Note: Descriptions are shown in the official language in which they were submitted.
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APPARATUS FOR AND METHOD OF DETERMINING PUMP
FLOW IN TWIN SCREW POSITIVE DISPLACEMENT PUMPS
CROSS REFERENCE TO RELATED APPLICATIONS
This application claims benefit to patent application serial no. 14/826,616,
filed
14 August 2015 (Atty dckt no. 911-002.074/F-GI-1504U5), which is hereby
incorporated by reference in its entirety.
This application is related to patent application serial no. 13/859,936, filed
10
April 2013, entitled "Method for determining pump flow for rotary positive
displacement pumps," which itself claims benefit to provisional patent
application
serial no. 61/623,155, filed 12 April 2012, which is hereby incorporated by
reference
in its entirety. Application serial no. 13/859,936, is directed towards
determining
pump flow for rotary positive displacement pumps, e.g., for gear and
progressive
cavity pumps; while the present application is directed towards determining
pump
flow for rotary positive displacement pumps, e.g., for twin screw pumps.
BACKGROUND OF THE INVENTION
1. Field of the Invention
This application relates to a rotary positive displacement (PD) 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
tuning
such a rotary PD pump in order to determine pump flow, including a twin screw
PD
pump.
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2. Brief Description of Related Art
Many different type or kinds of pumps, including rotary positive displacement
pumps, are known in the art. By way of example, some known pumps and
shortcomings associated with the same are set forth below:
U.S. Patent no. 6,591,697, entitled "Method for Determining Pump Flow Rates
Using Motor Torque Measurements," which is hereby incorporated by reference in
its
entirety, discloses a methodology that is based on the relationship of torque
and
speed versus pump flow rate and the ability to regulate pump flow using a
Variable
Frequency Drive (VFD) to adjust centrifugal pump speed. The technique used by
Mr. Henyan in the '697 patent relies on calibrating pump flow at several
speeds and
determining a flow value based on calibrated torque vs flow curves at several
speeds. An interpolation method is used to determine flow between calibrated
curves. Data for the calibrated flow curves are taken at zero flow (closed
valve
condition) at several speeds. A positive displacement pump cannot be operated
at
closed valve condition without pressure relief valves or bypass piping as the
pump
will continue to increase pressure and power until either a shaft or gear
breaks or
rupture occurs either in the system piping or pump casing. Another shortcoming
is
that the '697 patent relies on taking calibrated data at the factory which
makes the
variable frequency drive specific to the pump tested. Also, the invention has
no
provision for adjusting flow accuracy as pump wear occurs. Mr. Henyan's
invention
relates to centrifugal pumps where torque is proportional to the square of the
speed
change. In a rotary positive displacement pump torque is constant regardless
of
speed. Therefore mathematical relationships and the equations governing flow
between centrifugal and rotary positive displacement pumps are completely
different.
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Therefore, Mr. Henyan's technique is applicable to centrifugal pumps only and
cannot be applied to positive displacement pumps.
U.S. Patent No. 7,945,411 B2, entitled "Method for Determining Pump Flow
Without the Use of Traditional Sensors" which is hereby incorporated by
reference in
its entirety, discloses a technique that samples speed and power data at
closed
valve condition to correct the published pump curve for actual performance.
Normalized power curves along with speed and power data taken from a Variable
Frequency Drive (VFD) are used to calculate flow. The technique used in the
'411
patent by Mr. Kernan utilizes speed and power data at closed valve condition
to
adjust published pump performance for actual performance. A positive
displacement
pump cannot be operated at closed valve condition without a pressure relief
valve or
bypass piping as it will continue to increase pressure and power until either
a shaft or
gear breaks or rupture occurs either in the system piping or pump casing. Pump
flow is calculated by a polynomial power equation and normalized power curves.
In
a centrifugal pump pressure varies as the square of the speed change and power
varies as the cube of the speed change. A centrifugal pump is not a positive
displacement machine and as such the capacity output will vary based on the
resistance at the pump outlet. Less resistance will give more flow; more
resistance
less flow. A rotary positive displacement pump is a positive displacement
machine
where a defined volume of flow is positively displaced for each revolution of
the
pump shaft regardless of pressure at the outlet (unless blocked). For a rotary
positive displacement pump flow is proportional to a speed change regardless
of
outlet pressure. There is slip which occurs which reduces the theoretical
displacement due to clearances, pressure, viscosity and speed. Power typically
is
proportional to a speed change in rotary positive displacement pumps at
constant
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pressure; in a centrifugal pump power varies as the cube of the speed change.
