U.S. patent application number 11/539513 was filed with the patent office on 2008-04-10 for variable capacity natural gas compressor.
This patent application is currently assigned to VAPORTECH ENERGY SERVICES INC.. Invention is credited to Derek MACKENZIE, Sami SAAD, John VAN MANEN.
Application Number | 20080085180 11/539513 |
Document ID | / |
Family ID | 39275061 |
Filed Date | 2008-04-10 |
United States Patent
Application |
20080085180 |
Kind Code |
A1 |
MACKENZIE; Derek ; et
al. |
April 10, 2008 |
VARIABLE CAPACITY NATURAL GAS COMPRESSOR
Abstract
The invention relates to a method and apparatus for gas
compression. More particularly the invention is directed to a
variable capacity screw compressor. The screw compressor has an
engine, an intake slide valve, and a programmable logic controller
for controlling the transmission speed of the engine and for
controlling the position of the slide valve. The compressor also
has means for monitoring the engine load. The transmission speed
and the slide valve position are adjusted in accordance with the
engine load. The invention attempts to match available horsepower
of the engine with available gas volume by adjusting the compressor
to match gas throughput with horsepower. This automated process
assists the user in shipping greater volumes of gas.
Inventors: |
MACKENZIE; Derek; (Calgary,
CA) ; SAAD; Sami; (Edmonton, CA) ; VAN MANEN;
John; (Edmonton, CA) |
Correspondence
Address: |
EDWARD YOO C/O BENNETT JONES
1000 ATCO CENTRE, 10035 - 105 STREET
EDMONTON, ALBERTA
AB
T5J3T2
US
|
Assignee: |
VAPORTECH ENERGY SERVICES
INC.
Edmonton
CA
|
Family ID: |
39275061 |
Appl. No.: |
11/539513 |
Filed: |
October 6, 2006 |
Current U.S.
Class: |
415/31 |
Current CPC
Class: |
F04C 18/16 20130101;
F04C 28/12 20130101; F04C 28/08 20130101 |
Class at
Publication: |
415/31 |
International
Class: |
F01B 25/06 20060101
F01B025/06 |
Claims
1. A variable capacity screw compressor comprising: (a) an engine
coupled to a variable speed hydrostatic transmission; (b) an intake
slide valve, and actuation means for varying the intake slide
valve; (c) means for monitoring engine load; (d) means for varying
transmission speed; and (e) control means comprising an engine load
sensor input operatively connected to the engine load monitoring
means, a transmission speed output operatively connected to the
transmission speed varying means, an intake slide valve output
operatively connected to the intake slide valve actuation means,
wherein said control means automatically adjusts transmission
speed, and intake slide valve position, according to engine
load.
2. The compressor of claim 1 further comprising a Vi control slide
valve, and actuation means for varying the Vi control slide valve
operatively connected to the control means, which automatically
adjusts Vi control slide valve position according to engine
load.
3. The compressor of claim 1 wherein the variable speed hydrostatic
transmission comprises a variable speed motor, and a variable speed
pump.
4. The compressor of claim 1 wherein the control means is
operatively connected to a speed control device on the variable
speed motor, or to a speed control device on the variable speed
pump, or speed control devices on both the motor and the pump.
5. The compressor of claim 1 wherein the control means comprises a
programmable logic controller.
6. The compressor of claim 1 wherein the control means responds to
a change in engine load during operation by first adjusting
compressor speed, then by adjusting intake slide valve position,
and lastly by adjusting Vi control slide valve position.
7. The compressor of claim 6 further comprising a gas recycle line
connecting a gas discharge end of the compressor to a gas suction
end, a recycle valve controlling flow through the gas recycle line,
and means for actuating the recycle valve, said actuation means
operatively connected to the control means.
8. The compressor of claim 1 further comprising a suction pressure
control system comprising a PID loop operatively connected to a
suction pressure control valve and a suction pressure transmitter,
wherein said PID loop operates independently of the control
system.
9. The compressor of claim 8 further comprising a discharge
pressure control system comprising a PID loop operatively connected
to a discharge pressure control valve and a discharge pressure
transmitter, wherein said PID loop operates independently of the
control system.
