Note: Descriptions are shown in the official language in which they were submitted.
TITLE: RECIPROCATING COMPRESSOR SYSTEM WITH LIQUID PUMPING
CAPABILITY
CROSS-REFERENCE TO RELATED APPLICATIONS:
[0001] This application claims priority of United States Provisional Patent
Application No.
62/633,694 filed February 22, 2018.
TECHNICAL FIELD:
[0002] The present disclosure is related to the field of gas compressors for
use in the oil
and gas industry where liquids may be present in the stream, in particular,
compressors
used for casing gas, vapour recovery and multi-phase boosting, as some
examples.
BACKGROUND:
[0003] The torque requirement for a conventional reciprocating compressor
varies
significantly throughout a single revolution due to the varying of the crank
arm effective
radius from zero at top and bottom dead centre to maximum at mid stroke, and
due to the
compression loads varying from negative at the beginning of the inlet stroke
(expansion
of gas in the clearance volume) to maximum at the end of the discharge stroke.
The
suction and discharge pressures also vary over time and circumstance, which
also affects
the torque requirement of the system. To minimize the size of the prime mover
to the
average power required by the compressor, the inertia of the system is used to
both
absorb and release power to the compression activity. When the compression
power
requirement is low, the excess power supplied by the prime mover speeds up the
compressor slightly adding energy to the rotating system and when the
requirement is
higher than the prime mover can supply, the inertia of the system releases
power to the
compression activity slowing rotation slightly. This inertia is composed of
moving masses
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of the system, the speed at which they are moving and, often, a large
flywheel. As a
faster rotating compressor has more inertia than a slower one, and given that
the faster
a compressor rotates, the displacement requirement is smaller and, thus,
reducing the
cost of the compressor, the industry trend in compressors is to rotate them as
fast as
possible subject to the mechanical and valve limitations of the compressor.
Typically,
such compressors are operated at 900 to 1800 rpm.
[0004] These high-speed compressors cannot tolerate small amounts of liquids
without
catastrophic failure because the pressure required to move liquids through the
compressor valve is an order of magnitude higher than for a gas and, in
addition, is higher
than the mechanical components of the compressor can tolerate. When liquids
are
present in the gas feed to such compressors, the typical result is often
catastrophic failure.
[0005] Natural gas produced from a gas well will, typically, contain some
liquid, therefore,
all compressors packaged for this service have mechanisms to separate liquids
before
the gas enters the compression element, and then pump the liquids back into
the gas
stream downstream of the compression element. The separation system consists
of
inertial, gravity and coalescing mechanisms coupled in series that are
expensive, bulky
and prone to control component failures and liquid pump failures. In addition,
such
systems are designed for the largest anticipated slug (100% liquid volume).
Operationally, the slug capacity is usually underspecified and the separation
system will
fail when attempting to process a larger than anticipated slug thus resulting
in the loss of
production revenue reduction in addition to the cost of repair to the
compressor caused
by an oversize slug.
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[0006] It is, therefore, desirable to provide a compressor that overcomes the
shortcomings
of the prior art and that can pump a 100% liquid phase indefinitely.
SUMMARY:
[0007] A reciprocating compressor system with liquid pumping capability is
presented
herein. In some embodiments, the compressor can comprise a slow-moving, low
inertia
system that can vary speed significantly over each revolution. In some
embodiments, the
compressor can comprise a variable frequency drive ("VFD") to vary the speed
of the
compressor motor to maintain a constant power input. When the VFD senses the
low
power part of the cycle, the VFD can speed up the compressor to maintain
constant power
at a higher speed and lower torque (that is, the VFD can generate a higher
frequency,
lower amperage output to the motor). When the torque requirement increases,
the VFD
can sense the increased requirement and can maintain constant power by
increasing
torque and reducing speed (that is, the VFD can generate a lower frequency,
higher
amperage output to the motor). In the event liquid is introduced to the
system, the VFD
can slow the motor down to a speed consistent with moving liquids out of the
compression
chamber at a rate that is consistent with the power available from the drive
and, thereby,
avoid mechanical harm to the system.
[0008] In some embodiments, the compressor disclosed herein can allow a user
to
compress and transport process gas that is randomly fouled with liquid, while
managing
speed and torque demands. This can provide the advantage of allowing the user
to
extract gas from fields more efficiently with less power, and with less down
time for
maintenance and catastrophic failures.
