Orifice meters are used in gas measurement to produce accurate readings of the volume of gas.

Being able to accurately determine the volume of gas is critical for maintaining the integrity of third-party contracts, ensuring the delivery of the requested amount of gas, preventing gas loss in a system, and other objectives.

To achieve company objectives, gas measurement professionals need to understand how to use an orifice meter, how to perform critical calculations, and how to test the orifice meter. Understanding these elements will help you get the most from your orifice measurement.

How to Use an Orifice Meter

An orifice meter consists of an orifice plate, an inlet section and an outlet section, the meter tube that runs along the length of the meter, and pressure sensor taps. Natural gas flows through the inlet section, passes through the orifice plate, and exits through the outlet section.

During this process, there is an increase in velocity and decrease in pressure as the natural gas passes through the restrictive hole in the plate. The orifice meter uses the Differential Pressure Measurement Principle to calculate the flow rate.

How to Perform Calculations Using an Orifice Meter

The flow rate calculation in an orifice meter relies on the Bernoulli Equation. This equation uses the differences between pressure upstream and pressure downstream measured as natural gas passes the orifice plate.

The key elements required to perform an orifice flow calculation include:

• Diameter of the orifice plate (in)
• Inside diameter of the meter tube running along the meter (in)
• Static or flowing pressure of the orifice meter (psi)
• Differential pressure across the orifice (inches of water column)

The result of this calculation is Flow rate (ft3/s)

Flow rate is then added to additional calculations to compensate for supercompressibility and to report volume at standard temperature and pressure. Performing these calculations is important to accurately report the amount of natural gas flowing through a system.
How to Test the Orifice Meter
AGA Report 3, Part 2 sets industry standards for how to authenticate, test, and maintain orifice meters used to perform custody measurement.

– One of the most important tests that should be performed is calculating the Diameter Ratio (or Beta Ratio). This is a measurement of the orifice plate bore diameter divided by the meter tube diameter.

The acceptable range of Beta Ratios is between 0.2 and 0.8 to perform an accurate calculation of the flow rate and meet the “uncertainty” guidelines in AGA Report 3, Part 2. If you test the orifice meter and find a beta ratio above or below the range, then consider changing the orifice plate to one with an appropriate bore size.

– Another important test is the Meter Tube Inspection. This test requires a visual inspection of the meter to find corrosion, leaks, defects, or other issues that could be impairing your ability to perform accurate measurement.

– Consider another important type of test — Orifice Plate Inspection. This test requires measurement of the orifice bore and a visual inspection of the plate find dirt, corrosion, pits, round edges, defects, or other issues that could be impairing your ability to perform accurate calculations.

You may discover that the differential pressure sensors need to be recalibrated, the meter tube needs to be cleaned, or the overall meter needs to be reconfigured.

It is important to produce proper documentation that captures the results of the meter test. This will ensure your ability to validate the accuracy of the orifice measurement.

Register for Fundamentals of Gas Measurement Certification

Measurement professionals need a firm grasp on these critical elements in gas measurement. Understanding the gas measurement fundamentals, including orifice measurements, will allow professionals to confidently support their company’s objectives.

Gas Certification Institute (GCI) offers the Gas Measurement Fundamentals certification course to ensure that gas measurement professionals are equipped with information, understanding, and tools to perform critical measurement tasks in the field.

View the GCI Schedule to find the next certification course and register yourself or your team. We look forward to helping you gain confidence in the fundamentals of gas measurement to get the most out of your orifice measurement.