Orifice plate is one of the most popular differential pressure type liquid flowmeter generally employed for detection (or measurement) of flow. It is one of the simplest and most cost-effective ways to restrict flow. Orifices are simply a flat piece of metal with a particular size of hole drilled in it. These flat plates are usually installed between a couple of flanges and in a straight run of smooth pipe. This is done to keep flow patterns turbulences away from fittings and valves. Orifice acts as a primary device. By means of an orifice, the liquid flow is obstructed in order to create a differential pressure across the plate. The fluid flow detection is done via measuring the pressure difference from the upstream side to the downstream side of a partially obstructed pipe.
Key features of an Orifice plate are mentioned below:
- Thickness of Orifice plates is approximately 1/16 to 1/4 inch.
- Orifice plates are simple and easy to use.
- Orifice meters are inexpensive.
- They can be arranged for more or less every application in any material, even though the stainless steel is most frequently employed material.
- Orifice plates offer a turndown ratio of less than 5:1.
- Accuracy of Orifice plates is reduced at low flow rates. Besides, wear affects the accuracy.
- Orifice plates which are in good shape and with a sharp edge to the upstream side tender increased accuracies.
- Orifice plates are suggested for clean and dirty liquids and a few slurry services.
- Rangeability of Orifice plates comes around 4 to 1.
- Orifice plates typically have an accuracy of 2 to 4% of full scale.
- Viscosity effect is high in Orifice plates.
- For Orifice plates, pressure loss is medium and it mainly depends upon the area ratio. If the area ratio is 0.5, the head loss will be approximately 70 – 75% of the orifice differential.
- Orifice plates are accessible for all pipe sizes. In case, the requisite pressure drop is free, these plates prove to be extremely cheap and efficient for measurement of flows in large size pipes i.e. pipes having diameter range beyond inches 6 inches.
Types of Orifice Plates
There are mainly three types of orifice plates in use. They are:
Concentric: This type of orifice plate is most common amongst all the three types. In this design, the orifice is equidistant i.e. concentric to the inside wall of the pipe. A concentric orifice plate is generally employed for clean liquids, gases, and steam flows specifically in pipes under six inches, where Reynolds numbers range from 20,000 to 107.
Segmental: These types of orifice plates are almost the same as concentric orifice as far as their functioning is concerned. In a segmental orifice, the circular section is concentric with the pipe while the segmental part is mounted in a horizontal pipe. This installation helps in eliminating damming of foreign materials on the upstream side of the orifice. Based upon the type of liquid, the segmental section is positioned on either the top or bottom of the horizontal pipe. This arrangement results in better accuracy of the measurement. Another variation of segmental orifice is segmental wedge which is essentially designed to detect the flow of liquids containing solids. This unit is easy to design and capable enough to measure flows at low Reynolds numbers. Furthermore, the pressure drop through the unit is merely around half that of customary orifices.
Eccentric: Eccentric orifice plates are designed in such a way that the edge of the orifice is reallocated towards the interior of the pipe wall. Owing to this design upstream damming gets avoided. It is employed in the same manner in the segmental orifice plate as well.
All the three kinds of orifice plates are shown in figure below:
Other Orifice Assemblies include conical and quadrant orifices which are quite new. These units have been developed mainly for the purpose of fluid flow detection with low Reynolds numbers. Another type of significant orifice assembly is integral wedge assembly in which the wedge element and pressure taps are combined into a one-piece pipe coupling bolted to a typical pressure transmitter. This type of assembly requires no special piping or fittings for mounting the device in a pipeline.
Two distinctive disadvantages of orifice plates are mentioned below:
- They result in a high permanent pressure drop i.e. outlet pressure will be about 60% to 80% of inlet pressure.
- Orifice plates are prone to erosion, which in due course can generate inaccuracies in the measured differential pressure.
Major advantages of orifice plates include:
- Orifice flow meters consist of no moving parts.
- Moreover, the cost of orifices does not rise drastically according to their pipe size.
Principally, an orifice plate is a precision instrument. In best circumstances, the inaccuracy of Orifice plates can possibly fall in the range of 0.75-1.5% AR. However, there are numerous error causing conditions which can terribly affect the accuracy of an Orifice plate. Following factors are used to judge the performance of an Orifice plate:
- Precision in the bore calculations
- Quality of the installation
- Condition of the plate itself
- Orifice area ratio
- Physical properties of the fluid flow under measurement
Further class of installation depends upon following factors
- Tap location and circumstance. Generally, there are three ways to position a pressure tap.
- Provision of the process pipe
- Competence of straight pipe runs
- Gasket intervention
- Misalignment of pipe and orifice bores
- Lead line design
Extra detrimental conditions consist of
- Dulling of the sharp edge or nicks caused due to corrosion or erosion
- Warpage of the plate because of waterhammer and dirt
- Grease or secondary phase deposits on any of the orifice surface
Any of the above said conditions has the tendency to affect the discharge coefficient of an orifice plate to a large extent.