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[0009] Broadly stated, in some embodiments, a reciprocating compressor system
with
liquid pumping capability can be provided, the system comprising: a low
inertia
reciprocating piston compressor further comprising at least one intake valve
and at least
one discharge valve; an intake flow line operatively coupled to the at least
one intake
valve; an output flow line operatively coupled to the at least one discharge
valve; a
compressor drive unit operatively coupled to the compressor, the compressor
drive unit
configured to operate the compressor; and a suction pressure transducer
operatively
coupled to the intake flow line and further operatively coupled to the
compressor drive
unit, the suction pressure transducer configured to measure suction pressure
within the
intake flow line and to generate a suction pressure data signal, the
compressor drive unit
configured to control speed of operation of the compressor in response to the
suction
pressure data signal.
[0010] Broadly stated, in some embodiments, a method can be provided for
compressing
gas and pumping liquids, the method comprising the steps of: providing a
reciprocating
compressor system with liquid pumping capability, the system comprising: a low
inertia
reciprocating piston compressor further comprising at least one intake valve
and at least
one discharge valve, an intake flow line operatively coupled to the at least
one intake
valve, an output flow line operatively coupled to the at least one discharge
valve, a
compressor drive unit operatively coupled to the compressor, the compressor
drive unit
configured to operate the compressor, and a suction pressure transducer
operatively
coupled to the intake flow line and further operatively coupled to the
compressor drive
unit, the suction pressure transducer configured to measure suction pressure
within the
intake flow line and to generate a suction pressure data signal, the
compressor drive unit
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configured to control speed of operation of the compressor in response to the
suction
pressure data signal; drawing in gas through the intake flow line and
compressing the gas
with the compressor; measuring the suction pressure in the intake flow line to
produce a
current suction pressure reading; comparing a historical suction pressure
reading to the
current suction pressure reading; and slowing the speed of operation of the
compressor
if the historical suction pressure reading is greater than the current suction
pressure
reading by a predetermined liquid warning threshold.
[0011] Broadly stated, in some embodiment, the method can further comprise
slowing the
compressor when a liquid is drawn into the at least one intake valve.
[0012] Broadly stated, in some embodiments, the compressor can comprise at
least one
double-acting cylinder.
[0013] Broadly stated, in some embodiments, wherein one or both of the at
least one
intake valve and the at least one discharge valve can comprise a check valve.
[0014] Broadly stated, in some embodiments, wherein the compressor can
comprise at
least one substantially horizontal cylinder wherein the at least one intake
valve is
disposed on a top surface thereof, and the at least one discharge valve is
disposed on a
bottom surface thereof.
[0015] Broadly stated, in some embodiments, the compressor drive unit can
comprise a
motor operatively coupled to a speed reducer, wherein the speed reducer is
operatively
coupled to the compressor.
[0016] Broadly stated, in some embodiments, wherein the motor can comprise an
electric
motor.
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[0017] Broadly stated, in some embodiments, the system can comprise a variable
frequency drive (VFD") unit configured for controlling speed of operation of
the electric
motor, wherein the VFD unit can be configured for operating the electric motor
in a
constant torque mode at motor speeds between 0 to 1800 revolutions per minute
("RPM")
and in a constant horsepower mode at motor speeds between 1800 to 3600 RPM.
[0018] Broadly stated, in some embodiments, the VFD unit can be configured to
slow the
speed of the electric motor when a liquid is drawn into the at least one
intake valve.
[0019] Broadly stated, in some embodiments, the system can comprise a
controller
operatively coupled to the suction pressure transducer and to the VFD, the
controller
configured to generate and transmit a VFD control signal to the VFD, the VFD
control
signal configured for controlling the speed of the electric motor in response
to the suction
pressure data signal.
[0020] Broadly stated, in some embodiments, wherein the controller can be
configured to
compare a current suction pressure reading with a historical suction pressure
reading and
to generate a slug protection speed limit control signal if the historical
suction pressure
reading is greater than the current suction pressure reading by a
predetermined liquid
warning threshold.
[0021] Broadly stated, in some embodiments, wherein the controller can further
comprise
input controls configured for one or both of setting and controlling pressure
limits and
temperature limits of the system.
