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| What You Need to Know to Have a Tank Calibrated |  |
Many people who are in the position of having to have a tank calibrated have little knowledge of what is involved in the calibration. Some may not even know why the tank needs calibrating at all.
To help you assess your requirements we have prepared this general outline of tank calibration.
Why do tanks need calibration at all? [top]
If you do not need to know what is in, or what has gone out of, a tank and if spillage or overfilling is of no concern, then you probably do not need to have a tank calibrated. If you require any indication of quantity then you need a calibration of some sort.At its simplest, a calibration of any vessel involves a knowledge of the dimensions of the vessel and a calculation of the total volume. For a water tank this may be enough and you may not even need to call in a specialist.
At the other end of the scale, a tank can be measured precisely and calculations can be made to take into account factors such as hydrostatic expansion of the tank during filling and thermal expansion of the tank shell in service.
For most situations in Australia where you are selling from a tank or where customs duties are levied on tank contents, you will need to have a tank calibrated by an organisation certified to do so by The National Organisation of Testing Authorities or by your local State Government Trade Measurement Department.
For the purposes of this exercise we shall assume that you have decided that you do require a precise calibration of your tank.
What are the options in calibrating a tank? [top]
For any tank, whether it is a stationary tank or on a road, rail or marine tanker, there are two broad options. It can either be calibrated volumetrically, i.e. by measuring in volumes of liquid, or by physical measurement and calculation of the volume. In some cases it is clear which method is appropriate and in others either method can be used.Generally, volumetric methods are suited to small tanks or tanks that are irregular in form where mathematical models cannot be applied. Volumetric methods include both metering volumes into or out of the vessel to be calibrated and addition of volumes of water from a certified prover.
Road tankers are generally always calibrated by volumetric methods, rail-cars can be done either way, whilst large vertical tanks are normally calibrated by physical measurement.
Food products, brewery and wine vats can be done either way or sometimes by a combination of the methods.
We will consider volumetric and physical methods of calibration separately.
Calibration of Tanks for Petroleum and Petroleum Products to ISO, IP and API Standards.
The above organisations have a range of Standards to cover the calibration of vertical, horizontal and spherical fixed storage tanks as well as rail tankers and barge tanks.
The available Standards are as follows: ISO 7507 - 1 Vertical Tanks Strapping method ISO 7507 - 2 Vertical Tanks Optical Reference Line Method ISO 7507 - 3 Vertical Tanks Optical Triangulation Method
IP 202 Part II Section 1 Vertical Tanks Measurement Method IP 202 Part II Section 2 Horizontal & Inclined Tanks Measurement Method IP 202 Part II Section 3 Liquid Calibration Method IP 202 Part II Section 4 Spheres & Spheroids Measurement & Volumetric IP 202 Part II Section 5 Ship & Barge Tanks IP 202 Part II Section 6 Road & Rail Vehicles
API 2550 Upright Cylindrical Tanks Measurement Method API 2551 Horizontal Tanks Measurement Method API 2552 Spheres & Spheroids Measurement Method API 2553 Barges Measurement Method API 2554 Tank Cars Measurement Method API 2555 Liquid Calibration of Tanks
Vertical Tanks: [top]
Although there are differences in the application of some of the above Standards, there is a lot of duplication. Even where there are differences in the measurement or calibration methods, the difference to the end user is often less than any measurement errors involved, either in the calibration or the measurement of the product.In Australia, being a nominally metric country, it makes sense to use the ISO Standard for upright cylindrical tanks. ISO 7505 is almost a re-write of IP 202 Part II Section 1. The only real difference is the calculation that involves temperature correction during tank calibration. The ISO and IP Methods score over the API method in that, for various reasons, they are more directly applicable to computerised calculation.
There are two methods in common use for the physical (measurement) calibration of the shell of vertical tanks.
The original method is by strapping. Strapping is the process of measuring a tank by physically measuring the tank strakes with a calibrated steel tape at several levels on each course of plating. Strapping is a good method for small tanks that have scaffolding.
The other method often employed in Australia is the Optical Reference Line Method. This method involves strapping the tank just once at an accessible level, called the reference level. In this method, an "optical plummet", a device used to establish a true perpendicular, is used to measure the radial offset at several levels on each course of plate. This procedure is repeated at approximately four metre intervals around the tank and the circumference at each level is calculated from the sum of the differences between the reference level and the measured level.
The optical reference line method has a lot of advantages over strapping. It also has a couple of disadvantages.
