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.

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.

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.

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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.

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