U.S. patent application number 12/128265 was filed with the patent office on 2009-12-03 for enhanced turbocompressor startup.
Invention is credited to Wayne Jacobson, Jeff McWhirter, Saul Mirsky.
Application Number | 20090297333 12/128265 |
Document ID | / |
Family ID | 40996634 |
Filed Date | 2009-12-03 |
United States Patent
Application |
20090297333 |
Kind Code |
A1 |
Mirsky; Saul ; et
al. |
December 3, 2009 |
Enhanced Turbocompressor Startup
Abstract
A control method and apparatus for startup of turbocompressors
to avoid overpowering a driver of the turbocompressor. In a first
embodiment, the control system monitors input signals from
transmitters of various control inputs. When the input signals
exceed threshold values, the control system begins to close the
antisurge valve. In a second embodiment, the antisurge valve begins
to close after a predetermined time measured from the time startup
is initiated. In both embodiments, the antisurge valve continues to
ramp closed until the compressor has reached its operating zone, or
until the compressor's operating point reaches a surge control
line, at which point the antisurge valve is manipulated to keep the
compressor from surging.
Inventors: |
Mirsky; Saul; (West Des
Moines, IA) ; Jacobson; Wayne; (Des Moines, IA)
; McWhirter; Jeff; (Spring, TX) |
Correspondence
Address: |
STURM & FIX LLP
206 SIXTH AVENUE, SUITE 1213
DES MOINES
IA
50309-4076
US
|
Family ID: |
40996634 |
Appl. No.: |
12/128265 |
Filed: |
May 28, 2008 |
Current U.S.
Class: |
415/26 ;
415/36 |
Current CPC
Class: |
F04D 27/0292 20130101;
F04D 27/0261 20130101 |
Class at
Publication: |
415/26 ;
415/36 |
International
Class: |
F04D 27/02 20060101
F04D027/02 |
Claims
1. A method of minimizing an energy required to start a
turbocompressor having an antisurge valve and a control system, the
method comprising: (a) opening the antisurge valve fully; (b)
initiating a startup of the turbocompressor; (c) increasing a
rotational speed of the turbocompressor from a zero rotational
speed; and (d) actuating the antisurge valve toward its closed
position before said turbocompressor reaches a minimum operating
rotational speed.
2. The method of claim 1 wherein an initiation of a closing of the
antisurge valve comprises: (a) sensing at least one signal by the
control system; (b) comparing a magnitude of the at least one
signal with a minimum threshold value; and (c) initiating the
closing of the antisurge valve when the magnitude of the at least
one signal exceeds the minimum threshold value.
3. The method of claim 1 wherein an initiation of a closing of the
antisurge valve comprises: (a) initializing a timer to an initial
value at a time of initiating the startup of the turbocompressor;
(b) measuring time elapsed from the initial value; (c) comparing
the measured time with a threshold value; and (d) initiating the
closing of the antisurge valve when the elapsed time exceeds the
threshold value.
4. The method of claim 3 wherein the minimum operating rotational
speed comprises a constant operating rotational speed of the
turbocompressor, the method additionally comprising: (a) permitting
a driver of the turbocompressor to reach the constant operating
rotational speed; and (b) operating the turbocompressor at the
constant operating speed.
5. The method of claim 1 wherein increasing a rotational speed of
the turbocompressor from a zero rotational speed comprises: (a)
increasing a turbocompressor rotational speed set point within the
control system; and (b) controlling the turbocompressor rotational
speed based on said turbocompressor rotational speed set point.
6. The method of claim 3 wherein the threshold value is equal to
the initial value.
7. The method of claim 1 wherein an initiation of a closing of the
antisurge valve comprises: (a) sensing a plurality of signals input
to the control system; (b) comparing magnitudes of each of said
plurality of signals with a respective minimum threshold value; and
(c) initiating the closing of the antisurge valve when the
magnitudes of the plurality of signals exceed the respective
minimum threshold values.
8. The method of claim 1 wherein actuating the antisurge valve
towards its closed position before said turbocompressor reaches a
minimum operating rotational speed comprises: (a) detecting a
signal indicating a turbocompressor operating point; (b) comparing
said signal to a value representing a surge control line; and (c)
manipulating the antisurge valve to keep the turbocompressor
operating point from residing nearer a surge region than the surge
control line.
