The effects of pulsation on Natural Gas Measurement are a major factor to consider when designing your meter installations. Pulsations created by compressor, flow control valves, and gas regulators, can produce pulsations in a magnitude that will effect accuracy.
Another problem can be created by some piping configurations that can produce vortices that will create pulsations which would also affect accuracy. The resulting error is called Square Root Error (SRE) which is always a positive error and is inherit to the piping system and can only be reduced by modifications to the piping system which can be costly.
A secondary effect of SRE is called Gauge Line Error (GLE) this effect is where the Differential pressure for your measuring instrument at the orifice fitting connection is not the same differential pressure that is at the measurement instrument’s connection. This error can be both a positive error or can be a negative error. A study by Pipeline and Compressor Research Council (PCRC) in cooperation with Southwest Research Institute (SWRI) in San Antonio, Texas has shown the gauge line error can be reduced mounting your measurement instrument closer to orifice fitting. The best results is a maximum distance of 18” (45 centimeters) and keep your measurement sensing lines a minimum internal diameter of .375″ (9.5 millimeters) this will allow the pulsations to move in and out of the sensing lines and not build up pressure giving an inaccurate reading. Another factor is keeping both sides of the sensing lines from the orifice fitting to the measuring instrument the same length, deviation or unequal lengths of the “Low Side” and the “High Side” sensing lines could result in a Gauge Line Error.
Another consideration is in Gas Production and Gas Gathering system designs. Designing an installation that will drain condensate and other liquids is important. These liquids will build up in small orifice valves which will affect your accuracy. In these applications it is important to install the measurement instrument above the orifice fitting connections to allow self-draining of the manifold and sensing lines.