Mathematical relationships and the equations governing flow between
centrifugal
and rotary positive displacement pumps are completely different. Therefore,
Mr.
Kernan's technique is applicable to centrifugal pumps only and cannot be
applied to
positive displacement pumps.
In the prior art, it is known to use calculations from resource material,
e.g., a
pump handbook for a positive displacement pump. However, one disadvantage with
this approach is that calculation techniques such as those presented in the
pump
handbook require knowledge of difficult to determine variables such as pump
geometry factor and slip coefficient. These calculation techniques cannot
compensate for pump performance which deviates from published performance
calculations.
In the prior art, it is known to use external flow meters; however, external
flow
meters can add cost and complexity to the overall drive system.
None of the aforementioned techniques described herein may be used for
determining pump flow in rotary positive displacement pumps, as set forth
below and
herein.
SUMMARY OF THE INVENTION
While the calculation techniques covering rotary gear and progressive cavity
pumps are similar to twin screw pumps, the calculation procedure has been
found to
be less accurate when used with twin screw pumps. The main difference is that
no
correction or compensation is required for published rated flow based on
corrected
rated power. In addition, rated slip factor is compensated based on corrected
rated
power only; no flow compensation is required. In view of this, the present
invention
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provides new and unique techniques for tuning twin screw positive displacement
pumps.
This invention overcomes the aforementioned shortcomings by introducing a
tune function which corrects the calculated flow value based on published
performance data to reflect actual pump performance. Slip coefficients may be
automatically adjusted based on slip rules which compensate changes in known
variables such as torque, speed and viscosity.
Published performance for certain types of rotary positive displacement
pumps have been found to differ from actual performance. Figures 1 and 2 show
a
comparison for capacity and power between published performance vs. test data
for
a typical progressive cavity pump.
In summary, the technique according to the present invention requires an
input of pump parameters which are readily accessible to pump users such as
pump
type, rated flow, rated speed, rated power, rated viscosity and no slip flow.
Speed
and power data received in signaling taken from a Variable Frequency Drive
(VFD)
along with specific gravity and viscosity data may be used to calculate and
compensate slip flow at varying conditions. The slip flow can then be deducted
from
the theoretical displacement flow to determine an actual flow value. In
constant
temperature applications where specific gravity and viscosity are constant,
the
method of flow calculation becomes sensorless. For applications with varying
temperature, a simple temperature measurement device is required to compensate
changing conditions.
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The Apparatus
By way of example, and 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 actual pump
performance data for actual rated conditions captured during a tuning function
related to the operation of a twin screw positive displacement pump; and
determine corrected pump performance data to operate the twin screw
positive displacement pump by compensating pump performance data being
used for operating the twin screw positive displacement pump based at least
partly on the actual pump performance data for actual rated conditions
captured during a tuning function.
According to some embodiments of the present invention, the corrected pump
performance data may include corrected published pump performance data having
a
corrected published rated power and a rated slip factor which is compensated
for
actual rated conditions.
According to some embodiments of the present invention, the actual pump
performance data may include information about actual power, actual specific
gravity
and actual viscosity related to the operation of the twin screw positive
displacement
pump, e.g., in the signaling received from a pump controller or controlling
device,
such as a variable frequency drive (VFD) or programmable logic controller
(PLC).
According to some embodiments of the present invention, the signal
processor may be configured to use a value for a rated value for the twin
screw
positive displacement pump based upon published pump performance data.
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According to some embodiments of the present invention, the signal
processor may be configured to provide a control signal containing information
about
the actual flow value to control the operation of the twin screw positive
displacement
pump.
According to some embodiments of the present invention, the signal
processor may be configured to determine the corrected published rated power,
e.g.,
by compensating a published rated power based at least partly on actual power,
actual specific gravity and actual viscosity at the actual rated conditions.
According to some embodiments of the present invention, the signal
processor may be configured to determine a rated slip factor, e.g., by
compensating
for the actual rated conditions based at least partly on the rated flow and a
corrected
rated torque.
According to some embodiments of the present invention, the signal
processor may be configured to determine an actual flow value based at least
partly
on deducting the corrected rated slip flow based on slip rules for the
operating
condition as shown in Table IB for the twin screw PD pump type from a
theoretical
displacement flow.
According to some embodiments of the present invention, the signal
processor may be configured to activate the tune function and replace the
published
values for rated power and the rated slip factor (calculated from published
data)
being used for operating the twin screw positive displacement pump with the
corrected rated power and rated slip factor compensated for the actual rated
conditions, and to use the corrected rated power and the rated slip factor
compensated for the actual rated conditions until another tune function is
initiated.
