African Petrochemicals May/June Edition 15.3 {2018}

UNDERSTANDING THE EFFECTS OF PULSATIONS ON GAS FLOW
A measurement problem that is mostly undetected with serious consequences for the user is that
caused by pulsation flow. It is, therefore, important to grasp the effects of pulsations on gas flow
meters to be able to formulate control techniques to effectively mitigate pulsation effects.
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To clearly explain the
effects of pulsations
on gas flow metering,
Hendrik van Huyssteen,
Managing Director of
Energas Technologies,
a leading supplier of
high-end and specialised
equipment to the oil
and gas industries in
Southern Africa, cites a
closer-to-home instance.
He says that natural gas
from Mozambique, for
instance, is supplied in
Gauteng, South Africa,
through a network
of distribution and
reticulation pipework.
He notes that the American Gas Association
gives recommendations for installation
of flow meters to minimise the negative
effect of line conditions that could result in
metering errors such as non-uniform flow
and swirl along the distribution pipework.
However, the installation requirements
cannot compensate for the effect of
pulsations in the line. Pulsations can be
caused by piston compressors, often used
to increase the pressure of the gas as in
Compressed Natural Gas (CNG) applications.
“Custody certified meters used here must
have errors not more than +/- 1% when
compared to a certified master meter,” notes
Van Huyssteen. “Such meters are equipped
with volume correctors which calculate
totalised gas flow corrected to standard
pressure (101.325 kPA) and temperature (15
°C) and compensate for the compressibility
of gas.”
The types of meters used and ranges at
typical operating pressures used in the
industry are:
• Diaphragm meters for small flow rates less
than 16 Sm3/h at 50 kPa
• Positive displacement meters up to about
1500 Sm3/h at 100 kPa
• Turbine meters up to 200 000 Sm3/h at
3000 kPa
• Ultrasonic meters in excess of 500 000
Sm3/h at 3000 kPa
Diaphragm meters for small flow rates.
Pulsating gas flow
In turbine meters, for example, pulsating
gas flow causes a cyclical slowing and
speeding of the main rotor. This can result
in considerable over-reading of the meter,
resulting in the client being billed for much
more than the gas used.
“Most meter types are adversely affected
by pulsations. A pulsation is a periodic
fluctuation in local pressure and velocity
that occurs throughout a piping system or
network. Pulsations typically move through
a piping system as travelling waves. The
travelling waves can be effected by closed
and open ends of a piping network,” explains
Van Huyssteen, adding that pulsating gas
flow can cause severe damage to pressure
components and supports in the pipework
and should, therefore, be minimised.
Pulsation dampeners
To reduce the effect of pulsations, pulsation
dampeners could be installed at inlet and
outlet of reciprocating compressors. A
well designed pulsation dampener would
consider the frequency of pulsations,
design pressure, gas properties and the
pipe network lay-out. The pulse frequency
is determined by the number of pistons and
the rotating speed of the compressor.
“The pulsation dampener is a pressure vessel
with a series of carefully positioned baffle
plates. The pulsation dampener must be
designed for the line
pressure and temperature
and comply with the PER
requirements,” says Van
Huyssteen. “Normally, it
is recommend to put the
dampener as close to the
compressor as possible,
to avoid resonances in
the pipeline. In a case
where you have two
compressors, it would be
safer to put a dampener
on each compressor.”
Using one dampener
with two compressors
will work from a
mathematical standpoint,
but you have to connect
it further from the pulsation source, which
risks having a harmonic length of pipe in the
line before the dampener.
The pulsation dampener reduces the
amplitude of the pulsation as it travels
through the vessel. The reduction is
dependent on the frequency and can be
reduced by as much as 97%.
Auto-Adjust Turbine meters
The AGA report No. 7 notes that pulsations
cause a positive error in turbine meter output
that is dependent on pulsation velocity
amplitude at the meter, flow rate, gas density
and meter and pulsation properties. This
error can be as large as 50%.
According to Van Huyssteen, the Auto-Adjust
Turbine (AAT) meter supplied by Sensus is
the only one of its nature that can detect
and compensate for pulsating flow. Energas
Technologies is the sole representative of
Sensus Meters in South Africa. Energas has
supplied AAT meters to the local natural gas
industry since 2001.
“The AAT meter has two rotors, a main
rotor and a second sensing rotor, typically
running at 10% of the speed of the main
rotor,” he explains. “The speed of the rotors
is monitored by a special volume corrector
using complex algorithms. Any deviations of
accuracy relative to the originally calibrated
accuracy are detected and displayed.”