Orifice plates are simple, inexpensive, and highly accurate flow-measuring devices. They are typically used for liquids, gases, and low-velocity steam flows.
Eccentric bore orifice plates have the bore offset from the center to prevent foreign material accumulation in solids-containing applications. Segmental bore orifice plates have a hole in the form of a segment of a concentric circle to facilitate colloidal and slurry flow measurements.
This orifice plate type features a sharp, square-edged concentric bore. It is the most common and is ideal for liquid, gas, and steam service. This design offers high accuracy at an economical price, with no regular maintenance needed.
The orifice is located 2% of the pipe’s internal diameter above the wall, which allows solids and slurries to pass through. This feature reduces the risk of inaccuracies due to partial upstream blockages, which are common with other orifice plates. In addition, the bore of a concentric orifice is not beveled, reducing the likelihood of corrosion.
Concentric orifice plates require a precision machined hole in the middle of a thin metal plate. The plate is then welded to the flange, and the gap is sealed by the ring/gasket. These plates are generally manufactured from Stainless Steel, Duplex Stainless Steel, Monel or Nickel. The standard square-edge concentric orifice plate has a leading edge that is sharp and burr-free with a short cylindrical section. This is a common orifice plate specification required by most standards and handbooks.
Another orifice plate design that offers higher accuracy than a square-edge concentric bore is the quadrant edge bore. Its inlet edge is rounded instead of beveled, and its diameter is a quarter of the bore size. This design is better suited for viscous fluids like heavy crudes, syrups, and slurries with Reynolds Numbers below 100,000.
Regardless of which orifice plate is used, a calibration curve should be developed for the specific media and application. The calibration curve must account for the effect of the varying fluid and operating conditions on the orifice flow measurement. If this is not done, the orifice plate will not provide a valid measurement.
When flowing through a sharp-edged orifice, the velocity profile converges at a point downstream of the orifice called the vera contracta. At this point, the fluid reaches maximum velocity and minimum static pressure. This condition is ideal for measuring differential pressure since it reduces the effects of pipe roughness.
As the name implies, these plates have an off-center or eccentric hole in their center. The location of the bore prevents solid materials from accumulating in it, which makes this type of orifice plate particularly useful for applications that use solid-laden liquids. It also minimizes the amount of fluid that can be lost around the bore’s edge, thereby improving the plate’s accuracy. It should be noted, however, that the eccentric design does not offer a high level of performance as compared to the concentric orifice plate, especially in gas flow applications.
Eccentric orifice plates also work well in multiphase liquid flow applications. When the hole is not centered, a secondary phase in the flow will tend to sweep any solids into the pipe downstream of the orifice plate rather than collect them on the upstream side. This prevents the accumulation of undesired material behind the orifice plate that could clog or change the flow pattern and lead to measurement errors.
In addition, the eccentric orifice plate does not have any sharp edges on its upstream or downstream faces, which means that it is less likely to be subject to degradation from harsh operating conditions. These include corrosion, erosion, water hammer, dirt, and grease buildup. All of these factors can decrease the discharge coefficient of a conventional orifice plate by as much as 10% over time.
The eccentric orifice plate’s design also allows for the installation of multiple pressure tappings on both the upstream and downstream sides of the plate. This can be very beneficial in situations where the upstream and/or downstream tappings might be blocked by solids, gas bubbles, or debris or when the discharge profile is uneven.
Traditionally, orifice plates have been installed by inserting them between two pieces of flange and using a bolt to attach the orifice plate to the flanges. This method of installation is cost-effective but requires a process shutdown whenever the orifice plate needs to be removed for maintenance. A flange tap is a quick and easy alternative that eliminates the need for a process shutdown when the orifice plate needs to be replaced.
The most common orifice plate is the square-edged, concentric bore design that is manufactured by machining a precise, straight hole in the center of a thin metal plate. This type of orifice plate is used in the measurement of clean liquids, gases, and low-velocity steam flows. The square-edged orifice plate is easily reproduced and a great choice for a variety of applications because it can be mounted between flanges.
The square-edged orifice plate can be installed in a direction that is opposite the fluid flow and still works properly because it appears to have the same diameter from either direction of approach. This also allows square-edged orifice plates to be used in bidirectional flow situations.
Other types of orifice plates are available to deal with certain process conditions that cannot be addressed using the standard square-edged orifice plate designs. For example, if the fluid has heavy solids, bubbles, or droplets in it, an eccentric or segmental orifice plate can be chosen. These types of orifice plates are used in sewage treatment, steel, chemical, water conditioning, and paper industries.
An orifice plate can have a number of defects that can affect its discharge coefficient, such as nicks from wear, erosion of the sharp edge from corrosion, and warping of the plate due to heat or pressure. These factors can change the orifice discharge coefficient by as much as 10%.
Another factor that can cause an orifice to become inaccurate is the presence of entrained air in the fluid flow. These unwanted air bubbles, droplets, and solids can block part of the orifice hole reducing the discharge area. This will result in a lower-than-expected flow rate and may cause the instrument to alarm.
To reduce the chance of these problems occurring, a rounded or radiused edge on the orifice plate can be used. This design is known as a quadrant edge orifice and is designed for use with viscous fluids like heavy crudes, syrups, and slurries with Reynolds numbers below 100,000. The quadrant edge orifice has a rounded entrance hole instead of the sharp-edged design of the standard square-edged orifice. This design prevents inaccuracies that partial upstream blockages of the orifice can cause.
Orifice plates are still one of the most popular differential pressure flow measurement devices available. They are simple to install, tolerate a wide variety of process conditions, and are very reliable. They provide a high level of accuracy for both full-scale and part-scale flow calibrations. The most common orifice plate is the square-edged, concentric bore design, which is used in most processes, including clean liquids, gases, and low-velocity steam flow.
A key feature of an orifice plate is its rounded upstream edge which acts as a flow nozzle allowing for a consistent coefficient of discharge (COD) even with turbulent or choppy process flows. This design also makes the orifice suitable for use in high-viscosity fluid applications such as syrups, slurries, and heavy crudes.
In order to maximize the efficiency of an orifice plate, it is recommended that the device be installed in a straight run of smooth pipe to avoid disturbance of the flow pattern caused by fittings and Valves. This is especially important in torturous piping systems where choking or cavitation of the flow can occur.
To prevent fluid from building up on the upstream side of the orifice plate and causing cavitation, it is recommended that the orifice be fitted with a special flange that has a square or conical bore surface. This type of flange is known as an OFU (Orifice Flange Unit).
Mac-Weld can manufacture OFUs in various types of materials, such as stainless steel, Hastelloy, or Monel, to suit your specific application. In addition, we can supply the appropriate RTJ orifice plate holder to mount your orifice onto.
RTJ orifice plate holders incorporate an integral gasket for mounting between ring-type joint flanges. This design allows the orifice to be installed at a higher temperature and pressure than would normally be possible for an orifice plate of the same size. The holder is machined from solid stock material such as 316 SS, Hastelloy, or Monel and is available in a range of sizes to suit your application. The holder is provided with a set of hold-down screws, which are screwed down over the orifice plate to secure it in place.