Note: Descriptions are shown in the official language in which they were submitted.
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METHOD FOR TWIN SCREW
POSITIVE DISPLACEMENT PUMP PROTECTION
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims benefit to provisional patent application serial no.
62/205,205, filed 14 August 2015 (Atty dckt no. 911-002.073/F-GI-1505), which
is
hereby incorporated by reference in its entirety.
This application is also a continuation-in-part of, and claims benefit to,
patent
application serial no. 13/859,899 (Atty dckt no. 911-002.047-1/F-GI-1103US),
filed
10 April 2013, entitled "Method for rotary positive displacement pump
protection,"
which itself claims benefit to provisional patent application serial no.
61/622,684, filed
11 April 2012, which are both hereby incorporated by reference in its
entirety.
The parent application serial no. 13/859,899 is directed towards rotary
positive displacement pump protection, e.g., for gear and progressive cavity
pumps;
while the present application is directed towards rotary positive displacement
pump
protection, e.g., for twin screw pumps.
BACKGROUND OF THE INVENTION
1. Field of the Invention
This application relates to a rotary positive displacement pump, such as a
twin
screw pump, an internal or external gear pump, a lobe pump, a vane pump or a
progressive cavity pump; and more particularly, relates to techniques for
protection,
e.g., for a dry run condition, for such a rotary positive displacement pump,
including
such a twin screw pump.
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2. Brief Description of Related Art
Many different types or kinds of pumps and external protective devices,
including rotary positive displacement pumps with external protection devices,
are
known in the art. By way of example, some known external protection device
disadvantages associated with the same and set forth below:
One known device PMP 25, provided by a company named Load Controls,
Inc. (Sturbridge, MA), uses a load monitor technique that provides pump
protection
by observing the motor amperage draw and speed and then correlating the
resulting
power reading to various operating conditions (e.g. dry running, closing
valves). See
U.S. Patent nos. 5,930,092 and 5,754,421, which are hereby incorporated by
reference in their entirety. One disadvantage of this known device is that it
is
suitable only for constant speed applications and fails to distinguish control
differentiation from various system upset conditions.
Another known device, provided by a company named ABB Industry Oy
(Helsinki, Finland), uses a technique based on a variable frequency drive that
has
parameters that allow maximum and minimum torque values to be configured to
prevent the load driver (motor) from operating outside of these parameters.
One
disadvantage of this variable frequency drive technique is that it does not
provide
logic for interpreting normal operating conditions from system upsets, such as
distinguishing between a higher power requirement due to increased system
resistance versus a higher torque condition caused by dry running.
Other known devices consist of flow or pressure switches or liquid
presence/absence detectors to identify undesired operating conditions.
However,
the use of additional process flow or pressure switches adds cost and
complexity to
the drive system, a potential failure point, and unnecessary cost.
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United States patent application serial no. 11/601,373, filed 17 November
2006, entitled "Pump Protection Without the Use of Traditional Sensors," by A.
Stavale et al., which was published as US 2007/0212229 Al and is incorporated
by
reference in its entirety, sets forth techniques for providing pump protection
for
centrifugal pumps. Centrifugal pumps have a very different principle of
operation
than do rotary positive displacement pumps. In centrifugal pumps power varies
as
the cube of the speed change (Figure 1) and torque varies as the square of the
speed change. In addition, the tune process for dry run protection of
centrifugal
pumps described in patent application 11/601,373 is performed at a closed
valve
condition. The tune process for dry run protection of rotary positive
displacement
pumps could not be performed at the closed valve condition, since rotary
positive
displacement pumps will quickly destroy itself if operated at closed valve
condition
without intervention. For these reasons, the techniques disclosed in patent
application serial no. 11/601,373 would not be applied to rotary positive
displacement pumps.
None of the aforementioned patents or publications teach or suggest the
technique described herein for providing pump protection for rotary positive
displacement pumps, as set forth below.
Furthermore, twin screw positive displacement (PD) pumps are quite different
in construction from other rotary PD pumps such as gear pumps and progressive
cavity pumps. For example, twin screw pumps do not have rotor to rotor rubbing
contact (like gear pumps) or rotor to stationary housing contact (like
progressive
cavity pumps) when operating under dry run or partial dry run conditions. As
such
twin screw pumps are able to operate for extended time periods before damage
can
occur. Since rubbing contact does occur in these other rotary PD pumps,
failure can
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occur quickly when operating in a dry run condition. In addition, torque
signatures
differ greatly between rotary PD pumps and twin screw pumps. For example,
rotary
PD pumps can provide a robust torque ripple signature when operating under dry
run
conditions. In contrast, twin screw pumps have a torque signature which
provides
little change between normal operating conditions and a distressed operating
condition. It was found that the algorithms created for rotary PD pumps cannot
reliably detect dry run conditions for twin screw pumps without modification.
SUMMARY OF THE INVENTION
The present invention provides new and unique techniques for protecting
rotary positive displacement pumps, including twin screw pumps, while
differentiating
between dangerous operating conditions such as dry running which can result in
catastrophic damage if left to operate without intervention. Examples of
rotary
positive displacement pumps are internal or external gear pumps, lobe pumps,
vane
pumps and progressive cavity pumps. The methodology relies on two types of
protection to increase robustness and response time. Providing a robust pump
protection solution while avoiding nuisance faults can be difficult. In order
to use
power, or torque measurements to detect a dry run condition the following must
be
considered: power and torque varies with specific gravity, viscosity,
differential
pressure and speed changes. Speed is the easiest parameter to contend with as
it
can be measured directly. For varying temperature systems the power and torque
comparisons must all be evaluated at a common specific gravity and viscosity.
