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TANK CALIBRATION METHODOLOGY :
 
METHODS FOR CALIBRATION OF VERTICAL STORAGE TANKS GENERAL SECTION
 
A. SCOPE:
This part prescribes the methods for calibration of vertical storage tanks with or without a tilt used for bulk storage of liquids of atmospheric pressure or under low or high pressure with or without heating or cooling mounted aboveground or underground or on ships or barges either by strapping method (SM) by internal ultrasonic distance ranging method (IUDRM) or by internal electro – optical distance ranging method (IEODRM).
B. DEFINATIONS:
(1) Storage Tank (thereafter referred to as ‘tank’) – Specified modes of the inclination including a vat exceeding on kilolitre in capacity used for bulk storage of liquids or liquefied gases at atmospheric pressure or under low or high pressure with or without heading or cooling mounted aboveground or underground or on ships or barges together with its necessary supports, manholes, piping valves, gauges, meters etc, which shall be calibrated as a capacity measure.
(2) Vertical Tank – A tank in the shape of a right circular or elliptical cylinder or of a frustum of a cone or of a rectangular parallelepiped, the axis of geometric symmetry of which is vertical to the base of mounting with or without a tilt.
(3) Floating Roof Tank – A tank in which the roof floats freely on the surface of the liquid contents except at low levels when the mass of the roof is taken through its supports by the tank bottom.
(4) Datum Plate – A horizontal metal plate located along the vertical axis descending from the dipping reference point, providing a fixed contact surface from which liquid dip measurements are made.
(5) Dip Hatch – An opening in the top of a tank through which dipping, ullaging or sampling operations are carried out.
(6) DIP Pipe – A perforated metal pipe fitted below the dip hatch which projects downwards, ending near the bottom of the tank, directly above the dipping datum point and acts as a guide for the dip weight, particularly, when obstructions have to be avoided and it is obligatory in the case of a floating roof tank.
(7) Dipping Datum Plate – A point of intersection of the vertical measurement axis with the upper surface of the datum plate which constitutes the origin or zero reference for the measurement of liquid dips.
(8) Dipping Reference Point – A point clearly marked on the dip hatch located along the vertical axis ascending from the dipping datum point to indicate the upper reference position to which ullage is measured.
(9) Overall Height – The vertical distance between the dipping reference point and the dipping datum point which shall be marked on the tank at the dip hatch.
(10) Dip – The vertical distance between the dipping datum point and the liquid level.
Note: the term ‘innage’ is synonymous.
(11) Ullage – The vertical distance between the liquid level and dipping reference point.
Note: the term ‘outage’ is synonymous.
(12) Equivalent Dip – A dip corresponding to a given ullage which is obtained by subtracting the observed ullage from the overall height.
(13) Course – One circumferential ring of plates in a tank.
(14) Step Over – A device of metallic or wooden frame holding two scribing points used in strapping for measuring the distance apart along the arc of two points on a tank shell where it is not possible to use a steel tape directly because of an intervening obstruction (e.g. a protruding fitting or an intruding dent). The difference between the apparent distance between two points on a tank shall as measured by a strapping tape passing over on obstruction and the true arc distance as measured by a step-over in termed as a ‘void’. If the apparent distance is greater than the true arc distance, the void is said to be ‘negative’. The corrected circumference is obtained by subtracting the algebraic sum of the voids from the measured circumference.
(15) Deadwood – Any tank fitting referred to as ‘positive’ when the capacity of the fitting adds to the effective capacity of the tank, or ‘negative’ when the capacity of the fitting subtracts from the effective capacity.
(16) Automatic Level Gauge – An instrument using mechanical and/or electronic devices intended to measure automatically the level of the liquid contained in a tank with respect to a fixed reference point.
(17) Open Capacity – The calculated capacity of a tank or part of a tank before any allowance has been made for deadwood.
(18) Capacity Table – A tabular representation often referred to as a tank table or a calibration table, showing the capacities of, or volumes in, a tank corresponding to various liquid levels measured from a stable reference point.
 
RECOMENDED RECORD FORM FOR MEASUREMENTS OF VERTICAL TANKS
CLAUSE 5(1)
 
A.
General Data …………………………………………………………………..
Report No. …………………..
Date …………………………..
 
1. Tank No.: …………………... 10. Angle or Tilt from Vertical: …………………………
2. Type of Tank Joints: Riveted/Lap-welded/Bun-welded 11. Name of liquid intended to be contained: ………………
3. Nominal Tank Capacity: …………………… litres 12. Temperature of Liquid required to be maintained in the tank if it is thermally insulated: …………… °c
4. Type of Roof: Fixed/Floating/Hybrid/Variable Volume 13. Density of liquid at the ambient or maintained temperature: …………………. Kg/m3
5. Mass of Floating Roof: ………………… kg 14. Average ambient temperature during calibration: ………………. °c
6. Type of Bottom: Flat/Cone-up/Cone-down/with or without knuckle radius/Spherical segment/Hemispherical/ Semi-ellipsoidal segment 15. Whether equipped with automatic level gauge: Yes/No
7. Height of Depth of Crown: …………………. mm 16. Whether equipped with computerised liquid stock accounting systems: Yes/No
8. Overall Height: ……………………… mm  
9. Height of Datum Plate: …………………….. mm  
 
