HCOTEG175250 - Measurement: preferred measuring methods
Introduction
In all cases the trader should use the most accurate method available to them. They are not allowed to pick and choose the method they use. If the trader wishes to use a less preferred measure for any reason such as a temporary failure, they must ask our permission.
Storage tanks (site fixtures)
The preferred measuring methods are:
- Automatic (tank) Level Gauges (ALGs).
- Dipping/ullaging.
For small storage tanks properly calibrated sight glasses (a clear tube up the side of the tank that looks like a large thermometer) are acceptable.
Road or rail tank wagons
The preferred measuring methods are:
- Loading by volumetric meter (the use of meters is mandatory for road tankers loading all light oils, derv, kerosene and gas oil from duty suspended installations).
- Automatic loading gauges (a probe that ‘dips’ into the tank).
- ‘Dipping’ and ‘ullaging’ (are no longer possible and not considered safe on most road tankers).
- Weighbridge.
Ship transport
The preferred measuring method is a volumetric meter. (Short sea shipping is the most cost efficient method for large amounts outside of a pipeline). It is possible to dip (and ullage where the tank is constructed to allow this) ships’ tanks but they present special problems that render them generally inaccurate:
- Ship not level - they list (to the side) or pitch (front to rear) and thus the ships attitude in the water will change all the time that product is being pumped on or off the ship. As product is loaded, the ships’ displacement that is the height at which it sits in the waterline will also change, affecting its attitude. The ship may also be deliberately ‘trimmed’ for sailing - its angle in the water altered by flooding ballast tanks, to compensate for the loads contained in its oil compartments.
- Heavy oils require heating (ships will have their own heating devices) and crude oil coats the internal faces and bracing of the tanks in such a way that ‘remnants’ of the oil are left behind. (This remnant is so significant that operators go to the trouble of ‘washing’ the product out of the tank with lighter oil, in order to recover it).
Accordingly, although a ship’s tanks will have appropriate calibration tables, using ‘dips’ or ‘ullages’ requires extreme care and their routine use as an alternative to metering (except in an emergency) should be questioned and invite audit.
Vessel experience factors
Vessel experience factors (VEF) may be used to adjust ships tank dip figures to obtain a more accurate reading. This is an ‘adjustment factor’ calculated for a particular ship that compares the ships’ own readings historically against alternative shore measurements.