Archived Commentary Index
Calibration: The key to accurate gas measurement
Flow Measurement Section
Manager Southwest Research Institute
Research Facility (MRF) is one of just a few facilities in the world
that can perform high-accuracy, gas flow meter calibrations under
field-like operating conditions. The close-loop, re-circulating,
gas flow calibration facilities at the MRF provide maximum controllability
of pressure, temperature and flow rate, resulting in stable flow
conditions over long testing times. A key feature of the MRF is
its primary calibration systems based on gyroscopically-balanced
weigh tanks, the most accurate means known for measuring gas mass
flow rate. The MRF can correct flow meter bias errors as small as
of meter calibration.
A well run gas
transmission pipeline network will typically have an overall system
imbalance of less than ±0.5% (system imbalance is the difference
between the quantity of gas metered into and out of a pipeline network).
Large pipeline operators can potentially save millions of dollars
annually if they can reduce their system imbalance by just one percentage
point. To illustrate the potential for cost savings, let's look
at the financial impact of bias error in a single flow meter installed
in a typical interstate transmission pipeline. In this case, we
will choose a 12-inch diameter ultrasonic gas flow meter. The industry's
recommended practice, American Gas Association (A.G.A.) Report No.
9, specifies that meters of this type and size may have a maximum
error of up to ±0.7% over the upper 90% of the rated flow range.
The report further states that measurement bias errors attributed
to piping configuration at the meter station must not exceed +0.3%.
Let's assume that gas continuously flows through our meter at a
rate comparable to the meter's mid-range. We'll also assume that
the meter has a bias of minus 0.5% (i.e., it consistently under-registers
the volumetric flow rate by 0.5%). This meter bias error is well
within the allowable limits of Report No. 9, but would still result
in a measurement error valued at over $2,500 per day or almost $1
million per year. Bias errors of this type can usually be eliminated
by flow calibrating the meter at a cost of on the order of $5,000
to $50,000, depending on the method of calibration.
meters be calibrated?
A flow meter
calibration should take into account operational effects (and, when
practical, installation effects) that may adversely affect meter
performance. Flow calibrations may be performed either in-situ,
using a reference test flow meter plumbed in series at the meter
station, or off site, at a flow calibration test facility like the
MRF. For in-situ calibrations, consideration should be given at
the design stage as to all valving and piping requirements necessary
for a reference test flow meter to be installed on site. There are
currently no gas industry standards or guidelines for in-situ or
field meter proving. Off site flow meter calibrations are preferred,
in most cases, because test conditions can be more precisely controlled
than in the field.
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