[0022] Broadly stated, in some embodiments, the controller can comprise a
programmable logic controller.
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BRIEF DESCRIPTION OF THE DRAWINGS:
[0023] Figure la is a block diagram schematic depicting one embodiment of a
reciprocating compressor with liquid pumping capability.
[0024] Figure lb is a block diagram schematic depicting an alternate
embodiment of the
reciprocating compressor of Figure la.
[0025] Figure 2a is a perspective view depicting one embodiment of the
compressor of
Figure la.
[0026] Figure 2b perspective view depicting one embodiment of the compressor
of Figure
lb.
[0027] Figure 3 is an exploded, perspective view depicting the internal
elements of the
compressor of Figure 2a.
[0028] Figure 4 is a side cross-section elevation view depicting the internal
elements of
Figure 3.
[0029] Figure 5 is a partial side elevation view depicting the compressor
crankcase and
associated drive of the compressor of Figure 2a.
[0030] Figure 6a is an elevation view depicting a portion of one embodiment of
the
compressor of Figure 2a.
[0031] 6b is an elevation view depicting an alternate embodiment of the
compressor of
Figure 6a.
[0032] Figure 7a is an X-Y data chart depicting plots of torque and motor
speed of the
compressor of Figure 2a when no liquids are being pumped.
[0033] Figure 7b is an X-Y data chart depicting plots of torque and motor
speed of the
compressor of Figure 2a when liquids are being pumped.
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[0034] Figure 8 is a block diagram depicting functional features of the
controller of Figure
la.
[0035] Figure 9 is a flowchart depicting a suction pressure control algorithm
carried out
by the controller of Figure 8.
[0036] Figure 10 is a flowchart depicting a liquid warning calculation
algorithm carried out
by the controller of Figure 8.
[0037] Figure 11 is a flowchart depicting a speed control algorithm carried
out by the
controller of Figure 8.
DETAILED DESCRIPTION OF EMBODIMENTS:
[0038] In this description, references to "one embodiment", "an embodiment",
or
"embodiments" mean that the feature or features being referred to are included
in at least
one embodiment of the technology. Separate references to "one embodiment", "an
embodiment", or "embodiments" in this description do not necessarily refer to
the same
embodiment and are also not mutually exclusive unless so stated and/or except
as will
be readily apparent to those skilled in the art from the description. For
example, a feature,
structure, act, etc. described in one embodiment may also be included in other
embodiments, but is not necessarily included. Thus, the present technology can
include
a variety of combinations and/or integrations of the embodiments described
herein.
[0039] Referring to Figure la, a functional schematic of one embodiment of the
basic
elements of wet gas compressor drive system 10 is illustrated (only left
cylinder shown in
schematic). In some embodiments, compressor drive system 10 can comprise
compressor 40 and compressor drive unit 42. In some embodiments, compression
cylinder 12 can enclose piston 14 connected to crosshead 22 by piston rod 20.
In some
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embodiments, crosshead 22 can reciprocate inside tube 24 by means of
connecting rod
28 attached to a pair of throw plates 32 rotated, via crankshaft 30, by
electric motor 46.
In some embodiments, torque produced by electric motor 46 can be transmitted
through
speed reducer 44. In some embodiments, electric motor 46 can be controlled and
regulated by means of variable frequency drive ("VFD") 48 wherein VFD 48 can
be
configured to receive and control an input supply of direct current ("DC")
electrical power
or multi-phase alternating current ("AC") electrical power to provide a supply
of output
electrical power to electrical motor 46, as well known to those skilled in the
art. As drive
42 operates compressor 40, piston 14 can reciprocate within cylinder 12, thus
drawing in
gas or liquid from inlet 50 through intake valves 16, and then expelling same
through
discharge valves 18, and out through output flow line 52. In some embodiments,
one or
both of intake valves 16 and discharge valves 18 can comprise check valves, as
well
known to those skilled in the art.
[0040] In some embodiments, system 10 can comprise suction pressure transducer
59
configured to measure the pressure of the gas/liquid mixture flowing through
inlet 50. In
some embodiments, suction pressure transducer 59 can be operatively coupled to
controller 60 to provide pressure data of substances flowing in inlet 50 to
controller 60.
