GUIDELINES HELP SELECT STORAGE TANK CALIBRATION METHOD

S. Sivaraman
Exxon Research & Engineering Co.
Florham Park, N.J.

Guidelines have been developed to help owners and operators of vertical, cylindrical storage tanks determine the best technology to calibrate their specific tanks, and the circumstances under which they are used.

For more than half a century, vertical, cylindrical storage tanks have been calibrated by the manual strapping method. In that method, calibrated measuring tapes are used to measure the circumference of the tank at several elevations.

But during the past decade, new technologies for tank calibration have emerged, and many of these methods are in the final stages of standardization by national and international standards organizations. These new technologies offer advantages toward improved safety, accuracy, and efficiency in the overall spectrum of tank calibration.

Guidelines are presented in this article for fixed-roof designs and for both internal and external floating-roof designs. The guidelines also take specific construction details into account.

BACKGROUND

The manual strapping method, the oldest method of tank calibration, was introduced as a standard in the early 1960's by standards organizations such as the American Petroleum Institute (API) and the Institute of Petroleum (IP).

The optical methods developed during the early 1970's are now commercially available for routine application. These new methods have attained a level of maturity comparable to, and in many instances better than, the manual method. Table 1 summarizes all available methods and the appropriate industry standards for their application.

Here is a brief overview of the different methods of calibration. It is a quick and easy reference and will help promote better understanding of the guidelines for their applications.

TECHNOLOGIES

Only those methods which are already standardized or which are currently in the process of standardization are discussed, although there are other methods such as the photogrammetric and the laser imaging technology which are also commercially available. These methods may well be considered for standardization later.

MANUAL STRAPPING

The manual strapping method involves strapping each ring of a tank with a certified tape. As the tank diameters and the tank heights increase it becomes difficult to tape the tank uniformly due to tape sag, tank indentations, and the limited length of tapes. Nonetheless, it is a popular method, and may still be a cost-effective application in many cases.'

LIQUID CALIBRATION

Liquid calibration is considered the most accurate and the most preferred method of calibration where and when possible. This method consists of metering a known quantity of liquid and gauging the tank at regular height intervals to develop a capacity table .2

It is time consuming and may not lend itself for easy and quick application. However, under certain circumstances this may be the only available option for calibration.

OPTICAL REFERENCE LINE

The optical reference line method, which can be applied internally or externally, uses an optical theodolite to establish a perpendicular ray in a vertical plane.'

With this optical ray, the deviations in the tank diameters at various course heights are measured with respect to a reference offset at the bottom course.

In addition, the reference circumference is measured at the bottom course. These deviations together with the reference offset and the reference circumference are then translated into appropriate tank diameters.

OPTICAL TRIANGULATION

Optical triangulation can be applied internally or externally.4 The principle of this method is that a tank profile can be defined by triangulation with two theodolites and a target point on the tank, or with one theodolite and two target points on the tank at a number of stations around the tank.

For external application, depending on the tank height, a minimum space around the tank will be required to apply this method. This method, already in use in some countries, is in the final stages of standardization within the International Standardization Organization (ISO).

ELECTRO OPTICAL DISTANCE RANGING

Electro-optical distance ranging currently under development towards standardization within ISO, is primarily intended for internal calibration. With 'a single optical laser, and a distance ranging device and on-line computer, circumferential target points on the tank shell wall, for any given course, are mathematically and statistically analyzed almost instantaneously to give the required course diameter. The calibration can be accomplished from the ground level by one person. This method is presently in use in Europe.

SELECTION FACTORS

The type of calibration method that one may select may often be dictated by a number of factors. These factors are broadly grouped into the following:

Type of tank: floating or fixed roof

Operational constraints: entry or no entry
Insulation or no insulation
Riveted or welded
Other parameters such as number/size of wind girders.

Selection of any specific method for each of the above factors is presented in the form of decision charts (Figs. 1, 2, and 3). In the development of these guidelines, it is assumed that the insulation requirements pertain only to the fixed roof tanks.

Also, the term entry, as applied to the floating roof tanks, refers to access onto the top of the floating roof with the roof resting on its legs. The bottom calibration requirements are not considered separately, because they would belong to the same category where entry is required or permitted.

For each category, technology selections are presented in a prioritized order. The priority recommended is based upon the most expedient way of calibration for a given set of conditions, ensuring overall accuracy.

However, the recommended priority is not intended necessarily to optimize the overall cost of calibration. The cost factor associated with any given method is dependent on many factors.

CALIBRATION COSTS

Generally, the on-site cost of calibration of one tank may be expensive compared to the conventional manual method when new technologies are considered. However, with routine application and with the support and encouragement of the petrochemical and refining industries, the new technologies can be cost effective in the long run. In general, the following factors can be expected to affect the cost of calibration:

Number of tanks available at any given time

Size of tanks: diameter and height

Cost of auxiliary and support services necessary such as power supply, scaffolding (if required)

Cost of stripping and reinstallation of sections of insulation where and when required

Ultrasonic thickness measurements, if required, as part of calibration

Cost of bottom surveys where and when specified

Availability of calibration contractor locally or on-site proximity to minimize travel costs

Calibration work done on a straight-time basis or on overtime basis.

In addition, there are indirect costs involved. These include such costs as the down time, cost of gas freeing and preparing a tank for safe entry, removal of insulation across the whole tank, and its reinstallation where needed.

The direct cost of calibration is dependent on the type of method and the factors detailed above, while the indirect costs are site dependent. In the selection of any specific method, both the direct and indirect costs may have to be considered in some cases. In most instances, however, only the direct cost may be the deciding factor. The indirect costs may often be treated as an integral part of the plant's operating or maintenance expenses.

It is appropriate to point out that the cost of calibration of tanks often becomes an issue for intense debate and lengthy discussions. It is important to relate the cost to the frequency of calibration.

Considering the fact that tanks undergo recalibration once every 10-15 years, the cost of calibration, when prorated over this period, will indeed prove to be small if not insignificant. While the cost factor is no doubt important, it may not necessarily be the most decisive factor in the selection of any one method. Other advantages a specific technology has to offer, such as safety, accuracy expediency, and efficiency, should also be considered in arriving at a final decision in the overall technology selection and its application.

New technologies using optical devices provide quicker, safer methods with accuracy comparable to, and most likely better than, the manual method. The tank owner now has a much wider choice and flexibility in the selection of any method.

Application of guidelines presented here will facilitate tank owners in selecting the most desirable if not the optimal technology for application.

Using the decision charts, once a set of options is chosen, a calibration contractor may be requested to bid for each of the options. Such a bid should be based on a sound and a well-documented technical specification. Depending on specific site conditions and local requirements, any one method could prove to be cost effective.

Finally, it is hoped that by assisting in the selection of the right technology, the guidelines will promote technology application in its entirety, and avoid combination of different methods that could lead to erroneous applications and compromise on recommended procedures. The guidelines developed here are based on the experience of the author and most certainly do not reflect the views of individual companies or standards organizations. The final decision toward selection of any one method may eventually be dictated by internal policies, local regulations, and budget constraints.