UNCERTAINTIES IN SHORE TANK MEASUREMENT
Flowmeter
manufacturers are
sometimes asked by
tank operators why there is a
difference in the cumulative
measurements from the
flowmeters, compared to the
inventory measurements from
the level gauges in the tank.
This article is based on a
study carried out for a
customer using a KROHNE
custody transfer metering
system alongside a tank
inventory system. It outlines
some of the issues, gives a
listing of where errors can
occur and can accumulate.
Which system do I choose? Ultrasonic flow, turbine flow, or level gauge inventory?
When installed flowmeters
are directly compared to
shore tank volumes a
significant amount of
uncertainties has to be
considered. Are we
comparing a singe batch
going into the tank, or a
single batch exiting the tank?
In this case, multibeam
ultrasonic meters are the
measurement of choice. They
do not drift, they are
repeatable, and offer a high
custody transfer accuracy of
0.15%, which is virtually
impossible with any tank
inventory system currently
installed.
Factors affecting the accuracy of a tank inventory system
The uncertainties most
commonly faced when bulk
liquids are being measured
can be divided into three
categories:
1. Spurious Error
2. Systematic error or bias
3. Random error
When a certain uncertainty
has been defined, the source
of the error has to be
investigated and its
significance or weighting
determined.
Sources of error
- The first obvious source of error is the product to be measured. Many influences can be taken into consideration for determining the actual volume of liquid. Generally the higher the viscosity the higher the uncertainty in the measurement.
- The second common source of error is the uncertainty of the individual instruments required for the measurement.
- The third potential source of error is the influence of the instrument operator.
- The fourth source of error is caused by environmental influences on the instruments.
- accuracy/uncertainty of the applied strapping tables
- accuracy/uncertainty of the applied level measurement procedure
- accuracy/uncertainty of the applied temperature measurement
- accuracy/uncertainty of the applied API tables
The following aspects of the tank have to be accurately measured:
- the exact dimensions of the tank shell;
- the contours of the tank floor;
- the size and location of “dead wood”, eg. struts, agitators and other mechanical obstructions;
- the weight (volume) of the floating roof.
To compensate for any human errors it is preferable to use automatic level gauges. Mechanical gauges have to be calibrated prior to instalment and have to be checked periodically using a certified measurement tape. The influence of the uncertainty of level measurement in shore tank operations has frequently been over-estimated; it is common practice to measure a single batch and try to compensate for the delay in start/stop of the level gauge. An accuracy of level measurement of +/- 2 mm is in most applications the best achievable figure. It is obvious that the influence of the uncertainty of the level gauge depends on the transferred volume, on a batch of 1 meter the influence will reach 0.4 % while for an 8 meters batch it can drop to 0.05 %.