In some embodiments, VFD 48 can be operatively coupled to controller 60
wherein
controller 60 can be configured to provide control signals to VFD 48 to
control the
electrical power being supplied by VFD 48 to electrical motor 46. Thus,
controller 60 can
receive pressure data from suction pressure transducer 59 to determine whether
the
substances flowing through inlet 50 comprise gas, liquid or a mixture thereof,
and then
provide control signals to the VFD 48 to control and regulate the electrical
power supplied
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to electrical motor 46 accordingly. In some embodiments, controller 60 can
comprise a
programmable logic controller. In a representative embodiment, controller can
comprise
a model Micro820TM programmable logic controller as manufactured by Rockwell
Automation of Milwaukee, Wisconsin, USA or a model Simatic S7-1200
programmable
logic controller as manufactured by Siemens AG of Nuremberg, Germany. In a
representative embodiment, VFD 48 can comprise a model PowerFlex 753 AC Drive
as
manufactured by Rockwell Automation of Milwaukee, Wisconsin, USA or a model
Sinamics G120 variable frequency drive as manufactured by Siemens AG of
Nuremberg,
Germany.
[0041] In some embodiments, there can be a potential risk of failure in system
10 if there
is a failure of suction pressure transducer 59. Therefore, in some
embodiments,
mechanical torque limiting device 61 can be disposed in the mechanical
coupling between
motor 46 and speed reducer 44. In other embodiments, mechanical torque
limiting device
62 can be disposed in the mechanical coupling between speed reducer 44 and
compressor 40. This can either allow for controlled slip between the motor
inertia and the
compressor, or rapidly decouple the high inertia of the motor from the
compressor. Either
of torque limiting devices 61 and 62 can comprise a slipping type protection
device, which
could be in the form of a clutch, a fluid coupling or a magnetic coupling. In
some
embodiments, either of torque limiting devices 61 and 62 can be a disconnect
type of
coupling devices, which can comprise a ball detent or a shear pin coupling. In
some
embodiments, the disconnect device can require human intervention to reset, or
can reset
automatically, depending on operational requirements. In some embodiments, VFD
48
can comprise drive software configured to program controller 60 so that it can
detect a
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slip condition of a disconnect device and, thus, bring motor 46 to a stop as
quickly as
possible. In some embodiments, controller 60 can be configured to either issue
an alarm
signal for reset or repair, or to automatically reset and resume operation of
system 10
from low speed. In some embodiments, either or both of torque limiting devices
61 and
62 can provides a final mechanical protection in the event of a control system
failure of
system 10. In some embodiments, motor 46 can be coupled to torque limiting
device 61
via belt drive 65 as shown in Figure lb.
[0042] Referring to Figure 2a, shown is a top isometric external view of one
embodiment
of wet gas compressor drive system 10 comprising crankcase 36 with top pan 54
and
bottom pan 56 attached thereto. As explained above, with the use of double-
acting
compressor 40, all left-hand sided elements are duplicated on their right,
such as right
sided cylinder 13, intake valves 17 and discharge valves 19. In some
embodiments,
pressure safety valve ("PSV") 58 can join the inlet 50 to output flow line 52
and can be
used to release excess pressure from the system. In some embodiments, motor 46
can
be coupled to speed reducer 44 via belt drive 65 as shown in Figure 2b.
[0043] Referring to Figure 3, shown is a top isometric view of the internal
elements of
compressor 40, comprising crankcase divider 38, left-sided crosshead tube 24
and right-
sided crosshead tube 25. Starting from common crankshaft 30, left-sided throw
plates
32 can be positioned 90 degrees from right-sided throw plates 33. On the left
side, throw
plates 32 can be attached to their connecting rod 28 by throw pin 34, which
can be
attached to crosshead 22 by wrist pin 26, which can be further connected to
piston 14 by
piston rod 20. Equivalent right-hand sided components are shown and numbered
accordingly, except for the right side crosshead pin 27, which can be disposed
insideright
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crosshead 23. When assembled and enclosed, drive 42 (see Figures 1 & 2) can
rotate
crankshaft 30, causing throw plates 32 and 33 to reciprocate pistons 14 and 15
and,
thereby, operate compressor 40.