The advantages of the optical method are:
1. Optical Measurement is safer than strapping. There is no need to access the outside of the tank, which is normally done in a bosuns chair. The optical plummet operator sits at the base of the tank whilst an assistant manoeuvres a magnetic scale trolley on the tank shell from behind the hand rail on top of the tank.
2. Optical calibration is faster than strapping in most cases, with less impact on other operations in the area. A typical 20 metre diameter tank could have the shell measured in only a few hours.
3. There is a better traceability of the field data from the calibration. The data from which the tank is calibrated is recorded for later calculation and verification, as are calibration records of the optical plummet. With strapping there is the problem of the tape sagging during measurement and the real difficulty of repeatable measurements whilst the operator is suspended off the side of the tank, trying to maintain 50 Newtons of tension on the strapping tape.
4. As a side benefit of the optical method, the data from which the circumferences are calculated will also yield information on the shape of the tank shell. It is a simple matter to process the data to determine the verticality of the tank shell and to determine the "roundness" of the tank. This is essential information to the engineer who wants to fit floating blankets or roofs to vertical tanks. It is also information that can be useful in determination of any settlement or subsidence of the tank in service.
5. It is simple to optically calibrate a lagged tank. After measuring tank diameters internally, the plummet can be used inside the shell.
6. It is probably possible to calibrate a tank in stronger wind conditions than is safe for strapping. (This may depend on how brave the tank strapper is!)
7. There are some disadvantages of the optical method: 1. The equipment used is more expensive than that required for strapping. Optical plummets cost thousands of $'s and are delicate instruments. Even with optical equipment, it is still necessary to use strapping tapes for the reference measurement.
8. It is difficult to use the optical plummet in wet weather, as rain on the lenses makes vision difficult.
9. The existence of scaffolding can prevent the effective use of the magnetic scale used for optical measurements. Strapping is often indicated in this case.
10. Calculation of the tank volume tables is more complex, although with computerised calculation routines, this is no longer a problem.
Calibration of tank bottom volumes: [top]
There are two methods that can be used to calibrate the volume below the dip-plate in a vertical tank:1. The tank floor profile can be surveyed physically, using an engineers level or theodolite, and the volume calculated mathematically.
2. The tank floor can be calibrated volumetrically, using a meter or volumetric prover and water.
As before, each method has advantages and disadvantages:
The advantages of the physical survey are: [top]
1. Physical survey is faster, cleaner and does not require the availability (and following calibration, the removal) of large quantities of water. A typical physical survey on a 20 metre diameter tank may take only a couple of hours. To fill a 20 metre tank with a bottom volume of 30 m3 may take two or three hours to fill and most of the day to empty and clean. A tank of 35 metres diameter and a bottom volume of 100 m3 may take five to six hours to fill and a day to empty and clean. Physical survey would still take only a couple of hours.2. During a physical survey it is simple to measure the precise relative height of all tank internal structures. These heights can be related to the dip-plate to give the tank operators accurate figures for when tank structures are covered with product.
3. The data obtained during the physical survey can be processed to give the tank owners data on floor profile, tank tilt and settlement. This can be invaluable for comparison with previous or subsequent data to determine tank shell settlement and stresses.
4. Physical survey data from the bottom survey can be related to external measurements during tank hydrostatic testing to determine ground loading response.
5. Physical survey is less intrusive than metered water methods. The tank is immediately available for use after the calibration and unlike the volumetric method, does not require any cleaning after removal of water.
The advantages of volumetric calibration of tank bottoms are: [top]
1. For extremely irregular tank bottoms, or for tanks with extreme tilts, a volumetric calibration can sometimes be more accurate. The accuracy of a physical survey can be improved by use of more measurement points, but if extreme accuracy is required, a volumetric calibration may be the best option.2. If the tank is not safe to enter, it is sometimes possible to calibrate volumetrically if the dip-plate is visible from the manhole, or even if it is not, by dipping the tank whilst filling.
It is sometimes thought that a volumetric calibration is more accurate that a physical survey because of tank floor sponginess or spring. Our experience and information that we have from Europe, is that a water column of 2 metres is required to eliminate 90% of tank floor spring. In reality, it is not practical to fill any but the smallest of tanks to this level and floor spring is not a major problem in small tanks.
We have performed some practical research on a number of 30 meter diameter tanks, calibrating by both meter and physical survey. We have found the results to be within 1% in all the tanks concerned, which amounted to less than 1000 litres.