Therefore power and torque readings are corrected to rated conditions for
specific
gravity and viscosity changes before any evaluation is done. This can be
achieved
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by entering the specific gravity and viscosity vs. temperature curves in the
controller.
A simple temperature measuring device can then be used to correct power
readings.
For constant temperature systems, corrections to power readings are not
required and the protection method does not require traditional sensors.
Preventing nuisance faults is another important problem to resolve. This can
occur when changes in power readings are due to a changing system condition;
e.g.
increases or decreases in discharge pressure. The change in power readings
must
be distinguished between normal system changes and increased or decreased
power draw due to internal rubbing contact or dry run conditions. This is
achieved in
part by the basic pump protection algorithm where a speed change associated
with
changing conditions is allowed to re-stabilize at a constant speed with a +/-
change.
Once stabilized new power readings are sampled.
For gear pumps, the enhanced pump protection algorithm can distinguish
between a torque ripple signature during normal operation and a torque ripple
signature during a condition where the pump is in distress. If the torque
ripple
exceeds a predefined set point, then a dry run fault is declared. For
progressive
cavity pumps it was found that torque ripple is not a reliable method for
determining if
a dry run condition exists. It has been found through testing that these types
of
pumps can have an unstable torque signature. Therefore, a different approach
was
taken for enhanced pump protection for this type of pump. The algorithm for
enhanced pump protection calculates a corrected high and low power ratio and
compares it to a high and low power ratio set point to determine if a dry run
condition
exists.
In comparison, for twin screw pumps the enhanced pump protection algorithm
can distinguish between a torque signature during normal operation and a
torque
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signature during a condition where the pump is in distress. If the torque
exceeds a
predefined set point, then a dry run fault is declared.
The enhanced pump protection methodology can protect against difficult to
detect dry run conditions which the basic pump protection algorithm cannot.
These
conditions occur at low operating speeds (e.g., down to 20:1 turndown from
full load
motor speed) and in systems operating at a low differential pressure.
One advantage of the new and unique basic pump protection is to provide a
faster and more robust response to a dry run condition when the corrected tune
ratio
is greater than the tune ratio set point. Tune ratios above the set point
value are
associated with higher differential pressures. In this case, a response to a
dry run
condition can be identified more quickly than in enhanced protection
methodology.
The logic for these algorithms, for example, can be embedded in a variable
frequency drive (VFD) or a programmable logic controller (PLC).
The Apparatus
According to some embodiments, the present invention may take the form of
apparatus comprising a signal processor that may be configured to
receive signaling containing information about power, torque, speed,
viscosity and specific gravity related to the operation of a twin screw
positive
displacement pump; and
determine whether to enter an enhanced pump protection mode for the
twin screw positive displacement pump based at least partly on a relationship
between an actual corrected tune ratio and a tuned ratio set point (Tune Ratio
SP).
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According to some embodiments of the present invention, the signal
processor may be configured to determine if the actual corrected tune ratio is
less
than or equal to the actual corrected tune ratio set point (Tune Ratio SP),
and if so,
then to enter the enhanced pump protection mode, else to continue to use a
basic
pump protection mode.
According to some embodiments of the present invention, the signal
processor may be configured to determine the actual corrected tune ratio based
at
least partly on a ratio of an actual corrected torque (TAcorr) divided by a
tuned
corrected torque (TTcorr) at a specific operating speed.
According to some embodiments of the present invention, the signal
processor may also be configured to determine the actual corrected torque
(TAcorr)
based at least partly on a relationship between an actual torque (TACT) at the
current speed, a rated specific gravity (SG RTD) of the fluid being pumped, an
actual
specific gravity (SGACT) of the fluid being pumped, a rated viscosity
(VISCRTD) of
the fluid being pumped, an actual viscosity (VISCACT) of the fluid being
pumped.
For example, the signal processor may be configured to determine the actual
corrected torque (TAcorr) based at least partly on the equation:
TAcorr = TACT x (SG RTD/SGACT)/(VISCACT/VISCRTD) .275.
According to some embodiments of the present invention, the signal
processor may be configured to determine the tuned corrected torque (TTcorr)
based at least partly on a relationship between a measured or interpolated
tuned
value torque (TMEAS) at the current speed, a rated specific gravity (SGRTD) of
the
fluid being pumped, an actual specific gravity (SGACT) of the fluid being
pumped, a
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rated viscosity (VISCRTD) of the fluid being pumped, an actual viscosity
(VISCACT)
of the fluid being pumped. For example, the signal processor may be configured
to
determine the tuned corrected torque (TTcorr) based at least partly on the
equation:
TTcorr = TMEAS x (SGRTD/SGACT)/(VISCACT/VISCRTD) 275.
According to some embodiments of the present invention, the tuned ratio set
point (Tune Ratio SP) may include a default setting, e.g., including one
default
setting of about 1.3 for the twin screw positive displacement pump.
According to some embodiments of the present invention, the signal
processor may be configured to provide a control signal containing information
to
control the operation of the twin screw positive displacement pump, including
shutting the twin screw positive displacement pump off when a dry run
condition is
determined in the enhanced pump protection mode.
According to some embodiments of the present invention, the signal
processor may also be configured as, or take the form of, a controller that
controls
the operation of the twin screw positive displacement pump.
According to some embodiments of the present invention, the apparatus may
include the twin screw positive displacement pump itself in combination with
the
signal processor.
Enhanced Pump Protection Mode
for Internal or External Gear, Lobe or Vane Pumps
The signal processor may also be configured to continuously compensate
torque measurements for specific gravity and viscosity changes in systems
where a
process temperature is not constant.
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The signal processor may also be configured to perform each evaluation while
the pump is, e.g., at +/- a constant speed in order to distinguish between
increasing/decreasing discharge pressure and an upset condition.