9. An apparatus for minimizing an energy required to start a
turbocompressor, the apparatus comprising: (a) an antisurge valve;
(b) a control system; (c) a first control system function to
generate a first signal to open the antisurge valve fully; (d) a
second control system function to initiate a startup of the
turbocompressor and increase a rotational speed set point of the
turbocompressor from a zero rotational speed set point; (e) a third
control system function to discern that conditions are appropriate
to initiate a closing of the antisurge valve before said
turbocompressor reaches a minimum operating rotational speed; and
(f) a fourth control system function to generate a second signal to
actuate the antisurge valve towards its closed position after the
third control system function has discerned conditions are
appropriate to initiate the closing of the antisurge valve.
10. The apparatus of claim 9 additionally comprising: (a) at least
one signal generated by a measurement system and sensed by the
control system; (b) a comparator in the control system to compare a
magnitude of the at least one signal with a minimum threshold
value; and (c) an antisurge actuation signal generating function
within the control system to initiate a closing of the antisurge
valve when the magnitude of the at least one signal exceeds the
minimum threshold value.
11. The apparatus of claim 9 additionally comprising: (a) a timer,
used to measure a time elapsed from an initiation of the startup of
the turbocompressor; (b) a comparator in the control system to
compare the elapsed time with a threshold value; and (c) an
antisurge actuation signal generating function within the control
system to initiate a closing of the antisurge valve when the
elapsed time exceeds the threshold value.
12. The apparatus of claim 9 wherein the second control system
function additionally comprises: (a) a rotational speed set point
generating function within the control system; and (b) a feedback
control system to control the turbocompressor rotational speed
based on said turbocompressor rotational speed set point.
13. The apparatus of claim 9 additionally comprising: (a) a
plurality of signals generated by a measurement system and sensed
by the control system; (b) a comparator in the control system to
compare a magnitude of each of the plurality of signals with a
respective minimum threshold value; and (c) an antisurge actuation
signal generating function within the control system to initiate a
closing of the antisurge valve when the magnitude of all the
plurality of signals exceed the respective minimum threshold
values.
14. The apparatus of claim 9 wherein the fourth control system
function additionally comprises: (a) a signal indicating a
turbocompressor operating point; (b) a comparator function to
compare said signal to a value representing a surge control line;
and (c) a closed-loop control system to manipulate the antisurge
valve to keep the turbocompressor operating point from residing
nearer a surge region than the surge control line.
15. An apparatus for minimizing an energy required to start a
turbocompressor, the apparatus comprising: (a) an antisurge valve;
(b) a constant speed turbocompressor driver having a constant
operating rotational speed; (c) a control system; (d) a first
control system function to generate a first signal to open the
antisurge valve fully; (e) a second control system function to
initiate a startup of the turbocompressor to increase a rotational
speed of the turbocompressor from a zero rotational speed; (f) a
third control system function to discern that conditions are
appropriate to initiate a closing of the antisurge valve before
said turbocompressor reaches the constant operating rotational
speed; and (g) a fourth control system function to generate a
second signal to actuate the antisurge valve towards its closed
position after the third control system function has discerned
conditions are appropriate to initiate the closing of the antisurge
valve.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] Not applicable.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates generally to a control scheme.
More particularly the present invention relates to a method and
apparatus for reducing a shaft power required when starting up a
turbocompressor by manipulating the compressor's antisurge recycle
valve.
[0004] 2. Background Art
[0005] As shown in FIG. 1, a turbocompressor 100, whether axial or
centrifugal, is driven by a driver such as a variable speed
electric motor 110. A recycle valve 120, used for antisurge
protection, is piped in parallel with the compressor 100. An inlet
throttling valve 130 may be used for compressor capacity or
performance control.
[0006] As all those of ordinary skill in this art know, surge is an
unstable operating condition of a turbocompressor encountered at
generally low flow rates. The surge region is shown in FIG. 2 to
the left of the surge limit curve 210. In FIG. 2, H.sub.p is the
polytropic head and Q is the volumetric flow rate, both associated
with the turbocompressor.
[0007] For the purposes of this document, including the claims, the
compressor's minimum operating speed is hereby defined as the
minimum rotational speed, greater than idle speed, at which the
compressor may be operated continuously. The minimum operating
speed is defined by the compressor manufacturer. It is generally
depicted as the lowest performance curve in a compressor
performance map such as shown in FIGS. 2 and 3. Lower speeds,
greater than idle speed, are experienced on startup and shutdown,
but the compressor is not operated continuously at these speeds.