We have found that it is difficult to determine levels in a tank to within 3 mm during a water bottom calculation, due to meniscus/ surface tension effects on the dip-plate and to splashing due to the operator being in the tank to observe the filling process. The previously mentioned 100 litres equates to 1.4 mm in a 30 metre diameter tank.
In some circumstances it can be unsafe to work in a tank during water bottom calibration, due to gases from contaminated water. In these circumstances it is not possible to obtain relative height measurements from the water surface to tank structures.
Calculations of tank capacities for vertical tanks: [top]
Tank capacity tables are calculated from tank circumferences. The following factors are taken into consideration during calculation:1. Tape rises during strapping (i.e. welds or laps, manhole doublers and other obstructions.)
2. Shell temperature and tape calibration temperature during strapping.
3. Physical characteristics of the tank shell material (Young's modulus, thermal expansion factors etc.)
4. Difference in temperature between calibration and working conditions.
5. Paint and plate thickness, inside and outside the tank.
6. Tank expansion during calibration (if the tank is not empty).
7. Tank expansion from liquid head pressure in service/ density of product stored.
8. Tank tilt.
9. Effects of positive and negative deadwood on tank volumes.
What conditions are required for a vertical tank calibration? [top]
Most vertical tanks can be calibrated in one working day if done by a physical measurement. There are some other considerations:1. If the tank is new, it should have a full hydrostatic test prior to the calibration.
2. The tank shell coating should be cured, as a soft coating may be damaged by the strapping tape or the wheels of the magnetic trolley. (Some trolleys have narrow alloy wheels that will even damage a cured coating. Our trolleys have soft plastic wheels to minimize the problem).
3. Ideally there should be no other work being performed on the tank, as welding, grinding, hammering and sandblasting all make the shell vibrate, which makes optical measurements difficult. However, we all exist in the real world and if the time window is tight (and it always is) we can usually work with or around other operators.
4. The tank should be clean and gas free for entry. Most terminals have their own requirements for confined space entry and issue their own work permits. We have our own atmospheric monitoring equipment and we have training in gas free inspection. We will issue our own permits if there is no satisfactory system in operation. If the tank is not clean for entry, there are alternative options for bottom calibration as previously mentioned.
When should a vertical tank be re-calibrated? [top]
A tank may change its calibration whenever the operating conditions are changed. This could be for a variety of reasons. For example:1. Change of product density - this will effect the expansion characteristics of the tank, hence the volume.
2. Change of operating temperature - most tanks are calibrated for a shell temperature of 15 degrees. A large change of operating temperature will alter the tank increment (i.e. its ltrs/ mm.)
3. Any modifications to the tank or the dip plate - i.e. new pipe inverts, fitting a stilling well, floor repairs, floating roof modifications, changes in deadwood, etc.
4. Settlement of the tank with resultant changes in the tank geometry.
It is generally recommended that a tank should be re-calibrated at not more than 10 year intervals, especially considering tank settlement. In areas where tank settlement or ground subsidence is known to be a problem, an external settlement survey can be performed without tank entry, to marked positions on the external protrusion of the annular plate. These heights can be referenced to a datum level in the terminal and also to the internal tank calibration data.
The calibration of horizontal tanks: [top]
Horizontal tanks can be calibrated to either the API or the IP Standards. There is little difference in the end result. The calculation routines for both Standards have been around for a long time and neither one takes advantage of the ability of computers to perform laborious calculations. The API Standard in particular, makes extensive use of tables for various corrections, rather than providing formulae. We use a method that follows the IP method, but exceeds the precision of the method in that we can process greater degrees of tank tilt and greater ranges of head shape with much better accuracy. The Standards make several assumptions on the volumes of tank heads, where we are able to calculate volumes precisely.Horizontal tanks can be calibrated either volumetrically or by physical measurement.
What is involved in the volumetric calibration of tanks: [top]
Most commonly, volumetric calibration is performed by adding liquid, usually water, to a vessel in small volumes. The liquid can either be metered in through a calibrated "Master Meter" or dropped from a calibrated volumetric prover. After the addition of each new volume, the level is measured and recorded. Any corrections required for temperature or meter factor may be applied to the levels and a table or dipstick is manufactured for the tank.No corrections are applied for tank expansion, as this occurs during the filling process. There is a potential problem here for pressurised tanks or tanks holding products having densities different to the measuring liquid. If the tank service conditions for pressure, either hydrostatic or otherwise, are different from the calibrating conditions, there will be a volume error.
The problem of similar conditions for calibration and use also applies to tanks that may be installed on different angles from which they were calibrated. If the tank inclination is different from the calibrated condition, there may be errors in the volume.
