HCOTEG175020 - Measuring the Content of Tanks: Terms relating to tank measurement
- MPE – Maximum permitted error - the maximum error a measuring system can vary from the proving meter and still be used. (MPE usually refers to Meter rather than to Tank measurement). Note that ‘MPE’ is not an allowable tolerance: it does not allow a trader to understate by the amount of the MPE because that’s the amount we allow.
- Dipping and ullaging – dipping measures the depth of liquid in a tank (the top of the liquid to the bottom of the tank) whereas ullaging measures the height from the top of the liquid to the roof of a tank. Dipping has to be from a known point to the bottom of the tank, whereas ullaging is from a fixed point on the tank roof. Note: in order to work out the volume of liquid in a tank using either method, you have to know how much the tank holds at any given height/level. You can’t just assume the tank construction of the tank is regular. Accordingly, each tank should have a tank calibration table.
- Tank calibration table – a table that tells you what the volume in a tank is for any particular height of liquid (measured by either dipping or ullaging). If the tank is measured for revenue accounting purposes, this can be affected by:-
- Deadwood – anything that protrudes into a tank (a brace or reinforcing strut) which takes up volume that would otherwise be measured as oil.
- Water bottom – is the water in a tank on top of which oil floats. As water falls to the bottom of the tank, a measurement made by ‘ullaging’, might not detect the water bottom, which could then be assumed to be oil.
- Floating roofs – the roof on top of large refinery tanks can literally be floating and move up and down with the level of oil in the tank. They usually have bottom stops or stays that stop them falling to the very bottom of a tank. Floating roofs can affect volume measures in the tank (by displacement) and should never be walked on.
- Automatic Level Gauges (ALGs) – are commonplace and alternatives to manual ‘dipping’ and ‘ullaging’ found on the fixed roof of large depot tanks. They measure the height and also usually the temperature (see below) of oil within the tank. The measurement given by the ALG is usually transmitted electronically to the refinery or depots’ stock control room/system.
- Automatic Temperature Compensating (ATC) – as previously mentioned, HMRC account for oil by volume, but volume varies with temperature. Any measurement system has to adjust for what the oil temperature is at the time the oil is measured, and to calculate what the volume would be if the oil temperature was 150C. (We prefer ALGs that can do this automatically).
- Vapour Recovery – because of EU emission regulations (EU Directive 94/63/EC implementation beginning in 1998), and to save money, tanks’ breathing pipes that were once open to the air, now have to be connected to a small plant that returns the vapour given off back into the tank as liquid fuel. This is especially important for the more volatile fuels such as petrol, which vaporises easily, but it also applies to crude oil. Vapour recovery can be hazardous (see HCOTEG173000 ‘Vapour Recovery Hazards’).
- Weighbridge – the compact Oxford Dictionary says ‘a machine for weighing vehicles, set into the ground to be driven on to’. All that is usually visible is a metal plate that is large enough for a truck to drive on (or it could be set into rails for rail tank wagons).
- Tank Reference Height – this is the height of the point from where all manual dipping takes place.