B.
Shell Circumferences or Diameters:
 
  • 1st Course: …….. mm
  • 2nd Course: …..... mm
  • 3rd Course: …….. mm
  • 4th Course: …….. mm
  • 5th Course: …….. mm
  • 6th Course: …….. mm
No.       Description Elevation. Top of Floor to Bottom of Connection mm
1    
2    
3    
 
C.
Bottom Course Shell Connections:
 
Course No.   Shell Plate Thickness (mm) Width of Lap of Strap (mm) Thickness of Strap (mm) No. of Vertical  Joints Exposed Course Height (mm) Inside Course Height (mm)
nth            
2nd            
1st            
 
D.
Deadwood And Remarks (Use Separate Sheets If Necessary):
 
No. Description Size
Elevation
From (mm) To (mm)
1.        
2.        
3.        
 
1. CONDITIONS FOR MEASUREMENTS:
(1) All tanks shall be calibrated in an empty condition and in gas-free state. All regulation covering entry into hazardous areas and explosive atmospheres shall be rigorously observed.
(2) All ladders shall be securely lashed in position before being used. When painters’ cradles or bo’suns’ chairs are used, any item of questionable calibration cannot be carried out without the use of staging, properly constructed steel tube or timbre scaffolding shall be worn by the operating personnel working above ground level.
(3) Measurement shall be taken only after the tank has been filled at least once at its present location with liquid to be stored to its working capacity or with water to its equivalent height, and such liquid or water has been held in the tank for at least 24 hours to allow for setting.
(4) All data and methods , whereby measurements are obtained, necessary for the preparation of tank capacity tables, shall be in accordance with sound engineering principles.
(5) When manufacturer’s drawings for the tank are available, all measurements shall be compared with those obtained from the drawings and measurements showing discrepancies grater than the tolerance specified in clause 7.
(6) In the case of tank mounted on ships or barges, all dip measurements shall be taken in a plane perpendicular to the even keel water line over minimum surface ripples.
(7) The strapping method shall be applied only to a tank in the shape of a cylinder or a conical frustum. The tanks of all other shapes shall be calibrated by any suitable internal or external measurement methods.

4. INTERRUPTED MEASUREMENTS
If the calibration of a tank is required to be interrupted, it may be resumed with minimum delay, without repetition of work previously completed provided that:
(a) There is no major change in equipment and as far as possible, no change in personnel
(b) All records of work done are complete and legible; and
(c) Some hydrostatic head as before its maintained in the tank

5. DESCRIPTIVE DATA
(1) Complete descriptive data shall be entered on the Tank Measurement Record Form being used. A schematic diagram of a typical fixed roof cylindrical tank of circular cross section is given in Fig. 55 and a recommended record form is shown in Table 50.
(2) Supplemental sketches or notations each completed, identified dated and signed, shall form an important part of field data. These shall be made to indicate typical horizontal and vertical joints, number of plates per course, locations of courses at which thickness of plates changes, arrangement and sizes of pipes and manholes, dents and bulges in shell plates, direction of lean from vertical, method used in bypassing a large obstruction, such as clean-out box or insulation box located in the path of a circumferential measurement, location of tape path, location and elevation of possible datum plate and all other items of interest and value which will be encountered.
(3) All measurements made by the calibration authority shall be recorded on site and shall not be subjected to subsequent correction.

6. DEGREE OF ACCURACY
In order to obtain maximum obtainable accuracy in tank capacity tables, adjustments for effects of the following variables shall be incorporated in the tables:-
(a) Expansion and contraction of steel tank shell due to liquid heads
(b) Tilt from upright position, and
(c) Tank bottoms that are irregular in shape
Note: The degree of accuracy desired or required in the completed tank capacity table for a specific tank shall be the governing factor in determining the procedure to be followed.

7. EXPANSION AND CONTRACTION OF STEEL TANK SHELLS DUE TO LIQUID HEAD AND TEMPERATURE
As the tank shells expand under liquid head contained therein, a liquid head correction shall be applied to the tank capacity table during normal service. The tanks are also affected by thermal changes, as are any measuring tapes used, such as strapping and dip tapes which are calibrated to be correct at the appropriate reference temperature, e.g. 20°c. if the tank capacity table is required.

8. TOLERANCES
Measurements shall be read to the nearest 1 mm and within tolerance given in Table 51, when readings are taken at the same point.
 