10. A method of efficiently compressing gas from a gas well using a
variable capacity compressor having an intake slide valve, and
driven by an engine coupled to a hydraulic transmission including a
variable speed pump and variable speed motor, said method
comprising the steps of: (a) sensing engine load; (b) attempting to
maintain engine load within a desired range by: i. adjusting
compressor speed by varying the hydraulic transmission in response
to changes in engine load; or ii. adjusting the intake slide valve;
or iii. both.
11. The method of claim 10 wherein compressor speed is first
adjusted, and then intake slide valve is adjusted, if engine load
is not brought within the desired range by adjusting compressor
speed.
12. The method of claim 11 wherein the compressor further comprises
a Vi control slide valve, and the Vi control slide valve is
adjusted in order to vary engine load.
13. The method of claim 10 further comprising the step of
controlling suction pressure independently of controlling engine
load.
14. The method of claim 13 further comprising the step of
controlling discharge pressure independently of controlling engine
load.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a method and apparatus for
gas compression.
BACKGROUND
[0002] In the production of natural gas, flow from the well may
vary substantially over the course of time, for various reasons. At
the same time, downstream pressures into which this gas must travel
to reach its market may also vary. It is often necessary to place
compression at or near the well site in order to generate both
sufficient suction differential on the well to enhance gas
production, as well as to provide sufficient energy to enable the
gas to enter downstream gas pipelines. These compressors may be
required at the well site, in booster applications where the
production of several or many wells will be joined, or in fixed gas
plant or hydrocarbon processing plants.
[0003] There are a number of compression technologies in use in gas
production today which all serve specific production needs.
Positive displacement compressors such as screw/reciprocating and
vane type compressors are common. The production capacity of these
compressors are critically dependent on three key variables:
suction pressure, operating speed and discharge pressure.
[0004] Suction pressure is generally a function of the performance
of the well or header in question, while discharge pressure is
generally dependent on the line pack or flowing pressure of sales
gas pipelines into which the gas is delivered. Compressor speed is
the only practical tool for variation in production for positive
displacement compression technologies.
[0005] Of the common positive displacement compressors, the screw
compressor is a preferred technology primarily because it is cost
effective. The screw compressor is so named as it consists of two
screw shaped rotors whose threads mesh during rotation along their
length to highly engineered tolerances, forming a compression
"chamber" as the clearance between the rotors at the beginning of
the thread declines to the end of the thread. A screw compressor
traps a fixed volume of gas on the suction side and increases its
pressure by reducing the internal volume of the compression
chamber, thereby raising its pressure at the discharge side.
[0006] Conventionally, a screw compressor is directly coupled to an
engine which may be powered by natural gas, and which may produce
30 to over 1,000 horsepower. Placed at the well site or on a header
pad receiving gas from a number of wells, these units run at a
fixed speed--offering a fixed throughput volume based on the
suction pressure available. Typical installations run at the engine
speed of 1,800 rpm, although there are geared units available
capable of taking advantage of the compressors capacity for higher
speeds and thus higher volumes. These gear units must be designed
for the expected suction/discharge pressures and volume inputs for
a given application, and suffer significant degradation in
efficiency when conditions change.
[0007] The key problem of the standard natural gas drive design is
that a direct-coupled machine is incapable of matching the best
compressor speed with available horsepower to maximize throughput
given the current suction and discharge pressure. The compressor
cannot turn at speeds greater than engine speed, unless a
speed-multiplying gearbox is used. The gearbox then restricts the
compressor speed to the gear ratio of the engine speed. Natural gas
engines generally rotate at 1,800 rpm, subsequently restricting the
compressor to this speed or multiples of this speed. As positive
displacement machines, compressors take a fixed "gulp" of natural
gas with each rotation. Obviously, increasing the number of
rotations provides more throughput, resulting in greater gas
sales.
[0008] When the critical conditions begin to change, then the fixed
speed model becomes increasingly inefficient. If suction pressure
drops, thereby freeing horsepower demand on a fixed setup, this
horsepower cannot be utilized, as the compressor continues to
operate at the same speed. Conversely, if the discharge pressure
climbs unexpectedly, the engine will run out of horsepower and the
unit will shut down on high discharge pressure.
