Note: Descriptions are shown in the official language in which they were submitted.
RATE CONTROL FOR AN INJECTION PUMP
Field of the Invention
[0001] The invention relates to methods and systems for controlling injection
rate of an
injection pump.
Background
[0002] During production of oil and gas, it is often necessary to inject a
treatment chemical
into a well, often into the annular space between the well casing and
production tubing. These
chemicals might include demulsifiers, corrosion inhibitors, scale inhibitors,
or paraffin
inhibitors. The various chemicals and their intended effects are well known in
the industry.
[0003] Rotary motors, typically electric or hydraulic motors, are used to
power injection
pumps which inject chemicals into a process. The motors are typically
connected by a
gearbox or transmission to a camshaft which reciprocates a plunger within a
cylinder to pump
fluid through an injection valve. The motors are controlled by electric motor
controllers.
Summary of the Invention
[0004] The present invention relates to a method and system for controlling
the injection rate
of an injection pump to maintain a desired average chemical injection rate
while also
satisfying the system's component constraints.
[0005] In one aspect, the invention may comprise a method of controlling
injection rates of an
injection pump actuated by an electric motor below a minimum electric motor
speed,
comprising the step of cycling the electric motor off and on in a duty cycle
ratio calculated to
provide a desired injection rate below the injection rate of the minimum
electric motor speed.
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Preferably, the electric motor is controlled to operate in continuous mode
above its minimum
speed and duty cycle mode below its minimum speed.
[0006] In another aspect, the invention may comprise an injection pump system
comprising
an electric motor and an electric motor controller, wherein the controller is
adapted to operate
the electric motor in continuous mode above its minimum speed and in an off
and on duty
cycle mode below its minimum speed.
[0007] The minimum motor speed may be mandated by the pump or a component
other than
the motor itself, such as a gear assembly or a flow meter.
Brief Description of the Drawings
[0008] In the drawings shown in the specification, like elements may be
assigned like
reference numerals. The drawings are not necessarily to scale, with the
emphasis instead
placed upon the principles of the present invention. Additionally, each of the
embodiments
depicted are but one of a number of possible arrangements utilizing the
fundamental concepts
of the present invention.
[0009] Figure 1 is a schematic representation of a motor and pump system, with
a motor
controller and a power source.
[0010] Figure 2 is a graph of average injection plotted against motor speed.
[0011] Figure 3 is a graph of an on/off duty cycle below a minimum speed and a
transition to
continuous mode above the minimum speed.
[0012] Figure 4 is a schematic diagram of a basic DC motor controller.
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Detailed Description of Preferred Embodiments
[0013] In one aspect, the invention may comprise a rate control system for
controlling a
positive displacement pump, actuated by an electric motor. The methods and
systems
described herein may be adapted to and added on to the methods and systems
described in co-
pending US Patent Application No. 14/974,709, entitled "Method and System for
Enhanced
Accuracy of Chemical Injection Pumps", the entire contents of which are
incorporated herein
by reference for all purposes, where permitted.
[0014] In the system and method according to the invention, an electric motor
(100), powered
by power source (110), is connected to and operates pump (120) that is
configured to pump a
chemical into process (130). Motor (100) may be, for example, one of the
following types:
DC Motor, AC Induction motor, permanent magnet synchronous motor (PMSM), or
brushless
DC motor (BLDC). Pump (120) may be, for example but not limited to, one of the
following
types of pumps: positive displacement piston driven, packed plunger,
centrifugal, gear, vane,
or diaphragm. Power source (110) may be for example, DC power from a battery
or solar
panel, DC power from battery, DC power from an engine skid, DC power from a
thermoelectric generator, DC power from a fuel cell, DC generator, or
alternator, or a
combination of the above. Power source (110) may also be AC power from a
generator, or a
power grid, or an inverter, which converts power from DC or another frequency.
Process
(130) may be, for example, an oil or gas well, pipeline, valve, choke,
separator, tank,
downhole, flowline, or wellhead.
[0015] In one embodiment, rate control for the pump (120) may be provided
below a
minimum electric motor speed. Generally, average flow rates vary linearly with
motor speed,
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as shown in Figure 2. However, motors have a minimum speed of operation shown
as the
black rectangle in Figure 2. That minimum speed is generally about 1.6% of
maximum
speed, below which the motor may stall or overheat. In order to provide flow
rate control
below this minimum speed, an alternative strategy is required. In one
embodiment, one
example of which is illustrated in Figure 3, the motor is cycled on and off
below the minimum
speed.
[0016] Additionally, mechanical limitations of the pump itself or associated
components such
as gear assemblies or flow rate meters may mandate a minimum electric motor
speed. For
example, the pump may require a certain minimum speed to operate correctly, or
a gear
assembly may require a minimum speed to maintain lubrication, which means that
the electric
motor should not go below a certain speed, which may be greater than the
minimum speed of
the motor itself. In another example, the transition from continuous to duty
cycle control can
be motivated by the need to maintaining a minimum flow rate through injection
control
devices such as flow meters. Often such devices have a calibration span that
does not extend
down to zero flow thus requiring a lower flow rate limit.
[0017] An on/off cycle of 1:1 would provide an injection rate of about 50% of
the set
minimum speed. Where the minimum speed is 1.6% of maximum, in order to provide
an
injection rate of 1% of maximum, the motor is cycled on and off in a ratio of
about 1:0.6. The
motor controller may automatically select continuous mode for higher rates,
switch
automatically to duty cycle mode if an injection rate below the continuous
minimum is
selected, and then switch automatically to continuous mode for higher
injection rates.
