Compressing gas flowing through a conduit

An apparatus (1) is provided for compressing gas, the apparatus comprising a conduit (3) provided with a non-return inlet valve (10) arranged at the upstream inlet (5) end of the conduit (3), a localized ignition source (20) arranged in the conduit (3) downstream of the non-return inlet valve (10), and means (23) for intermittently supplying secondary fluid into the conduit (3) between the non-return inlet valve (10) and the localized ignition source (20).

FIELD OF THE INVENTION 
The present invention relates to compressing gas flowing through a conduit. 
The gas can be an inert gas or an oxidant or a combustible gas such as 
natural gas. 
BACKGROUND OF THE INVENTION 
A suitable application of the present invention is downhole compression of 
natural gas; this is done to enhance the production from an underground 
natural gas reservoir. Natural gas is typically produced from an 
underground reservoir containing natural gas through tubing arranged in a 
well drilled to the reservoir. During production, the cumulative amount of 
natural gas produced increases and consequently the reservoir pressure 
will decrease. As a result of the decrease in reservoir pressure, the 
production rate decreases and, in order to maintain the production rate at 
an economically acceptable level the gas has to be compressed. The 
compression is preferably done downhole. This compression also results in 
an increased economic cumulative production. 
An alternative application of the present invention is compression of a gas 
flowing through a pipeline to increase the rate of gas transported through 
the pipeline. 
U.S. Pat. No. 2,899,287 discloses an apparatus for producing combustible 
gas from fuel oil or coal comprising a conduit provided with a non-return 
inlet valve arranged at the upstream inlet end of the conduit, a localized 
ignition source arranged in the conduit downstream of the non-return inlet 
valve, and means for supplying continuously fuel into the conduit between 
the non-return inlet valve and the localized ignition source. 
The known apparatus is an apparatus for partially combusting fuel to 
generate a combustible gas. During normal operation (a) fuel is supplied 
continuously into the conduit with oxidant that has entered in the conduit 
through the non-return valve; (b) the combustible mixture is allowed to 
ignite, which ignition yields a high pressure wave front closing the 
non-return inlet valve and pushing gas out of the outlet end of the 
conduit and a low pressure wave front; and (c) the non-return inlet valve 
is allowed to open on arrival of the low pressure wave front to allow 
oxidant to enter into the conduit. 
In the known method the interval between two successive combustions is 
determined by the geometry of the conduit which may not always have the 
most suitable form for an optimal interval. The pressure increase for the 
compression stage of the known apparatus is very small, the pressure at 
the outlet end of the conduit is about 2 or 3% above the pressure at the 
inlet end. For pumping natural gas such a pressure increase for a 
compression stage is unacceptably small. Therefore it is an object of the 
present invention to provide an apparatus for compressing a gas which will 
give a larger pressure increase than the known apparatus. 
SUMMARY OF THE INVENTION 
To this end, the apparatus for compressing gas according to the present 
invention comprises a conduit provided with a non-return inlet valve 
arranged at an upstream inlet end of the conduit, a localized ignition 
source arranged in the conduit downstream of the non-return inlet valve, 
and means for intermittently supplying secondary fluid into the conduit 
between the non-return inlet valve and the localized ignition source. 
The invention further relates to a method of compressing gas flowing 
through a conduit having an inlet end and an outlet end and being provided 
with a non-return inlet valve at its inlet end, which method comprises the 
steps of 
(a) supplying intermittently a combustible mixture in the conduit 
downstream of the non-return inlet valve; 
(b) allowing the combustible mixture to ignite, which ignition yields a 
high pressure wave front closing the non-return inlet valve and pushing 
gas out of the outlet end of the conduit and a low pressure wave front; 
and 
(c) allowing the non-return inlet valve to open on arrival of the low 
pressure wave front to allow gas to enter into the conduit. 
Applicant has found that it is advantageous to supply intermittently a 
combustible mixture in the conduit, so that the interval between two 
successive combustions is determined by the interval between successive 
supplies of combustible mixture in the conduit. The geometry of the 
conduit can now be selected to minimize the resistance to flow. 
By supplying intermittently a combustible mixture in the conduit the 
time-averaged amount of combustible mixture is small compared to the 
amount of gas flowing through the conduit.