TABLE 51 – SPECIFIED TOLERANCES
Tank Circumference, C (m)       Tolerance (mm)
C < 25                                         ± 2
25 < C < 50                                 ± 4
50 < C < 100                               ± 6
100 < C < 200                             ± 8
200 < C                                       ± 10
 
At a reference tank shell temperature offer than the reference temperature of the tape, the linear measurements shall be adjusted by temperature correction.
 
 
FIG. 56 – A SCHEMATIC DIAGRAM OF TYPICAL FIXED ROOF CYLINDRICAL TANK
 
SECTION 1 – CALIBRATION BY STRAPPING METHOD

9. GENERAL
(1) The method is based on the measurement of external circumference which are subsequently corrected to yield the true internal circumference.
(2) The stipulated number external circumference measurements together with subsidiary deviation of the tape from the true circular path shall be obtained as described under clause 11.
(3) An internal diameter may be measured at approximately, the same height as that at which a circumferential measurement is desired.
(4) If may be necessary in practice to refer all tank dips to a datum point other than the datum point used for the purpose of tank calibration. If so, the difference in level between these datum points shall be determined either by normal surveying methods or by other suitable means.

10.
EQUIPMENT
(1) Steel Tapes – Shall comply with the specifications under Part VII of the Sixth Schedule. The tape shall be greased well before use.
(2) Spring Balance – Reading up to 10 kg. with 0.1 kg. graduations is necessary for measuring the tension applied to the tape. It is preferable to have two balances. Spring balance shall comply with specifications given under Part IV of the Seventh Schedule – Heading A.
(3) Step Over – This is used to correct deviation of the tape from its normal circular path, namely passing over fittings or joints between plates.
(4) Dip Tape and Dip Weight – Complying with the specifications given under Part IX of the Sixth Schedule.
(5) Loops and Cords – One a more metal loops which can slide freely on the tape and to which are attached two cords, each of sufficient length to reach from the top of the tanks to ground. The tape is positioned and its tension evenly distributed by passing these loops around the tank.
(6) Accessory Equipment – Rope, Hooks, Safety Belts, Ladders, Painters’ Cradles etc.
(7) Miscellaneous Equipment – Steel Rule, Spirit Level, Awl and Scriber, Marking Crayon, Plumb Line, Dumpy Level, Positive Displacement Bulk Meter, Water Meters, Proving Measures etc.
 
11. CIRCUMFERENCE MEASUREMENTS
(A) STRAPPING LEVELS
Circumferences shall be measured by a minimum of two strapping per course at the following levels:
(a) For riveted tanks -
(1) At 7 per cent to 10 per cent of the height of exposed portion of each course above the level of the upper edge of each horizontal overlap between courses (see A of Fig. 57(a)), and
(2) At 7 per cent to 10 per cent of the height of edge of each horizontal overlap between courses and below the level of the lowest part of the top angle iron of the rank (see B of Fig 57(a)).
(b) For welded tanks –
(1) Two levels (see A and B of Fig. 57(b) for Lap-welded tanks and of Fig. 57(c) for butt-welded tanks), the upper and the lower levels, at the top and bottom of courses shall be 20 per cent of the height of the exposed portion of the respective courses away from the angle iron seams.
(2) Circumferential tape paths, having been located at elevations as under 11(A)(a) above shall be examined for obstructions and type of vertical joints. Projections of dirt and scale shall be removed along each path.
(3) Occasionally, some feature of contraction such as a manhole or insulation box, may make it impracticable to use a circumference evaluation at the prescribed location. If the obstruction can be spanned by a step-over, then the circumference shall be measured at the prescribed elevation, using a a suitable method given under 11(C)(2)(d). If the obstruction cannot be conveniently spanned by a step-over, then a substitute path located nearer to the centre of the course may be chosen. The strapping record shall include the location of the substitute path and the reason for the departure.
The type and characteristics of vertical joints shall be determined by close examination in order to establish the method of measurement and equipment required. If the tape is not in close contact with surface of the tank throughout its whole path owing to the vertical joints a step-over shall be applied so that a correction may be calculated to adjust the gross difference for this effect.
 