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[0018] As a result, a very wide turndown in injection volume may be
implemented without
any mechanical changes to the pump. This method may be applied to any variable
speed
electric motor running an injection pump. Controllers for such variable speed
electric motors
are well known in the art, and adapting a motor controller to implement the on-
off duty cycle
described herein is well within the capability of one skilled in the art. A
basic DC motor
speed controller is shown below in Figure 4. Below the minimum speed of the
motor, the
controller may be adapted to cycle on and off to achieve sub-minimum injection
rates.
Definitions and Interpretation
[0019] Aspects of the present invention may be described with reference to
flowchart
illustrations and/or block diagrams of methods, apparatus (systems) and
computer program
products according to embodiments of the invention. It will be understood that
each block of
the flowchart illustrations and/or block diagrams, and combinations of blocks
in the flowchart
illustrations and/or block diagrams, can be implemented by computer program
instructions.
These computer program instructions may be provided to a processor of a
general purpose
computer, special purpose computer, or other programmable data processing
apparatus to
produce a machine, such that the instructions, which execute via the processor
of the
computer or other programmable data processing apparatus, create means for
implementing
the functions/acts specified in the flowchart and/or block diagram block or
blocks.
[0020] The flowchart and block diagrams in the Figures illustrate the
architecture,
functionality, and operation of possible implementations of systems, methods
and computer
program products according to various embodiments of the present invention. In
this regard,
each block in the flowchart or block diagrams may represent a module, segment,
or portion of
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code, which comprises one or more executable instructions for implementing the
specified
logical function(s). It should also be noted that, in some alternative
implementations, the
functions noted in the block may occur out of the order noted in the figures.
For example, two
blocks shown in succession may, in fact, be executed substantially
concurrently, or the blocks
may sometimes be executed in the reverse order, depending upon the
functionality involved.
It will also be noted that each block of the block diagrams and/or flowchart
illustration, and
combinations of blocks in the block diagrams and/or flowchart illustration,
can be
implemented by special purpose hardware-based systems that perform the
specified functions
or acts, or combinations of special purpose hardware and computer
instructions.
[0021] The corresponding structures, materials, acts, and equivalents of all
means or steps
plus function elements in the claims appended to this specification are
intended to include any
structure, material, or act for performing the function in combination with
other claimed
elements as specifically claimed.
[0022] References in the specification to "one embodiment", "an embodiment",
etc., indicate
that the embodiment described may include a particular aspect, feature,
structure, or
characteristic, but not every embodiment necessarily includes that aspect,
feature, structure, or
characteristic. Moreover, such phrases may, but do not necessarily, refer to
the same
embodiment referred to in other portions of the specification. Further, when a
particular
aspect, feature, structure, or characteristic is described in connection with
an embodiment, it is
within the knowledge of one skilled in the art to affect or connect such
module, aspect,
feature, structure, or characteristic with other embodiments, whether or not
explicitly
described. In other words, any module, element or feature may be combined with
any other
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element or feature in different embodiments, unless there is an obvious or
inherent
incompatibility, or it is specifically excluded.
[0023] It is further noted that the claims may be drafted to exclude any
optional element. As
such, this statement is intended to serve as antecedent basis for the use of
exclusive
terminology, such as "solely," "only," and the like, in connection with the
recitation of claim
elements or use of a "negative" limitation. The terms "preferably,"
"preferred," "prefer,"
"optionally," "may," and similar terms are used to indicate that an item,
condition or step
being referred to is an optional (not required) feature of the invention.
[0024] The singular forms "a," "an," and "the" include the plural reference
unless the context
clearly dictates otherwise. The term "and/or" means any one of the items, any
combination of
the items, or all of the items with which this term is associated. The phrase
"one or more" is
readily understood by one of skill in the art, particularly when read in
context of its usage.
[0025] The term "about" can refer to a variation of 5%, 10%, 20%, or 25%
of the
value specified. For example, "about 50" percent can in some embodiments carry
a variation
from 45 to 55 percent. For integer ranges, the term "about" can include one or
two integers
greater than and/or less than a recited integer at each end of the range.
Unless indicated
otherwise herein, the term "about" is intended to include values and ranges
proximate to the
recited range that are equivalent in terms of the functionality of the
composition, or the
embodiment.
[0026] As will be understood by one skilled in the art, for any and all
purposes, particularly in
terms of providing a written description, all ranges recited herein also
encompass any and all
possible sub-ranges and combinations of sub-ranges thereof, as well as the
individual values
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making up the range, particularly integer values. A recited range includes
each specific value,
integer, decimal, or identity within the range. Any listed range can be easily
recognized as
sufficiently describing and enabling the same range being broken down into at
least equal
halves, thirds, quarters, fifths, or tenths. As a non-limiting example, each
range discussed
herein can be readily broken down into a lower third, middle third and upper
third, etc.
[0027] As will also be understood by one skilled in the art, all language such
as "up to", "at
least", "greater than", "less than", "more than", "or more", and the like,
include the number
recited and such terms refer to ranges that can be subsequently broken down
into sub-ranges
as discussed above. In the same manner, all ratios recited herein also include
all sub-ratios
falling within the broader ratio.
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