VARIABLE CAPACITY NATURAL GAS COMPRESSOR
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.
The present invention relates to a method and apparatus for gas compression.
BACKGROUNDIn 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.
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.
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.
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.
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.
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.
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.
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.
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 INVENTIONIn one aspect, the invention comprises a variable capacity screw compressor comprising:
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- (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;
- (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.
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.
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.
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.
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.
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:
-
- (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 in response to changes in engine load; or
- iii. both.
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.
Suction pressure may be controlled independently of controlling engine load, and as well, discharge pressure may also be controlled independently of controlling engine load.
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:
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.
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.
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.
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.
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.
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.
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.
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.
In one aspect, as shown in the general schematic of
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.
As shown in
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).
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.
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.
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).
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).
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.
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.
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.
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
As shown in
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.
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.
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.
The recycle valve (7) may also throttle open if the discharge pressure approaches the maximum allowable working pressure (MAWP) of the unit.
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.
Engine shutdown will be controlled by the control system by killing the ignition, as well as de-energizing the fuel supply solenoid.
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).
Shutdown Summary
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.
1) Low Suction Pressure
2) High Suction Pressure
3) Low Lube Oil Level
4) High Lube Oil Level
5) High Discharge Temp
6) High Vibration
7) Suction Scrubber High Level
8) High Discharge pressure
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/
1) Low Oil Pressure
2) High Coolant Temp
3) Overspeed
4) High Vibration
5) High Manifold Pressure
Start-Up Sequence
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.
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.
The sequence of events in a start-up is as follows:
-
- a) Engine is started and the control system checks to ensure the compressor is unloaded with the intake slide valve at 0%.
- b) Engine runs up to operating speed, such as 1800 rpm.
- 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%.
- d) When the pump is at full stroke, the control system will begin to move the intake slide valve and load the compressor.
- 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%.
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.
Shutdown Sequence
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:
-
- a) Operator initiates stop command via control panel.
- b) The control system will drop the signals to the hydraulic motor and to the hydraulic pump effectively stopping the compressor.
- c) The controls will then fully unload the compressor (slide valve to 0%).
- d) The engine will throttle down to minimum speed, (approx 1000 Rpm), until the operator shuts the engine off via the control panel.
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.
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.
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.
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.
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.
Type: Application
Filed: Oct 6, 2006
Publication Date: Apr 10, 2008
Applicant: VAPORTECH ENERGY SERVICES INC. (Edmonton)
Inventors: Derek MACKENZIE (Calgary), Sami SAAD (Edmonton), John VAN MANEN (Edmonton)
Application Number: 11/539,513
International Classification: F01B 25/06 (20060101);