Volumetric calibration is ideal for tanks which require dipsticks, as there is no need to apply other corrections, and a dipstick can be made directly from the recorded levels.
The process does require a volume of water equal to the tanks capacity to be available during calibration and requires that the vessel be clean enough to allow the water to be disposed of after calibration.
It can also be quite a time consuming process for a large tank and it may interfere with other work that is being done in the area.
What is involved in the physical calibration of horizontal tanks: [top]
Horizontal tanks are generally manufactured in a workshop and transported to site. With physical measurement methods tanks can be calibrated either in the workshop or after installation, as long as the tank if installed on the designed inclination.Horizontal tanks can be calibrated by either internal or external measurement.
External measurement is suited to tanks that are situated above ground, with clear access to the tank shell heads. Tanks do not need to be empty or clean for external calibration, although it is not generally possible to measure tank deadwood on tanks that are in service. The tank dimensions are determined by a series of circumferences taken along the tank barrel and by measuring the profile of the heads. We determine the head profile by erecting a reference plane across the end of the tank and measuring a series of points across the head. A problem with external measurements on tanks in service is that it is difficult to establish the height of the dip-plate/ striker plate. This is not an insurmountable problem, but it is the source of calibration errors on horizontal tanks.
Internal measurement is our preferred method, as it enables more precise measurements to be taken and less assumptions made on the tank construction. A series of internal diameters are measured with an internal tank gauge (a device similar to a large internal micrometer), barrel lengths are measured and the head profile measured with a purpose built head measuring gauge.
Deadwood can be measured precisely, as can the heights of the dip-plate etc. Obviously for internal measurement, the tank must be clean and gas free for entry.
For either internal or external measurement, the reference height must be measured and if the tank is pressurised, special measurements for tank gauging systems, slip tubes etc., must be taken.
Calculation of tank capacities for horizontal tanks: [top]
Tank capacity tables are calculated from tank diameters, barrel length and head profile data. The following factors are taken into consideration during calculation.1. Tape rises during strapping (i.e. welds or laps and other obstructions.)
2. Shell temperature and tape calibration temperature during strapping.
3. Difference in temperature between calibration and working conditions.
4. Paint and plate thickness, inside and out of tank.
5. Tank expansion during calibration (if tank is not empty).
6. Tank expansion from pressure in service.
7. Tank tilt - this is critical in horizontal tank calibration.
8. Location of dip tube position with regard to barrel.
9. Effects of positive and negative deadwood on tank volumes.
Notes: [top]
1. There are no "average values" for millimetres of product on tank tables for horizontal tanks. Obviously the ltrs/mm value changes for every level in the tank.2. In the case of pressurised tanks, tables can be issued for different working pressures or factors can be given for pressures different from the usual working pressure.
What you should look for in a tank calibrator: [top]
Whilst the mathematics of tank calibration are relatively straightforward, to have a thorough understanding of all the principals and factors involved requires some study.We believe that the person who measures your tank requires a thorough knowledge of the Standards, the measurement procedures and the calculations involved to ensure that the correct measurements are taken.
NATA is the organisation responsible in Australia for regulating the Standards of metrology laboratories. Any tank calibration company accredited to NATA is audited annually to ensure that their quality system and equipment is maintained to the required standards and that the companies personnel are competent to perform the work.
What tends to happen in Australia is that a company may hold NATA registration in one location and use any staff available in another location to perform field measurements. This does not automatically mean that the work performed is shoddy or inferior, but it does mean that there is less control over the training of staff, the calibration of equipment and the accuracy of data.
The calibration company that you choose should also be able to demonstrate a knowledge of safe practices in your terminal. In particular, they should be trained in confined space entry procedures and should have the necessary equipment to work safely in confined spaces and at heights.
The calibration company should be able to offer prompt service. To calculate an average tanks calibration table may take eight hours, but there is no reason why you should not be able to request a faxed calibration table within 24 hours of the field operator leaving your terminal. You should also be able to expect to receive the finished table within a week.
Obviously, we hope that you will come to us. We believe strongly that tank calibration is an important part of terminal stock control and we treat it accordingly.
Liquid Measure Australia is a division of a global network, Intertek Testing Services - Caleb Brett.
Liquid Measure Australia , is committed to providing all aspects of tank calibration. We will not send out inexperienced or unqualified operators to measure tanks.
If you choose to use Liquid Measure Australia for your tank calibration, the field operator that you see calibrating your tanks in your terminal will be able to discuss all aspects of the calculation of your tables and answer all your questions.