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According to some embodiments of the present invention, the signaling may
contain information about pump parameters, including some combination of a
pump
type, a rated flow, a published rated speed, a published rated power, a
published
rated viscosity, a published rated specific gravity and no slip flow, and
information
about actual speed and power from a variable frequency drive (VFD) along with
actual specific gravity and viscosity data; and the signal processor is
configured to
determine a corrected slip flow (determined from slip rules) or factor based
at least
partly on the signaling. The signal processor may also be configured to
determine
an actual flow value based at least partly on deducting the corrected rated
slip flow
or factor from a theoretical displacement flow or factor.
According to some embodiments of the present invention, the signal
processor may be configured to determine the corrected rated slip flow based
on slip
rules for the operating condition as shown in Table IB below for the twin
screw PD
pump type or factor without using sensors based at least partly on a constant
temperature application where specific gravity and viscosity are substantially
constant. Alternatively, in applications with varying temperature, the signal
processor may be configured to receive a temperature measurement and determine
the corrected rated slip flow (determined from slip rules) or factor by
compensating
changing conditions, including specific gravity and viscosity, based at least
partly on
the same.
According to some embodiments of the present invention, the signal
processor may be configured to determine the corrected published rated power
based at least partly on the following equation:
RTD HPCORR = HPACT X (SGRTD / SGACT) / (VISCACT / VISCRTD)AN,
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where:
RTD HPcoRR is the corrected published rated power in the form of rated hp
corrected for specific gravity and viscosity,
HPAc-r is the actual power at rated conditions, e.g., captured during the tune
process,
SGRTD is the rated specific gravity of the pumped liquid,
SGAcT is the actual specific gravity of the pumped liquid, e.g., captured
during
the tune process,
VISCRTD is the rated viscosity of the pumped liquid,
VISCAcT is the actual viscosity of the pumped liquid, e.g., captured during
the
tune process, and
N is an exponent which varies by the type of pump.
For twin screw PD pumps, the exponent N may equal about 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.
According to some embodiments of the present invention, the signal
processor may be configured to determine the rated slip factor compensated for
the
actual rated conditions based at least partly on the following equation:
KS=
(VISCRTD X QN0 SLIP X (QN0 SLIP - QRATED)/(75.415 X KG X (N RATED) X TRTD
CORR),
where:
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KS is the rated slip factor compensated for the actual rated conditions,
VISCRTD is the published pump rated viscosity,
QNO SLIP is the flow in gpm at rated speed and rated viscosity at 0 psid
differential pressure,
QRTD is the rated flow for the application from the rating curve. No
correction
or compensation is used.
KG = 0.004329, a design constant,
NRATED = Rated Pump Speed for the application, and
TRTD CORR = Corrected Rated Torque in Ft-Lbs (US), which is determined as
follows: TRTD CORR = (5252 X RTD H PcoRR)/N RTD.
According to some embodiments of the present invention, the signal
processor may be configured to determine an actual flow value for the rotary
positive
displacement pump based at least partly on the following equation:
QACT CORR =
(QN0 SLIP X ((NMOTOR/RATIO)/NRATED)) (((75.415 X KG X KScoRR) X
(NmOTOR/RATIO) X TACTCORR)/ (VISCOSITYAcT X QNO SLIP),
where:
QNO SLIP is the flow in gpm at rated speed and rated viscosity at 0 psid
differential pressure,
NmOTOR = the current motor speed,
RATIO = the ratio of speed reduction if a gear reducer is used,. If no gear
reducer is used than the value of the RATIO = 1Ø
NRATED = the Rated Pump Speed for the application,
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KG = 0.004329, a design constant,
KScoRR is the corrected rated slip factor based on the slip rules shown in
Table IB (See Section B below for twin screw pumps),
TACT CORR = Torque in Ft-Lbs (US), where TACT CORR = (5252 x HPAcT
coRR)/NAcT, and
H PACT CORR = H PACT X (SGRTD/SGACT),
H PACT = Actual motor power,
NAcT = Actual pump speed, and
VISCAcT is the actual viscosity of the pumped liquid.
According to some embodiments of the present invention, the signal
processor may also be configured as, or take the form of, a controller or
control
module that controls the operation of the rotary 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, including where the twin screw positive displacement pump
takes
the form of twin screw positive displacement pumps either now known or later
developed in the future.
