Pressurewhat pressure? - Coastal Flow

1 pressure ? Among the most misunderstood and misapplied terms used in natural gas measurement are pressure Base, Atmospheric pressure , Gauge pressure , and Absolute pressure . Not only are these expressions easy to confuse, when their values are incorrectly applied within the flow equation, measurement error is introduced and oftentimes that error can be severe. Understanding these fundamental terms and how they are applied is of vital importance to those involved in every phase of gas measurement, accounting, and regulatory reporting. The following is a brief definition for each of these terms: pressure Base is the pressure at which a volume occupies one standard cubic foot (scf), as defined by contract, expressed in pounds per square inch absolute (psia).

2 State regulatory agencies will often require that volumes are calculated at different pressure base conditions ( , psia for Texas and Oklahoma, and psia for Louisiana and Mississippi). Atmospheric pressure is the amount of pressure created due to the weight of the atmosphere (air and water vapor), expressed in pounds per square inch (psi). These values vary with elevation above sea level. For example, the average atmospheric pressure at sea level is usually assumed to be psi while the average atmospheric pressure at higher elevations is less ( , psi in the Rocky Mountain region). Gauge pressure is a measure of the amount of pressure which is greater than atmospheric pressure , expressed as pounds per square inch gauge (psig).

3 Absolute pressure is the sum of the gauge pressure and the atmospheric pressure used in all gas law equations, expressed as pounds per square inch absolute (psia). Because these values can be similar, it's very easy to enter incorrect data for configuration, calibration, or meter set- up. Accordingly, it is important to know how they are used in the flow equation and how they should be correctly applied to avoid systematic measurement error, as indicated in the following simplified volume calculation: As an example, using a Coefficient (C') of , Absolute pressure (Pabs) of psia [Atmospheric pressure of psi + Gauge pressure of 20 psig], and Differential pressure (?)

4 P) of 50 inches of H2O in the flow equation above produces a calculated rate (Q) of 417 Mcf per day at a pressure Base of psia. However, with a simple misunderstanding of how Absolute pressure is determined, a 24% error (understatement) would be introduced if the calculation were performed using Gauge pressure rather than Absolute pressure . As this demonstrates, it is critical that these pressure values are properly applied to avoid costly errors which could affect natural gas measurement, accounting, and regulatory reporting. The real definition of Absolute pressure ' is third- day close- out after a holiday weekend! ~ Anonymous, Gas Measurement Analyst