TANK STORAGE •
CALIBRATION
November/December 2009
Inaccurate capacity tables can negate even the most accurate gauging equipment and cost the terminal thousands of dollars every month
Why calibrate a storage tank?
L
arge storage tanks are not perfect cylinders. There are many variables that change the shape during construction and as the tank ages. For example heat input during welding can create distortions at the weld seams and tank settlement can cause various changes to the shape of the bottom shell and can tilt the tank in certain circumstances. Tank strapping or calibration is the process of gathering accurate measurements for the purpose of producing tank capacity tables. The tables express the volume measurement for a specific product height in the storage tank. These are tied to the strike point and serve as the reference for all elevations within the tables. The strike point is positioned on the tank bottom plates below the gauge pole or under a roof mounted xxx
gauge point. It is also tied to the floating roof and serves to establish when the roof is fully floating in product. Volumetric strapping and the capacity tables derived from these measurements are the most economical and accurate way of determining the true volume of a containment system in relation to the liquid level. The only way that any gauging system can accurately measure the amount of product in a containment system is to calibrate that system with accurate capacity tables. Inaccurate or out of date tables may negate even the most accurate tank level gauging systems. With the today’s high commodity prices the potential costs of out of date or inaccurate capacity tables can add up to significant loses. A small error in
the diameter of the tank can add up a significant amount of money every month, especially if the tank is frequently cycled.
Measurements Strapping table measurements not only include accurate circumference and height measurements but also all internal deadwood (internal piping and appurtenances), shape of bottom plates, displacement of floating roof if applicable and tilt measurements. The maximum fill height should also be recorded as a maximum limit to run the tables. It is not recommended to run strapping tables above the roof angle of a fixed roof tank or above the overflows of an internal floating roof tank. Tables with run heights above tank design heights can encourage
overfills and potential tank damage. Strapping table run heights should reflect the tank liquid design heights. In addition to external measurements, it is necessary to measure and record the size and position within the tank of all internal piping / piping piping supp supports, orts, floor patch plates, heating coils, floating roof drains, etc. These measurements also include tank appurtenances that increase the volume of the tank such as manways and nozzles which protrude outside of the tank shell. These measurements are referred to as deadwood as they reflect material in the tank which subtracts volume and add space outside the tank shell thereby increasing the total volume. For tanks fitted with floating roofs an accurate roof weight measurement is required, because a roof
November/December 2009 •
TANK STORAGE
that is fully floating displaces an amount of product equal to the weight of the roof. Accurate roof weight measurements combined with a survey (steel roofs) and dimension measurements provide an accurate volume and displacement weight of the roof. On floating roof tanks, calibration tables usually contain a highlighted portion referred to as the critical zone. This area represents the vertical section where the roof is not fully landed and not fully floating. It is important to know that calibration tables may not be accurate while the roof is in the critical zone. Inaccuracies in the critical measurements such as tank diameter (reference circumference or floating roof displacement) can cause large errors in the tables. Critical errors such as these can be responsible for losses of thousands of dollars per transaction. The costs related to tank strapping are minimal when compared to the potential costs of inaccurate tables. Strapping measurements are performed as a repeatable procedure in order to provide as accurate as possible results. The customer decides on the presentation of data for their tank calibration table. These tables can run data including variable such as innage or ulage (also known as outage). The customer also dictates the units for calculating the tables, density of product on the tables and the reference temperature of the product.
tank measurements, settlement and tilt 2. Variables affecting tank structures, changes to gauge height, alterations or repairs which change the deadwood inside the tank or change the shape of the tank
When to perform strapping: External measurements can be obtained while the tank is in-service provided that the product height, temperature and density are taken into account. Internal measurements are not possible while the tank is in-service. In these instances, a less accurate table may be calculated based on as-built drawings. The ideal time to perform strapping measurements is in conjunction with an out of service inspection after any repairs have been performed. Three variables generally affect the volume of storage tanks: 1. Variables affecting
CALIBRATION dramatic effect on the volume calculations in a storage tank. Codes and standards applicable to strapping include MPMS Chapter 2 (API Manual of Petroleum Measurement Standards) and ISO standards. It is important to note that all standards require
Costs related to tank strapping are minimal when compared to the potential costs of inaccurate tables 3. Variables affecting tank operations, changes in product density, temperature. These changes may only require a table recalculation rather than complete re-strapping. New strapping measurements and tables should be developed when: • new construction (post hydro test) • after any structural modifications • in conjunction with out of service API 653 inspection (post repair) • every 15 years as per MPMS Chapter 2. Existing tables should be recalculated when there is a: • change in service • change in roof configuration • change in gauge height • change in reference temperature. Storage tanks may change shape as they age, particularly tanks which have new or improperly prepared foundations. Foundations which are subject to settlement either because of the soil conditions or an improperly compacted foundation tend to settle into the soil. This changes the shape of not only the bottom plates but the shell as well. Severely settled tanks often have buckles and distortions in the shell plates which change the shape of the tank. This is important for volume measurements and floating roof seal performance. Edge or differential settlement also affects the amount of product stored above or below the strike point and can affect the plumb of the tank. All of these conditions can have a
that strapping measurements on new tanks be performed after the hydro test.
Methods: Manual strapping: This method involves setting a steel tape on each shell course usually at 20 and 80% level on each shell course. This method is rarely done anymore due to the difficulty of accessing the upper shell courses. Optical reference line method: This uses the same steel tape as the manual strapping method to determine the reference circumference on first course shell. Ideally that level is at 80% up the shell plates. An optical plummet is then used to relate the strap on the first course with the upper courses. Optical triangulation method: This uses the manual strapping method to determine the reference circumference of the first course shell plate, also ideally at 80% up the shell plates. This reference circumference is combined with triangulation measurements from a theodolite or total station for the upper shell courses. Each of these methods has advantages and disadvantages but the most popular methods are now the optical reference line method and the optical triangulation method.
Future outlook The future of volumetric strapping measurements may well lie in high definition surveying techniques using a laser scan system. A reference circumference can be used to compare to the scanned information. The
scanning system replaces the shell, bottom surveys and can even gather deadwood information. These systems have several advantages over traditional methods such as: 1. Accuracy – laser scan systems are the most accurate method to gather information about the true dimensions of a storage tank. A laser scan system gathers thousands of data points on every scanned surface of the tank. Traditional methods gather information at predetermined points. For example: a 150’ (45.72m) tank would have 16 stations around the tank where the shell would be surveyed and the bottom survey would be performed. This is roughly every 29.4 feet (8.96m), which means there is 29.4 feet (8.96m) between each station where information is not gathered.) 2. Speed – laser scan systems gather millions of points in a matter of minutes. 3. A 3D image of the tank is created which contains all of the dimensional data required for strapping tables. This image can be compared to later scan images in future tank service outages to provide a comparison of the condition of the tank. These scanned images also serve as a functional record for everything present in the tank at the time of strapping and the dimensions of the tank itself and everything inside can be pulled from these images at anytime. Strapping tables will become more and more accurate as laser scanning technology improves and software is developed specifically for tank volume tables. This is mutually beneficial to tank owners and companies providing strapping tables. Whatever the method or purpose of strapping storage tanks, it is important that it is performed with a high degree of repeatability and accuracy. With general upward pressure on commodity prices it has never been so important to obtain accurate tables to track product volumes in tanks. n For more information: This article was written by Michael Pfister from Streamline Inspection, mpfi
[email protected]
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