[0044] Referring to Figure 4, shown is a side cutaway view of compressor 40,
with the
figure elements as described above. Figure 5 shows a partial side view of
compressor
crankcase 36 and associated drive 42. Figure 6a shows a side view of drive 42
and an
end view of crankcase 36 without its throwplate pans 54 and 56. Figure 7 shows
data
plots of torque and motor speed performance results with the system under
heavy load.
In some embodiments, motor 46 can be coupled to speed reducer 44 via torque
limiting
device 61 as shown in Figure 6b.
[0045] Conventional compressors are high speed devices, with typical speeds
ranging
from 400 to 1800 rpm. In any piston compressor, the torque required to provide
the force
on the piston required to compress gas varies significantly over a revolution
of the crank
shaft. In a conventional high-speed compressor, the inertial nature of the
system is such
that it absorbs power by speeding up slightly during the part of the cycle
when
compression power requirements are low and releases this power to the
compression
activity when compression requirements are high thus slowing the system down
slightly.
Typically, these systems run with a speed and torque variation of less than
2%. Because
of their high speed and high inertia, the introduction of relatively
incompressible liquids
can create a much higher pressure drop moving through the valves. This creates
a very
high-pressure spike that can be greater than the mechanical strength of the
compressor
components and catastrophic failure is often the result.
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[0046] The disclosed wet gas compressor drive system 10, and its method of
operation,
is different from conventional gas compressors in that system 10 can comprise
a low
inertia reciprocating piston compressor by compressor 40 not having a
flywheel. In some
embodiments, system 10 can operate at a low speed as compared to conventional
gas
compressors so that the speed of the compressor can be varied significantly
during a
single revolution of the crank shaft as demonstrated by the torque vs. speed
data shown
in Figures 7a and 7b. The masses of the components and the rotational speed of
the
compressor can be minimized as follows: Rotational power can be supplied by a
conventional induction motor 46 through a low inertia planetary speed reducer
44, with
motor 46 driven by VFD 48 that can be programmed for constant horsepower
delivery
from motor 46 through the 1800 to 3600 rpm range, and constant torque delivery
from
motor 46 during the 0 to 1800 rpm range. In a representative embodiment, a
Teco
Westinghouse PHD0154 15 horsepower, 480 volt 3-phase AC induction electric
motor
can be used although functionally equivalent motors that can be operated with
a VFD, as
well known to those skilled in the art, can be used. In some embodiments, the
resulting
output at crankshaft 30 can be less than 300 rpm depending on the amount of
liquid in
the gas flow. With normal gas flow, VFD 48 can sense lower torque demand of
the system
and adjust motor 46 speed to maintain either constant horsepower or constant
torque.
This torque-motor speed relationship is shown in Figure 7a. If a large
quantity of liquid
enters the gas compressor, the torque required to move the liquid through
discharge valve
18 will rise, and the compressor speed can drop until an equilibrium is
reached, with the
speed of liquid moving through the valve being reduced to a point where
compressor 40
has enough torque to move the liquid through discharge valve 18. This torque-
motor
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speed relationship is shown in Figure 7b. One should note that the phase
difference
between torque and speed is due to minor inertial effects of the dynamic
system.
[0047] In some embodiments, a permanent magnet AC motor or a DC motor could be
used as motor 46 and directly coupled to a single stage planetary gear reducer
or directly
coupled to the input shaft of the compressor. In this embodiment, the inertia
of the high
speed induction motor described above would be significantly reduced, thereby
making it
easier for the drive to change speeds in response to changes in torque
required.
[0048] In some embodiments, when there is no electrical power available to
power an
electrical motor embodiment of motor 46, an internal combustion engine driven
pressure
compensated or constant horsepower control on a hydraulic variable
displacement pump
and motor combination can be used as motor 46. This simpler torque limiting
control
system, such as a pressure compensated pump, can reduce speed quickly to
maintain
torque below operating limits to protect the machine components (bearings,
crankshaft,
connecting rods, etc.), and increase speed quickly to maximize production of
gas.
[0049] In some embodiments, double acting compression cylinders, crosshead and
piston
rod can be used. In other embodiments, single acting pistons can be used, with
or without
a crosshead. In some embodiments, a crankshaft driving any number of
reciprocating
pistons an be used. In some embodiments, a low speed high torque driver can be
used
as speed reducer 44 in place of a high-speed motor and planetary gear reducer.