[0009] A screw compressor may be manually adjusted to match current
conditions. The process generally requires human intervention to
adjust the throughput. When production conditions change through
well depletion, or more importantly, new drilling or current well
optimization, the standard design requires human intervention to
either resize the compressor, the engine or the gearbox to rematch
hardware with throughput. Such intervention is expensive,
time-consuming and inconvenient.
[0010] Therefore, there is a need in the art for a variable output
engine driven screw compressors which are not currently known to
those skilled in the art.
SUMMARY OF THE INVENTION
[0011] In one aspect, the invention comprises a variable capacity
screw compressor comprising: [0012] (a) an engine coupled to a
variable speed hydrostatic transmission; [0013] (b) an intake slide
valve, and actuation means for varying the intake slide valve;
[0014] (c) means for monitoring engine load; [0015] (d) means for
varying transmission speed; [0016] (e) control means comprising an
engine load sensor input operatively connected to the engine load
monitoring means, a transmission speed output operatively connected
to the transmission speed varying means, an intake slide valve
output operatively connected to the intake slide valve actuation
means, wherein said control means automatically adjusts
transmission speed, and intake slide valve position, according to
engine load.
[0017] The compressor may further comprise a Vi control slide
valve, and actuation means for varying the Vi control slide valve
operatively connected to the control means, which automatically
adjusts Vi control slide valve position according to engine
load.
[0018] In one embodiment, the variable speed hydrostatic
transmission comprises a variable speed motor, and a variable speed
pump. The control means may be operatively connected to a speed
control device on the variable speed motor, or to a speed control
device on the variable speed pump, or speed control devices on both
the motor and the pump.
[0019] In one embodiment, the compressor may further comprise a gas
recycle line connecting a gas discharge end of the compressor to a
gas suction end, a recycle valve controlling flow through the gas
recycle line, and means for actuating the recycle valve, said
actuation means operatively connected to the control means.
[0020] In one embodiment, the compressor may comprise a PID loop
operatively connected to a suction pressure control valve and a
suction pressure transmitter, wherein said PID loop operates
independently of the control system. Furthermore, the compressor
may also comprise a PID loop operatively connected to a discharge
pressure control valve and a discharge pressure transmitter,
wherein said PID loop operates independently of the control
system.
[0021] In another aspect of the invention, the invention may
comprise a method of efficiently compressing gas from a gas well
using a variable capacity compressor having an intake slide valve,
and driven by an engine coupled to a hydraulic transmission
including a variable speed pump and variable speed motor, said
method comprising the steps of: [0022] (a) sensing engine load;
[0023] (b) attempting to maintain engine load within a desired
range by: [0024] i. adjusting compressor speed by varying the
hydraulic transmission in response to changes in engine load; or
[0025] ii. adjusting the intake slide valve in response to changes
in engine load; or [0026] iii. both.
[0027] In one embodiment, the intake slide valve is first adjusted,
and then compressor speed is adjusted. If engine load is not
brought within the desired range by adjusting compressor speed and
intake slide valve, then the Vi control slide valve may also be
adjusted in order to vary engine load.
[0028] Suction pressure may be controlled independently of
controlling engine load, and as well, discharge pressure may also
be controlled independently of controlling engine load.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] The invention will now be described by way of an exemplary
embodiment with reference to the accompanying simplified,
diagrammatic, not-to-scale drawings. In the drawings:
[0030] FIG. 1 is a schematic representation of one embodiment of
the present invention.
[0031] FIG. 2 is a schematic representation of one embodiment of a
screw compressor.
[0032] FIG. 3 is a schematic representation of one embodiment of a
hydraulic pump and motor.
[0033] FIG. 4 is a schematic representation of one embodiment of an
engine.
[0034] FIG. 5 is a schematic representation of one embodiment of a
PLC.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0035] The present invention provides for a variable output gas
compressor. When describing the present invention, all terms not
defined herein have their common art-recognized meanings. To the
extent that the following description is of a specific embodiment
or a particular use of the invention, it is intended to be
illustrative only, and not limiting of the claimed invention. The
following description is intended to cover all alternatives,
modifications and equivalents that are included in the spirit and
scope of the invention, as defined in the appended claims.
