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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 straps around 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 at the 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 in detail 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 Strapping Method 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 be found 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.

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MANUAL TANK STRAPPING METHOD Circumference Measurement Before taking any measurements, it must be ascertained that the tank has been filled at least once with a 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. If this 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 a specific 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 at the 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 the tank, if high enough, or lay them out on a railroad line or paved highway, whichever is available. Secure both tapes at their top ends. Pull the master tape to the tension specified on its calibration certificate. There are two methods of calibrating the working tape from this point: a) Pull the working tape with the same tension as the master tape and then compare the reading of the two tapes. Apply a correction to the working tape to make its reading the same as the master tape. This tension must now be used on working tape throughout the calibration and the same tape correction applied at each reading. b) Pull the working tape with sufficient tension to make it read the same as the master tape. Note: the amounts of tension used and use this pull throughout the strapping procedure. With this procedure, there is no tape correction to be applied to the readings but again this determined tension must be used throughout the strapping operation.

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Chapter 2.2A details the location and number of strappings to be taken on each tank ring for different types of tanks. The tape is laid around the vertical walls of the tank, parallel to and a measured distance from a particular ring. The position of the tape can be adjusted either by using jointed aluminum poles with a special guide fitting on the top pole or by use of a slotted ring-like guide fitted with two 1/4" ropes. The tape is read after sliding to distribute surface tension and applying the predetermined pull at the tape ends using a spring balance. When measuring tanks the tape is read to the nearest 0.005' for single measurements and 0.001' for multiple measurements. Heights and Other Measurements All height measurements should be recorded to the nearest 0.01'. The total shell height should be measured together with the heights of the plates in each ring. The total gauge height along with the height of the liquid level in the tank is required, also recorded should be the maximum fill height. If the tank contains liquid, the temperature and a sample for API Gravity determination must be taken, together with the ambient temperature. Additional measurements required on the tank shell are the plate thickness and the paint thickness. Tilt Test measurements of the tank should be made to find out whether the tank is tilted out of the vertical. This is easily carried out by hanging a plumb line from the top of the shell to the ground, from various positions around the top of the tank. If tilt is present, it should be related back to the datum plate. Tanks tilted less than one part in seventy parts can be disregarded, as the correction is negligible. Deadwood In addition to external measurement, it is necessary to determine the amount of space taken up inside the tank by pipes, heaters, manholes, mixers, ladders, etc. This is called deadwood and is generally described as items that subtract from or add to the volume of liquid in the tank. Deadwood measurements are normally made to the nearest 1/4". The strapping report must include the dimensions of the deadwood and its location in the tank relative to the datum plate. Bottom Survey Once the measurement of deadwood is completed, the final operation is to carry out a bottom survey. This is usually determined by means of a surveyor's level or transit measuring the height differences between the datum plate and various selected points on the tank bottom. Starting at the datum plate a survey is taken to the center. Then, from the datum plate, take survey levels around the circumference, and at each 45-degree position again take another survey into the center of the tank. It follows that the more level readings that are taken, the more accurate will be the bottom calculations. Another method of determining the bottom volume of the tank is by metering quantities of water into the tank and recording the relative heights above and below the datum plate. Floating Roof When afloat, floating roofs displace a volume of liquid equal in weight to the weight of the roof, therefore an accurate assessment of the total weight of the roof is very important. The physical measurement of the roof should be made detailing each piece of metal used in the construction, together with its exact position in the roof. Using these measurements, calculations are made which give both volume and weight. Floating roofs are also fitted with drain lines, seal tensioners, support legs and roof ladder, which must also be measured and taken into account when calculating the roof weight. All measurements and calculations should be checked against the maker's design specifications and construction drawings.

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One very important measurement is the height of the lowest part of the roof above the datum plate. This height determines the lower position of what is known as the "Critical Zone", the higher position being determined by calculation. In large floating roof tanks the roof legs can be set in a high or low position which gives rise to two "Critical Zones". This critical zone is the part of the tank where the roof is not fully supported by its legs or afloat. Measurements of oil quantities with the roof in this position are virtually impossible to make accurately unless the critical zone has been liquid calibrated. OPTICAL CALIBRATION METHODS Introduction In the past five years, there have been major procedural and technological advances in the area of tank calibration. 1. OPTICAL REFERENCE LINE METHOD The Optical Reference Line Method was originally perfected in Belgium and had much exposure in Europe before being established in the United States. It has gained acceptance and popularity here and is generally considered the standard method of tank calibration. The Optical Reference Line Method (ORLM) is simply an alternative method for determining tank diameters using an optical device. The primary difference between the ORLM and the Strapping Method is the procedure for the determination of tank diameter. The reference diameter is measured on the bottom course by manual strapping and deviations in the diameter are then measured at other predetermined horizontal and vertical stations by ORLM. The ORLM can be used both internally and externally, but is not always suitable for abnormally deformed tanks. Equipment In addition to the regular strapping equipment, the following additional equipment will be required for the ORLM method: (a) Optical device (i.e., optical plummet consisting of a theodolite and a precision level mounted on a tripod providing a 90 degree perpendicular line of vision). (b) A traversing magnetic trolley with a graduated slide to measure offsets at different vertical stations. Preparation Selection of the number of horizontal stations is made according to the tank diameter:

