API MPMS 14.8:1997(R2011) pdf download.Manual of Petroleum Measurement Standards Chapter 14—Natural Gas Fluids Measurement
3 Application
This publication does not set tolerances or accuracy limits. The application of the information here should be adequate to achieve acceptable measurement performance using good measurement practices, while also considering user require- ments and applicable codes and regulations. Systems for measuring liquefied petroleum gases use either volumetric or mass determination methods, and both methods apply to either static or dynamic conditions. Mass determination methods of measurement are most commonly used where conditions in addition to temperature and pressure will affect the measurement. Such conditions include compositional changes, intermolecular adhesion, and volumetric changes caused by solution mixing. Mass mea- surement is applicable to liquefied petroleum gas mixtures where accurate physical correction factors have not been determined, and to some manufacturing processes for mass balance determination. Volumetric methods of measurement are generally used where physical property changes in temperature and pressure are known and correction factors can be applied to correct the measurement to standard conditions. 6,7 Volumetric measure- ment is applicable to most pure components and many com- mercial product grades. Many of the measurement procedures pertaining to the measurement of other products are applicable to the measure- ment of liquefied petroleum gases. However, certain charac- teristics of liquefied petroleum gas require extra precautions to improve measurement accuracy.
Liquefied petroleum gas will remain in the liquid state only if a pressure sufficiently greater than the equilibrium vapor pressure is maintained (see Chapters 5.3 and 6.6). In liquid meter systems, adequate pressure must be maintained to pre- vent vaporization caused by pressure drops attributed to pip- ing, valves, and meter tubes. When liquefied petroleum gas is stored in tanks or containers, a portion of the liquid will vaporize and fill the space above the liquid. The amount vaporized will be related to the temperature and the equilib- rium constant for the mixture of components. Liquefied petroleum gas is more compressible and has a greater coefficient of thermal expansion than the heavier hydrocarbons. The application of appropriate compressibility and temperature correction factors is required to correct mea- surements to standard conditions, except when measurement for mass determination is from density and volume at meter- ing temperatures and pressures. Meters should be proven on each product at or near the normal operating temperature, pressure, and flow rate. If the product or operating conditions change so that a significant change in the meter factor occurs, the meter should be proven again according to Chapters 4 and 5.