There is no substitute for well maintained, properly installed, and properly performing measurement equipment. Industry- standard measurement equipment installation, operation, and maintenance provide the raw data necessary for those dealing with natural gas liquids (NGLs) to transact business. Then, this raw data can be adjusted or converted to values suitable for transactions to take place and for proper accounting.
To determining which adjustments or conversions to use, begin by understanding the desired results. Some measurement processes and accounting software applications require volumetric quantities. Others require mass quantities. For many NGL applications, the preferred outcomes are liquid volumes of pure components. This is because most NGLs are eventually fractionated into pure products and market prices for these are readily available. Sometimes, the gas equivalent values of liquids are useful for operations.
Knowing the starting point is equally important. The starting point can vary depending on the type of measurement. Different resources and applications provide for different means of measuring NGLs. Quantity measurements can be made on a mass basis or a volumetric basis.
MEASUREMENT BY VOLUME
When NGLs are measured volumetrically, the measured volume is converted to contract base conditions. Conditions consist of a specified absolute pressure and temperature. Contract base conditions are pressure and temperature conditions specified in the contract between the buyer and seller. The American Petroleum Institute (API) Manual of Petroleum Measurement Standards (MPMS), Chapter 12, 1996 Edition , a.k.a. API-12, provides guidance for this calculation process.
For volume calculations, the desired result is Net Standard Volume (NSV). NSV is the measured volume of the liquid adjusted for all conditions, instrument performance, and sediment and water quantities. NSV is the product of two factors: Indicated Volume (IV) and the Combined Correction Factor (CCF).