The Method
According to some embodiments, the present invention may take the form of
a method comprising steps for receiving with a signal processor signaling
containing
information about actual pump performance data for actual rated conditions
captured
during a tuning function related to the operation of a twin screw positive
displacement pump; and determining with the signal processor corrected
published
pump performance data to operate the twin screw positive displacement pump by
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compensating published pump performance data being used for operating the twin
screw positive displacement pump based at least partly on the actual pump
performance data for the actual rated conditions captured during the tuning
function.
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 (HP) versus discharge pressure (PSIG) related to
a power comparison of published versus test data at 200 Cp, 1750 Rpm for a
rotary
positive displacement pump.
Figure 2 is a graph of capacity (GPM) versus discharge pressure (PSIG) for a
capacity comparison of published versus test data at 200 Cp, 1750 Rpm for a
rotary
positive displacement pump.
Figure 3 is a block diagram of apparatus according to some embodiments of
the present invention.
DETAILED DESCRIPTION OF THE INVENTION
By way of example, as shown in Figure 3, according to some embodiments,
the present invention may take the form of apparatus 10 that includes a signal
processor 12 that may be configured to control and protect the operation of 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.
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By way of example, the signal processor 12 may be configured to receive
signaling containing information about actual pump performance data for actual
rated
conditions captured during a tuning function related to the operation of the
rotary
positive displacement pump 14 and determine corrected published pump
performance data to operate the rotary positive displacement pump by
compensating
published pump performance data being used for operating the rotary positive
displacement pump based at least partly on the actual pump performance data
for
the actual rated conditions captured during the tuning function.
For rotary PD, magnetic drive, progressive cavity and magnetic drive
progressive cavity pumps disclosed below in relation to Section A, the signal
processor 12 may also be configured to determine an actual flow value, e. g.,
based
at least partly on the corrected published pump performance data. In contrast,
for
twin screw positive displacement pumps disclosed below in Section B, the
signal
processor 12 may be configured to use a value for a rated flow, e.g., based
upon
published pump performance data. In either case, the signal processor 12 may
be
configured to provide a control signal containing information about a
respective flow
control value to control the operation of these types of rotary positive
displacement
pumps.
The rotary positive displacement pump 14 may include a module 16
configured to provide the signaling containing information about actual pump
performance data for actual rated conditions captured during a tuning function
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 these types of rotary positive displacement pump 14.
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By way of example, for rotary PD, magnetic drive, progressive cavity and
magnetic drive progressive cavity pumps, the signal processor 12 may be
implemented consistent with that set forth below in Section A, and for twin
screw
pumps, the signal processor 12 may be implemented consistent with that set
forth
below in Section B, as follows:
A. Implementation for Rotary PD, Magnetic Drive, Progressive Cavity
and
Magnetic Drive Progressive Cavity Pumps
For gear and progressive cavity pumps, the control logic according to the
present invention works by compensating published values of rated power, rated
flow and rated slip factor for actual rated conditions, as follows:
The published rated power may be compensated for actual power,
actual specific gravity and actual viscosity at rated conditions. This becomes
the corrected rated power.
The published rated flow may be compensated for actual rated
conditions based on the corrected rated power. This becomes the corrected
rated flow.
The rated slip factor, calculated from published data, may be
compensated for actual rated conditions based on the corrected rated power
and corrected rated flow.
Once these values are calculated and a tune function is activated the
published values for Rated HP, Rated Flow and Rated Slip Factor are replaced
by
the corresponding compensated or corrected values. These compensated or
corrected values are saved and do not change unless another tune function is
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initiated. Note the tune function is typically activated while the pump is
operating at
rated speed and rated conditions.
By way of example, the technique of compensation and flow calculation may
consist of the following steps:
a) Rated HP Compensation (RTD HPcoRR)
For instance, the signal processor 12 may be configured to determine the
corrected published rated power based at least partly on the following
equation:
RTD HI3cORR = HPACT X (SGRTD / SGAcT) / (VISCAcT / VISCRTD)AN,
where:
RTD HI3cORR is the rated hp corrected for specific gravity and viscosity,
HPACT is the actual power at rated conditions,
SGRTD is the rated specific gravity of the pumped liquid,
SGAc-r is the actual specific gravity of the pumped liquid,
VISCRTD is the rated viscosity of the pumped liquid,
VISCAcT is the actual viscosity of the pumped liquid, and
N is an exponent which varies by the type of pump.
By way of example, for rotary PD pumps such as gear, vane and lobe, the
exponent N equals about 0.10, and for progressive cavity pumps, the exponent N
equals about 0.275.
