Orifice Flange Unions, or Orifice Pairs, are specialized flanges used for flow/rate metering of liquids and gases. They come with pairs of pressure taps, typically on two sides and directly opposite each other.
As fluid passes through the orifice plate, it gains velocity but loses pressure. This is because kinetic energy is converted to potential energy.
Flow Measurement
As the name implies, orifice flanges are used to measure the rate of flow of liquids, gases, or steam in various industrial processes. The ability to accurately measure the rate of flow is critical for ensuring that the piping system functions smoothly and efficiently. Typically, orifice flanges are installed in a length of pipe known as a meter run. This section of pipe is often a part of the process and is isolated from other areas in the facility to allow for independent testing and calibration.
The principle behind orifice flanges is the conversion of potential energy into kinetic energy as the fluid passes through a restriction. As the liquid travels downstream of the orifice plate, it experiences a drop in pressure due to friction and loses potential energy. As the liquid moves away from the orifice, it develops velocity and gains kinetic energy, and the pressure drops further until it recovers to its original value upstream of the orifice plate. This is referred to as a Bernoulli equation relationship.
Orifice flanges have tapping holes or tapped ports that allow monitoring equipment to be attached to record the differential pressure of a commodity traveling through the plate. The tapped port locations must be spaced far enough apart to prevent pressure recovery between the two plates. A typical design places the taps about one orifice plate diameter upstream and 0.3 to 0.8 of a pipe diameter downstream, depending on the type of orifice plate and layout configuration.
The accuracy of flow measurement through an orifice flange depends on the proper construction and installation and the conditions under which it is operated. It must be free from any turbulence, and the flow must be acceptably smooth and straight for stipulated distances upstream and downstream of the plate. Sometimes, to achieve this requirement, flow conditioners are inserted into the line to develop a proper flow profile.
There are many different types of orifice plates and flanges that can be utilized in measurement applications. For example, segmental wedge orifice plates are primarily designed to measure flows in low Reynolds numbers and have a constant flow coefficient based on the square root of the wedge gap size. Other orifice flanges include conical and quadrant orifice plates, which are designed to be used in a variety of flow control applications and have varying degrees of pressure drop.
Pressure Measurement
Orifice flanges are used in the industrial world to measure how fast liquids or gases move through a pipeline. This is essential for many reasons, including regulating ship fuel oil flows and measuring water consumption in cities. It also helps improve operational efficiencies and reduce energy costs. In addition, orifice flanges can help detect leakages by monitoring fluid characteristics.
The most important thing to remember when using orifice flanges is that the accuracy of a flow measurement depends on how much turbulence there is in the pipe. Flow turbulence is created by obstructions in the configuration from items like fittings and valves, and it can cause inaccurate readings. To ensure an accurate measurement, it’s necessary to have a straight run of the pipe before and after the orifice plate. This length is referred to as the meter run, and it’s determined by a combination of factors, such as the diameter of the orifice plate, the type of fluid flowing through the pipeline, and the desired measuring accuracy.
Another critical factor that determines the accuracy of an orifice flange is the coefficient of discharge of the orifice plate. This is calculated by dividing the orifice diameter by the square of the flow rate, and it can be affected by a variety of conditions, such as the dulling of the sharp edges due to erosion or corrosion, warping of the orifice plate caused by water hammer, or grease or secondary phase deposits on either orifice surface.
When calculating the coefficient of discharge, it’s also necessary to take into account the pressure drop across the orifice plate. This is determined by the ratio of the pressure differential downstream to the upstream pressure at a specific location in the pipe, called the vena contracta. The vena contracta can be measured by tappings placed one pipe diameter upstream of the orifice plate and at a position 0.3 to 0.9 diameters downstream.
Orifice flanges are available in a variety of sizes and materials, and they can be designed with threaded or welded necks. They are often found in high-pressure applications, so it’s important to choose the right ones for your application. Orifice flanges are an integral part of any piping system, and installing them correctly is essential for accurate measurements, optimizing performance, and reducing downtime.
Temperature Measurement
Orifice Flanges are used in oil and gas pipelines to measure the rate of flow of a liquid or gas media. These specialized dual-pressure measuring devices work on the same principle as Venturi-style meters, using Bernoulli’s equation to determine volumetric and mass flow rates from the difference in fluid velocity and pressure downstream of the orifice plate.
The key is that the orifice plate restricts the diameter of the passage of a flowable medium, creating a differential in both velocity and pressure downstream of it and upstream. This difference in pressure can be measured with pressure taps that are installed on the orifice flange.
The vena contracta point is the location in the pipe where the maximum difference in the fluid pressure between normal pipe sections occurs. It occurs just downstream of the physical orifice and is related to the beta ratio of the orifice plate (which varies by size and material) and the Reynolds number of the flowing fluid. It is a significant measurement point because it indicates the rate of fluid flow.
Typical orifice flanges come in either raised face or ring-type joint styles, with the latter more commonly used for higher-pressure applications. Usually, they’re also equipped with a welded carrier ring/gasket that holds the orifice plate in place. Common carrier ring/gasket materials include soft iron and Stainless Steel.
When properly installed, orifice flanges provide highly accurate measurements of the flow rate of a liquid or gas based on the difference in pressure between two tapped ports. However, there are certain requirements that must be met to achieve this accuracy. The most important is a stable flow pattern. A general rule of thumb is that a straight-run length of pipe equal to 10-15 piping diameters is sufficient to establish this condition.
Another criterion is that the orifice plate must be located at a constant distance from both ends of the piping system. If the orifice is located too close to either end, it may create unreliable flow rate readings. This is because the fluid may be impeded in its flow by the flange, or it may be subject to water hammer, air pockets, or solids clogging.
Pressure Drop Measurement
The measurement of the rate of flow of a commodity in a pipe is a critical step for most industrial processes. This data is used to adjust equipment and controls and is also required to produce billing and accounting information for the commodity consumed by the plant. In order to produce accurate results, it is important to maintain a smooth and consistent flow pattern throughout the system. In order to do this, the piping design typically includes a section of straight pipe called a meter run.
In most meter runs, the rate of flow is measured by a set of orifice plates mounted between a pair of orifice flange unions. The orifice plates are then connected to instrumentation that records the differential pressure drop across the orifices. During operation, several adverse conditions can affect the discharge coefficient of an orifice plate, which in turn can negatively impact the overall accuracy of the measurement.
These adverse conditions can include clogs in the fluid that cause secondary phase build-up on the upstream edge of the orifice plate, sharp edges eroded by fluid and vibrations (water hammer) that lead to dulling of the edges of the plates, and warpage of the plate due to changes in temperature, humidity, and other process variables. They can also include a change in the discharge coefficient caused by local pressure tappings that are not on the same plane as the orifice flanges (corner taps, flange taps, and D+D/2 taps).
For these reasons, orifice flange unions are usually designed to permit the easy removal of the orifice plate for servicing or inspection. They are most commonly constructed of stainless steel to withstand the harsh and corrosive operating conditions found in many applications. They can be provided with a variety of taps that can be fitted for monitoring instruments, as well as a series of tapped holes for connecting hoses to the flanges for flushing.
Typically, the orifice flange unions are constructed with jack screws that allow them to be removed and replaced without shutting down the flow line. This is in contrast to the standard orifice plate that requires a complete process shutdown whenever it needs to be removed for maintenance and inspection. In addition, the orifice flange unions can be made with a venturi design that allows for much easier access to the orifice plate. This design also helps to recover the pressure drop more effectively than the standard orifice plate while requiring less upstream straight pipe diameter.
