Recommended Practice in the Use of Orifice Plates

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Orifice plates are the most common flow device you will find in any process plant. Although not very high in accuracy but they are rugged, have low maintenance cost and have been applied successfully in process instrumentation to measure flow. 

Listed in the table below are recommended practices when it comes to industrial application of Orifice plates:

	Recommended Practice in the Use of Orifice Plates
1	Orifice plates, with flange taps, square edge and concentric orifice are normally used on pipe sizes from 2” and above.
2	Eccentric or Segmental orifice plates are typically specified on fluids with suspended solids, sediments and on biphasic streams.
3	Quarter Circle orifice plates are typically specified for high viscosity fluids with a Reynolds number between 2500 and 10000.
4	The full-scale value for measurements is normally selected equal to the maximum flow rounded off at 1-5 % above or adjusted to 1.2 times the normal operating flow if the maximum flow is unknown.
5	Standard calibrated ranges of differential pressure are typically used for the calculation with a preference for 250 mbar.
6	The recommended values of Beta ratio will be selected within 0.2 - 0.75, preferably between 0.4 – 0.75.
7	Orifice bore will be rounded off to the nearest 0.1 mm.
8	Bleed and vent holes are usually specified for orifice plates in horizontal pipe and with a bore diameter of at least 20 mm.
9	Orifice plates for pipe sizes of ½”, ¾”, 1” and 1”1/2 are usually installed between flanges in meter runs prefabricated.
10	Meter sections will be preferably flanged. Meter runs and relevant orifice bores are usually sized using the Manufacturer proprietary algorithms.

How Conditioning Orifice Plates Work

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Over the years, in many industries especially the natural gas industry, the orifice plate has become the de facto flow meter for measuring flow. In many liquid, steam or gas application, the standard orifice plate with a bore at the centre of the plate is being used with considerable operational reliability and accuracy.

However, standard orifice plates performance are affected by flow distorters such as elbows, tees which introduces flow distortions in the flow stream necessitating some flow conditioning to maintain accuracy. Flow conditioning in standard orifice plate applications can either be  by way of a flow conditioner or lengthy straight pipe runs requirement that are often difficult to achieve in practical situations. 

Conditioning Orifice Plates Standards & Technology
Conditioning Orifice Plates are differential producers used to measure flow in a pipe. Conditioning Orifice Plate Technology is based on the same Bernoulli streamline energy equation

Basics of Permanent Pressure loss in Differential Pressure Flow Meters

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The standard primary flow sensors commonly used in differential pressure flow meters are the orifice plates, flow nozzles and venturi tubes. These flow meters are often called "head loss" meters because there is a permanent pressure loss downstream these meters. In other words, upstream pressure never recovers to its original value downstream these meters. Various designs of these flow sensors are available which can provide the optimal meter for the desired operating conditions and requirements of the user. A critical factor in choosing a differential pressure flow meter is the pressure loss of the flow sensor. As a rule, when applying differential pressure devices, pressure loss must be small. This is because pressure loss means energy loss and higher pumping/compression costs.

The different installation versions of the primary flow sensors (orifice plates, flow nozzles, venturis) of differential pressure flow meters commonly used in flow measurement are tabulated below:

How to Specify an Orifice Plate

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Orifice plates are commonly used in flow measurement applications. After prolonged use, there may be need to order a new orifice plate to replace the one in service. In most industrial applications for large flow measurement, the orifice plate is often contained in a dual orifice chamber fitting and is usually replaced online in these types of fittings. In flange types, the orifice plate is sandwiched between flanges. The main reason for replacing an orifice plate is often due to deterioration and corrosion in service. Therefore to order a new orifice plate, the following specification checklist tabulated below should be handy so that mistakes are not made in the ordering process:

AGA 3 Gas Flow Equation for Orifice Plates

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Recall that we have discussed the ISO 5167 Orifice Meter Equation. While some companies use the ISO 5167 orifice gas flow equation, a vast majority of companies use the American Gas Association committee report 3 gas flow equation otherwise referred to as AGA 3 Gas Flow Equation for Orifice Plates. The flow rate computed by this equation is in standard units i.e Standard Cubic Feet per Hour or SCFH.

Any equation computing flow in standard units must predict the effective expansion of the gas as if

How to Size an Orifice Plate Flow Meter with Software

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Orifice plate sizing is normally done these days with software provided by the manufactures of the device. This tutorial will use the Daniel Orifice Flow Calculator version 3.1.
Before going through this Orifice sizing tutorial please read:
•The Orifice Flow Meter Equation
•A Guide to Sizing Orifice Plate Flow Meters

In these articles, you will familiarise yourself with the Orifice plate flow equation and the various correction factors applied when determining flow from Orifice plate flow meter installations. You will also learn about the parameters that affect Orifice plate sizing the most.

To size an orifice plate with software, you require the following process parameters:

A Guide to Sizing Orifice Plate Flow Meters

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The Orifice plate is a very robust flow measurement device. It is one device that is very easy to use and can easily be adaptable to many flow measurement applications. Its cost of operation is minimal and familiarity with the device is near universal. All these pluses make the Orifice plate the first choice measurement device in almost every flow application. It does however have some limitations which makes the sizing process a little tricky. Two key limitations include:

(a) Limited Turn Down – See Flow Meters Accuracy and Terminology
(b) Non-linear loss of accuracy at low flow rates as shown by the graph below:

The Orifice Flow Meter Equation

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The orifice flow meter is one of the most popular flow devices for measuring flow. It has proven its mettle in the both liquid and gaseous applications. In the Natural gas industry, the orifice plate continues to play a dominant role in flow measurement applications.

In the orifice flow measurement application, changes in static pressure, temperature and density are critical. In liquid systems static pressure changes have no effect on liquid density but in gaseous systems, a change in static pressure significantly impacts density due to the compressible nature of gaseous systems. Change in temperature affects both liquid and gaseous densities and as such are

Basics of Flow Measurement with the Orifice Flow Meter II

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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

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:

$Q = K\sqrt{\frac{\Delta {P}}{ρ}}$

Where
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.

Basics of The Orifice Plate Flow Meter

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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.

Types of Orifice Plates Used in Flow Measurement

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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: