Vertical Storage Tanks

Vertical Storage Tanks The Vertical TanK

A vertical storage tank is in the shape of right circular or elliptical or of a rectangular parallelepiped, the axis of geometric symmetry of which is vertical to the base. A tank may have a fixed or floating roof. A tank may be mounted aboveground or underground or on ships and barges. The tank includes necessary supports, spiral stair cases, manhole, piping, valves, gauges, meters etc. It is treated as a capacity measure and calibrated as such. Calibration and Verification

Calibration is the method of accurately determining the capacity of a tank or a part of it and expressing the capacity as a volume for a given height of the liquid. Calibration is usually done by scrapping method following the ISO standard ISO 7507-1, AP 202 Part II, Section I and API MPMS Chapter 2, Section 2A In case of large tanks, strapping method is difficult to follow when the physically measuring the tank strakes with a calibrated steel tape at several levels on each course of platting is required. In such cases, it is better to follow Optical Reference Line Method (ORLM) or Optical Triangular Method (OTM) Verification, is the process of ascertaining when the capacity table of the tank is within the prescribed limit of Maximum Permissible Error (MPE. For verification Manual Tank Scrapping Method is followed as prescribed under the Ninth Schedule (Part II and III) of the Legal Metrology (General) Rules, 2011. Maximum Permissible Error for a vertical tank is ±0.2% of the capacity. Concepts

Datum Plate A metal plate fixed at the bottom of the tank along the vertical axis descending from the dip reference point. It provides a fixed contact surface from which the dip measurement is made.

Dip Hatch An opening at the top of the tank through which dipping or ullaging operation is carried out.

Dip Pipe A perforated metal pipe fixed below the dip hatch. It extends vertically downward directly above the datum plate and ends near it. It acts as guide for dip tape fitted with a weight (to add tension.)

Dipping Datum Point A point on the upper surface of the datum plate which serves as a zero reference for measurement of liquid dips.

Dipping Reference Point A point marked on the dip hatch located along the vertical axis from the dipping datum point. It serves as the upper reference point from which ullage is measured.

Overall Height The distance between the dipping reference point and the datum point.

Dip The vertical distance between the liquid level and the dipping datum point

Ullage The vertical distance between the dipping reference point and the liquid level.

Course One circumferential ring of plates in a tank.

Deadwood Any fitting inside the tank which decreases (negative deadwood) or increases (positive deadwood) the tank capacity.

A schematic Diagram (Fixed Roof)

Verification Verification is done in two stages:  

examination of the tank in situ calibration

Calibration Basics

Conditions for measurement   

Measurements shall be taken only after the tank has remained filled to its working capacity at least for 24 hours at the present location. All data and methods used for measurements should be based on sound engineering principles. All measurements should be checked with measurements given in the manufacturer's drawing (when available) and discrepancies greater than tolerance limit should be carefully verified. Circumference

Up to 30 metres

Tolerance ±2 mm

Over 30 and up to 50 metres ±4 mm Over 50 and up to 70 metres ±6 mm

Over 70 and up to 90 metres ±8 mm Over 90 metres

±10 mm

Descriptive Data  



Complete descriptive data shall be entered in the Tank Measurement Record. Supplemental sketches or notations should be drawn and included in the field data with proper attestation. The items to be included are - typical horizontal and vertical joints, number of plates per course, location of courses where plate thickness changes, arrangement of and size of angles at top and bottom of shell, location and sizes of pipes, man holes, dents etc, direction of lean from vertical, methods used in bypassing large objects of obstruction, location of tape path, elevation of datum plate and its location and other items as may be considered important. All data should be recorded in the field and there must not be any subsequent correction.

View | Download Suggested Format for Recording Vertical Storage Tank Measurements (General Data) (Table 1/5)

Tools and equipment     

 

Verified Steel Tapes. It should be well greased. Spring Balances. Two verified spring balances suitable for reading up to 10 kg with 0.1 kg graduations. They are required for measuring the tension applied to the tape. Step-over. Verified Dip-tape and dip-weight. Loops and cords. Metal loops which can slide freely on the tape and to which are attached two cords each of sufficient length to reach from top to bottom of the tank. The tape is positioned and its tension is evenly distributed by passing the loops around the tank. Accessory Equipment - Rope, hooks, safety belts and ladder. Miscellaneous - Steel Ruler, spirit level, awl and scriber, marking crayon, plumb line, positive displacement bulk meter/proving measures/water meter etc.

Stages of Calibration Stage 1: Measurements The following measurements shall be obtained and recorded.      

Circumference Measurement Step-over measurement Measurement of shell-plate thickness Vertical measurements Dead-wood measurement measurement of tank bottom (Bottom Calibration)

For details: Measurements of Vertical Storage Tank

Stage 2; Preparation of Tank capacity Tables Tank Capacity Tables are obtained from following operations:        

Computation of capacities Temperature Correction Liquid-head correction Tilt correction Dead -wood correction Systematic calculation Preparation of linear measurement table Calculation of corrected internal circumference. Preparation of Open Capacity Table



Preparation of Dip/Capacity Relationship Table.

