Note: Descriptions are shown in the official language in which they were submitted.
CA 02461086 2004-03-09
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TITLE OF THE INVENTION:
Method of power generation from pressure control stations of a natural gas
distribution system
FIELD OF 'THE INVENTION
The present invention relates to method of generating; power from pressure
control
stations of a natural gas distribution system.
BACKGROUND OF THE INVENTION
Natural gas distribution systems use three types of natural gas pipeline
networks:
high pressure (approximately 1,000 psig), medium pressure (approximately 100
psig) and
low pressure (approximately 5 psig). Where the high pressure pipeline network
feeds into
the medium pressure pipeline network, the pressure must be reduced from 1000
psig to
100 psig. Where the medium pressure pipeline network feeds into the low
pressure
pipeline network, the pressure must be reduced from 100psig to Spsig. This is
done
through a series of pressure reducing control valves at facilities known as
Pressure Control
Stations.
2 0 As the pressure of natural gas is reduced, it expands. As the natural gas
expands,
the temperature of the natural gas decreases. This dramatic drop in
temperature leads to
the formation of hydrates, which damage the pressure reducing control valves.
In order to
avoid the formation of hydrates, the natural gas is pre-heated at the Pressure
Control
Stations before pressure is reduced, with a view to maintaining an outlet
temperature of ~
2 5 degrees Celsius. The mode of preheating the natural gas upstream of the
pressure control
valves is by consuming some of the natural gas in a hot water or low pressure
steam
boiler, which supplies heat to a heat exchanger. The heat exchanger is then
used to
preheat the incoming natural gas.
3 0 In this typical mode of pipeline pressure control arrangement, the energy
lost
across the pressure reducing control valves in bringing the prE;ssure down
from 1000psig
to 100psig is significant. Similarly, there is energy lost across the pressure
reducing
CA 02461086 2004-03-09
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control valves in bringing the pressure down from 100psig to 5psig. If this
energy could
be captured; there potentially could be a net energy gain, as opposed to a net
energy loss
realized from the Pressure Control Stations.
SUMMARY OF THE INVENTION
What is required is a method of generating power from pressure control
stations of a
natural gas distribution system.
According to the present invention there is provided av method of generating
power
from a pressure control station of a natural gas distribution system. A first
step involves
channelling natural gas entering the pressure control station unto a turbine
which is powered
by expansion of the natural gas as the pressure of the natural l;as is
reduced. A second step
involves capturing the output of the turbine for application for useful
purposes.
The method described above utilizes energy that is presently lost across the
stem of
the pressure control valves and utilizes it in a form of turbine which is
powered by
expanding gases, commonly known as a Turbo-expander.
2 0 BRIEF DESCRIPTION OF THE DRAWINGS
These and other features of the invention will become more apparent from the
following descriptian in which reference is made to the appended drawings, the
drawings are
for the purpose of illustration only and are not intended to in any way limit
the scope of the
invention to the particular embodiment or embodiments shown, wherein:
2 5 FIGURE 1 labelled as PRIOR ART is a schematic diagram of a Pressure
Control
Station.
FIGURE 2 is a schematic diagram of a Pressure Control Station constructed in
accordance with the teachings of the present invention.
3 0 DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The preferred method of generating power from a press~.ure control station of
a natural
gas distribution system will now be described with reference to (FIGURES 1 and
2.
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In order to place the present method in context, the PRIOR ART system will
first be
described. Referring to FIGURE 1, a high pressure pipeline network is
indicated by
reference numeral 12 and a low pressure pipeline network is indicated by
reference numeral
14. Interposed between high pressure pipeline network 12 and low pressure
pipeline network
14 is a Pressure Control Station generally indicated by reference numeral 16.
High pressure
natural gas flowing from high pressure pipeline network 12 passes through a
first line shut off
valve 18, a course control valve assembly, generally indicated by reference
numeral 20, a ~.ne
control valve assembly 22, before encountering a second line shut off valve
24. A boiler 26
with an associated heat exchanger 28 is positioned on a diversion loop 30.
Three valves 32
are provided which control the feed of natural gas into and out of heat
exchanger 28. Natural
gas is pre-heated in heat exchanger 28. The pre-heated natural gas is then
directed through a
series of pressure reducing control valves 34. A third line shut off valve 36
enables Pressure
Control Station 16 to be isolated from low pressure pipeline network 14. A
fuel gas supply
conduit 38 diverts some of the processed low pressure natural gas for use in
fuelling boiler 26.