The signal processor may also be configured to detect a speed change and
restart a protection mode algorithm.
Enhanced Pump Protection Mode for Progressive Cavity and Twin Screw Pumps
According to some embodiments of the present invention, when in the
enhanced pump protection mode for the twin screw positive displacement pump,
the
signal processor may be configured to determine a corrected high and low
torque
ratio; and compare the corrected high and low torque ratio to a high and low
torque
ratio set point to determine if a dry run condition exists.
According to some embodiments of the present invention, the signal
processor may be configured to determine if either
For Progressive Cavity Pumps:
PACT2CORR/PACT1CORR >= HI P RATIO SP
or
PACT200RR/PACT100RR <= LO P RATIO SP; and
For Twin Screw Pumps:
TACT2CORR/TACT1CORR >= HI T RATIO SP
or
TACT2CORR/TACT1CORR <= LO T RATIO SP; and
if so, then to declare a dry run fault, else to operate the twin screw
positive
displacement pump in a normal condition, where
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TACT1CORR is a corrected torque reading for specific gravity and
viscosity and is a mode value over an initial sample period,
TACT2CORR is a continuously updated corrected torque reading for
specific gravity and viscosity and is a mode value after the initial sample
period,
HI T RATIO SP is a default high torque ratio set point, and
LO T RATIO SP is a default low power ratio set point.
According to some embodiments of the present invention, the signal
processor may be configured to determine the corrected torque reading for
specific
gravity and viscosity based at least partly on the equation:
TACT1CORR = TACT x (SGRTD/SGACT)/(VISCACT/VISCRTD) .275.
According to some embodiments of the present invention, the signal
processor may be configured to update the value of TACT1CORR under, e.g., the
following conditions: when +/- a predetermined rpm speed change occurs, during
pump start-up and after a predetermined operating time elapses.
According to some embodiments of the present invention, the signal
processor may be configured to detect an inadvertently closed suction valve
during
startup by implementing the following;
during start-up, once the speed set point has been reached, take an
initial torque reading at some point after a timer begins;
take subsequent torque readings at subsequent periodic intervals until
the timer expires;
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compare each subsequent torque reading to the initial torque value;
and
determine if a current torque value/initial torque value <= some
predetermined default setting, then issue a Dry Run Fault.
According to some embodiments of the present invention, the signal
processor may be configured to determine a momentary peak P that exists
directly
after suction valve closure during dry run conditions and takes the form of a
distinguishing characteristic between a dry run torque signature and a
condition
normal system change.
According to some embodiments of the present invention, the signal
processor may be configured to determine if a dry run condition exists for
Enhanced
Pump Protection using a comparison of a corrected high and corrected low
torque
ratio to a high and low torque ratio set point.
According to some embodiments of the present invention, prior to making the
comparison, the signal processor may be configured to determine if the
momentary
peak P exists by performing a sample torque check to see if the dry run
condition
exists due to a control valve being opened/closed.
According to some embodiments of the present invention, the signal
processor may be configured to take sample torque readings at a minimum sample
rate, and to compare an initial torque reading at constant speed to each
successive
torque reading.
According to some embodiments of the present invention, the signal
processor may be configured to determine if greater than a predetermined
number of
comparisons are negative, and if so determined, then
start a timer;
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evaluate at a given rate until the timer expires equations, as follows:
TACT2CORR/TACT1CORR >= HI T RATIO SP,
and
TACT2CORR/TACT1CORR <= LO T RATIO SP, and
either declare the dry run condition if one or both equations is true, or
declare a normal run condition if both equations are false,
where
TACT1CORR is a corrected torque reading for specific gravity and
viscosity and is a mode value over an initial sample period,
TACT2CORR is a continuously updated corrected torque reading for
specific gravity and viscosity and is a value after the initial sample period,
HI T RATIO SP is a default high torque ratio set point, and
LO T RATIO SP is a default low torque ratio set point.
The Basic Pump Protection Mode
According to some embodiments of the present invention, when in a basic
pump protection mode the signal processor may be configured to determine at
the
current operating speed if the actual corrected torque (TAcorr) is less than
or equal
to a dry run factor (KDR) multiplied by the tuned corrected torque (TTcorr),
where the
dry run factor (KDR) has a default setting, including about 0.95 and can be
adjusted
if nuisance trips occur; and if so, the signal processor is configured to
declare a dry
run fault, else to operate the twin screw positive displacement pump in a
normal
condition.
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According to some embodiments of the present invention, the signal
processor may be configured to keep the basic pump protection mode always
active.
The Method
According to some embodiments, the present invention may take the form of
a method comprising: receiving with a signal processor signaling containing
information about power, torque, speed, viscosity and specific gravity related
to the
operation of a twin screw positive displacement pump; and determining whether
to
enter an enhanced pump protection mode for the twin screw positive
displacement
pump based at least partly on a relationship between an actual corrected tune
ratio
and a tuned ratio set point (Tune Ratio SP).
According to some embodiments of the present invention, the method may
also include implementing one or more of the features set forth above.
BRIEF DESCRIPTION OF THE DRAWING
The drawing includes the following Figures:
Figure 1 is a graph of power (BHP) versus speed (RPM) for a centrifugal
pump protection tune at a closed valve condition that is known in the art.
Figure 2 is a block diagram of apparatus according to some embodiments of
the present invention.
Figure 3 is a graph of capacity (GPM) versus discharge pressure (PSIG) for a
pump protection tune.
Figure 4 is a graph of power (BHP) versus speed (RPM) for a rotary positive
displacement pump protection tune at rated conditions.