 
FIG. 57 – LOCATIONS OF STRAPPING LEVELS FOR DIFFERENT TYPES OF TANK JOINTS
 
(B) STRAPPING PROCEDURE
(1) The tank shall be strapped by either of the methods described under (2) and (3) below. In either case a tension of 4.5 ±0.5 kg shall be applied to the tape and if necessary, transmitted throughout its length by suitable means, namely, by means of metal loops sliding freely on the tape, the loops being passed around the tank by operators with the aid of light chain or cords. The tape path shall be parallel with circumferential seams of the tank.
(2) If the tape to be used is not long enough to encircle the tank completely, then after the level of the tape path has been chosen, fine lines shall be scribed perpendicular to this path to allow the circumference to be measured in sections. The scribed lines shall be drawn in the middle circumferential third of any plate at such distances as will ensure that the whole of the length of the tape used is under the observation of one or other of the operators. Subject to the conditions under 11(A)(b)(3) and 11(A)(b)(4), the external circumference of the tank is then the sum of the lengths between the scribed lines.
(3) If the tape to be used can encircle the tank completely, then after the level of the tape path has been chosen, the tape is passed around the circumference and held so that the first graduated centimeter lies with the middle circumferential third of any plate. The other end of the tape shall be brought alongside. The tension is then applied through the spong balance and transmitted throughout the length of the tape.
(4) After a a circumference has been measured (see (9) above), the tape shall be shifted a little around the tank, brought to level and tension as above, and the reading repeated. The final reading shall be the arithmetic average of the readings.
C. STEP-OVERS
(1) Principles
If the tape crosses obstructions, such as projections, deformities, fittings or lapped joints, it well deviate from a true circular path and an erroneous circumferential measurement will result. In order to avoid such errors a ‘step-over’ is used to measure the correction to be applied for such obstructions, a suitable design of which is given in Fig. 58.
(2) User of Step-Over
(a) For obstructions, the strapping tape shall be stretched as if in measurement of a circumference on the tank which is being calibrated, but not within 300 mm of any horizontal seam. The scribing points shall then be applied to the tape near the middle of a plate where the tape is fully in contact with the tank surface. The length between the points, as measured on the curved tape is then read off as closely as possible, fractions of tape deviations being estimated. The reading shall be repeated on a minimum of two and maximum of four plates equally spaced around the circumference, and the average of the results taken, as the step-over will vary with tank diameter and the course concerned since they are made on surface differently curved.
 
FIG. 58 – STEP OVER
 
(b) With, the tape still in position and under the tension used in strapping, the step-over shall be applied to the tape on either side of each obstruction lying on the tape path, and readings shall be taken of the lengths of tapes included between the scribing points. All step-over readings shall be recorded  for subsequent use in calculation.
(c) Care shall be taken in placing the instrument in a truly level position at each obstruction to avoid distortions in circumferential path, in the case of a step-over of relatively long space, the use of a spirit level is recommended as an aid in determining its correct position before scribed marks are stuck off on the places.
(d) When the butt-strap or lap joint, or tank shell, include rivets or other features which exert uneven effects on the resultant void between tape and tank from joint to joint, then a step-over will be required. The span of the instrument should be measured prior to use in accordance with 11(C)(2)(a) and above. The two legs should be separated by a distance sufficient to spar each void between tape and shell encountered. The legs shall be of sufficient length to prevent contact between the inter-connecting member and the tank plate or obstruction. Stretch the tape over the joints and place the step-over in position at each location of void between tape and shell, completely spanning the void so that the scribing points, with the tape maintained in proper position and tension, should be estimated to the nearest 0.5 mm. At each step-over location, therefore, the difference between the length of tape encompassed by the scribing points and the known span of the instruments is the effect of the void, at that point, on the circumference as measured. The algebraic sum of such differences in an given path, subtracted from the measured circumference, will give the corrected circumference.
 
12. SHELL PLATE THICKNESS
(1) Where the type construction leaves the plate edges exposed, minimum of four thickness measurements shall be made on each course of points approximately equally spaced about the circumference. The arithmetical average of the measurements for each course shall be recorded; all thickness measurements, property identified, shall be noted on a supplemental data sheet which shall form a part of the measurements record. Care shall be taken to avoid plate thickness measurements at locations where edges have been distorted by coulking.
(2) Where plate edges are concealed by the type of construction, the strapping record shall be marked ‘not obtainable at rank’. Alternatively plate thickness measurement may be obtained as described under (3) below.
(3) Plate thickness measurement obtained before or during construction, and recorded on a properly identified strapping record may be acceptable.
In the absence of any direct measurements of plate thickness obtained and recorded before or during construction either those shown on the manufacturer’s drawings may be accepted and so identified in the calculation records or any other practicable methods may be used for measurements of plate thickness like ultrasonic or electronic thickness gauges pre-verified with a known thickness.
13. VERTICAL MEASUREMENTS
(1) A tape shall be suspended internally along the wall of the shell from the top curb angle to the bottom course and the height of the course measured to the nearest millimetre. The difference in height between the datum plate at which dip is taken and the bottom course shall be transferred to the datum plate by applying the correction (Fig. 59).
Example: In Fig. 59, the difference in height between bottom course and datum plate is 1520-1505 mm) = 15 mm i.e. height of the datum plate is 15 mm and hence, A=1505 mm. Applying this correction the corrected height of the course at
(2) When it is inconvenient to measure the source height internally, then they shall be computed from external measurements, due allowance being made for the effect of horizontal seam overlaps. The heights obtained shall be the vertical distances, measured to the nearest 5 mm, between successive edges of the courses as exposed internally in the tank. For this purpose, in the cases of lap joints, it will be necessary to determine the width of lap In each course.

