Many measurement errors can be prevented long before any gas is purchased or delivered by simply adhering to industry-accepted best practices for orifice meter installation. Sometimes a rushed or careless installation may cause unintended changes that can create unnecessary and avoidable problems.
One essential design element is to position pressure taps in a vertical plane facing upward (vertical taps
) to help prevent accumulation of free liquids in the gauge lines. Additionally, the gauge lines should be sloped upward (at ¼" per foot) from the taps to the secondary device. Often, the erroneous assumption is made that since dry gas is being measured the orientation of the taps is not a concern. As a result (and in the absence of specific installation instructions), the operator may choose to install the orifice fitting with the plate-access in a more convenient vertical position, thereby causing the pressure taps to be horizontal. Extensive field reviews of custody-transfer measurement installations have found that liquid traps are frequently the cause of unacceptable "Lost and Un-Accounted For"
(LUAF) quantities. This type of error is not easily detected and can create gains or losses with inconsistent, unpredictable amounts.
In one documented case, there was an ongoing difference between delivery and purchase meters that ranged from a 10% gain to a 40% loss on nominally dry gas measured downstream of a dehydrator (view the graph here
). The purchase meter was installed with horizontal taps and it was noted that the difference would substantially decrease immediately following each meter inspection. After several days the difference would gradually increase until the next meter inspection when, again, the difference would decrease. Based on this, recommendations were made to roll the meter tube 90 degrees to provide for vertical taps. Once the meter tube was rolled, the measurement differences did not reoccur.
Among the many sources of measurement error, the presence of liquids in the meter, as described above, is quite common. For more information on common sources of measurement error, gas sales verification, and related technical topics, you can click here
, and be sure to consult with your organization’s measurement experts, whether in-house or contract servicer.
"Even the best planning is not so omniscient as to get it right the first time."
~ Fred Brooks, American Computer Scientist & Software Engineer