[0036] Production flow from a conventional natural gas well is
typically not constant, and as a result, the suction pressure of a
gas compressor package constantly varies. With a constantly varying
suction pressure, a compressor package is subjected to high recycle
rates in order to maintain the suction throughput. This leads to
wasted horsepower, fuel gas and increased heat loads. For example,
in a well where a plunger lift is installed as an optimization
technique to unload unwanted water, gas production and thus suction
pressure can vary significantly. While plunger lift device design
is varied, a typical design consists of a metal cylinder which
rises and falls in the production tubing in the well, thereby
carrying water out of the well bore. During the plunger down cycle,
gas production will cease from the well. During an up cycle, gas
flow will resume.
[0037] The use of a plunger lift pose significant problems for well
site compression. The varying suction pressure and volume
conditions do not allow efficient compressor operation. A
conventional compressor must go to high recycle rates during
periods where gas flow ceases, which leads to overheating problems,
and eventual shutdown. It is possible to manually unload the
compressor, but requires human intervention. As some plungers might
cycle a dozen or more times a day, this approach is simply
impractical.
[0038] The present invention may compensate for these changes by
making the aforementioned control changes and ensure driving
horsepower is efficiently utilized, without the attendant problems
of human intervention or interrupted operation of the
equipment.
[0039] This invention comprises a variable output screw compressor
package designed to maximize the amount of gas that can be moved to
sales as the operating conditions change. The screw compressor
comprises a control system which is designed to utilize more
horsepower available from the engine when it is advantageous to do
so, by varying one or more of the compressor speed, suction
pressure, discharge pressure, and slide valve position in a
dynamically controlled environment.
[0040] A capacity control or intake slide valve is available on
most screw compressors, and is effectively a device which can vary
the volume of gas permitted to enter the compression chamber with
each rotation. In addition, a screw compressor may have a volume
ratio control slide valve, which varies the volume index, or Vi, of
the compressor. The volume index is the ratio of the volume of
trapped gas at the start of compression, versus the volume of gas
as it is discharged. Some slide control valves are equipped to
decrease discharge port area and allow some discharge gas to shunt
back to the suction side. This approach allows a single slide valve
to achieve both capacity control and volume ratio control.
[0041] In operation, the present invention attempts to continuously
match available horsepower with available gas volume by adjusting
the compressor to match gas throughput with horsepower. This
process is automated, requires no human intervention, and assists
the producer to ship a greater volume of gas to market, given the
physical limitations of the equipment available.
[0042] This adjustment and control process comprises the use of
variable speed hydraulic coupling devices, as well as hydrodynamic,
electrical and instrument controls to adjust one or more of engine
speed, motor speed, pump output, compressor speed, discharge
pressure, and slide valve position to match gas pressure conditions
and available driver horsepower.
[0043] In one aspect, as shown in the general schematic of FIG. 1,
the invention comprises a variable output compressor comprising a
gas compressor (10), an engine (12), a transmission connected
between the compressor and the engine, and a control system (14)
for controlling the output of the compressor in response to
variable conditions. In one embodiment, the transmission is a
hydrostatic transmission involving a hydraulic pump (16)
operatively connected to a hydraulic motor (18), where the
hydraulic pump is driven by the engine, and the hydraulic motor
drives the compressor. In one embodiment, one or both of the
hydraulic pump and motor is a variable displacement hydraulic pump
or motor. Variable displacement hydraulic pumps and motors are well
known in the industry and need not be further described here.
[0044] In another aspect, the invention comprises a method of
maximizing compressor efficiency by hydrodynamic coupling of the
engine to the compressor and using a control algorithm linked to
electro-mechanical systems to control the hydrodynamic coupling,
and various other parameters, to optimize the speed of the
compressor. These controls may regulate the inlet volume of gas to
the compression chamber in the compressor, by controlling the
capacity control slide valve and as well may regulate the
compression ratio in the compression chamber, by controlling the
volume ratio control slide valve. Hydraulic motor speed, as well as
hydraulic pump fluid volumes may also be controlled to maximize
compressor efficiency. The control system monitors engine load and
activates system components to relieve excess engine load, or to
increase engine load, where appropriate. In one embodiment,
compressor capacity may be primarily controlled by two methods:
compressor speed and slide valve position, either or both capacity
control and volume ratio control.