For details: Preparation of Tank Capacity Tables

Preparation of Tank Capacity Tables Capacity tables should be set out to show litres at intervals of 5 cm in dip with proportional part tables for intermediate dips in millimetres.

Computation of Capacities Open capacity in litres per centimetres = (C2 X 10) / 4Π = 0.795778 X C2. When the tank is tilted by an angle Ø, the open capacity in litre per cm = 0.795778 X C2 X sec Ø (C = internal circumference of the tank in metres and Π = 3.14158

Calculating Internal Circumference Internal Circumference of a tank may be calculated by measuring internal diameters at different heights or by obtaining the external circumference by strapping method and applying following corrections:

1. Temperature correction

The measuring tape is calibrated at 20°C but the reference temperature for tank calibration is 15° C. So, the external circumference in metre will have to be multiplied by correction factor of 0.99991

2. Step-over correction   

By choosing tape courses to avoid obstructions, step-overs can be avoided to a great extent. Step-over correction is necessary for all vertical seams. Step-over less than 2 mm need not to be included.

3. Shell Plate Thickness Correction the correction will be deduction of 2Π X d where d is the thickness of shell-plate.

4. Correction for Liquid Head and Temperature Theses effects are eliminated by strapping the tank when it is filled not less than 2/3 rd capacity with the product or water.

Open Capacity Calculation Table 3/5

This table has been divided into four parts for the sake of space and clarity. Practically, there should be a combined table to include all data in a single sheet. It is also to be noted carefully that circumference measurement is always in metre and height measurements in cm.

Part I: Course 1

Height

Effective Height Tape External Mean External (Cumulative) Path Circumference Circumference

2 in cm 3 in cm

1st Bottom 150.0 133.6* Course

2nd Bottom 150.0 283.6 Course

4

5 in metre

Top

47.121 47.1185

Bottom 47.116 Top

-------

47.114 47.1145

Bottom 47.115 Top

----------

6 in meter

------

-------

------Bottom -------

8th Bottom 150.0 1184.4 Course

9th Bottom 148.5 1332.9 Course

Top

47.150 47.1510

Bottom 47.152 Top

47.124 47.1285

Bottom 47.133

* Height of datum plate 16.4 cm

Part II: Mean External Circumference after temp. correction

Mean External Circumference

No of Vertical welds

Step-over correction for welds

Net correction

Net External Circumference

6 in meter

7=(6 X 0.99991)

8

9 in mm

10 in metre

11

47.1185

47.1142

8

1

0.008

47.1062

47.1145

47.1102

8

1

0.008

47.1022

----------

----------

----------

----------

----------

------

47.1510

47.1467

8

1

0.008

47.1387

47.1285

47.1242

8

1

0.008

47.1162

Part III: Net External Circumference

Shell/Plate thickness

Correction for shell/Plate thickness t

External Circumference

Open Capacity in litre/cm

11

12 in mm

13 in metre 14 = (11 - 13) (2 X 3.1416 X t) in metre

15 in litre/cme

47.1062

12

0.0754

47.0308

1760.125

47.1022

10

0.0628

47.0394

1760.769

----------

----------

----------

----------

----------

47.1387

6

0.0377

47.1010

1765.383

47.1162

6

0.0377

47.0785

1763.697

Part IV: Bottom Calibration at 0.00 cm: 28230 litre

Total Capacity Calculation Table 4/5 Dead-wood Calculated effected range Capacity in Course of height in l/cm cm

Dead-wood Corrected Actual Total Capacity in Correction in Capacity in Height Litres l/cm l/cm in cm

Bottom Calibration at

0.0

28,230

1760.125

23.7

69,944.9625**

1762.601

69.0

191,564.4315***

0.0 to 23.7

1760.125*

23.7 - 92.7

1760.125

92.7 - 133.6

1760.125

1760.125

40.9

263,553.544

2nd

133.6 -283.6

1760.769

1760.769

150

527,668.894

...

...

...

...

...

...

8th

1034.4-1184.4 1765.383

1765.383

150

2116,108.8118

9th

1184.4-1332.9 1763.697

1763.697

148.5

2378,017.8163

1st

+ 2.476

...

Hints: * from Table 3/5 col 15 ** 1760.125 X 23.7 + 28,230 *** 1762.601 X 69.0 + 69,944.962

Dip Height - Capacity Relationship Table (with Part Tables for intermediate ranges) Table 5/5   

Main table shows the capacity at 5 cm interval. Part table shows intermediate readings between 0.1 to 5 cm. Readings are correspondent to respective course height measurement. Always refer to value of corrected capacity for a particular course height from Table 4/5.

