API MPMS 3.3:1996(R2017) pdf download.Manual of Petroleum Measurement Standards Chapter 3—Tank Gauging
1 Scope
This standard provides guidance on the installation, cali- bration, and verification of automatic tank gauges (ATGs) used in custody transfer for measuring the level of liquid hydrocarbons having a Reid vapor pressure of 15 psi (103 kilopascals) or greater, stored in stationary, pressurized storage tanks. The standard also provides guidance on the requirements for data collection, transmission, and receiving. This standard is not applicable to the following: a. Conversion of tank level to liquid volume. b. Measurement of free water or sediment lying under the liquid hydrocarbon. c. Measurement of temperature, density, or sediment and water (S & W). These measurements are discussed in the API Manual of Petroleum Measurement Standards (MPMS), Chapters 7, 9, and 10. d. Sampling for determination of the properties of the liquid hydrocarbon. This is discussed in API MPMS Chapter 8. e. Detection of tank leaks. f. ATGs used for inventory control and plant operations. Note: Converting net vapor space volumes to equivalent liquid volumes or mass is described in API MPMS Chapter 12. (The section covering calcu- lation of vapor to liquid equivalent was in preparation at the time this stan- dard was written.) This standard may involve hazardous materials, opera- tions, and equipment. This standard does not purport to address all of the safety problems associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
3 Accuracy Requirement
The accuracy of automatic level measurement is affected by inherent (intrinsic) error of the ATG, the effect of installation, and the effect of changes in operating condi- tions. Some of the factors described below also affect manual measurement, which may be used as part of the ATG setting and calibration. 3.1 INHERENT ERROR OF ATGS The level measurement accuracy of all ATGs is affected by the inherent error of the ATG; for example, the error of the ATGs when tested under defined referenced conditions as specified by the manufacturers. 3.2 CALIBRATION PRIOR TO INSTALLATION The reading of the ATG in custody transfer service should be calibrated prior to installation (in the factory or testing laboratory under controlled conditions) to agree with a certi- fied reference (for example, a certified gauge tape) within ± 1 ⁄ 16 inch or 1 millimeter over the entire range of the ATG. The certified reference should be traceable to the National Institute of Standards and Technology standards and should be provided with a calibration correction table. The uncer- tainty of the certified reference should not exceed 1 ⁄ 32 inch or 0.5 millimeter with the calibration correction applied. Note: ATGs used for internal inventory control or operational purposes (such as tank filling and emptying, and level control) should be calibrated to ensure safe operation as determined by the tank owner.