In some
embodiments, a low-speed, low-inertia driver coupled with a torque or power
limiting
device can be used.
[0050] In some embodiments, system 10 can comprise a power limiting drive
system with
a torque sensing variable speed drive to reduce motor speed quickly and to
maintain
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power below operating limits when torque rises due to hazardous multiphase
process
conditions.
[0051] In some embodiments, system 10 can comprise a constant power drive so
that as
torque requirements change over time, the drive system can respond by
increasing or
decreasing speed to maintain constant power draw. This can be important, as
power
infrastructure is frequently limited at remote sites, thus, it is desirable to
use the available
power as efficiently as possible by minimizing the maximum power draw for a
given
operating condition.
[0052] In some embodiments, system 10 can comprise piston types with a
geometry
similar to a commonly available reciprocating compressors, with seals and
materials
compatible with both gas and liquid phases. This can increase utility and
maintenance
options in remote environments and, ultimately, increased operational
longevity.
[0053] In some embodiments, system 10 can operate at low speeds (low gas
velocities)
and employs large ports (large valves) which make the novel compressor more
tolerant
to viscous liquids and solid particles that can be found in a typical
operating environment.
[0054] In some embodiments, system 10 can comprise an intake/exhaust geometry
that
can eject liquids prior to the discharge of gas, unlike other prior art
equipment on the
market which retains liquids for sealing. In some embodiments, intake valves
16 and 17
can be disposed on a top surface of horizontal cylinders 12 and 13, and
discharge valves
18 and 19 can be disposed on a bottom surface thereof, wherein liquids can
pool on the
bottom of cylinders 12 and 13 due to gravity and flow out through discharge
valves 18
and 19 with the gas during compression cycle instead of coming out all at once
at the end
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of the compression cycle, which is more difficult to manage. In some
embodiments,
system 10 can run completely dry if conditions call for it.
[0055] In some embodiments, system 10, as described herein, can comprise a
potential
shortcoming when the load of gas flowing therethrough is light and VFD 48 is
consequently controlling electrical motor 46 to operate at maximum motor speed
throughout much of the cycle. Introduction of large amounts of liquid under
these
conditions would require VFD 48 to slow motor 46 down significantly to be able
to process
the liquid as it has a much higher pressure drop through discharge valve 18
than gas.
Under these conditions, compressor 40 is a low inertia device as it is turning
slowly,
whereas electric motor 46 is turning quickly, with its speed being reduced
through speed
reducer 44. Electric motor 46, thus, can comprise considerable stored energy,
which is
proportional to the square of the motor speed and it is possible that a
breaking resistor
operatively coupled to electrical motor and controller 60 can't react fast
enough to the
increase in torque which will occur due to liquid ingestion. In some
embodiments, this
could result in high forces and perhaps failure of the compressor components.
To avoid
this case, a method of determining that a large influx of liquid is in the
piping moving
towards the compressor (commonly called a slug) is provided. In some
embodiments,
the liquid moving towards compressor 40 can experience a much larger pressure
drop
traveling through the piping than an equivalent volume of gas and this can
result in a
decrease in the inlet pressure at the inlet to compressor 40 as the forcing
pressure
remains constant. This rapid reduction in pressure can be monitored via
suction pressure
transducer 59 and the software controlling VFD 48, either directly or through
controller
60, can be configured to cause VFD 48 to start slowing motor 46 down prior to
the liquid
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entering compression chamber 44 or 46, thus reducing the kinetic energy stored
in motor
46 to a level low enough to prevent overstressing of the compressor components
when
discharging the liquid. Lower speed can also reduce the pressure required to
pump
liquids, further reducing the stress on components and risk of damage.
[0056] Referring to Figure 8, one embodiment of the functional aspects or
elements of
controller 60 is shown. In some embodiments, suction pressure data measured by
suction pressure transducer 59 at regular intervals can be relayed as suction
pressure
data signal 63 to controller 60 to measure the current suction pressure, as
shown at
current suction pressure function block 70. In operation, the suction pressure
can range
from 0 to 50 pounds per square inch ("PSI") when only gas is flowing into
system 10.