The principle of operation is to preheat the natural gas in heat exchanger 28
to avoid the
production of hydrates when the natural gas passes through the series of
pressure reducing
control valves 34. The energy generated as the pressure of the natural gas is
produced is lost
at pressure reducing control valves 34. In addition, energy input is needed in
the form of gas
2 0 consumption to power boiler 26. There is, therefore, a net energy loss.
Referring to FIGURE Z, a configuration in accordance with the present method
is
illustrated as being super-imposed upon the PRIOR ART Pressure Control Station
of
FIGURE 1. It is envisaged that the existing infrastructure will be kept in
place to maintain
2 5 redundant systems for reasons of public safety.
In accordance with the teachings of the present method, natural gas is
diverted by
passing heat exchanger 28 and series of pressure reducing control valves 34. A
key aspect of
the present method is channelling natural gas entering Pressure Control
Station along line 50
3 0 and into a turbine 52 which is powered by expansion of the natural gas as
the pressure of the
natural gas is reduced. The output of turbine 52 is then captured for
application for useful
purposes. It is preferred that the turbine be used to power an elE:ctrical
generator 54. The use
CA 02461086 2004-03-09
of turbine 52 can be done either with or without the natural gas being pre-
heated, as will
hereinafter be further described.
Turbine 52 is preferably a turbine known as a "turbo-expander". It is a radial
inflow
turbine with variable inlet guide vanes for flow control, which are used to
extract energy
from a gas stream. The method uses the turbo-expander (turlbine 52) to
generate power in
Pressure Control Stations 16 in a natural gas distribution system. The
expansion across the
inlet guide vanes and expander wheel produces torque and therefore shaft power
that can
be used to turn power generator 54.
Where the natural gas specifications of the working stream permit, turbine 52
can
be used without preheating the natural gas. The natural gas is channelled into
turbine 52,
with a view to intentionally generating cold temperatures. A heat exchanger 56
is provided to
capture the cold temperatures generated for use in either refrigeration or air
conditioning. A
fluid circulation can then be provided through heat exchanger 56 which can be
used for air
conditioning of nearby facilities or refrigeration of nearby cold storage
warehouses. The
refrigeration achieved by expansion of the gas is usually nnuch more than
achieved by
3oule-Thompson (J-T) expansion across a valve.
2 0 Where preheating of the natural gas is required, boiler 26 replaced by a
gas
fuelled turbine power generator 58, sometimes referred to as a "micro-
turbine". A portion
of the high pressure natural gas is diverted along conduit 60 and passed
through a gas
conditioning system 62 to condition the natural gas so that the natural gas is
suitable to power
gas fuelled turbine power generator 58. Exhaust gases from gas fuelled turbine
power
2 5 generator flow along conduit 64 and are passed through a first heat
exchanger 66. A hot water
circulation circuit is provided which includes expansion tank circulation
conduit 68,
expansion tank 70, a pump 74 and valves 76. Expansion tank 7U provides make up
water for
circulation conduit 68, as required. Pump 74 is used to circulate hot water
through circulation
conduit 68. Water is circulated through conduit 68, which passes heat
exchanger 66, so that a
3 0 heat transfer takes place with the hot exhaust gases from gas fuelled
turbine power generator
58 and heating the water. The exhaust gases are then released to atmosphere. A
secondary
heat exchange then takes place in a second heat exchanger 78 between the hot
water and the
CA 02461086 2004-03-09
natural gas. The natural gas, which has been preheated in seco~ad heat
exchanger 78 is then
channelled through line 50 to t«rbine 52. The output of gas fuelled turbine
power generator
58 is also captured for useful purposes of power generation through generator
portion 54.
The intent of the method is to capture and use energy that is currently being
wasted.
5 Depending upon the circumstances, it may be desirable to position a
dehydrator upstream
of second heat exchanger 78, to dry the natural gas. Suitable dehydrators
which use
absorbent medium are well known in the art. Normally two <~re used. One is
always in
service, while the absorbent medium in the other is being regenerated. Of
course, where
the objective is to generate low temperatures for the purpose of air
conditioning or
refrigeration, the hot water circulation circuit will not be used.
In this patent document, the word "comprising" is used in its non-limiting
sense to
mean that items following the word are included, but items not specifically
mentioned are :not
excluded. A reference to an element by the indefinite article "a" does not
exclude the
possibility that more than one of the element is present, unless the context
clearly requires that
there be one and only one of the elements.
It will be apparent to one skilled in the art that modi',fixcations may be
made to the
2 0 illustrated embodiment without departing from the spirit and scope of the
invention as
hereinafter defined in the Claims.
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