Orifice Plate – Primary Flow Element

An orifice plate, at its simplest, is a plate with a machined hole in it. Carefully control the size and shape of the hole, mount the plate in a fluid flow path, measure the difference in fluid pressure between the two sides of the plate, and you have a simple flow measurement setup. The primary flow element is the differential pressure across the orifice. It is the measurement from which the flow rate is inferred. The differential pressure is proportional to the square of the flow rate.

An orifice plate is often mounted in a customized holder or flange union that allows removal and inspection of the plate. A holding device also facilitates the replacement of a worn orifice plate or the insertion of one with a different size orifice to accommodate a change in the process. While the device appears simple, much care is applied to the design and manufacture of orifice plates. The flow data obtained using an orifice plate and differential pressure depend upon well-recognized characteristics of the machined opening, plate thickness, and more. With the pressure drop characteristics of the orifice fixed and known, the measuring precision for differential pressure becomes a determining factor in the accuracy of the flow measurement.

There are standards for the dimensional precision of orifice plates that address:

• The circularity of the bore
• Flatness
• Parallelism of the faces
• Edge sharpness
• Surface condition

Orifice plates can be effectively “reshaped” by corrosion or by material deposits that may accumulate from the measured fluid. Any distortion of the plate surface or opening has the potential to induce measurable error. This being the case, flow measurement using an orifice plate is best applied with clean fluids.

Certain aspects of the mounting of the orifice plate may also have an impact on its adherence to the calibrated data for the device. Upstream and downstream pipe sections, the concentric location of the orifice in the pipe, and the location of the pressure measurement taps must be considered.

Properly done, an orifice plate and differential pressure flow measurement setup provide accurate and stable performance. Share your flow measurement challenges of all types with a specialist, combining your own process knowledge and experience with their product application expertise to develop an effective solution.

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