When a slug of liquid is approaching inlet 50 of system 10, the suction
pressure can be
reduced by 5 to 10 PSI. Each successive suction pressure data signal 63
measured by
transducer 59 can be relayed as suction pressure data signal 71c and stored in
a
computer memory disposed in suction pressure history function block 72. At
liquid
warning calculation function block 76, the current suction pressure data
reading,
represented as suction pressure data signal 71b, can be compared to a previous
or
historical suction pressure data entry, represented as previous suction
pressure data
signal 73. If the historical pressure reading minus the current pressure data
reading is
greater than a preset or predetermined liquid warning threshold, function
block 76 can
then generate slug protection speed limit control signal 77 that can be
relayed to speed
controller function block 78. In a representative embodiment, the preset or
predetermined
liquid warning threshold can be 10 PSI below the observed average pressing
reading,
although this can be selected to be higher or lower by those skilled in the
art depending
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on the size of the compressor cylinders and on the relative composition of
fluids and gas
being processed from a particular well. At suction pressure control function
block 74, the
current suction pressure data entry at function block 70, represented as
suction pressure
data signal 71a, can be used to send speed control signal 75 to speed
controller function
block 78. Function block 74 can comprise a scaler controller or a proportion-
integral-
differential ("PID") controller to generate the speed control signal or any
other speed
controller method or process as well known to those skilled in the art, which
can be
configured to operate system 10 so as to maintain a minimum suction pressure
as
measured by suction pressure transducer 59. Speed controller function block 78
can
receive speed control signal 75 from function block 74 in addition to
receiving slug
protection speed limit control signal 77 from function block 76. In some
embodiments,
controller 60 can comprise an additional control function block 80, which can
comprise
additional features, elements and/or input controls configured for setting
and/or controlling
additional pressure and temperature limits of system 10. These additional
features, limits
and/or controls can comprise controls or limits of subcomponents of system 10
or of other
equipment used in combination with system 10 as would be understood by those
skilled
in the art. Thus, control function block 80 can generate additional control
signal 81 that
can then be relayed to speed controller function block 78. Thus, with input
signals 75, 77
and 81 from any or all of function blocks 74, 76 and 80, speed controller
function block
78 can be configured to generate VFD control signal 79 that can be relayed to
VFD 48 so
as to cause VFD 48 to set a minimum motor speed in motor 46.
[0057] Referring to Figure 9, a flowchart is shown representing one embodiment
of
suction pressure control algorithm 900 that can be carried out by suction
pressure control
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function block 74. In some embodiments, algorithm 900, starting at step 902,
can
compare suction pressure 908 to operator selected pressure set point 906 at
decision
step 904. If suction pressure 908 is less than set point 906, control function
block 74 can
then set the pump ("P") speed to zero at step 910, and then exit algorithm 900
at step
916. For the purposes of this specification, "pump speed" shall mean the
rotational speed
of compressor 40 as shown in Figures la to 6b. If suction pressure 908 is not
less than
set point 906, control function block 74 can then instruct RID control 912 to
set the P
speed to the output as generated by RID control 912 at step 914, and then exit
at step
916.
[0058] Referring to Figure 10, a flowchart is shown representing one
embodiment of liquid
warning calculation algorithm 1000 that can be carried out by liquid warning
calculation
function block 76. in some embodiments, algorithm 1000, starting at step 1002,
can
update section pressure 1004 at update suction pressure history step 1006 to
keep an
ongoing record or log of suction pressure at step 1010, which can be stored in
a computer
memory. At step 1008, a comparison of current suction pressure 1004 with a
previous or
historical suction pressure reading from suction pressure history 1010 can be
undertaken.
At decision step 1012, if the difference between a current suction pressure
reading and a
previous suction pressure does not exceed a predetermined liquid warning
pressure
differential threshold, then control function block 76 can exit algorithm 1000
at step 1016.
If the difference between a current suction pressure reading and a previous
suction
pressure does exceed a predetermined liquid warning pressure differential
threshold,
then control function block 76 can then generate a liquid warning alarm
condition at step
1014, and then exit at step 1016.
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[0059] Referring to Figure 11, a flowchart is shown representing one
embodiment of
speed control algorithm 1100 that can be carried out by speed control function
block 78.
