Orifice meters are perhaps the most commonly used d/p flow meters in refineries, production facilities, and chemical plants. The accuracy of an orifice meter depends on many factors, such as fluids, upstream piping configuration, Beta ratio (d/D, or b, the ratio of orifice bore to pipe size), Reynold’s number limits, and correction factors used. The overall accuracy ranges from ±0.5 to ±5%.
The overall accuracy of orifice metering can be affected by:
• improper meter run design (i.e. insufficient flow conditioning or straight run of pipe)
• incorrect installation of the plate (e.g., backward) and sensing line (e.g., trapped vapor in liquid service)
• rough meter tube surface
• problems of the plate (as described later)
• high or low beta ratio
• pulsating flow
• liquid entrained in gas flow
• transmitter problems (e.g., drift)
• inadequate secondary element (e.g., should it be a chart recorder or an electronic flow computer)
• gas sampling and analysis for specific gravity measurement used in the calculation
• the inherent accuracy limitation (“uncertainty”) of the orifice discharge coefficient in the calculation. The orifice coefficients used in the industry standards were developed based on limited lab and field test data.
Improperly designed and installed orifice meter systems tend to under-measure flow because of the inherent characteristics and imperfections of the current industry standards. Orifice meters tend to over-measure flow in a pulsating-flow situation.