The scope of the invention is not intended to be limited to the specific
aforementioned equation and parameters set forth above to determine the
corrected
published rated power. For example, embodiments are envisioned in which
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variations of the aforementioned equation and/or parameters may be used to
determine the corrected published rated power consistent with that now known
or
later developed in the future.
b) Rated Flow Compensation (QRATEDCORR)
For instance, the signal processor 12 may be configured to determine the
corrected published rated flow based at least partly on the following
equation:
QRATEDCORR = (RTD HPcoRR / RTD HP) X Q RTD,
Note that QRATEDCORR is calculated at rated speed.
Where:
QRATEDCORR is the corrected rated flow
RTD HI3cORR is the rated hp corrected for specific gravity and viscosity,
RTD HP is the rated hp for the application
Q RTD is the rated flow for the application
The scope of the invention is not intended to be limited to the specific
aforementioned equation and parameters set forth above to determine the
corrected
published rated flow. For example, embodiments are envisioned in which
variations
of the aforementioned equation and/or parameters may be used to determine the
corrected published rated flow consistent with that now known or later
developed in
the future.
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C) Rated Slip Factor Compensation (KS):
For instance, the signal processor 12 may be configured to determine the
rated slip factor KS compensated for the actual rated conditions based at
least partly
on the following equation:
KS=
(VISCRTD X ONO SLIP X (ONO SLIP - ()RATED CORR )/ (75.415 X KG X (NRTD) X TRTD
CORR),
where:
KS is the rated slip factor compensated for the actual rated conditions,
ONO SLIP is the flow in Gpm at rated speed and rated viscosity at 0 psid
differential pressure,
ORATEDCORR is the corrected rated flow,
KG = 0.004329, a design constant,
NRTD = Rated Pump Speed for the application, and
TRTD CORR = Corrected Rated Torque in Ft-Lbs (US), which is calculated as
follows: TRTD CORR = (5252 X RTD HI3coRF)/NRTD, where
RTD HI3cORR is the rated hp corrected for specific gravity and viscosity.
The scope of the invention is not intended to be limited to the specific
aforementioned equation and parameters set forth above to determine the rated
slip
factor. For example, embodiments are envisioned in which variations of the
aforementioned equation and/or parameters may be used to determine the rated
slip factor consistent with that now known or later developed in the future.
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d) Tune Function Activation
To calculate and save values calculated in steps a ¨ c, the signal processor
12 is configured to activate the tune function process while the pump is
stable and
operating at rated conditions. The tune function process is seamless to the
user.
Tuning samples the actual rated conditions without changing operating
conditions or
pump speed. Once tuning is completed, the values for RTD H PcORR, ()RATED CORR
and
KS are saved. These values do not change unless another tune function process
is
re-initiated.
Periodically as wear occurs, pump flow accuracy can be restored by re-
activating this parameter when operating at rated conditions.
e) The Actual Flow Calculation
For instance, the rated slip factor, KS, may be corrected for changing
variables due to operating conditions by the slip rules for rotary and
progressive
cavity pumps as shown in Tables IA and IIA. The corrected slip factor becomes
KSooRR.
The signal processor 12 may be configured to determine an actual flow value
for the rotary positive displacement pump based at least partly on the
following
equation:
OACT CORR =
(ONO SLIP X ((NMOTOR/RATIO)/NRTO) ¨ (((75.415 X KG X KSCORR) X (NmoToR/RATIO)
X
TACT CORR)/ (VISCACT X ONO SLIP),
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where:
QN0 SLIP is the flow in Gpm at rated speed and rated viscosity at 0 psid
differential pressure,
NmoT0R = current motor speed,
RATIO = the ratio of speed reduction if a gear reducer is used (If no gear
reducer is used than the value of the RATIO = 1Ø),
NRTD = Rated Pump Speed for the application,
KG = 0.004329, a design constant,
VISCAcT is the actual viscosity of the pumped liquid,
TACT CORR = Torque in Ft-Lbs (US), which is calculated as follows: TACT CORR =
(5252 X H PACT CORR)/NACT, where H PACT CORR = HPAcT X (SGRTD/SGACT), and
KScoRR = corrected slip factor based on slip rules for the operating condition
as shown in Tables IA and IIA.
For constant temperature applications, a specific gravity value is not
required
and H PACT CORR = H PACT, where
H PACT = Actual motor power, and
NAcT = Actual pump speed.
Adjustments to the rated slip factor, KS, may be made by the slip rules shown
below, which results in a corrected slip factor, "KScoRR".