(3) If necessary, heights at more than one vertical around the tank may be taken, and for each course, an average of the results obtained.



FIG. 59 – AN ILLUSTRATION OF VERTICAL MEASUREMENTS


14. DEADWOOD
(1) Deadwood shall be accurately accounted for, as to size and location to nearest millimetre in order to permit:
(a) Adequate allowance for volumes of liquid displaced or admitted by the various parts, and
(b) Adequate allocation of the effects at various elevations within the tank.
(2) Deadwood should be measured, if possible, within the tank. Dimensions shown on the manufacture’s drawing may be accepted if actual measurement is impractical.
(3) Measurements of deadwood should show the lowest and highest levels, measured from the tank bottom adjacent tot the shell, at which deadwood affects the capacity of the tank. Measurements should be in increments which permit allowance for its varying effect on tank capacity at various elevations.
(4) Large deadwood of irregular shape may have to be measured in separate sections suitably chosen.
(5) Work sheets on which details of deadwood are sketched, dimensioned and located, should be clearly identified and should become part of the strapping record.
(6) For variable deadwood, such as nozzles and manholes, encountered in the bottom one or two courses of the tanks, an average deadwood correction shall be made.
 
15. TANK BOTTOMS: (a) Flat Type
(1) Tank bottom which are flat and stable under varying liquid loads will have no effect on tank capacity depressed on the basis of geometric principles.
(2) Where tank bottom conditions of irregularity, slope and instability exist, and where correct capacities cannot be determined conveniently from linear measurements alone, it shall be necessary to resort either to liquid calibration or to floor survey.
(3) Liquid Calibration – The procedure in carrying out the liquid calibration is to fill into the tank quantities of known volume of water or other non-volatile liquid until the datum point is just covered and the total quantity recorded. Additional quantities shall then be added until the highest point of the bottom is just covered. This may be done in one or more stages as desired and the dip reading and quantity at each stage recorded. It is convenient for dip readings to be taken at intervals of approximately 30 mm, the successive intervals not necessarily being identical. This liquid may conveniently be measured into the tank by a positive displacement bulk meter which should be previously calibrated for the liquid and rate of flow to be used. Alternatively, and accurately calibrated proving measure may be used.
(4) Volumes for the tank capacity table above this elevation shall be computed from linear measurements.
(5) Floor Survey – The floor survey consists in recording levels of the floor by means of a dumpy sections and the longitudinal sections of the entire floor may be computed, the levels when plotted with define the profile and the geometric pattern of the bottom of the tank. Thus the capacity of tank may be calculated.
(6) During the tank bottom calibration the circumference in height between the datum plate and the bottom of the bottom course should be recorded, wherever possible.
(b) Conical, Hemispherical, Semi-ellipsoidal and Spherical Segment
Tank bottoms conforming to geometrical shapes have volumes which may either be computed from linear measurements or measurements by liquid calibration by incremental  fining or by floor survey, as desired. Any appreciable differences in shape affecting volume, such as knuckle redil etc. shall be measured and recorded in sufficient detail to permit computation of the true volume.
16. MEASUREMENT OF TILT
Measurements shall be taken to determine the amount, if any, by which the tank is fitted. This can conveniently be done by suspending a plump line from the top angle and measuring the offset at the bottom angle (see Fig. 60). Alternatively, if the tank bottom is being calibrated by floor survey with a dumpy level as in 15(a)*5(, the tilt can be estimated by tanking reading along the periphery of the tank bottom. Also, if a liquid calibration of bottoms is being made as outlines in 15(a)(5), the tilt can be determined by taking measurements from the surface of the liquid to the bottom of the tank.
 
FIG. 60 – A SECTION OF A TILED TANK
 
In any of these methods, a sufficient number of measurements shall be taken at different points of the circumference to determine the maximum offset.
17. FLOATING ROOF AND HYBRID ROOF TANKS
(1) All calibration measurements shall be made exactly as for tanks with fixed roofs. A hybrid roof tank comprised of a covered floating roof where a panroof is installed within a fixed roof tank. A schematic sectional view of a typical open top floating roof tank is shown in Fig. 61.
(2) Liquid Calibration for Floating Roof Displacement.
(a) Corrections for floating roof displacement arising associated with it shall be allowed for in the calibration measurement.
(b) If the mass of the floating roof is accurately known, correction for the displaced liquid may be applied knowing density and temperature of the tank contents, at the time of determining the actual inventory.
(3) alternately, displacement due to the floating roof and deadwood may be determined by admitting liquid to the tank until the dip reading is just bellow the lowest point of the roof. Known quantities accurately determined (for example by bulk meter or delivery from portable proving measure which has been accurately calibrated are then admitted to the tank and the corresponding dip readings recorded of a number of suitable intervals until the point is reached when the roof just becomes liquid-borne. Record the density and temperature of liquid used.
(a) It is advisable to use a liquid of nearly the density as that for which the tank is tntended. If this is not practical, water may be used and suitable corrections applied.
(b) During liquid calibration any space under the roof that will trap gas should be vented to the atmosphere.
(c) Before liquid calibration the height of the lowest joint of the roof with reference to datum point should be recorded, wherever possible.
(d) To assess the point at which roof becomes liquid-borne the following procedure may be followed:
 