DETAILED DESCRIPTION OF THE INVENTION 
The apparatus 1 comprises a conduit 3 having an inlet end 5 and an outlet 
end 8, which conduit 3 is provided with a non-return inlet valve 10 
arranged at the inlet end 5 of the conduit 3. 
The conduit 3 is provided with a localized ignition source 20 arranged in 
the conduit 3 downstream of the non-return inlet valve 10, and with means 
23 for intermittently supplying secondary fluid into the conduit 3 between 
the non-return inlet valve 10 and the localized ignition source 20. 
The means 23 for intermittently supplying secondary fluid includes a 
container 26 having an inlet 27 and an outlet 29 debouching into the 
conduit 3 provided with a non-return outlet valve 32, and a supply conduit 
35 which is connected to the inlet 27. 
When the gas to be compressed is a combustible gas, corn pressing such a 
gas using the apparatus according to the present invention comprises 
allowing combustible gas to pass the non-return inlet valve 10 and 
supplying intermittently oxidant into the conduit through outlet 29 of the 
means 23 for intermittently supplying secondary fluid so that a 
combustible mixture is supplied intermittently in the conduit 3 downstream 
of the non-return inlet valve 10. The combustible mixture is allowed to 
ignite, which ignition yields a high pressure wave front closing the 
non-return inlet valve and pushing gas out of the outlet end of the 
conduit and a low pressure wave front. The non-return valve 10 is allowed 
to open on arrival of the low pressure wave front to allow combustible gas 
to enter into the conduit 3. A new cycle then starts. 
The time-averaged amount of combustible gas required for the combustion is 
small compared to the amount of combustible gas flowing through the 
conduit 3. 
This application is useful to compress natural gas for pumping the gas 
through a pipeline or through a well tubing arranged in a well extending 
from an underground gas reservoir to the surface. 
The apparatus according to the invention can as well be used to compress 
oxidant, in which case the method comprises allowing oxidant to pass the 
non-return inlet valve 10 and the supplying intermittently fuel into the 
conduit through outlet 29 so that a combustible mixture is allowed to 
ignite, which ignition yields a high pressure wave front closing the 
non-return inlet valve and pushing gas out of the outlet end of the 
conduit and a low pressure wave front. The non-return valve 10 is allowed 
to open on arrival of the low pressure wave front to allow combustible gas 
to enter into the conduit 3. A new cycle then starts. 
When the localized ignition source 20 is sufficiently heated, the localized 
ignition source acts as a hot spot so that supply of electric power can be 
interrupted. 
The oxidant is a gas containing free oxygen, an example of suitable oxidant 
is air, a further example is air enriched with oxygen. The oxidant can 
furthermore contain water, which upon vaporizing will furthermore increase 
the pressure. 
Each of the non-return inlet valve 10 and the non-return outlet valve 32 
can be provided with a control device (not shown) allowing opening of the 
each of the valves 10 and 32 at a pre-determined pressure difference 
across the valve. By adjusting the control device the opening 
characteristics of the non-return valves can be adjusted. An example of 
such a control device is a spring, an other example of such control device 
is a magnetic latch. 
The interval between two successive supplies of combustible mixture in the 
conduit is determined by the ratio of the diameter of the supply conduit 
35 and the inlet 27 of the container 26, the volume of the container 26, 
and the opening characteristics of non-return outlet valve 32. 
More than one apparatus according to the present invention can be arranged 
in series, each following apparatus compressing the gas compressed by the 
previous one. If required, the compressed gas can be cooled between two 
successive compressive stages. 
The conduit 3 has a constant inner diameter; in an alternative embodiment 
of the invention, the conduit comprises, in the direction of flow, a 
narrowing section upstream of the non-return valve 10, a section having a 
constant diameter, and a widening section downstream of the localized 
ignition source 20.