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Figure 5 is a graph of torque (in-lbs) versus time (sec) for enhanced pump
protection - torque ripple condition normal.
Figure 6 is a graph of torque (in-lbs) versus time (sec) for enhanced pump
protection - torque ripple dry run condition.
Figure 7 is a graph of torque ( /0) versus time (second) showing a function
that
includes a peak P in relation to a suction valve closure.
Figure 8 is a flowchart of a pump protection mode, according to some
embodiments of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
By way of example, as shown in Figure 2, according to some embodiments,
the present invention may take the form of apparatus 10 that includes a signal
processor 12 configured to protect the operation a rotary positive
displacement pump
14, e.g., which may include, or take the form of, a twin screw pump, an
internal or
external gear pump, a lobe pump, a vane pump or a progressive cavity pump.
The signal processor 12 may be configured to receive signaling containing
information, e.g., about power, torque, speed, viscosity and specific gravity,
related
to the operation of the rotary positive displacement pump 14, and determine
whether
to enter an enhanced pump protection mode for the rotary positive displacement
pump based at least partly on a relationship between an actual corrected tune
ratio
and a tuned ratio set point (Tune Ratio SP) else remain in the basic
protection mode.
The signal processor 12 may also be configured to provide a control signal
containing information to control the operation of the rotary positive
displacement
pump 14, including shutting the rotary positive displacement pump off when a
dry run
condition is determined in the enhanced or basic pump protection mode.
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The rotary positive displacement pump 14 may include a module 16
configured to provide the signaling containing information, e.g., about power,
torque,
speed, viscosity and specific gravity, related to the operation of the rotary
positive
displacement pump 14, and may also be configured to receive the control signal
containing information to control the operation of the rotary positive
displacement
pump 14, including shutting the rotary positive displacement pump off when the
dry
run condition is determined in the enhanced or basic pump protection mode.
In operation, the signal processor 12 may be configured to determine if the
actual corrected tune ratio is less than or equal to the actual corrected tune
ratio set
point (Tune Ratio SP), and if so, then to enter the enhanced pump protection
mode,
else to continue to use a basic pump protection mode. By way of example, for
gear
and progressive cavity pumps, the signal processor 12 may be configured to
determine the actual corrected tune ratio based at least partly on a ratio of
an actual
corrected power (PAcorr) divided by a tuned corrected power (PTcorr) at a
specific
operating speed, consistent with that set forth in relation to section A
below. By way
of further example, for twin screw pumps, the signal processor 12 may be
configured
to determine the actual corrected tune ratio based at least partly on a ratio
of an
actual corrected torque (TAcorr) divided by a tuned corrected torque (TTcorr)
at a
specific operating speed, consistent with that set forth in relation to
section B below.
The logic for the basic and enhanced algorithms, for example, can be embedded
in a
variable frequency drive (VFD) or a programmable logic controller (PLC).
By way of example, implementations of the basic pump protection mode and
the enhanced pump protection mode for gear and progressive cavity pumps and
twin
screw pumps are set forth in detail below:
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A. Implementation for Gear and Progressive Cavity Pumps
In effect, the present invention consists of two types of positive
displacement
pump protection control logic which utilize the direct feedback of power,
torque,
speed, viscosity and specific gravity to calculate an actual corrected tune
ratio
consisting of the actual corrected power divided by the tuned corrected power
at a
specific operating speed. The power measurements are continuously compensated
for specific gravity and viscosity changes in systems where process
temperature is
not constant. The corrected actual tune ratio is then compared to a tune ratio
set
point in a decision tree algorithm. If the calculated tune ratio is greater
than the tune
ratio set point basic pump protection becomes active.
The process for activating pump protection is to first do a protective tune
which samples speed and power data at three or more speeds (e.g., five (5)
speeds)
while operating at rated conditions. (In contrast to techniques related to the
tune
process at the closed valve condition re centrifugal pumps, the tune process
for dry
run protection of rotary positive displacement pumps as described in this
application
is performed at rated conditions.) The protection functionality must be
disabled
during this process. If the pump is operating on a system with multiple system
curves the protection tune should be performed with the pump operating on the
system curve having least resistance. For the pump and system shown in Figure
3
the protection tune would be performed while operating on system curve
labelled A.
This is necessary to avoid nuisance dry run faults when transitioning between
higher
to lower discharge pressures.
Once the protection tune is completed the pump protection functionality can
be enabled.
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In a positive displacement pump, the torque remains substantially constant for
a constant differential pressure regardless of speed, and power will vary
proportionally to the change in speed as shown in Figure 4. The power curve in
Figure 4 for PD pumps varies directly with the change in speed (provided there
is
adequate suction pressure) for a given differential pressure. For centrifugal
pumps
power varies as the cube of the speed change (Figure 1). Centrifugal pumps may
operate at closed valve condition for short periods. It is not acceptable for
positive
displacement pumps to operate against a closed valve. Pressure will continue
to
build until pump damage occurs or the pump housing and/or piping ruptures.
After the protective tune has been completed and pump protection has been
enabled the decision tree algorithm for basic pump protection becomes active
as
follows:
The Basic Pump Protection Mode
The following is an example of steps for the basic pump protection mode for a
rotary positive displacement pump, including an internal or external gear
pump, a
lobe pump, a vane pump or a progressive cavity pump:
Pump Running
If true, then
Pump at Constant +/- Speed: If False, then ¨> T
If true, then
PAct Corr/PTune Corr <= Tune Ratio SP
If False, then ¨> go to basic pump protection
If true, then ¨> go to enhanced pump protection
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The tune ratio at current operating speed is determined or calculated by the
following set of equations:
PAct Corr/PTune Corr;
PACTCORR = PACT x (SGRTD/SGACT)/ (VISCACT/VISCRTD) "0.275; and
PTUNECORR = PMEAS x (SGRTD/SGACT)/ (VISCACT/VISCRTD) ^0.275,
where:
PACT = actual power at current speed,
PMEAS = measured or interpolated tuned value power at current speed,
SGRTD = rated specific gravity,
SGACT = actual specific gravity,
VISCRTD = rated viscosity, and
VISCACT = actual viscosity.