FIG. 61 – A SCHEMATIC SECTIONAL VIEW OF A TYPICAL OPEN TOP FLOATING ROOF TANK
 

With the roof resulting fully on its supports, point four short horizontal white lines about 30 mm wide on the tank sides in such a position that, viewed from some definite point, their lower edges are just above four similar lines marked on the roof edges or shoes. Then slowly pump liquid into the tank; when all roof markings are seen to have moved upwards, regard the roof as liquid-borne, and take the dip reading of the liquid at this level. Alternatively, from some chosen view point on the dipping platform, note the position of the roof against river heads on vertical seam or other markings on the tank walls instead of point marks. In both cases extend the points of reference round the greater path of the tank interior and see movement relative to all points.

(4) Floating Mass – The floating mass of the entire roof shall include mass of roof plus half the mass of the rolling ladder and other hinged and flexibility supported accessories that are carried up and down in the tank with the roof. These are calculated by the tank manufactures and given on the drawing and on the roof name plate.
(5) Deadwood
(a) Fixed deadwood shall be measured as described in clause 14. the drain lines and other accessories attacked to the  to the underside of the roof shall be treated as fixed deadwood in the position they occupy when the roof is a treated on the supports.
(b) When all or part of the mass of the roof is resting on its supports, the roof itself is deadwood and as the liquid rises around the roof, its geometric shape will determine how it should be deducted. The geometric shape should be taken from the manufacture’s drawings or measured in the field with aid of and engineers level while the roof is resting on its supports.
 
18. VARIABLE VOLUME ROOFS
(1) Roofs such as lifter, flexible membrane, breather or balloon, may require special deadwood measurements for roof parts that are sometimes submerged. When these parts, such as columns, are fixed relative to the tank shell, they should be measured as deadwood in the usual way. When these parts move with roof and hang down into the liquid, they should be deducted as fixed deadwood with roof in the lowest position. Details may be secured from the manufacture’s drawings or measured in the field.
(2) Some variable volume roofs have flexible membrane which may float on the surface when the membrane is deflated and liquid level is high. The floating mass of the membrane displaces a small volume of liquid. Data on the floating mass should be secured from the manufacture's drawings and supplemented, if necessary, by fields observation and measurement.
(3) Some variable volume roofs have liquid seal troughs or other appurtenances with make the upper outs depart of the shell inaccessible for calibration of this position of the shell may be made, or (a) theoretical dimensions may be taken from the measurable circumferential measurement may be used  as a basis for the portion if the tank that cannot be measured. When the method (a) or (b) is used, it shall be so indicated on the tank capacity table.
SECTION 2 – CALIBRATION BY INTERNAL TAPE MEASUREMENT METHOD
19. GENERAL
(1) This method is based on the measurement of internal dimensions of all vertical tanks of any geometric shape.
(2) In the case of a cylindrical tank is simulated number of internal diameters shall be obtained as described under clause 21.
(3) Where practicable, an external circumference shall be measurement approximately the same as that at which a set of diameters of which a internal diameter shall be compared and if a discrepancy is found, the measurements shall be verified.
(4) It may be necessary in practice to refer all tank dips to a datum point other than the datum point used for the purpose of tank calibration, so, the difference in levels between these datum points shall be determined either by normal surveying methods or by other suitable means.
20. EQUIPMENT
(1) Dynamometer – This is used for applying tension to steel tape.
(2) Other equipment as referred to under clause no. 10.
21. SELECTION OF PREDETERMINED POINTS
(1) A predetermined (PD) point is one of a series of points marked clearly on the inside
surface of the tank shell wall to which diameters are measured by the use of tape measurement method. Two sets of predetermined points per course, one at 1/5 to 1/4  of course height above the lower horizontal seam, the other at 1/5 to 1/4 of course height below the upper horizontal seam, shall be selected.
(2) The number of predetermined points per set, on each course of the tank shell wall is dependent on the tank circumference. The minimum number of predetermined points per set as  function of tank circumference in given in Table 52 and illustrated in Fig. 62.
(3) The predetermined points shall be at least 300 mm from any vertical seam.
22. DIAMETER MEASUREMENTS
(1) All diameter measurements shall be made with a tension of 4.5 ±0.5 kg applied to the tape as indicated by the dynamometer.
(2) All tape measurements shall be recorded as read, that is without including the length of the dynamometer.
(3) The dynamometer length at 4.5 kg shall be taken accurately before it is put into commission, and subsequently checked before and after calibrated of each tank, the final check being made before leaving the site.
(4) Measurements shall be taken with the zero end of the steel tape attached to the dynamometer on the predetermined point and the second operator placing the rule end-on to a point diametrically opposite. The tape with graduated side wholly upwards is then pulled along the rely until the requisite tension is registered by the sounding of the buzzer in the dynamometer. The relative position of tape and rule is maintained by a film grip until the rule is removed from the side of the tank and the measurement read on me tape at the end of the rule which was previously in contact with the tank side. The operation shall be repeated at the various positions at which measurements are required throughout the tank. The measurements shall be recorded clearly in white chalk on the steel plates in such a manner as to indicate the positions at which they were taken.
 