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Table IA: Slip Rules for Rotary PD Pumps and Magnetic Drive Rotary PD
Pumps:
Changing Corrected Slip Factor "KScoRR" used in step "e"
Variable
Torque SLIP FACTOR is Constant; KSooRR = KS
Speed KScoRR = KS * (= NRATED/ NACT )
Speed and SLIP FACTOR is Constant; KScoRR = KS
Torque
Viscosity KScoRR = KS * (ViscAcTNi5cRATED)^0.5
Speed and KScoRR = KS * (= NRATED/ NAcT)*
Viscosity (ViscAcTNi5cRATED)^0.5
Torque and KScoRR = KS * (ViscAcTNi5cRATED)^0.5
Viscosity
Speed, Torque KScoRR = KS* (= NRATED/ NACT ) *
and Viscosity (ViscAcTNi5cRATED)^0.5
Table IIA: Slip Rules for Progressive Cavity Pumps and Magnetic Drive
Progressive Cavity Pumps:
Changing Corrected Slip Factor "KScoRR" used in step "e"
Variable
Torque SLIP FACTOR is Constant; KSooRR = KS
Speed KScoRR = KS * (= NRATED/ NACT )
Speed and SLIP FACTOR is Constant; KSooRR = KS
Torque
Viscosity KScoRR = KS * (ViscAcTNi5cRATED)^0.5
Speed and KScoRR = KS * (= NRATED/ NAcT)*
Viscosity (ViscAcTNi5cRATED)^0.5
Torque and KScoRR = KS * (ViscAcTNi5cRATED)^0.5
Viscosity
Speed, Torque KScoRR = KS* (ViscAcTNi5cRATED)^0.5
and Viscosity
The result of the flow calculation in step (e) is as follows:
QACT CORR = QACT, and
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the QACT CORR is displayed as the actual flow (QAcT) in Gpm.
In the tables, the parameters for actual and rated pump speed are as follows:
NADT = Actual pump speed, and
NRTD = Rated pump speed of the application.
For magnetic drive rotary and magnetic drive progressive cavity PD pumps:
TACT CORR = Torque in Ft-Lbs (US), which is calculated as follows: TACT CORR =
(5252 x
H PACT CORR)/NACT, where:
H PACT CORR = (H PACT ¨ (PMAG CORR X (NADT/NRTD)2) X SGRTD/SGACT),
H PACT = Actual motor power, and
PMAG CORR is the eddy current loss in Hp at rated speed for the containment
shell material used. Note for non-metallic containment shells the eddy current
loss is
0 hp.
The scope of the invention is not intended to be limited to the specific
aforementioned equation and parameters set forth above to determine the actual
flow value. For example, embodiments are envisioned in which variations of the
aforementioned equation and/or parameters may be used to determine the actual
flow value consistent with that now known or later developed in the future.
In effect, this tuning methodology calculates flow for rotary positive
displacement pumps such as gear pumps and progressive cavity pumps by
introducing a tune function which corrects the calculated flow value based on
published performance data. Additionally, slip coefficients are automatically
adjusted
based on slip rules which compensate changes in known variables such as
torque,
speed and viscosity. The approach below takes a different approach for twin
screw
pumps.
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B. Implementation for Twin Screw Pumps
For twin screw pumps, the control logic according to the present invention
works by compensating published values of rated power and rated slip factor
for
actual rated conditions, as follows:
The rated power may be compensated for actual power, actual specific
gravity and actual viscosity at rated conditions. This becomes the corrected
rated power.
The rated slip factor, calculated from published data, may be
compensated for actual rated conditions based on the corrected rated power.
Once these values are calculated and a tune function is activated the
published values for Rated HP, and Rated Slip Factor are replaced by the
corresponding compensated or corrected values. These compensated or corrected
values are saved and do not change unless another tune function is initiated.
Note
the tune function is typically activated while the pump is operating at rated
speed and
rated conditions.
By way of example, the technique of compensation and flow calculation may
consist of the following steps:
a) Rated HP Compensation (RTD HPcoRR)
For instance, the signal processor 12 may be configured to determine the
corrected published rated power based at least partly on the following
equation:
RTD HI3cORR = HPACT X (SGRTD / SGADT) / (VISCADT / VISCRTD)AN,
where:
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RTD HI3cORR is the rated hp corrected for specific gravity and viscosity,
HPAcT is the actual power at rated conditions, e.g., captured during the tune
process,
SGRTD is the rated specific gravity of the pumped liquid,
SGAc-r is the actual specific gravity of the pumped liquid, e.g., captured
during
the tune process,
VISCRTD is the rated viscosity of the pumped liquid,
VISCADT is the actual viscosity of the pumped liquid, e.g. captured during the
tune process, and
N is an exponent which varies by the type of pump.
By way of example, for twin screw pumps, the exponent N equals about
0.275.