The exponent of 0.275 is a default value although the scope of the invention
is
intended to include embodiments having a different exponent consistent with
that
now known or later developed in the future.
By way of example, for internal or external gear, lobe or vane PD pumps, the
Tune Ratio SP (i.e. set point) has a default setting of 2.0; while for
progressive cavity
PD pumps, the Tune Ratio SP has a default setting of 1.3, although the scope
of the
invention is intended to include embodiments having a different default
setting for the
Tune Ratio SP consistent with that now known or later developed in the future.
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If the basic pump protection is active, the following relationship is
evaluated at
the current operating speed by the equation:
PACTCORR <= KDR x PTUNECORR,
where KDR is a dry run factor with a default setting of 0.9; although the
scope of the
invention is intended to include embodiments having a different default
setting
consistent with that now known or later developed in the future . Note the KDR
value
can be adjusted by the user if nuisance trips occur.
If PACTCORR <= KDR x PTUNECORR is false, then the condition of the PD
pump is normal.
If PACTCORR <= KDR x PTUNECORR is true, then a dry run fault condition
for the PD pump is declared.
Enhanced Pump Protection Mode
For rotary PD pumps, the enhanced pump protection mode may be used if the
following condition is true:
PAct Corr/PTune Corr <= Tune Ratio SP
Consistent with that set forth below, one type of an enhanced pump protection
mode is used for internal or external gear, lobe or vane PD pumps, and another
type
of an enhanced pump protection mode is used for progressive cavity PD pumps.
In
either enhanced pump protection mode, the basic pump protection may also
remain
active.
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The Enhanced Pump Protection Mode
for Internal or External Gear, Lobe or Vane Pumps
For an internal or external gear, lobe or vane PD pump, the enhanced pump
protection mode is based at least partly on the following torque ripple
condition:
Torque Ripple Ratio >= Torque Ripple Set Point.
If the torque ripple condition is true, then a dry run fault is declared for
the
internal or external gear, lobe or vane PD pump.
In contrast, if the torque ripple condition is false, then the internal or
external
gear, lobe or vane PD pump has a normal condition.
Consistent with that set forth above, in this enhanced pump protection mode,
the basic pump protection is always active, but enhanced pump protection
(torque
ripple) is only active when the tune ratio is less than or equal then the tune
ratio set
point.
In the enhanced pump protection mode, highest /lowest torque values may be
compared to the torque ripple set point, e.g., during a 20 sample period. The
sample
period will typically depend on the monitor update rate. For example, for a
100 msec
update rate the sample period is 2 sec. Note the torque measurements may be
continuously compensated for specific gravity and viscosity changes in systems
where the process temperature is not constant.
According to some embodiments of the present invention, the default setting
for the torque ripple set point may be about 1.10, although the scope of the
invention
is intended to include embodiments having a different default setting
consistent with
that now known or later developed in the future.
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Each evaluation may be performed while the pump is at +/- a constant speed
in order to distinguish between increasing/decreasing discharge pressure and
an
upset condition. If a speed change is detected the algorithm restarts.
In rotary positive displacement pumps the torque ripple during normal
operating
conditions is substantially less than in a dry run condition. As the rotor
begins to lose
lubrication and friction increases the torque begins to spike as the rotor
goes in and
out of lubricating conditions.
Figure 5 shows a graph of torque (in-lbs) versus time (sec) as an example for
enhanced pump protection - torque ripple condition normal. In Figure 5, the
normal
operating conditions are shown at 88 rpm (20:1 turndown in maximum speed). For
normal operation, the torque ripple is less than 1%. Figure 6 also shows a 2
second
snapshot of a dry run condition also at 88 rpm which quickly exceeds the
torque
ripple set point of 1.10. In contrast, and by way of comparison, Figure 6
shows a
graph of torque (in-lbs) versus time (sec) as an example for enhanced pump
protection - torque ripple dry run condition.
Enhanced Pump Protection Mode
for Positive Displacement Progressive Cavity Pumps
For progressive cavity pumps, the algorithm for basic pump protection is very
similar to other rotary positive displacement pumps including the requirement
for a
protective tune . However, the default setting for the tune ratio set point is
1.3 for this
type of pump; although the scope of the invention is intended to include
embodiments having a different default set consistent with that now known or
later
developed in the future. . For progressive cavity pumps it was found that
torque
ripple is not a reliable method for determining if a dry run condition exists.
It has
been found through testing that these types of pumps can have an unstable
torque
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signature. Therefore, a different approach was taken for enhanced pump
protection
for this type of pump. The algorithm for enhanced pump protection calculates a
corrected high and low power ratio and compares it to a high and low power
ratio set
point (HI P RATIO SP and LO P RATIO SP) to determine if a dry run condition
exists.
By way of example, the enhanced pump protection mode is based at least
partly on the following high/low power condition:
PACT2CORR/PACT1CORR) >= HI P RATIO SP
or
PACT2CORR/PACT1CORR) <= LO P RATIO SP.
If either high/low power condition is true, then ¨> a dry run fault is
declared for
the progressive cavity PD pump.
In contrast, if the high/low power condition is false, then ¨> the progressive
cavity PD pump has a normal condition.