Table 52 – Minimum number of predetermined points per set
Tank Circumference, cm
Minimum number of PD points
C < 50
8
50 < C < 100 12
100 < C < 150 16
150 < C < 200 20
200 < C 24
Note: A number of PD points greater than the minimum number of points as a specified above may be selected depending on specific circumferences and tank conditions.
 
 
FIG. 62 – PREDETERMINED POINTS POSITIONING OF TANK SHELL WALL
 
23. OTHER MEASUREMENTS
All other measurements shall be followed in accordance with Section 1.
SECTION 3. CALIBRATION BY INTERNAL ULTRASONIC DISTANCE RANGING METHOD
24. GENERAL
This method is based on the measurement of internal dimension so f all vertical tanks of any geometric shape by means of an ultrasonic distance ranging (UDR) instrument and subsequent compilation of tank capacity tables.
25. EQUIPMENT
(1) Ultrasonic distance ranging (UDR) instrument: The instrument is low-power ultrasonic wave emitter as well as received for reflected waves under for direct determination of distances and shall have a scale interval not greater than 1 mm and an uncertainly equal to or less than ±2 mm.
(2) Other equipment as referred to under clause 20.
26. CONDITIONS FOR MEASUREMENTS
(1) Calibration shall be carried out without interruption.
(2) The UDR Instrument shall be verified prior to calibration against a known reference length comparable to the diameter of the tank.
(3) The tank as well as the UDR instrument shall be free from external vibration.
(4) The UDR instrument shall be set perpendicular to the tank shell wall, thus ensuring the circularity of the plane of measurements and minimizing the overall uncertainty of the diameter measurements.
(5) The PD points shall be so selected that the path of transmitted waves from the UDR instrument to the diametrically opposite tank shell wall shall not be obstructed.
(6) Other conditions shall comply with the provisions described in clause 3.
27. SELECTION OF PREDETERMINED (PD) POINTS
The selection of PD points shall comply with provisions described in clause 21.
28. CALIBRATION PROCEDURE
(1) Diameter shall be measured at all PC points at each course location as illustrated in Fig. 62 in clause 21. An average of five maximum readings shall be recorded.
(2) The measurements to the PD points on each course by internal tape measurement method. The resulting diameters shall be completed prior to moving to the next course.
(3) A reference diameter shall be measured approximately in middle of each course by internal tape measurement method. The resulting diameters shall be compared, and if a discrepancy is found, the measurements shall be verified.
29. OTHER MEASUREMENTS
All other measurements shall be followed in accordance with Section 1 and Section 2.
SECTION 4 – CALIBRATION BY INTERNAL ELECTRO-OPTICAL DISTANCE RANGING METHOD
30. GENERAL
This method is based on the measurement of internal diameters of vertical cylindrical tanks having diameters equal to or greater than 5 metres by means of an electro-optical distance ranging (EODR) instrument and subsequent compilation of tank capacity tables.
 
31. EQUIPMENT
(1) electro-optical distance ranging (EODR) Instrument:
(a) The angular measuring part of the instrument shall have an angular graduation equal to or less than 0.0002 degree and uncertainly equal to or less than ±0.001 degree.
(b) The distance – measuring part of the instrument, used for direct determination of distances, shall have scale interval equal to 1 mm and an uncertainly equal to or less than ±2 mm.
(2) Laser beam emitter – This instrument is a low-power laser beam emitter which is either on integral part of the EODR instrument or a separate device. If the laser beam emitter is a separate device., it may be fitted with a fibre optic light transmitter system and a theodolite telescope eyepiece  connection, be which the laser beam may be transmitted through the theodolite, or such that it may be fitted to a theodolite with its axis parallel to the axis of the theodolite. The laser beam may be coincident with the optical axis of the telescope. The laser beam emitter is used to position target points on the tank shell.
(4) Instrument Mounter – The instrument mounter consists of a tripod shall be held firm, and steadled by suitable devices such as magnetic bearers.
(5) Metalic Length Measure of 1 Metre – Complying with the specifications given under Path IV of the Sixth Schedule.