Main Table

Part Table (in nearest rounded litre)

Dip in cm Litre 0.0 5* 10 15 20

70

151553

75

160366

80

169179

130

0.1

176

176

176

0.2

352

353

352

0.3

528

529

528

0.4

704

705

704

0.5

880

881 **

880

0.6

1056

1058

1056

0.7

1232

1234

1232

0.8

1408

1410

1408

0.9

1584

1586

1584

1

1760

1763

1760

2

3520

3525

3520

3

5280

5288

5280

4

7041

7050

7041

5

8801

8813

8801

63433

142740

125

92.8-133.6 cm

54632

65

120

23.7-92.7 cm

45831

--

115

0.0-23.7 cm

37031

--

--

Dip in cm 28230

-230815 239616 248416 257217

* 1760.125 X 5 +28230 = 37 030.625 = 37 031 litres ** 1762.601 X 0.5 = 881.3005 = 881 litre

The tables should continue cover all heights up to 1332.9 cm Part Tables will cover intermediate heights of each course height ( as per Table 4/5). For example, 133.7 to 283.6 cm (second course), 283.7 to 433.6 cm (third course), 1034.4 to 1184.4 cm for the 8th course, etc. /div>

Calculation of Capacity from Table 5/5

Let the dip reading be 127.7 cm. Now 125 + 2 + 07 = 248416 + 3520 + 1232 = 253,168 Litres. Note that readings for 2 cm and 07. cm will be taken from column 4 of the Part Table, as the dip height falls within the range of 92.8 to 133.6 cm.

Measurements of Vertical Storage Tank Before initiating any measurement, it must be ensured that the tank has been filled at least once with the liquid to be stored or with static water and kept for 24 hours. Normally strapping method is employed and it is the only method approved by the SoWM (General) Rules, 1987.

Circumference Measurement Strapping Method The normal tape used is made of steel, 1/8" to 1/4" width , and of enough length so that it may cover the entire circumference. If the tape is not long enough, the measurements may be made in sections, sum of which will give the measurement of the circumference. The tape should run parallel to the seam of the tank. Loops and cords should be used to ensure this. A tension of 4.5 ±0.5 kg should be applied to the tape through a spring balance. Three measurements, at the bottom, middle and top of each course shall be taken.

Measurement of Step-over During the measurement of circumference obstructions such as projections, deformities, fitting or joints may be found on the surface of the tank course. Obstructions force the tape to deviate from its true circular path and giving erroneous measurements. Step-over is used to correct the measurement of circumference.  The strapping tape should be stretched over the tank surface including the obstruction.  Now place the legs of the step-over across the obstruction on the tape where it is in contact with the





surface and note the reading between two legs of the step-over on the tape. Let the distance be 5 mm. Lift the step-over and place it again on the tape where it is in full contact with the surface, on the left of the obstruction. Let the distance on the tape between the legs be 4.1 mm. Then place the step-over on the tape on the right side of the obstruction. Now, the distance is 3.9 mm. The average of two readings is 4 mm. The step-over correction will be 5-4 = 1 mm.

Measuring Shell Plate Thickness 



Now a days, plate thickness is measured by using ultrasonic or electronic thickness gauges. But, before using, the gauge should be tested with a preverified thickness. If any direct measurement is not possible, the course-wise thickness data supplied by the manufacturer may be used.

Vertical Measurement 

a tape should be suspended internally along the wall of the shell from the top curve angle to the bottom course and the height of course is taken in mm.  The difference in height between the datum point and the bottom course (the thickness of the datum plate) is calculated.  Let the difference be 15 mm  This difference should be applied to the height of a course as correction In the figure, the actual height of 1st course at A is 1520-15 = 1505 mm = 150.5 cm 2nd course at B is 3057-15 = 3042 mm = 304.2 cm 3rd course at C is 4685-15 = 4670 mm = 467.0 cm 4th course at D is 6235-15 = 6220 mm = 622.0 cm and so on. Height measurements should always be converted into cm. It may not be possible to measure course height internally. In such cases, external measurement may be taken and allowance may be made to neutralize the effect of horizontal seam overlapping.

Dead Wood Measurement Table 2/5



 

Deadwood should be measured internally. If not feasible, dimensions given in the manufacturer's diagram may be accepted.  Measurements should show the lowest and highest level of deadwood from the tank bottom adjacent to the cell.  The total volume of each piece of deadwood should be calculated.  Measurements should be made in increments which permit allowance to be given to tank capacity at various heights. Work sheet on which details of deadwood are sketched dimensions and located, should be clearly mentioned in the strapping record. For variable deadwood such as nozzles and manholes, located at the bottom of a tank, an average deadwood correction shall be made.

Tank Bottom Measurement 

  

The method most employed is "Liquid Calibration". The procedure is to fill into the tank quantities of known volume of water until the datum plate is just covered and the total quantity recorded. (say 28230 litres) Measure the height of the water level from the bottom when dip at datum plate was 0.0 cm. (say 16.4 cm) Measure the height of the bottom course. (say 150 cm) Effective height of the bottom course will be the difference between these two heights. 150.0 cm - 16.4 cm = 133.6 cm

Tilt Measurement  This is done by suspending a plump line from the top angle and measuring the offset at the bottom angle. Let the angle be Ø  The tilt may be ignored if it is less than 1 in 50, this representing a maximum error 0f 0.2 per cent.  Open capacity of the tank in litre per cm will have to be multiplied by sec Ø