta is where the fluid flow has the smallest cross-sectional area, and also has the lowest pressure. Vena contracta taps are designed around achieving the maximum differential pressure.
The vena contracta is dependent on the flow rate and pipe size, and can vary should either of these parameters change. The vena contracta taps will therefore be affected should the flow rate or pipe size change.
Pipe taps are located 2.5 pipe diameters upstream and 8 pipe diameters downstream of the orifice plate. Pipe taps are used typically in existing installations, where radius and vena contracta taps cannot be used. They are also used in applications of greatly varying flow, as the measurement is not affected by flow rate or orifice size. Accuracy is reduced, as they do not measure the maximum available pressure.
Corner taps measure the pressure in the corner between the orifice plate and the pipe wall. Uses for corner taps are found in installations with pipe diameters less than 2 inches.
Radius taps are a modification on the vena contracta taps, where the downstream tap is located one-half pipe diameter from the orifice plate. This is to avoid the unstable region that occurs immediately after the orifice plate. Radius taps are generally preferred to vena contracta as the pressure tap location is simplified.
The Secondary Device:
As noted earlier, the other major component of the orifice meter is the secondary device. The secondary device is often a differential pressure transmitter called a flow transmitter when installed to measure flow. The flow transmitter measures the differential pressure created by the orifice plate in the flow stream. This differential pressure is measured via impulse lines located upstream and downstream the orifice plate. The impulse lines are connected to the high and low pressure ports of the transmitter which then converts the differential pressure measurement into an analogue (4 -20 ma) or digital signal which can be processed to provide a display of the instantaneous rate of flow.
Advantages of the Orifice Plate Flow meter
· They have a simple construction.
· They are inexpensive.
· They are easily fitted between flanges.
· They have no moving parts.
· They have a large range of sizes
· They are suitable for most gases and liquids.
· They are well understood and proven.
· Price does not increase dramatically with size.
Disadvantages of the Orifice Plate Flow meter
· Inaccuracy is typically in the range of 1%.
· They have Low range-ability, typically 4:1.
· Their accuracy is affected by density, pressure and viscosity fluctuations.
· Erosion and physical damage to the restriction affects measurement accuracy.
· Their installation causes some unrecoverable pressure loss.
· Fluid viscosity limits measuring range.
· Require straight pipe runs to ensure accuracy is maintained
· Pipeline must be full especially for liquid flow measurement
Types of Orifice Plates Used in Flow Measurement
Orifice plates are the most widely used type of flow meters in the world today. They offer significant cost benefits over other types of flow meters, especially in larger line sizes, and have proved to be rugged, effective and reliable over many years. Where a need exists for a rugged, cost effective flow meter which has a low
installation cost and a turn-down of not more than 4:1, the orifice plate continues to offer a very competitive solution for flow rate measurement.
Here we introduce the most common types of orifice plates used in industrial process plants for flow measurement applications.
Types of Orifice Plate Designs
The orifice plate is a relatively inexpensive component of the orifice flow meter. Orifice plates are manufactured to stringent guidelines and tolerances for flatness bore diameter, surface finish, and imperfections in machining such as nicks and wire edges on the bore. Specific tolerances applicable to orifice plates in use in industrial applications are detailed in the American Gas Association (AGA) Reports especially report 3. The common designs of orifice plates available are:
Square-Edge, Concentric Orifice Plates
The most common orifice plate is the square-edged, concentric bored design shown below. This type of orifice plate is manufactured by machining a precise, straight hole in the middle of a thin metal plate. The concentric bored orifice plate is the most used design because of its proven reliability in a variety of industrial applications and the extensive amount of research conducted on this design. The concentric orifice plate is also easily reproduced at a relatively low cost. The concentric orifice is used to measure a wide variety of single phase, liquid and gas products, typically in conjunction with flange taps.
As shown in the diagram above, a side view of a square-edged concentric orifice plate reveals sharp edges (90 degree corners) at the hole. The square edge on the hole in an orifice plate helps to minimize contact with the fast-moving fluid stream going through the hole. Square-edged orifice plates may be installed in either direction, since the orifice plate looks exactly the same from either direction of fluid approach. In fact, this feature allows the square-edged orifice
plate to be used for measuring bidirectional flow rates.
A text label printed on the paddle or handle of any orifice plate customarily identifies the upstream side of that plate. In the case of the square-edged orifice plate it does not matter which side of the plate is upstream. It is vitally important to pay attention to the paddle’s text label on the orifice plate as this is the only sure indication of which direction an orifice plate needs to be installed.
Also shown in the above diagram is the side view for a bevelled square edge concentric orifice plate. The downstream side of the orifice plate is bevelled if the plate is relatively thick (1/8” or 1” or more). This bevelling helps to minimize contact with the fluid stream. Bevelled orifice plates are obviously uni-directional, and must be installed with the paddle text facing upstream while the bevelled end faces downstream.
Square-Edge, Eccentric Orifice Plates
The eccentric square-edged orifice plate has a hole that is located off-center to allow the undesired portions of the fluid to pass through the orifice rather than build up on the upstream face as shown below:
Square edge, eccentric orifice plate
These are generally used when the process material contains foreign matter that may block the orifice in the case of a concentric configuration. Eccentric orifice plates are used to measure the flow of fluids that carry solids and are also used to measure gases which carry liquids. With the eccentric orifice at the top of the plate, it can measure liquids that carry gas. It should be noted that the eccentric orifice has a higher degree of uncertainty as compared to the concentric orifice.
Square-Edge, Segmental Orifice Plates
The segmental Orifice plate has a hole that is not circular but rather a segment of a concentric circle as shown below:
Square edge, segmental orifice plate
Segmental orifice plates are used to measure the flow of light slurries and fluids with high concentration of solids. The design of segmental orifice eliminates the damming of foreign matter and provides more complete drainage than the eccentric orifice plate. The segmental orifice is considerably more expensive than the eccentric orifice and has slightly greater uncertainty.
Quadrant Radius Orifice Plates
Quadrant radius orifice plates have a special bore design as shown below:
Quadrant radius orifice plate
The upstream side of the bore is shaped like a flow nozzle while the downstream side acts as a sharp edge orifice plate. They are recommended for measurement of viscous fluids which have pipe Reynolds numbers below 10,000. An increase in the viscosity of a fluid flowing through a sharp edge orifice will increase the diameter of the vena contracta (point of lowest static pressure downstream the orifice bore), which results in a decrease in differential pressure. However, an increase in the viscosity of a fluid flowing through a flow nozzle increases the friction drop in the flow through the nozzle, which results in an increase in the differential pressure. The quadrant radius orifice plate combines these two effects to produce a constant coefficient.
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