The parameter PACT1CORR is a corrected power reading for specific gravity
and viscosity as shown by the equation below:
PACTCORR = PACT x (SGRTD/SGACT)/ (VISCACT/VISCRTD) ^0.275.
The exponent of 0.275 is a default value, although the scope of the invention
is
intended to include embodiments having a different exponent consistent with
that
now known or later developed in the future.
For constant temperature systems no corrections are required.
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By way of example, the value of PACT1CORR may be updated under the
following conditions: when +/- an rpm speed change occurs, during a pump start-
up
and after a 1 hr operating time elapses, although the scope of the invention
is
intended to include embodiments having a different +/- rpm speed change and/or
a
different operating time elapsing consistent with that now known or later
developed
in the future. The value of PACT1CORR may be the mode value, e.g., over a
predetermined sample period, e.g., a 20 sample period. The sample period will
depend on the monitor update rate.
The value of PACT2CORR may be continuously updated using the
aforementioned equation. The value of PACT2CORR may be the mode value, e.g.,
over a predetermined sample period, e.g., a 20 sample period.
The ratio of PACT2CORR/PACT1CORR may be continuously updated and
compared to the high power ratio set point HI P RATIO SP and the low power
ratio
set point LO P RATIO SP. The calculated value of the ratio
PACT2CORR/PACT1CORR may be based on the mode value, e.g., over a
predetermined sample period, e.g., a 20 sample period.
The default set point for the high power ratio set point HI P RATIO SP may
be, e.g. about 1.2, although the scope of the invention is intended to include
embodiments having a different default setting consistent with that now known
or
later developed in the future
The default set point for the low power ratio set point LO P RATIO SP may be,
e.g. about 0.80, although the scope of the invention is intended to include
embodiments having a different default setting consistent with that now known
or
later developed in the future.
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Consistent with that set forth above, the above algorithms for the basic pump
protection mode may always be active, but the enhanced pump protection mode is
only active when the tune ratio is less than or equal then the tune ratio set
point.
B. Implementation for Twin Screw Pumps
This invention consists of three modules of twin screw positive displacement
pump protection: a startup module, basic pump protection module and enhanced
pump protection module.
Start-up Module
The intent of the startup module is to detect an inadvertently closed suction
valve during startup which can occur due to operator error. The start-up
module is
an optional module which offers protection as a one-time check during initial
start-up.
During start-up, once the speed set point has been reached an initial torque
reading is taken, e.g., 12 sec after a timer begins. A torque reading may be
taken,
e.g., every 30 sec thereafter, until the timer expires. Each subsequent torque
reading may be compared to the initial torque value. If the current torque
value/initial
torque value <= 0.97, then a Dry Run Fault may be issued. During the Start-up
Module check, no changes should to be made to the system (e.g. speed changes,
valve changes) until the timer expires. In constant temperature applications,
e.g.,
such as unloading applications, a drop in torque would likely indicate a
temperature
increase in the suction line if the suction valve is closed prior to startup.
As
temperature increases specific gravity is reduced and the torque requirement
is
lowered. If a Dry Run Fault is detected, then the drive faults immediately and
does
not wait for the timer to expire. If the drive is shutdown before the timing
cycle is
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completed, then the Start-up Module will reset itself, if active. In contrast,
if the
current torque value/initial torque value > 0.97, then the condition is normal
and the
start-up module is disabled. The default timer value at rated speed is, e. gõ
2
minutes. The timer duration may be extended for speeds less than rated. It is
not
the intent of the present invention to limit either the timer value or the
current torque
value/initial torque value ratio to any specific value. The 2 minute timer
value at
rated speed is settable by the user via parameter. Additionally, the default
torque
ratio value of 0.97 is also settable via parameter. At minimum speed, the
torque
ratio value of 0.97 may be reduced slightly to 0.975 to avoid prolonged timer
values.
During the Start-up Module check, a user message may appear on the HMI "Start-
up
Check". Once the timer expires and the condition is normal, a user message may
appear on the HMI "Start-up Check Success" and the start-up module is
disabled. If
a speed change is detected >= 2% during the "Startup Check", then the Start-up
algorithm is aborted and a message "Startup Protection Aborted" is displayed.
The
Twin Screw Pump then resumes normal operation.
Protective Tune Module
The protective tune may be implemented similar to that set forth above. For
example, the process for activating pump protection is to first do a
protective tune
which samples speed, torque or power data, e.g., at five speeds, while
operating at
rated conditions. The protection functionality must be disabled during this
process.
If the pump is operating on a system with multiple system curves, then the
protection
tune should be performed with the pump operating on the system curve having
least
resistance. For the pump and system shown in Figure 1, the Protection Tune
would
be performed while operating on system curve labeled A. This is necessary to
avoid
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nuisance dry run faults when transitioning between higher to lower
differential
pressures.
Consistent with that set forth above, once the protection tune is completed
the
pump protection functionality can be enabled.
As shown in Figure 2 the power curve for PD pumps varies directly with the
change in speed (provided there is adequate suction pressure) for a given
differential
pressure. For centrifugal pumps, power varies as the cube of the speed change
(Figure 3). Centrifugal pumps may operate at closed valve condition for short
periods. However, noted that it is typically not acceptable for positive
displacement
pumps to operate against a closed valve. Pressure will continue to build until
pump
damage occurs or the pump housing and/or piping ruptures.
The control logic for basic and enhanced pump protection utilizes the direct
feedback of power, torque and speed to calculate an actual corrected tune
ratio
consisting of the actual corrected torque divided by the tuned corrected
torque at a
specific operating speed. The torque measurements may be continuously
compensated for specific gravity and viscosity changes in systems where
process
temperature is not constant. The corrected actual tune ratio may then be
compared
to a tune ratio set point in a decision tree algorithm. If the calculated tune
ratio is
greater than the tune ratio set point, then the basic pump protection becomes
active;
otherwise enhanced pump protection becomes active.