Number of Stations -----------------------------------------------------Tank diameter (ft) Min. Number of Stations 50 8 100 12 150 16 200 20 250 26 300 32 350 36

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The location of each horizontal station must allow the vertical traverse of the trolley to be at least 12" from any vertical weld seam. Two vertical station per course, high and low, must be established approximately 20% from the horizontal weld seam of each course. For example, as the trolley moves upwards approximately 20% of the plate width into the course, past the horizontal weld seam, a measurement reading of the scale on the trolley is taken. The trolley is then raised to within 20% of the next horizontal weld seam and a measurement is again recorded. Instrument Verification It is necessary that the optical device be leveled along the three axes at each horizontal station and that the perpendicularity of the device be verified. Verification is normally accomplished by raising the trolley/slide to the uppermost level. A note is made of the reading, the optical device is rotated through 180 degrees and the reading is again noted. The difference between the readings should not be greater than 0.005 ft.

Procedure The ORLM is currently dependent upon the reference circumference determined by manual strapping of the first ring, 20% below the top horizontal weld seam. This line of measurement, in turn, becomes the first vertical station. After positioning the optical device, the trolley is placed on the first vertical station and the reference offset is recorded. The trolley is then moved upwards vertically to each predetermined vertical station so that readings are recorded sequentially. After reading the uppermost offset, the trolley is lowered back to the first vertical station to verify that the latest reading is within 0.005' (ft) of the first reading. This verifies that there have been no physical changes in the trolley or scale. The procedures and methods are then performed in the same manner at each succeeding horizontal station. Additional measurements needed to accurately calibrate a tank are taken as described in the Tank Strapping Method, Chapter 2.2A. The ORLM is simply an alternative to manual strapping each ring. Calculation As the distance from the tank center to the vertical reference line is constant for each given horizontal station, the following is true:

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(r ′ + m ) = (r + a ) r ′ = r + (a − m )

where: r r' a m C n

r′ =

C + (a − m ) 2π

r′ =

C (a − m ) + 2π n

= Reference radius, as circumference. = Radius at given vertical station. = Reference offset. = Offset at given vertical station. = Reference circumference (Measured). = Number of Horizontal Stations.

While it is true that measurements are taken at two vertical stations per ring, which in turn determines two radii per ring, it is the mean radius which is ultimately used to determine the volume of any given course.

R= R r'1 r'2

(r1′ + r2′ ) 2

= Mean Course Radius = Course Radius at Higher Vertical Station = Course Radius at Lower Vertical Station

Once the mean course radius is known, it is easy to convert to circumference, diameter and ultimately volume. It should be noted that this method merely determines circumference, and that there are numerous additional measurements, which must be taken in order to complete the tank calibration. 2. OPTICAL TRIANGULATION METHOD France has developed and been using an Optical Triangulation Method (OTM) which uses the measurement of tank angles to determine the tank diameter. This method again provides a means for calibrating vertical cylindrical tanks using external measurement of angles with a theodolite or a laser theodolite. The method also requires a measured reference circumference (same as ORLM), determined

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by manual strapping at its bottom course. The theodolite being used must have an angular graduation and inaccuracy equal to or less than 0.022 degrees to ensure the required accuracy of measurement is achieved. Preparation As in the ORLM, the number of horizontal stations should be selected according to tank diameter. The minimum number of stations as shown in the table should be used, but the number of maximum stations is left to the choice of the person doing the work. Obviously, the more stations used the better accuracy of the calibration. Number of Horizontal Stations -------------------------------------------------------------Tank diameter (ft) Min. Number of Stations 50 4 100 6 150 8 200 10 250 13 300 15 350 18 The horizontal stations should be approximately spaced, at equal distances, along a circle concentric to the tank. The point of tangency sighting line to the tank should not be closer than 12" to any vertical weld seam. The weld seam will introduce errors in measurement because it does not react to tank movement in the same way as the tank shell moves. The number of vertical stations are the same as in the ORLM Method (two per ring) and are established at 20% of the distance from the upper and lower horizontal weld seams for each tank course. Procedure