The scope of the invention is not intended to be limited to the specific
aforementioned equation and parameters set forth above to determine the
corrected
published rated power. For example, embodiments are envisioned in which
variations of the aforementioned equation and/or parameters may be used to
determine the corrected published rated power consistent with that now known
or
later developed in the future.
b) Rated Flow Compensation
It is note that Q RTD is the rated flow for the application from the rating
curve,
e.g., consistent with that set forth above on section B(b). For twin screw
pumps, no
correction or compensation is used.
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C) Rated Slip Factor Compensation (KS):
For instance, the signal processor 12 may be configured to determine the
rated slip factor KS compensated for the actual rated conditions based at
least partly
on the following equation:
KS=
(VISCRTD X ONO SLIP X (ONO SLIP - ()RATED)/ (75.415 X KG X (NRTD) X TRTD
CORR),
where:
KS is the rated slip factor compensated for the actual rated conditions,
ONO SLIP is the flow in Gpm at rated speed and rated viscosity at 0 psid
differential pressure,
As stated above, QRTD is the rated flow for the application from the rating
curve. No correction or compensation is used.
KG = 0.004329, a design constant,
NRTD = Rated Pump Speed for the application, and
TRTD CORR = Corrected Rated Torque in Ft-Lbs (US), which is calculated as
follows: TRTD CORR = (5252 x RTD HPcoRF)!NRTD, where
RTD HI3cORR is the rated hp corrected for specific gravity and viscosity.
The scope of the invention is not intended to be limited to the specific
aforementioned equation and parameters set forth above to determine the rated
slip
factor. For example, embodiments are envisioned in which variations of the
aforementioned equation and/or parameters may be used to determine the rated
slip
factor consistent with that now known or later developed in the future.
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d) Tune Function Activation
To calculate and save values calculated in steps a ¨ c, the signal processor
12 is configured to activate the tune function process while the pump is
stable and
operating at rated or near rated conditions. The tune function process is
seamless to
the user. Tuning samples the actual rated conditions without changing
operating
conditions or pump speed. Once tuning is completed, the values for RTD H
13cORR
and KS are saved. These values do not change unless another tune function
process is re-initiated.
Periodically as wear occurs, pump flow accuracy can be restored by re-
activating this parameter when operating at rated conditions.
e) The Actual Flow Calculation
For instance, the rated slip factor, KS, may be corrected for changing
variables due to operating conditions by the slip rules for twin screw pumps
as
shown in Table IB. The corrected slip factor becomes KScoRR.
The signal processor 12 may be configured to determine an actual flow value
for the rotary positive displacement pump based at least partly on the
following
equation:
QACT CORR =
(QNO SLIP X OMOTOR/RATIOYNIRTO) ¨ (((75.415 X KG X KSCORR) X (NMOTOR/RATIO) X
TACT CORR)/ (VISCACT X QNO SLIP),
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where:
QN0 SLIP is the flow in Gpm at rated speed and rated viscosity at 0 psid
differential pressure,
NmoT0R = current motor speed,
RATIO = the ratio of speed reduction if a gear reducer is used (If no gear
reducer is used than the value of the RATIO = 1Ø),
NRTD = Rated Pump Speed for the application,
KG = 0.004329, a design constant,
VISCAcT is the actual viscosity of the pumped liquid,
TACT CORR = Torque in Ft-Lbs (US), which is calculated as follows: TACT CORR =
(5252 X H PACT CORR)/NACT, where H PACT CORR = HPAcT X (SGRTD/SGACT), and
KScoRR = corrected slip factor based on slip rules for the operating condition
as shown in Table IB.
For constant temperature applications, a specific gravity value is not
required
and H PACT CORR = H PACT, where
H PACT = Actual motor power, and
NAcT = Actual pump speed.
Adjustments to the rated slip factor, KS, may be made by the slip rules shown
below, which results in a corrected slip factor, "KScoRR".
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Table IB: Slip Rules for Twin Screw PD Pumps:
Changing Corrected Slip Factor "KScoRR" used in step "e"
Variable
Torque SLIP FACTOR is Constant; KScoRR = KS
Speed KScoRR = KS * (= NRATED/ NACT )
Speed and KSCORR = KS * (NRATED/ NACT )
Torque
Viscosity KScoRR = KS * (ViscAcTNi5cRATED)^0.4
Speed and KScoRR = KS * (= NRATED/ NAcT)*
Viscosity (ViscAcTNi5cRATED)^0.4
Torque and KScoRR = KS * (ViscAcTNi5cRATED)^0.4
Viscosity
Speed, Torque KScoRR = KS* (= NRATED/ NACT ) *
and Viscosity (ViscAcTNiscRATED)^0.4
The result of the flow calculation in step (e) is as follows:
QACT CORR = QACT, and
the QACT CORR is displayed as the actual flow (QACT) in Gpm.