Once the protection tune is completed the pump protection functionality can
be enabled.
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Once pump protection has been enabled the decision tree algorithm for basic
pump protection is as follows:
Basic Pump Protection Module
Pump Running
If true
Pump at Constant +/- Speed: If False ¨> T
If true
Tact Corr/Ttune Corr <= Tune Ratio SP
If False, then go to basic pump protection.
If true, then go to enhanced pump protection.
The tune ratio at current operating speed may be determined or calculated by
the following set of equations:
Tact Corr/Ttune Corr
TACTCORR = TACT x (SGRTD/SGACT)/(VISCACT/VISCRTD) ^0.275
TTUNECORR = TMEAS x (SGRTD/SGACT)/(VISCACT/VISCRTD) ^0.275,
where:
TACT = actual torque at current speed,
TMEAS = measured or interpolated tuned value torque at current speed,
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SGRTD ¨ rated specific gravity,
SGACT = actual specific gravity,
VISCRTD = rated viscosity, and
VISCACT = actual viscosity.
The exponent of 0.275 is a default value, although the scope of the invention
is
intended to include embodiments having a different exponent consistent with
that
now known or later developed in the future.
By way of example, for twin screw pumps the Tune Ratio Set point may have
a default setting of 1.3. However, it is not the intent of this invention to
limit the value
of the Tune Ratio Set Point to a specific value. The Tune Ratio Set Point
value may
be changeable by the user, e.g., as a selected parameter.
Once pump protection is enabled basic pump protection is always active, and
the following relationship is evaluated at the current operating speed:
TACTCORR <= KDR x TTUNECORR,
where KDR is a dry run factor that may have a default setting of 0.95. It is
not the
intent of this invention to limit the value of KDR to a specific value. The
KDR value
may be changeable by the user, e.g., as a selected parameter.
If nuisance trips occur the KDR value can be adjusted, based upon the
following:
TACTCORR <= KDR x TTUNECORR
If False, then the condition is normal.
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If True, then a dry run fault is declared.
Enhanced Pump Protection Module
The enhanced pump protection module may be used if the following decision
tree argument is true:
Tact Corr/Ttune Corr <= Tune Ratio SP.
If true, then go to Enhanced Pump Protection.
Note in the above decision tree argument the Basic Pump Protection Module
is always active but the Enhanced Pump Protection Module is only active when
the
Tune Ratio is less than or equal to the Tune Ratio Set Point.
For twin screw pumps, the algorithm for Basic Pump Protection is similar to
other rotary positive displacement pumps.
However, it was found thru testing that torque signatures can differ greatly
between rotary PD pumps and twin screw pumps. For example, rotary PD pumps
can provide a robust torque ripple signature when operating under dry run
conditions
due to rotor to rotor rubbing contact (gear pumps) or rotor to stationary
housing
contact (progressive cavity pumps). In contrast, twin screw pumps can have a
torque
signature which provides little change between normal operating conditions and
a
distressed operating condition. Therefore, it was found that the algorithms
created
for rotary PD pumps like gear and progressive cavity pumps, e.g., as set forth
above,
cannot reliably detect dry run conditions for twin screw pumps.
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As shown in Figure 7, when simulating dry run conditions it was found that a
momentary peak labelled P exists directly after suction valve closure. This
peak P
was found to be consistent at all speeds and differential pressures tested.
This peak
P was found to be the only distinguishing characteristic between a dry run
torque
signature and a condition normal system change.
The algorithm for Enhanced Pump Protection determines or calculates a
corrected high and corrected low torque ratio and compares it to a high and
low
torque ratio set point to determine if a dry run condition exists. However,
prior to
evaluating these arguments, a check may be made to determine whether the
condition is due to a control valve being opened/closed or a dry run
condition. This
may be done by determining if the aforementioned peak exists by doing a sample
torque check as follows:
By way of example, 200 sample torque readings may be taken at a minimum
sample rate of 100 msec. An initial torque reading at constant speed (+/- 5
Rpm) is
compared to each successive torque reading as follows: (initial torque reading
-
torque reading "N") ...N+1, N+2, N+199. If <= 4 evaluations are negative,
then it
is assumed all readings are positive (decreasing) or equal to zero (constant);
this
can occur if the control valve is opening and the evaluation block as shown in
figure
8 stating Are Sample Readings Constant or Decreasing?" is true and the
Enhanced
Pump Protection Module is deactivated and the Basic Pump Protection Module
only
becomes active. Note if the control valve is closing then the Tune Ratio Set
Point
may be exceeded (under the Basic Pump Protection Module) and the Enhanced
Pump Protection Module is then deactivated. If > 4 evaluations are negative,
then
the evaluation block as shown in figure 8 stating "Are Sample Readings
Constant or
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Decreasing?" becomes false. A 2 minute timer may then be started, and the
arguments
TACT2CORR/TACT1CORR >= HIT RATIO SP
TACT2CORR/TACT1CORR <= LO T RATIO SP
are evaluated at a minimum rate, e.g., of every 100 msec, until the two minute
timer
expires. If the 2 minute timer expires with both arguments being false, the
condition
is normal and the algorithm resets itself. If one or both of the arguments are
true,
then a dry run fault is immediately issued. The default values of HI T Ratio
SP and
LO T Ratio SP are 1.10 and 0.9, respectively. It is not the intent of this
invention to
limit the HI or LO T Ratio Set Points or the timer setting to a specific value
consistent
with that now known or later developed in the future.
The HI or LO T Ratio setpoint value is changeable by the user via parameter.