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The base length is determined by manually strapping the bottom course circumference, 20% below the horizontal weld seam. This measurement is the reference circumference. The tank is then sighted from the first horizontal station using a theodolite. Two sightings must be made tangentially to the tank, on the left and right from each station, recording the angle subtended between the two sightings. The first vertical sighting should be made at the same height as the reference circumference was taken. This measurement will determine the reference angle. The theodolite is then angled upwards to sight at the next vertical station. In order to prevent any correction for tilt in the tank, the vertical angle for each pair of sightings should not be changed during the measurement. After the angle between each pair of sightings has been recorded for all vertical stations at the first horizontal station, the theodolite is relocated to the next predetermined horizontal station. All measurements and procedures are then repeated, beginning at the first vertical station. Calculation The distance between the vertical centerline of the tank and the vertical line of any horizontal station is constant to the height of the tank. The course radii are calculated as follows: Let T be the horizontal station site of the theodolite. The sighting T --> B and T --> B' at the exact location of the manual strapping determines the reference horizontal angle 2. Therefore:

TZ = r × TZ =

1 sinθ

C 1 × 2π sinθ

Sightings T --> A and T --> A' to any ring or vertical station give the horizontal angle 2'. Therefore:

r ′ = TZ × sin θ ′

r′ =

C sinθ ′ × 2π sinθ

The arithmetic mean of all the radii (r') for a given vertical station will determine the tank radius at that vertical station. As there will be two average radii per ring, the mean value of the two will be the average radius for that course. CAPACITY TABLES When developing gauge tables for storage tanks, the incremental volume and unit of volume must be considered.

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The unit of volume will have a wide range based on the product to be stored. For molten sulfur the unit may be 2240 lb. Tons or 1,000 kilogram Metric Tons, gasoline may be in U.S. Gallons or Liters, and Crude Oil in U.S. Barrels or Cubic Meters. The increments will also vary. Usually for storage tanks, the increment is generally 1" for the main table with a side fraction table of 1/4", 1/8", or 1/16" increments. Gauge tables may be prepared on an innage or ullage (outage) basis. On an innage basis, the quantities are expressed as increasing from the bottom datum plate to the liquid level. On an outage basis, the quantities are expressed as decreasing from the gauge reference point at the top of the tank to the liquid level. Innage gauges may be converted to outage gauges by subtracting the innage gauge from the total gauge height of the tank. RECALIBRATION OF TANKS The question is often asked when should storage tanks be recalibrated. Verification of the diameter of the bottom ring of the tank, tank course plate thickness changes and examination of tilt on a regular basis, will all suggest if recalibration is necessary. In addition, changes in the physical properties of the petroleum liquids stored can also affect the calibration. It is important to differentiate between the terms recalibration and recalculation of a tank's capacity table. Recalibration simply means that the tank diameters are re-determined through physical measurements and based on these measurements a new capacity table is developed. In contrast, recalculation involves development of a revised capacity table based on previously established tank diameters. While recalibration includes recalculation, the process of recalculation by itself does not involve new physical tank measurements. Generally, three variables affect tank volume determinations. These factors may be classified as follows: Variables affecting Measurement: - These include tank diameter, tank plate thickness, and tilt. Variables affecting Tank Structure: - These include tank deadwood, reference height, tank structure (both internal and external, including floating roof), and repairs or alterations to the tank and tank bottom. Variables affecting Operations: - These include product temperature, ambient temperature, density of the product stored in the tank, and reference height. As a general rule, changes to the Measurement Variables and the Structural Variables may dictate the need for the tank to be recalibrated, while changes to the Operating Variables may require a recalculation of the capacity table. For all of these parameters, variations greater than established acceptable limits, indicate the need for recalculation or recalibration of the tank volume. CONCLUSION We have looked briefly at the three main methods used in tank calibration. All have their merits and disadvantages. Tank Strapping is the traditional method used in this country. It is well known and the equipment is cheap. If accurately performed there can be no doubts as to its accuracy and validity. Of the disadvantages, the method is labor intensive, has some serious safety hazards, and always runs the risk of a misreading of the strapping tape. For these reasons, the strapping method is slowly disappearing, as the industry these days prefers to go with the Optical Methods.

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Optical Calibrations are the easiest methods to use. They reduce the manpower required to generally two men for the ORLM and often one man for the OTM. Optical readings are quickly made reducing the time of calibration to a minimum. Their big disadvantage is that both methods require a reference circumference be measured. Should this be determined in error then the whole tank table will be incorrect. Both the ORLM and the OTM methods have been accepted as API Standards and tank calibration companies offer both as alternative methods. Whatever the method of tank calibration is selected, it is important that it has a high degree of repeatability. When selecting which method to use consideration must be given to your company standards, the requirements of fiscal bodies and local authorities; primary, secondary or back-up measurement systems; tank throughput and cost. Questions of volume are eventually resolved most often by the accurately calibrated storage tank.

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