In some embodiments, algorithm 1100 can, starting at step 1102, can determine
if system
is in pause, alarm condition or shutdown mode at decision step 1104. If any of
those
conditions do exist, then control function block 78 can set the outputspeed of
system 10
to zero at step 1106, and then exit algorithm 1100 at step 1116. If none of
these
conditions exist, then algorithm 1100 can determine whether a liquid warning
alarm has
been generated by algorithm 1000 at decision step 1108. If no liquid warning
alarm has
been generated, control function block 78 can maintain the pump speed at its
current
setting at step 1114, and can then exit algorithm at step 1116. If a liquid
warning alarm
has been generated, control function block 78 can determine whether the pump
speed is
greater than a liquid warning limit at decision step 1110. If the pump speed
is not greater
than the liquid warning limit, then control function block 78 can maintain the
pump speed
at its current setting at step 1114, and can then exit algorithm at step 1116.
If the pump
speed is greater than the liquid warning limit, then control function block 78
can reduce
the pump speed to the speed limited by the liquid warning limit at step 1112,
and can then
exit algorithm 1100 at step 1116.
[0060] The various illustrative logical blocks, modules, circuits, and
algorithm steps
described in connection with the embodiments disclosed herein may be
implemented as
electronic hardware, computer software, or combinations of both. To clearly
illustrate this
interchangeability of hardware and software, various illustrative components,
blocks,
modules, circuits, and steps have been described above generally in terms of
their
functionality. Whether such functionality is implemented as hardware or
software depends
CA 3034391 2019-02-21
upon the particular application and design constraints imposed on the overall
system.
Skilled artisans may implement the described functionality in varying ways for
each
particular application, but such implementation decisions should not be
interpreted as
causing a departure from the scope of the embodiments described herein.
[0061] Embodiments implemented in computer software may be implemented in
software, firmware, middleware, microcode, hardware description languages, or
any
combination thereof. A code segment or machine-executable instructions may
represent
a procedure, a function, a subprogram, a program, a routine, a subroutine, a
module, a
software package, a class, or any combination of instructions, data
structures, or program
statements. A code segment may be coupled to another code segment or a
hardware
circuit by passing and/or receiving information, data, arguments, parameters,
or memory
contents. Information, arguments, parameters, data, etc. may be passed,
forwarded, or
transmitted via any suitable means including memory sharing, message passing,
token
passing, network transmission, etc.
[0062] The actual software code or specialized control hardware used to
implement these
systems and methods is not limiting of the embodiments described herein. Thus,
the
operation and behavior of the systems and methods were described without
reference to
the specific software code being understood that software and control hardware
can be
designed to implement the systems and methods based on the description herein.
[0063] When implemented in software, the functions may be stored as one or
more
instructions or code on a non-transitory computer-readable or processor-
readable
storage medium. The steps of a method or algorithm disclosed herein may be
embodied
in a processor-executable software module, which may reside on a computer-
readable or
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processor-readable storage medium. A non-transitory computer-readable or
processor-
readable media includes both computer storage media and tangible storage media
that
facilitate transfer of a computer program from one place to another. A non-
transitory
processor-readable storage media may be any available media that may be
accessed by
a computer. By way of example, and not limitation, such non-transitory
processor-
readable media may comprise RAM, ROM, EEPROM, CD-ROM or other optical disk
storage, magnetic disk storage or other magnetic storage devices, or any other
tangible
storage medium that may be used to store desired program code in the form of
instructions or data structures and that may be accessed by a computer or
processor.
Disk and disc, as used herein, include compact disc (CD), laser disc, optical
disc, digital
versatile disc (DVD), floppy disk, and Blu-ray disc where disks usually
reproduce data
magnetically, while discs reproduce data optically with lasers. Combinations
of the above
should also be included within the scope of computer-readable media.
Additionally, the
operations of a method or algorithm may reside as one or any combination or
set of codes
and/or instructions on a non-transitory processor-readable medium and/or
computer-
readable medium, which may be incorporated into a computer program product.
[0064] Although a few embodiments have been shown and described, it will be
appreciated by those skilled in the art that various changes and modifications
can be
made to these embodiments without changing or departing from their scope,
intent or
functionality. The terms and expressions used in the preceding specification
have been
used herein as terms of description and not of limitation, and there is no
intention in the
use of such terms and expressions of excluding equivalents of the features
shown and
described or portions thereof, it being recognized that the invention is
defined and limited
only by the claims that follow.
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