[0045] As shown in FIG. 2, a gas compressor (10) has a gas inlet
(1) and a gas discharge (2). The capacity control slide valve (3)
is actuated by a slide valve load solenoid (SY100) and a slide
valve unload solenoid (SY101). The slide valve position is reported
by sensor (RPT120). Suction gas pressure is reported upstream from
the compressor by pressure transmitter (PT100) and is controlled by
a suction pressure control valve (4), which is actuated by a
suction pressure control valve controller (PY100). Discharge
pressure is reported by pressure transmitter (PT140), and is
controlled by discharge pressure control valve (5), which is
actuated by a discharge pressure control valve controller
(PY140).
[0046] In one embodiment, a variable speed hydraulic motor (18) is
driven by a hydraulic pump (16) which itself may be a variable
speed unit. The motor (18) is stroked by a hydraulic motor control
(PY500), while the pump may be varied by hydraulic pump control
(PY600).
[0047] The engine (12) may be a natural gas engine, as is well
known in the art. Speed of the engine, for display purposes or
control purposes, may be reported by a speed transmitter (ST400).
Intake manifold pressure, which is representative of engine load,
may be reported by pressure transmitter (PT400). An engine fuel
shutoff solenoid (FY410) may be provided to cut off fuel to the
engine in order to effect a shutdown.
[0048] A control system (14) of the present invention reads the
data inputs and actuates the control devices described herein. The
control system may comprise a programmable logic controller or PLC.
A PLC is a computer typically used for automation of industrial
process, and may run software stored in memory. The controller may
comprise a microprocessor or a microcontroller with on-chip
resources, such as an A/D converter, ROM (EPROM), RAM. The
microprocessor or microcontroller is suitably programmed, for
example in software or firmware, to perform the operations
described below as will be within the understanding of those
skilled in the art.
[0049] In one embodiment, the electro-mechanical controllers are
driven from a pressure sensor PT400 located in the engine inlet
manifold which gives a direct indicator of engine load by measuring
manifold pressure. In normally aspirated engines, as load increases
manifold pressure declines. In turbocharged engines, as load (HP
demand) increases, manifold pressure increases. The control system
will monitor engine load and may adjust one or more of compressor
speed, the capacity control slide valve; or the Vi control slide
valve, in an effort to stabilize the manifold pressure at a preset
level, thus engaging as much available horsepower as possible to
the compression of gas. Speed range for the compressor may be in
the 1,500 rpm to 5,000 rpm range. Speed adjustments may be
accomplished by repositioning the swashplate in the hydraulic pump
(16) or the hydraulic motor (18), or both, through control signals
from the control system (14).
[0050] Thus, in one embodiment, the first response to a change in
engine load is to adjust compressor speed by varying the hydraulic
motor (18). If speed control is insufficient to relieve excessive
load on the engine, then the capacity control intake slide valve
(22) is repositioned to reduce the inlet gas volume into the
compressor, thereby reducing load. In addition, or alternatively, a
Vi control slide, if so equipped, may be repositioned to reduce the
volume index of the compressor, thereby reducing load. Should this
control adjustment prove insufficient to reach optimal load, then
the hydraulic pump (16) will begin to reduce its fluid contribution
to the hydraulic motor (18).
[0051] In one embodiment, a compressor may have a gas recycle line
(6) and control valve (7) actuated by controller PY150, which
provides another means of controlling compressor capacity. Opening
the recycle control valve (7) will have the effect of reducing load
on the engine.
[0052] The controls are capable of a continuous and unattended
adjustment of the package, and its controls entirely dependent on
compressor load conditions. In booster applications, where the
number of wells producing into a given compressor suction might
vary significantly over a number of hours or days, conventional
prior art screw compressor packages require constant human
intervention in order to ensure the compressor is loaded
correctly.
[0053] An important advantage of this design is on the discharge
side of compressor performance. On conventional screw compressor
packages, the fixed speed permits a very limited window of
discharge operation. When the compressor is configured for a given
suction/discharge regime, any variation is this regime leads, at
best, to operating problems with human intervention and, at worst,
to equipment shutdown, and lost gas sales.
