3 Understanding Bulk and Commingled Stock

This chapter contains these topics:

Section 3.1, "About Bulk Stock"
Section 3.2, "How Does Temperature and Density Affect Volume?"
Section 3.3, "How Does the System Convert Volume?"
Section 3.4, "How Do You Measure Volume?"
Section 3.5, "How Do You Measure Temperature and Density?"

3.1 About Bulk Stock

The volume of a bulk product changes in relation to ambient temperature. Ambient temperature is the temperature of the surrounding environment that a product is in, such as a tank or a compartment of a vehicle. To record volume at a common base for all stock movements, you need to convert volume that you have calculated at ambient temperatures to volume calculated at a standard temperature. The system uses only standard volumes to make adjustments to bulk inventory.

This section contains the following:

How Does Temperature and Density Affect Volume?
How Does the System Convert Volume?
How Do You Measure Volume?
How Do You Measure Temperature and Density?

3.2 How Does Temperature and Density Affect Volume?

Temperature has a rather unique effect on liquids. A liquid product expands when its temperature rises and contracts when its temperature declines. How much the product expands or contracts is its relative density. The more dense the liquid, the less the liquid expands or contracts.

When you measure liquid products, you need to convert the volume measured at the ambient temperature of the liquid to its volume based on a standard temperature. The system performs this conversion using standard tables or algorithms. You define the standard temperature to which you want to convert for each product by depot.

3.3 How Does the System Convert Volume?

When you record an intra-depot stock movement, a receipt of stock, or other volume entry for bulk stock, you can enter volumes calculated at ambient or standard temperatures. The system uses the temperature and density table indicated on the Bulk Product Information form to calculate a volume correction factor (VCF). It then multiplies the VCF by the ambient quantity to get the standard quantity. If the depot's standard temperature is different from the temperature used in the table, a secondary conversion is made to convert from the base table temperature to standard.

If no table is indicated, the system uses the co-efficient of expansion to calculate the standard volume.

The system also calculates the weight of the product and converts the standard quantity to the Primary Stock Accounting Unit (PSAU) quantity for the product.

To calculate volume for asphalt and bitumen products or other products stored in heated tanks, the system also applies an expansion factor to account for the tank temperature.

The system records the following for volume conversion:

Ambient volume
Standard volume
Weight
PSAU quantity, either weight or volume

The following graphic illustrates the process that the system uses to calculate volume and weight.

3.4 How Do You Measure Volume?

To measure the volume of bulk products, you can perform various types of dips or use a weighbridge to weigh the product. Tank dip readings include the pipeline and discharge volumes, plus the initial dip volume.

3.4.1 Dip Measurement

A wet dip (innage) measures the liquid height in the tank. You measure the liquid height by lowering the innage tape and bob to the gauge striking point of the tank, pulling it out, and noting where the liquid marks the tape.

Figure 3-1 Innage Tape and Bob

Description of "Figure 3-1 Innage Tape and Bob"

A dry dip (ullage) measures the space between the liquid and a reference point at the top of the tank. You perform this type of dip when the product is too thick to be accurately measured. You measure the space between the top of the liquid and the reference point, and then determine the liquid height by subtracting the dry dip measurement from the reference height.

Tank Gauging and Strapping

After you measure the height of the liquid in the tank, you refer to the strapping table computed specifically for that tank. The strapping table converts tank dip readings to gross volumes for a particular tank.

Figure 3-2 Tank Measuring Points

Description of "Figure 3-2 Tank Measuring Points"

Strapping tables might be set up in the following ways:

Measurement	Description
Shell Height	Distance between the bottom of the bottom angle of the tank and the top of the top angle of the tank.
Gauging Height or Reference Height	Distance from the striking point on the tank floor (or strike plate) to a designated reference point on the gauge hatch.
Effective Inside Tank Height	Distance from the strike plate to the top angle, or where the product would begin to overflow. This height defines the upper and lower limits of the tank table.

Water and Sediment Height

When determining the gross amount of product in a tank, you must subtract the water and sediment from the total amount of product in the tank. To do this, you cover the innage tape with a water-finding paste, and then perform a wet dip. The paste reacts with the sediment, dissolves, and turns the tape red.

Floating Roofs

Floating roof tanks are normally used for aviation fuels or other products where it is critical to minimize the amount of water in the product.

Floating roof tanks have a moveable roof that floats on top of the product in the tank. A tank with a floating roof displaces a certain amount of liquid around its edges and up into the tank hatch. With such tanks, you must make a correction to the product measurement. The amount of displacement depends on the weight of the roof.

Heated Tank

A depot uses heated tanks for bulk products that need to be kept heated, such as asphalt and bitumens. A depot might also use a heated tank for any bulk product that might be abnormally expanded, as would occur in a cold environment.

3.4.2 Weighbridge Measurement

You can use a weighbridge to weigh the product in a tank, for example, a tank on a truck. To do so, you subtract the weight of the vehicle from the total weight. The system uses the weight to calculate volume. Weight is typically standard. It is not subject to volume changes in relation to temperature and density.

3.5 How Do You Measure Temperature and Density?

There are many types of thermometers for measuring the temperature of liquid in a tank and some that are standard for a particular type of tank. The temperature of liquid in a tank might vary throughout its depth, so you might need to perform readings at various depths and calculate an average reading.

You use a hydrometer to measure the density of liquids. A hydrometer floats vertically in liquid petroleum. Its buoyancy depends on the density of the liquid. You first take a sample of the liquid from the tank and put it in a glass cylinder. Then, lower the hydrometer into the tank and take a reading, as well as a temperature reading (using a thermometer). You use the temperature reading to convert from the density at the liquid's ambient temperature to the density at its standard temperature.