Effects Of Abnormal Conditions On Orifice Measurement Accuracy

By Dean Graves, Devon Energy, Oklahoma City | July 2010 Vol. 237 No. 7

Figure 1: Simple diagram of a measurement station and the dynamic elements.

Whenever one focuses on gas or fluid measurement, he or she will eventually discover an abnormal condition at a measurement station. Invariably someone will ask, “What effect will this have on measurement?” A student of measurement may spend years answering this question.

This and similar questions have generated many research studies. These studies have enabled us to better understand measurement abnormalities and to improve measurement procedures and standards. Even though we have made great strides in measurement, we will continue to ask this question. It is this question that has led to the development of this article. Instead of focusing on certain specific abnormalities, this article addresses the overall subject of measurement abnormalities and presents some investigative tools to assist attempts to answer the question. However, before we can understand measurement abnormalities, it is important to review proper or accurate measurement.

One of the oldest types of fluid measurement is based on the concept of forcing a pressure drop and velocity change with a specifically designed obstruction, and measuring the pressure change on either side of the obstruction, then applying these readings to a formula to calculate the fluid flow. This obstruction is called an orifice plate and the overall process is called orifice measurement. Orifice measurement includes an orifice, a meter tube, and sensing equipment. An orifice is installed in the meter tube. The sensing equipment measures the pressure change on both sides of the plate. The readings are used in a formula to calculate gas or fluid volume. The fluid flow changes caused by orifice measurement can be referred to as “critical dynamic elements of orifice measurement.” Figure 1 is a simple diagram of a measurement station and the dynamic elements.

The oil and gas industry quickly learned that not just any orifice or meter tube would produce accurate and repeatable results. Different pieces of equipment produce different results. Therefore, the industry conducted many studies to better understand these varying results. The studies were conducted at various flow laboratories under very controlled conditions to produce very repeatable results. They determined it was necessary to establish standards for the manufacture and installation of the equipment. The industry has developed a standard published by American Gas Association, “Orifice Metering of Natural Gas and Other Related Hydrocarbon Fluids, Parts 1 -4” (AGA 3). When the equipment was standardized, various orifices and meter tubes would produce the same dynamic element results.

Additional studies proved that abnormal conditions affect measurement because they alter one or more of the critical dynamic elements of the measurement process. Therefore, to understand what effect abnormal conditions have on the elements it is necessary to understand the four different critical dynamic elements of fluid measurement: (1) fully developed entry flow profile, (2) exit flow profile, (3) readings at the sensing taps, and (4) gas characteristics.