ECHONews - SOCON Sonar Control Kavernenvermessung GmbH

The gas drying unit (GDU)
When gas flows through the GDU the
pressure and temperature is reduced.
The pressure drop depends upon the volumetric
flow and real values are used for
the calibration. For a given rate the pressure
drop can be interpolated by a spline.
A „good“ approximation is yielded from
a large number of accurate measured values
and provides a „reasonable“ assessment
of the pressure drop. When multiple
GDU‘s are working in parallel the gas
stream is split in line with the number of
used GDU’s. The volumetric flow leads to
a smaller pressure drop.
The pressure reduction stream and
heat exchanger
If the combined cavern pressure compared
to the transmission pipeline (national
grid) pressure is very high, then
the pressure is reduced to the pressure
which is necessary to transfer the gas
into the transmission pipeline plus pressure
losses in the station piping and the
meter streams. The heat exchanger has
the job of raising the gas temperature to
achieve a minimum temperature of 18°C
downstream the pressure reduction station.
Temperatures are generally calculated
using the Joule-Thomson algorithm.
The calculations have been taken from
the worksheet G499 published by the
German Technical Association for Gas
and Water (DVGW) [5].
Possible scenarios during injection /
withdrawal
Essentially four cases can be differentiated
in the calculations for the above
ground procedures:
1. Withdrawal without compression
2. Withdrawal with compression
3. Injection without compression
4. Injection with compression
By way of example case 1 is described
in more detail. The procedures and cal-
10
Fig.4: Flow chart for the situation: Withdrawal without compression
culation routes with the individual components
are summarised in Fig.4. The
circled numbers (x) in the figure indicate
the order of the calculations. The dynamic
input data is: Cavern head pressures
for three caverns, total flow volume as
well as pressure and temperature in the
transmission pipeline (national grid).
Procedure for case 1:
Withdrawal without compression
Pressure and temperature losses in the
field pipelines from the cavern heads to
the manifold (1) up to the inlet separators
(2) are calculated. Pressure and temperature
after the separators and before the
heat exchangers as well as the pressure
reduction streams are determined (3). At
this point the simulation result defines
whether withdrawal is to take place with
or without a compressor. If withdrawal
can be done without a compressor then
generally the pressure must be reduced
and the heaters must raise the temperature
of the gas so that after the pressure
reduction it is equal to or higher than
18°C or 301.15°K. If the pressure drop is
low and /or the gas temperature is high,
the 18°C temperature will be achieved
without heaters so they will not be used
in the simulation. In order to decide if a
compressor must be used the simulation
calculates the pressure loss from the
national grid to point (3). This is done by
a retrograde simulation of the pressure
and temperature losses that occur from
the national grid via the meter streams
(4), the station piping (5), the gas drying
units (6) and up to just before the heat
exchangers (7). Points (4) to (7) do not represent
flow paths but rather calculation
paths. The 7.2 MPa pressure at point (7)
is about 0.6 MPa less than the pressure
at point (3), where it is 7.8 MPa. This means
withdrawal can take place without
recompression, but pressure reduction
is needed. The heat exchanger need not
to be used because the downstream
temperature due to pressure reduction
(8) is more than 11.92 K above 301.15
K (setpoint temperature). The pressure
and temperature losses in the station
piping (11) and in the meter stream
(12) indicate a pressure of 7 MPa at the
transmission pipeline (national grid).