Q
&
A
Flow
Proponents of the use of live fluids (i.e. natural gas, crude
oil and brine) argue that flowloops using such fluids are more
representative of the conditions that meters will encounter in
service. However, research shows that the use of live fluids, such
as natural gas, crude oil and brine, increases uncertainty on the
calculated fluid phase properties. This is due to the issues of
dealing with essentially unknown partitioning of hydrocarbon
components between the vapour and liquid phases.
In order to achieve the lowest overall uncertainty, it is
necessary to control all of the parameters as accurately as
possible. This would therefore justify that the best metrological
approach is to eliminate this issue by the use of suitable, stable,
well-characterised reference fluids. It is for this reason that NEL
has recently switched from using crude oil to refined oil as part of
it multiphase flowloop meter testing.
This is also consistent with the approach recommended in the
current issue of the Department of Energy and Climate Change’s
Guidance Notes for Petroleum Measurement, Issue 9, July 2014
(section 9.7), which makes specific mention of the need to account
for possible transfer of components between phases and a
preference for ‘model’ fluids to minimise additional uncertainties.
Fluid mechanics, computational
fluid dynamics and flow research
One of the great advantages of CFD is that it can be used in
design optimisation for flowmeters and their installation, saving
significant amounts of time and money associated with testing.
Using CFD to undertake computer simulation before physical
testing means that a model can be tested using multiple variables,
to fully assess a product’s potential before an investment is made
to build a prototype.
However, competitive pressures to bring products to
market faster and at reduced cost means that mechanical CAD
designers and engineers, in addition to specialists, are now able
to perform CFD analyses. With CFD tools now readily available,
the temptation is to think that anyone can perform CFD. But, the
dilution of CFD expertise increases the likelihood that ill trained,
inexpert end-users will introduce the element of human error.
For high value markets, even the smallest prediction errors are
unacceptable. For example, oil and gas are both bought, sold and
taxed based on the measurements from flowmeters, and a 1 or 2%
error could be the difference of £100 million/yr.
While CFD brings many advantages, it is important to
remember that it is still just a prediction method. To be assured
of accuracy, these predictions must be verified by an excellent
engineering understanding of the systems being modelled, and
the degree of accuracy of the simulation result must be fully
understood by the engineers. As the work required to deliver a
CFD prediction is complex, an expert CFD engineer must be relied
on to exercise their ‘engineering judgement’ to use it effectively.
Flowmeter technology, selecting a
flow measurement tool
There are many flow measurement technologies available as
commercial products in the oil and gas industry, each having
there own advantages and disadvantages. Some of these can
be subjective and many end users disagree on which is the best
meter for a particular application. What people do agree upon
though is that there is no perfect meter that can be considered
optimal in every application. Instead, when selecting a meter for a
process, careful consideration has to be given to the application in
question. The end user must always ask themselves the questions
that will narrow down their selection process. This will also
highlight their selection criteria.
Why is the measurement necessary? This could be due to
government regulations, for safety and control of a process, or
used in the custody transfer or allocation of produced fluids.
Whatever the application, there will be a maximum allowable
measurement uncertainty that has to be achieved by the selected
technology.
What is being measured? Consider the range of process
conditions of the stream; fluid, temperature, pressure, acidity,
viscosity, flowrate. All may play an important part in the decision
process through achievable turn down, fouling, pressure drop or
even damage to the meter.
What is the physical installation? Many meters are susceptible
to flow profile distortions causes by installation effects – are
there enough upstream and downstream straight lengths of pipe
for a fully developed flow profile? The space for the meter itself,
the space for removal, working at heights, power, communications
etc, can all play a part in meter selection.
Are there any additional selection criteria? Some applications
must limit pressure drop, some dictate the use of non-invasive
technologies. A criterion can be considered obscure to one end-
user but may be crucial to another.
Once all the correct questions have been asked then a
decision can made on which technology to use. However, it is
sometimes an iterative approach and there may be no technology
available to match all the users needs. Often, a compromise is
sought in order to make any measurement at all, where only the
most important selection criteria are satisfied.
Diagnosing meter performance
Modern meters can now record and store a vast amount of
flow measurement-related data. This has allowed the detailed
monitoring of all the recorded data to be used as a diagnostic
Figure 1.
Substitute fluids offer a better solution when testing
flowmeters.
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