[0054] In addition to the primary compressor speed control, and
control of the suction slide valve and Vi control position, it is
possible to separately control both suction pressure and discharge
pressure. Thus, in one embodiment, as shown in FIG. 2, the
compressor suction pressure is controlled by the control system
(14) via a PID loop with an adjustable setpoint. Suction gas
pressure is reported upstream from the compressor by pressure
transmitter (PT100) and is controlled by a suction pressure control
valve (4), which is actuated by a suction pressure control valve
controller (PY100). As the suction pressure falls below a desired
setpoint or range, the control valve (4) will open. As the suction
pressure rises above the setpoint or range, the control valve (4)
will close.
[0055] As shown in FIG. 2, the compressor discharge pressure is
controlled via PID loop with an adjustable setpoint. Discharge
pressure is reported by pressure transmitter (PT140), and is
controlled by discharge pressure control valve (5), which is
actuated by a discharge pressure control valve controller (PY140).
As the discharge pressure falls below the setpoint or range, the
control valve (5) will throttle closed. As the discharge pressure
rises above the setpoint, the control valve (5) will throttle
open.
[0056] In one embodiment, in operation and upon startup, the
control system it will try to achieve full engine load by first
loading the compressor, after the initial warm-up period.
Compressor load may be achieved by actuating the intake slide valve
and increasing the compressor speed, as described above. When full
engine load is achieved, then the system will stop loading the
compressor. If the intake slide valve is at 100% and compressor
speed is at 100%, and the engine is still not fully loaded, then
the Vi control slide valve may be adjusted to increase Vi.
[0057] Conversely, if there is a need to reduce engine load, such
as when the engine load begins to increase due to increased
discharge pressure, the control system will first drop compressor
speed until engine load stabilizes or a minimum desired compressor
speed is reached. If compressor speed is dropped to a minimum, and
engine load is still increasing, then the intake slide valve, or
the Vi control slide valve, or both, will begin to unload until
engine load reaches the desired level.
[0058] Intake suction control pressure may be separately monitored
and controlled. If suction pressure is lower than the desired
setpoint, then the suction pressure control valve may open until
suction pressure stabilizes. If suction pressure continues to fall
after the valve is at 100% open, the compressor speed will begin to
drop. If the suction pressure continues to drop after achieving
minimum speed on the compressor, the intake slide valve will begin
to unload. As well, the recycle valve may throttle open to maintain
minimum suction pressure if minimum suction pressure cannot be
maintained by slowing compressor speed and unloading the intake
slide valve. preferably after a timed delay, in order to reduce the
engine load and save fuel. In one embodiment, the opening of the
recycle valve may be delayed using a timer. Also, if all other
controls have been implemented, i.e. with the unit running 0% on
the slide valve, minimum compressor speed, and the recycle valve
open to maintain suction pressure, engine speed may also be
reduced, preferably after another timed delay. For example, the
engine may kick down to 1000 rpm from the normal speed of 1800
rpm.
[0059] The recycle valve (7) may also throttle open if the
discharge pressure approaches the maximum allowable working
pressure (MAWP) of the unit.
[0060] In one embodiment, the control system is operatively
connected to the engine, allowing control over engine parameters
such as ignition timing, air/fuel ratio and engine speed. Thus, the
control system may handle all shutdowns as well as the starting of
the engine. In one embodiment, speed feedback to the PLC will be
for display purposes only, with the exception of the start-up
sequence. The gas starter will disengage once the minimum start
speed has been exceeded.
[0061] Engine shutdown will be controlled by the control system by
killing the ignition, as well as de-energizing the fuel supply
solenoid.
[0062] Engine speed may be controlled with the use of a solenoid
incorporated into a throttle linkage. Conventional means to sense
engine speed may be used such as a magnetic pick-up on the flywheel
and the use of a signal converter to provide an input signal
(ST400) to the control system (14).
[0063] Shutdown Summary
[0064] There are many scenarios where a compressor shutdown is
necessary or desirable, examples of which follow. In each case, the
control system will react to an input from a sensor and initiate
compressor shutdown.
