Tank strapping tables, innage and ullage: how tank volume is calculated

A tank strapping table is a list that converts liquid height into volume for one specific tank, built from measurements of the tank geometry and, in Indonesia, validated through verification (tera) by the legal metrology authority.

Every stock figure in a terminal passes through this table. Whether the height comes from a dip tape or from an automatic tank gauge (ATG), the volume is only as good as the table it is looked up in. This guide explains how the table is made, how heights are measured, and when the table must be renewed.

On paper, the volume of a cylinder is π × r² × h. A real storage tank departs from that formula in ways that matter at the scale of a terminal.

The shell is built from courses of plate with different thicknesses, so the inside diameter steps at each course. Under load, the shell bulges outward as it fills. The bottom may be coned up or down, and it settles over the years. Inside the tank, pipes, heating coils, roof supports and the floating roof take up or displace volume. A strapping table captures all of this as a height-to-volume list made for that tank alone.

That is why a table can never be copied from a sister tank, even one built from the same drawing. Two tanks of equal nominal size will differ in bottom shape, plate tolerance and internal fittings.

Manual sounding (dipping) is a person lowering a weighted tape into the tank and reading where the liquid cuts it. It looks simple, and that is why it is often done badly.

API MPMS Chapter 3.1A (3rd edition, 2013, reaffirmed 2018) sets the practice. It covers gauging the product level and the free water, verifying tape length under field conditions, and the influence of the bob weight, temperature and the reference point on the result. Indonesian sites usually add their own work instruction on top. The steps below summarise the common sequence; the standard and the site procedure govern the details.

The depth tape itself must be re-verified every year under Permendag No. 24 Tahun 2024, so check its verification mark before use.

Making a table means finding the average inside radius of the tank at every height, then summing the volume slice by slice from the bottom up.

API MPMS Chapter 2.2A (2nd edition, 2019) describes manual tank strapping for upright cylindrical tanks: a tape is run around each course to measure circumference, plate thickness and deadwood are recorded, and incremental volumes are computed. It is also the reference for the other Chapter 2.2 methods, which find the radius by optical or electro-optical means. A bottom survey adds the volume below the lowest course. The result is the capacity table that the certificate refers to.

The choice of method depends on the tank: whether it can be emptied, whether it is insulated, and what equipment the calibration contractor holds. Internal methods need an empty, clean tank, while external methods disturb operations less.

A valid certificate does not guarantee a valid table. Physical changes to the tank can make the table wrong long before its date.

• Bottom or shell repair. Replacing bottom plates, patching the shell or adding a new course changes the geometry.
• New internal fittings. Heating coils, new suction lines or roof supports change the deadwood.
• Roof work. A new or modified floating roof changes its weight and the roof correction.
• Settlement. A tank that has settled unevenly tilts the bottom and shifts the datum plate.
• Reference height drift. A reference height that no longer matches the certificate points to movement of the hatch or the datum plate.

Any of these warrants a discussion with the metrology authority, ideally before the repair starts so the calibration can share the tank shutdown. For the ATG, the same table sits in the TMS configuration, so a new certificate must be loaded there on the day it takes effect.

A strapping table looks like a long list of heights and volumes, but it carries notes and corrections that are easy to skip.

Most tables give volume at coarse height steps and add a fraction table for the finer steps in between, so the reader adds the fraction to the main entry instead of interpolating by eye. The table states the unit and the reference temperature of the steel shell. API MPMS Chapter 12.1.1 includes a correction for shell temperature, because a warm shell holds slightly more than the table says.

Floating-roof tanks have a roof correction that applies only above the level where the roof floats freely, and a zone below it where readings should not be used for transfer. The certificate also names the dip point and the reference height that the table assumes. If the gauger dips through a different hatch, the table no longer matches the measurement.

Write these notes into the TMS configuration and the site work instruction, and have a second person check the table entries after they are typed. A table typed in without its notes produces numbers that look right and are not.

Most table problems Citia sees are not calculation errors. They are version problems: the right table exists on paper, but the system uses another one.

The TMS tank configuration page in Citia's application holds product, volume, height, diameter, tape and bob values for each tank, and the INTEGRA TMS calculation depends on them. The ATG integration checklist asks sites to prepare the tank table in use, its units and dimensions before any integration starts, so the new system and the old records use the same basis.

Comparisons with manual readings expose a wrong table quickly. In the Redeco ATG assessment, ATG levels were checked against stick readings, and the Cikampek tank-monitoring upgrade closed with functional tests before handover. When the level agrees but the volume does not, look at the table and the volume correction factor before suspecting the gauge. The tank management system guide explains where these values sit in the stock calculation.