In the tables, the parameters for actual and rated pump speed are as follows:
NACT = Actual pump speed, and
NRTD = Rated pump speed of the application.
The scope of the invention is not intended to be limited to the specific
aforementioned equation and parameters set forth above to determine the actual
flow value. For example, embodiments are envisioned in which variations of the
aforementioned equation and/or parameters may be used to determine the actual
flow value consistent with that now known or later developed in the future.
In effect, the approach for the twin screw pump does not require flow
compensation (correction) in the calculation and the slip factor calculations
are
different, e.g., consistent with that set forth herein. These differences
improve the
accuracy of the underlying tuning function for twin screw pumps, especially
when
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compared to the tuning function used for the other rotary PD pumps disclosed
in
Section A.
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. Some examples are variable frequency drives and
programmable logic controllers. 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 actual pump performance data for actual rated
conditions captured during a tuning function related to the operation of the
twin
screw positive displacement pump 14; and to determine corrected pump
performance data to operate the twin screw positive displacement pump 14 by
compensating pump performance data being used for operating the twin screw
positive displacement pump based at least partly on the actual pump
performance
data for actual rated conditions captured during a tuning function.
By way of still further example, the functionality of the signal processor 12
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
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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 Periodic Tuning Functions
Moreover, as a person skilled in the art would appreciate based upon that
disclosed in the present invention the twin screw positive displacement pump
may be
initially operated using pump performance data that may include one or more
values
based upon published pump performance data. In operation, the signal processor
12 may receive the signaling containing information about the actual pump
performance data for the actual rated conditions, e.g., captured during a
first tuning
function, related to the operation of the twin screw positive displacement
pump, and
determine the corrected pump performance data to operate the twin screw
positive
displacement pump by compensating the pump performance data being used for
operating the twin screw positive displacement pump based at least partly on
the
actual pump performance data for the actual rated conditions captured during a
first
tuning function.
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The twin screw positive displacement pump will continue to operate using the
corrected pump performance data determined, which may include one or more
compensated values for the one or more values of the published pump
performance
data initially used.
Subsequent to the first tuning function, the signal processor 12 may receive
subsequent signaling containing information about subsequent actual pump
performance data for subsequent actual rated conditions, e.g., captured during
a
subsequent tuning function, related to the operation of the twin screw
positive
displacement pump, and determine subsequent corrected pump performance data to
operate the twin screw positive displacement pump by compensating the
corrected
pump performance data being used for operating the twin screw positive
displacement pump based at least partly on the subsequent actual pump
performance data for actual rated conditions captured during a subsequent
tuning
function. The twin screw positive displacement pump will continue to operate
using
the subsequent corrected pump performance data determined, which may include
one or more subsequently compensated values for the one or more values of the
published pump performance data initially used and/or subsequently
compensated.
At some point during the operation of the pump, it is understood that most, if
not all,
of the values of the published pump performance data initially used will be
replaced
by the subsequent corrected pump performance data, e.g., by compensating the
corrected pump performance data being used for operating the twin screw
positive
displacement pump based at least partly on the subsequent actual pump
performance data for actual rated conditions captured during periodic repeated
tuning functions.
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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 of
positive
displacement machine thereof that is either now known or later developed in
the
future. By way of example, such rotary positive displacement pumps are
understood
to include a motor or motor portion for driving a pump or pump portion, and
may
include a module like element 16 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:
Rotary positive displacement pump flow calculations - flow estimations for
rotary positive displacement pumps rely upon accurate power curves to estimate
pump flow. Published performance for certain types of positive displacement
pumps
such as progressive cavity pumps have been found to differ from actual
performance
based on anticipated wear in the stator liner. The tune function described
above will
correct the calculated flow value based on published performance to reflect
actual
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pump performance for rotary positive displacement pumps. By way of example,
for
gear and progressive cavity pumps, the tune function can also restore flow
accuracy
by compensating for pump wear.
In comparison, twin screw positive displacement pump flow calculations - flow
estimations for twin screw positive displacement pumps rely upon accurate
rating
curves to estimate pump flow. Published performance for certain types of
positive
displacement pumps have been found to differ from actual performance. The tune
function described above will correct the calculated flow value based on
published
performance to reflect actual pump performance for twin screw positive
displacement
pumps.
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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