Introduction
Coriolis flow meters have been used for Custody Transfer
of Liquid Hydrocarbons in the upstream, downstream and pipeline segments
for single-phase flow measurement of petroleum products such as Crude
Oil, White Oil, Black Oil, NGL Liquids etc.
Historically, these applications were done using a variety of methods,
adopted for verification of these meters performance in the field
or laboratory for custody transfer applications.
In Oct 2002, Committee on Petroleum Measurement (COPM) of the API
published the first edition of the measurement standard that provides
significant recommendations for the usage of this technology for custody
transfer of Crude Oil and finished hydrocarbon. This document was
added and listed as Chapter 5 Section 6 to the API MPMS document.
The intent of this article is to give an overview of the recommendations
from this standard for the application of Coriolis flow meters for
custody transfer with interpretation of the insitu proving requirements.
But first, it is important to have a quick overview of some operational
issues that are unique to these devices.
Factors Affecting Coriolis Meter Outputs
Coriolis meter is unique as it measures both mass
and density of the fluid. It is necessary to evaluate both the accuracy
of the mass measurement and density measurement, when considering
the accuracy of the volume output. Coriolis meters can differ dramatically
in their specification of density accuracy and therefore, would differ
dramatically in their volume accuracy.
The Coriolis meter has two main parts - a sensor and a transmitter.
The sensor is made up of single or dual tube configuration through
which the fluid passes. The transmitter for each sensor is typically
programmed with two unique numeric values: The manufacturer flow calibration
factor and density calibration factor. The factors are a result
of the sensor calibration that is normally performed in the laboratory
during the manufacturing process.
These calibration factors interpret the sensor signals into measurement
data and helps ensure that the meter performs to its stated specification.
These factors should not be confused with K-Factor or Meter factor
(or Density Meter factor). After programming the transmitter with
the flow calibration factors, the transmitter output signals are configured.
PSF (Pulse Scaling Factor) pulses per unit mass or volume; a coefficient(s)
is then entered in the Coriolis meter transmitter that defines the
relation between pulse output of the transmitter and quantity.
This is usually expressed in Hz (pulses per second) for a given flow
rate. For example, 10000 Hz = 1000 meter cube per hour. A similar
Kfactor is entered into the accessory equipment (such as a flow computer)
and is used to translate the pulses back to a quantity (flow rate).
The K-factor for the above example can be determined from the PSF
by converting the flow rate into units per second. The Hertz value
for that flow rate is then divided by the flow rate in units per second
which, for this example is: 10000 Hz/(10/36th of meter cube per sec)
= 36000 pulses per meter cube....