[0065] 1) Low Suction Pressure
[0066] 2) High Suction Pressure
[0067] 3) Low Lube Oil Level
[0068] 4) High Lube Oil Level
[0069] 5) High Discharge Temp
[0070] 6) High Vibration
[0071] 7) Suction Scrubber High Level
[0072] 8) High Discharge pressure
[0073] There are other scenarios where engine shutdown is necessary
or desirable. In these cases, the control system will react to a
sensor input, and initiate engine shutdown/
[0074] 1) Low Oil Pressure
[0075] 2) High Coolant Temp
[0076] 3) Overspeed
[0077] 4) High Vibration
[0078] 5) High Manifold Pressure
[0079] Start-Up Sequence
[0080] Prior to starting the unit, the operator should ensure that
all fluid levels are within their normal ranges, all valves are in
their operating positions, and all necessary safety devices are in
place and active.
[0081] If the unit was shutdown due to a fault condition or
shutdown, the operator should ensure that the fault condition has
been repaired or removed. The alarm will have to be acknowledged on
the control panel, and then reset prior to start up.
[0082] The sequence of events in a start-up is as follows: [0083]
a) Engine is started and the control system checks to ensure the
compressor is unloaded with the intake slide valve at 0%. [0084] b)
Engine runs up to operating speed, such as 1800 rpm. [0085] c) Once
the engine reaches operating temperature, the compressor is cleared
to start and the hydraulic pump begins to stroke and the compressor
begins to rotate. The pump continues to stroke until it reaches
100%. [0086] d) When the pump is at full stroke, the control system
will begin to move the intake slide valve and load the compressor.
[0087] e) Once the compressor reaches full load (slide valve to
100%), the control system will begin to stroke the hydraulic motor
to increase the compressor speed towards 100%.
[0088] Concurrently, the suction valve controller will be active,
trying to maintain its setpoint. As the compressor begins to rotate
and move gas, suction pressure will decrease, and the valve will
begin to throttle open to maintain its setpoint. Also, the
discharge valve controller will be active, although it will not
begin to open until the discharge pressure upstream of the
discharge valve reaches the controller setpoint. The discharge
pressure controller setpoint is typically set to maintain the
minimum pressure required by the internal lubrication system of the
compressor to function properly.
[0089] Shutdown Sequence
[0090] A shutdown can be initiated by either the operator, or by
one of the protective shutdown devices on the unit. Sequence is as
follows for an operator shutdown: [0091] a) Operator initiates stop
command via control panel. [0092] b) The control system will drop
the signals to the hydraulic motor and to the hydraulic pump
effectively stopping the compressor. [0093] c) The controls will
then fully unload the compressor (slide valve to 0%). [0094] d) The
engine will throttle down to minimum speed, (approx 1000 Rpm),
until the operator shuts the engine off via the control panel.
[0095] In the event of a protective shutdown, the control system
will close the fuel supply to the engine, effectively stopping the
entire unit almost immediately.
[0096] In the event of an Emergency Shut Down, (ESD), the controls
will close the fuel supply to the engine as well as grounding out
the ignition system to ensure the engine/compressor stops
immediately.
[0097] In an example of field use of one embodiment of the present
invention, the discharge pressure for normal operation in a booster
application was 225 psi discharge into a sales gas line. This line
led to the suction of a large reciprocating compressor owned by the
gas utility. When the downstream reciprocating compressor breaks
down, then line pressure in the sales line will increase. All of
the producer's regular booster compressors automatically shut down
when the line pressure reached 275 psi, as this load exceeded the
available horsepower for these units. A variable output compressor
of the present invention may, unattended, adjust the rotational
speed and load on the compressor downward to ensure there is
sufficient horsepower available to keep gas moving into the sales
line--even at discharge pressures above 325 psi. This compressor
was able to support operations throughout the day and night until
the downstream reciprocating compressor was repaired. This enabled
the producer to continue producing sales gas and generating
revenue, where other compressors were required to shut down.
[0098] The preceding detailed description of specific embodiments
of the present invention does not limit the implementation of the
invention to any particular programming language or signal
processing architecture. In one embodiment, the present invention
is implemented, at least partly, using a digital signal processor
operating under stored program control. It will be understood that
the present invention may be implemented using other architectures,
including a microprocessor, a microcontroller, a field programmable
logic device such as a field programmable gate array, discrete
electronic and logic components or combinations thereof. Any
limitations described herein as a result of a particular type of
architecture or programming language are not intended as
limitations of the present invention.
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