Moreover, by way of example, the number evaluations used is four (4), although
the
scope of the invention is not intended to be limited to the number of
evaluations, and
embodiments are envisioned using a number of evaluations greater than 4, or
less
than 4, within the spirit of the present invention.
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Note in the above algorithms the Basic Pump Protection Module is always
active but the Enhanced Pump Protection Module is only active when the Tune
Ratio
(TACT CORR/T TUNE CORR) is equal to or less than the TUNE RATIO SP.
TACT2CORR/TACT1CORR) >= HI T RATIO SP
or
TACT200RR/TACT100RR) <= LOT RATIO SP,
where
TACT1CORR is a corrected torque reading for specific gravity and viscosity
as shown below, and
TACTCORR = TACT x (SGRTD/SGACT)/ (VISCACT/VISCRTD) ^0.275.
The exponent of 0.275 is a default value, although the scope of the invention
is
intended to include embodiments having a different exponent consistent with
that
now known or later developed in the future.
For constant temperature systems, no corrections is typically required.
The value of TACT1CORR may be updated under the following conditions:
a +/- 2 rpm speed change occurs, during pump start-up and after, e.g., 1 hr of
operating time elapses.
The calculated value of TACT2CORR/TACT1CORR may be based on the
mode value, e.g., over a 20 sample period.
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Figure 8 shows a flowchart generally indicated as 100 for implementing the
aforementioned pump protection mode, e.g., having steps a through n.
The Signal Processor 12
The signal processor 12 performs the basic signal processing functionality of
the apparatus for implementing the present invention. The signal processor 12
may
be a stand alone signal processing module, form part of a controller,
controller
module, etc., or form part of some other module of the apparatus 10. Many
different
types and kind of signal processors, controllers and controller modules for
controlling
pumps are known in the art, for example, including programmable logic
controllers
and variable frequency drives. By way of example, based on an understanding of
such known signal processing modules, controllers and control modules, a
person
skilled in the art would be able to configure the signal processor 12 to
perform the
functionality consistent with that described herein, including to receive the
signaling
containing information about power, torque, speed, viscosity and specific
gravity
related to the operation of a twin screw positive displacement pump; and to
determine whether to enter an enhanced pump protection mode for the twin screw
positive displacement pump based at least partly on a relationship between an
actual
corrected tune ratio and a tuned ratio set point (Tune Ratio SP) else remain
in the
basic protection mode. By way of further example, based on an understanding of
such known signal processing modules, controllers and control modules, a
person
skilled in the art would be able to configure the signal processor 14 to
perform
functionality consistent with that described herein, including to determine if
the actual
corrected tune ratio is less than or equal to the actual corrected tune ratio
set point
(Tune Ratio SP), and if so, then to enter the enhanced pump protection mode,
else
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to continue to use a basic pump protection mode, as well as to determine the
actual
corrected tune ratio based at least partly on a ratio of an actual corrected
torque
(TAcorr) divided by a tuned corrected torque (TTcorr) at a specific operating
speed.
By way of still further example, the functionality of the signal processor may
be implemented using hardware, software, firmware, or a combination thereof,
although the scope of the invention is not intended to be limited to any
particular
embodiment thereof. In a typical software implementation, such a module would
be
one or more microprocessor-based architectures having a microprocessor, a
random
access memory (RAM), a read only memory (ROM), input/output devices and
control, data and address buses connecting the same. A person skilled in the
art
would be able to program such a microprocessor-based implementation to perform
the functionality described herein without undue experimentation. The scope of
the
invention is not intended to be limited to any particular implementation using
technology known or later developed in the future.
The signal processor, controller or controller module may include other
modules to perform other functionality that is known in the art, that does not
form
part of the underlying invention, and that is not described in detail herein.
The Rotary Positive Displacement Pump 14
The rotary positive displacement pump like element 14, and rotary positive
displacement pumps in general, are known in the art, e.g., which may include a
twin
screw pump, an internal or external gear pump, a lobe pump, a vane pump or a
progressive cavity pump, and not described in detail herein. Moreover, the
scope of
the invention is not intended to be limited to any particular type or kind
thereof that is
either now known or later developed in the future. By way of example, such
rotary
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positive displacement pumps are understood to include a motor or motor portion
for
driving a pump or pump portion, as well as some module like element 16 for
example
a programmable logic controller (PLC) or variable frequency drive (VFD) for
implementing some functionality related to controlling the basic operation of
the
motor for driving the pump 14. By way of example, and consistent with that set
forth
herein, the motor is understood to receive control signals from the signal
processor
in order to drive and control the rotary positive displacement pump to pump
fluid.
The motor is also understood to provide the signaling containing information
about
power, torque and speed related to the operation of the pump.
Other Possible Applications
Other possible applications include at least the following:
Pump Protection Algorithms ¨ sensorless dry run protection can provide a
reliable method for positive displacement pump fault tolerance during system
upset
conditions or operator error. In constant temperature systems this can be
achieved
without the added cost and complexity of external sensors. By way of example,
such
possible applications are envisioned for positive displacement pumps, such as
a twin
screw pump, an internal or external gear pump, a lobe pump, a vane pump or a
progressive cavity pump, consistent with that set forth herein.
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The Scope of the Invention
It should be understood that, unless stated otherwise herein, any of the
features, characteristics, alternatives or modifications described regarding a
particular embodiment herein may also be applied, used, or incorporated with
any
other embodiment described herein. Also, the drawings herein are not drawn to
scale.
Although the invention has been described and illustrated with respect to
exemplary embodiments thereof, the foregoing and various other additions and
omissions may be made therein and thereto without departing from the spirit
and
scope of the present invention.
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