Medicion Hidrocarburos

CALIBRATION OF STORAGE TANKS Class #2070 Michael Yeandle Consultant Troy, Ohio

SCOPE This paper will discuss several field measurement methods that are presently in use to calibrate upright, above ground, cylindrical, cone and floating roof steel storage tanks. INTRODUCTION Tank calibration is often referred to as "tank strapping" derived from to the old method of placing metal bands or strapsaround wooden containers used for the storage of oils. Over the years, as the price of crude oil and petroleum products has increased, storage facilities, and the accurate measurement of oil in storage, has become very important. We now have storage tanks as large as 2,000,000 barrels in volume and therefore one can see how importance the calibration of a storage tank can be. Any errors made atthe calibration stage will cause errors in the final tank table. TANK CALIBRATION STANDARDS The API Measurement Committee on Petroleum Measurement have issued a Manual of Petroleum Measurement Standards (MPMS) containing all the present individual measurement standards, indexed, revised to the present state of the art and rewritten to a standard format. The section on tank calibration is covered indetail in Chapter 2. METHODS OF CALIBRATION Calibration is the process of accurately determining the capacity or partial capacities of a tank and expressing this capacity as a volume for a given linear increment or height of liquid. Above ground cylindrical storage tanks are usually calibrated by placing a measuring tape around the tank shell. This procedure, known as the Manual Tank StrappingMethod is the original tank calibration technique, and can be found described in detail in the API MPMS Chapter 2, Section 2A. Present day tank calibration techniques have taken the tank strapping method and refined it into two optical methods of measurement. These are known as: 1. Optical Reference Line Method (ORLM). 2. Optical Triangulation Method (OTM). The "Optical Reference Line Method" can befound in the API Manual of Petroleum Measurement Standards, Chapter 2, Section 2B. The "Optical Triangulation Method" can be found as a standard method in the API Manual of Petroleum Measurement Standards, Chapter 2, Section 2C.

174

MANUAL TANK STRAPPING METHOD Circumference Measurement Before taking any measurements, it must be ascertained that the tank has been filled at least once witha liquid of a density equal to or greater than the normal service liquid. Generally, the static water test carried out on the completion of construction satisfies this condition. NOTE: This requirement is common to all tank calibration methods, whichever is used. The normal tape for tank strapping is made of steel, 1/8" to 1/4" wide, of a length great enough to measure the total circumference. Ifthis tape is not available, circumferential measurement can be made in sections, the sum of all the sections being used as the circumference. Standard tapes are usually 100', 200', 500' and 1,000' in length, marked at each foot, with one or both ends marked in 0.01' increments. A Master Tape is a tape measured by the National Institute for Standards and Technology, and reported as having aspecific length, for example: 100.0023 feet at a temperature of 68 degF, and a tension of 10 pounds. 100.0198 feet at a temperature of 68 degF, and a tension of 30 pounds. The Master Tape length may be adjusted to the petroleum industry base temperature of 60 degF, by using the thermal coefficient of expansion shown on the calibration report. All tapes being used in the calibration should be checked atthe tank site against an NIST Master Tape. The tank is strapped using a working tape that has previously been checked against the Master Tape. Never use the Master Tape to perform the actual strapping operations. There are two methods of checking the calibration of the working tape. Stretch out both the working tape and the master tape, alongside each other, by either hanging them from the top of...