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.