32. CONDITIONS FOR MEASUREMENTS
(1) Calibration shall be carried out without interruption.
(2) The tank as well as the EODR instrument shall be free from external vibration and the tank shall be free from air-borne dust particles.
(3) The EODR instrument shall be verified prior to calibration according to procedures described in clause 35.
(4) Other conditions shall comply with the previous described in clause 3.

33. SELECTION OF TARGET POINTS
(1) A target point is one of the series of points marked clearly on the inside surface of the tank shell wall to which slope distance, vertical and horizontal angles are measured by use of the EODR instrument. The distance measured from the EODR instrument to a target point on any given course of tank shell wall is said to be slope distance. Any fixed target point with reference target point. Two sets of target points per course, one 1/5 to 1/4 of course height above the lower horizontal seam, the other at 1/5 to 1/4 of course height below the upper horizontal seam, shall be selected.
(2) The number of target points per set, on each course of the tank shell wall is dependent on the tank circumference. The minimum number of target points per set, as a function of tank circumference is given in Table 53 and illustrated in Fig. 63.
 
Table 53 – Minimum number of predetermined points per set
Tank Circumference, cm
Minimum number of PD points
C < 50
8
50 < C < 100 12
100 < C < 150 16
150 < C < 200 20
200 < C 24
Note: A number of target points greater than the minimum number of points as a specified above may be selected depending on specific circumferences and tank conditions.
 
 
FIG. 63 – TARGET POINTS POSITIONING OF TANK SHELL WALL
 
(3) The target points shall be at least 300 mm from any vertical seam.
(4) Two reference target points shall be selected and marked clearly on the tank shell wall approximately 90° apart and preferably on the same horizontal plane as the instrument.

34. EQUIPMENT SETUP PROCEDURE
(1) The instrument shall be set up so as to be stable, if necessary, the tank bottom in the vicinity of the instrument shall be made firm and steady by placing heavy weights in the area. The legs of the tripod on which the instrument is mounted shall be steadied by use of suitable devices, such as magnetic bearers, to prevent slippage on the tank bottom.
(2) The instrument shall be located at, or near the centre of the tank. This will ensure that the measured slope distances, at an horizontal level, do not vary significantly and minimizes the overall uncertainty of slope distance determination.
(3) The instrument shall be set horizontal, thus ensuring that the standing axis is vertical.
(4) The sighting lines from the instrument to the tank shell wall shall not be obstructed.

35. TOLERANCES AND FIELD VIREFICATION OF EQUIPMENT
(a) Angular measurement verification
(1) The EODR instrument shall be set up according to procedures described in clause 34 and shall be switched on and brought to operating temperature, allowing at least the minimum warm up time.
(2) Each EODR instrument will have manufacture’s instructions concerning collimation of both the vertical and horizontal angular measurement components of the instrument. These instructions shall be followed exactly, and the uncertainly of both the vertical and horizontal angular measurement calculated and recorded.
(3) The collimation uncertainly of both vertical and horizontal components of the instrument shall not exceed the tolerance of ±0.001 degree.
(b) Distance measurement verification
(1) The metre bar shall be mounted on a tripod horizontally and perpendicular to the to the aiming axis of the EODR instrument and shall be locked in position.
(2) The metre bar shall be placed at a distance of approximately 5 metres from the instrument and perpendicular to an imaginary line between the instrument and the centre of the metre bar, as illustrated in Fig. 64.
(3) The horizontal angle 29 subtended at the instrument by the zero mark and the 1 m mark of the metre bar shall be measured, using the EODR instrument.
(4) The horizontal distance D shall be computed from the equation.
D = B/2 x cote where B = 1 m.
 
FIG. 64 – DISTANCE VERGICATION SETUP
 
(5) The slope distance r : measured by the EODR instrument and the average computed distance D shall agree to within a tolerance of ±2 mm.

36. CALIBRATION PROCEDURE
(1) All of the target points along the horizontal plane at each course location shall be sighted and the slope distance, horizontal angle and the vertical angle to each, shall be measured as illustrated in Fig. 65.
(2) The slope distance, horizontal angle and vertical angle to each of the reference target points shall be measured and recorded.
(3) The measurements to the target points on each course shall be completed prior to moving to the next course. Measurements shall begin at the bottom course and extend, course by course, to the top of the tank.
(4) The measurements to the reference target points shall be repeated after all measurements on a course being completed.
(5) If the repeated slope distance, horizontal angles and vertical angles to the reference target points do not agree within the tolerances given in clause 35, then the procedure 1 – 5  shall be repeated.
(6) If statistical agreement is not obtained between the original and repeated measurements of slope distances, horizontal angles or vertical angles, then the reasons for such disagreements shall be determined, the cause eliminated and the tank calibration procedure repeated.

37. OTHER MEASUREMENTS
All other measurements shall be followed in accordance with Section 1.
 
 
FIG. 65 – MODE OF MEASUREMENTS OF TARGET POINTS ON TANK SHELL WALL
 
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