Gas compressor

Details Gas compressor Gas compressor

2001-06-05

2003-08-19

Verdier, Christopher (Department: 3745)

Rotary kinetic fluid motors or pumps

Method of operation

C415S104000, C415S106000, C415S112000, C415S113000, C415S230000, C415S231000, C277S408000, C277S431000

Reexamination Certificate

active

06607348

ABSTRACT:

This invention relates to a gas compressor and finds particular, though not exclusive, application to gas liquefaction, e.g. liquified nitrogen gas, ethylene and ammonia, refining, gas production and gas reinjection for enhanced oil production.
By way of background prior art, reference is directed to U.S. Pat. No. 3,420,434 and U.S. Pat. No. 5,421,593.
The problem that the present invention solves will now be described with reference to
FIGS. 1 and 2
of the accompanying drawings. In
FIG. 1
, there is shown a conventional system including gas compressor
1
used for compressing natural gas, for example from a gas production field. For simplicity, the portion of the compressor located below the axis of its main shaft
2
is indicated diagrammatically, whereas the portion above the shaft axis is depicted in some detail.
The compressor
1
has a main housing
3
, a gas inlet
4
, a delivery line
5
delivering production gas at production pressure (low pressure) to the compressor inlet
4
, and a gas outlet
6
discharging compressed (high pressure) gas along gas discharge line
7
. Within the housing
3
are successive, axially separated, gas compression stages or impellers. In
FIG. 1
are shown, by way of example, three compression stages
1
a,
1
b,
1
c,
but it is to be understood that any number of such stages may be used. Typically, the compressor will have between one and ten gas compression stages. The compression stages
1
a,
1
b,
1
c
progressively compress the low-pressure inlet gas, for discharge from the compressor as high-pressure gas.
As is well-known in the art, the compressor comprises a balance drum
8
with associated labyrinth seal
8
a,
separating the high-pressure region within the compressor housing from a balance chamber
9
, which is maintained at the same pressure as the inlet pressure to the compressor. For this purpose, a pressure equalization line
10
connects the compressor inlet
4
to the balance chamber
9
, as diagrammatically depicted in FIG.
1
. By means of this standard arrangement, net axial force acting on the compressor rotor in either axial direction can be significantly reduced, there being a double effect thrust bearing (not shown for simplicity) at the inlet end of the compressor for withstanding such reduced axial force, in whichever direction it acts.
The main shaft is supported at each end by a sealing arrangement which will now be described. Only the sealing arrangement at one end, i.e. that where the balance chamber
9
is located, will be described, but it will be appreciated that the description applies correspondingly to the sealing arrangement at the second end.
As shown, a labyrinth shaft seal
11
is provided adjacent the balance chamber
9
, but is not sufficient in itself to provide a sufficiently effective and reliable seal. Accordingly, an additional shaft sealing arrangement is provided by tandem inboard and outboard gas seals
12
,
13
respectively. Such seals are well known in the art and need not be further described herein. By way of example, the seals may be constructed in accordance with the disclosure of International Patent Applications PCT/IB94/00379, PCT/GB96/00939 or PCT/GB96/00940, all belonging to the present applicants.
An inlet port
12
a
of inboard gas seal
12
is supplied with gas by the delivery gas pressure in gas discharge line
7
, by way of a branch line from discharge line
7
comprising a common line
14
and a branch section
15
. The common line
14
also supplies gas to the inboard gas seal at the other end of the compressor in corresponding fashion. Each outboard seal
13
has an inlet port
13
a
which, as shown, is blocked off. Alternatively, no inlet port is provided at all. A filter system
16
is incorporated in line
14
for removing solid and liquid particulates from the high-pressure gas flow and thereby cleans the gas before it reaches the tandem gas seals (
12
,
13
). The outboard face of labyrinth seal
11
communicates via a small gap between the stationary and moving parts of gas seal
12
with the gas pressure at the port
12
a,
which is slightly above the pressure (compressor inlet pressure) in the balance chamber
9
, so that there is a small flow of gas along this route, past the labyrinth seal
11
, between the seal and shaft surface, and into the interior of the compressor. The remainder of the gas entering port
12
a
flows through the inboard gas seal
12
and arrives in a gas chamber
17
between the inboard and outboard seals
12
,
13
, a proportion of this gas being conveyed from this chamber
17
to a discharge line
18
leading to a flare system, which burns the discharged gas. The flare system operates at a pressure slightly above atmospheric pressure, say a few hundred millibars (e.g. 0.2 to 0.3 bar above atmospheric pressure).
The remaining proportion of gas in chamber
17
passes through the sealing region of gas seal
13
, from where it is conveyed along discharge line
19
to an atmospheric vent system.
The compressor system also includes various control valves, specifically an automatic on/off valve
20
connected in gas delivery line
5
, a further automatic on/off valve
21
connected in gas discharge line
7
, and a control valve
22
connected in common line
14
. The function of control valve
22
is, under normal operation, to reduce the gas discharge pressure in line
7
to a pressure just above that in line
5
and also to reduce the flow rate (and thereby increase the gas residence time in the filter), so as to ensure adequate filtering performance. Automatic on/off valves
20
,
21
are operated from a central control panel. In addition, an anti-surge valve
32
and cooler
33
are included in a bypass line
31
, connecting delivery line
5
to discharge line
7
. The anti-surge valve
32
is responsive to the inlet flow through line
5
so as to open when the gas flow falls to a predetermined value, say 70% of nominal flow, below which there would be a risk of compressor operation becoming unstable (surging) due to reverse flow through the compressor, in turn causing shaft vibration. When the anti-surge valve is open, the cooler
33
serves to cool the gas passing through connecting line
31
from its high pressure end to its low pressure end, to keep the gas inlet temperature to the compressor at an acceptable level. The compressor operates as follows.
In normal operation when the compressor is running, on/off valves
20
,
21
are both open and anti-surge valve
32
is closed. The compressor
1
compresses the low-pressure inlet gas in its successive stages and delivers high-pressure gas through gas discharge line
7
. A proportion of this gas is branched off through common line
14
and solid and liquid particles in the line are removed by filter system
16
. The gas pressure in common line
14
is then reduced by control valve
22
to a value just slightly above the gas inlet pressure to the compressor. This establishes the sealing pressure (SP) of the inboard gas seal
12
.
Referring now to
FIG. 2
, this is a pressure-enthalpy diagram, from which the operation of the compressor will be understood. The sealing pressure of the inboard gas seal
12
is denoted by the value “SP” on the pressure abscissa. Because this sealing pressure is very slightly larger than the inlet pressure maintained in balance chamber
9
, there will be a small flow of gas from the outboard side of labyrinth seal
11
to the inboard side, typically 1% of the compressor delivery. The remaining proportion of the gas passes through the inboard gas seal
12
to gas chamber
17
, from where a proportion of the gas passes to flare and the remainder flows, via second gas seal
13
, to vent, as described above.
In
FIG. 2
, the inlet gas pressure or sealing pressure SP to the gas seal
12
of the gas sealing arrangement is indicated by operating point A, that in the region of the inboard seal
12
communicating with gas chamber
17
being denoted by B and that in the region of the outboard gas seal
13
communicating with the vent line
19
by C. The reason why the

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