For turbocompressors operated at a constant speed, such as those
driven by constant speed electric motors, the minimum operating
speed is simply the constant operating rotational speed.
[0008] As those of ordinary skill know, the accepted startup
procedure for a turbocompressor is to increase the rotational speed
of the compressor with the antisurge valve 120 wide open until the
compressor reaches the compressor's minimum operating speed (if the
compressor is operated at variable speed) or the compressor's
operating speed (if the compressor is driven by a constant speed
driver). At this point in the startup procedure, the antisurge
valve 120 is ramped closed and the compressor's 100 automatic
performance control takes control of the compressor's rotational
speed, inlet throttling valve 130, or variable guide vanes to
control the compressor's 100 capacity.
[0009] As is recognized by all those of ordinary skill, this
startup procedure provides the most safety for the compressor
because surge will be avoided, as depicted in FIG. 3. The
compressor's 100 operating point trajectory 320 is shown as a
dot-dashed line. Curves of constant compressor rotational speed
310a-310e are shown as solid lines. The curve 310a represents the
minimum operating rotational speed, while the curve 310e represents
the maximum operating rotational speed. Because the recycle valve
120 is maintained in its fully open position until minimum
rotational speed has been achieved, the compressor operating point
trajectory 320 tends to give wide berth to the surge limit curve
210 in the region below the minimum rotational speed curve
310a.
[0010] Additional impetuses for startup with the antisurge valve
120 fully open are that the surge limit curve 210 is usually
unknown for rotational speeds less than the minimum operating
speed, and that pressure and flow sensor signals of reasonable
magnitude must be achieved before a valid compressor operating
point may be determined. The compressor's operating point must be
calculated to compare its location to the surge limit line 210, or
surge control line 220 to avoid having the compressor's operating
point cross the surge limit line 210. Antisurge control algorithms
are described in the Compressor Controls Series 5 Antisurge Control
Application Manual, Publication UM5411 rev. 2.8.0 December 2007,
herein incorporated in its entirety by reference.
[0011] Due to the large flow through the compressor 100 during
startup using the above standard procedure, the shaft power
required to drive the compressor 100 is large. This results in
slower startup and, possibly, tripping of the driver due to power
overload.
[0012] A gas turbine driver may experience high exhaust gas
temperatures during the startup of a turbocompressor. An electric
motor driver may trip on thermal overload due to a current being
too high for too long a duration.
[0013] There is, therefore, a need for an improved control strategy
for the startup of turbocompressors to reduce the loading of the
compressor while maintaining the compressor flow out of the
unstable, surge region.
BRIEF SUMMARY OF THE INVENTION
[0014] An object of the present invention is to provide a method
and apparatus for safely starting a turbocompressor while
minimizing an overall energy required to accomplish the
startup.
[0015] Compressors having gas turbine drivers and variable
frequency drive electric motors tend to have long startup times--on
the order of several minutes. For this class of compressors, a
first embodiment of this invention prescribes that the compressor's
antisurge valve be maintained at its fully open position until
predetermined signal strengths are realized from the compressor's
suction and discharge pressure sensors, and the flow sensor. At
this point, the antisurge valve is ramped closed at a predetermined
rate under control of the antisurge control system to keep the
compressor's operating point from crossing the surge control curve.
Startup continues independently of the antisurge controller's
operation.
[0016] A second class of compressors comprises constant-speed
electric motor driven compressors. The startup times for this class
of compressors tend to be on the order of less than a minute. In
this case, the control system starts the antisurge valve in a fully
open position, and begins to ramp the antisurge valve closed at a
predetermined rate after a predetermined time has elapsed after the
initiation of the startup of the compressor. Because of the rapid
startup, the pressure and flow sensor signals become viable very
quickly, so antisurge control may be carried out before the
compressor's operating point reaches the surge control curve.
[0017] The novel features which are believed to be characteristic
of this invention, both as to its organization and method of
operation together with further objectives and advantages thereto,
will be better understood from the following description considered
in connection with the accompanying drawings in which a presently
preferred embodiment of the invention is illustrated by way of
example. It is to be expressly understood however, that the
drawings are for the purpose of illustration and description only
and not intended as a definition of the limits of the
invention.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
[0018] FIG. 1 is a schematic of a compressor, driver, and antisurge
recycle valve;
[0019] FIG. 2 is a first representative compressor performance
map;
[0020] FIG. 3 is a second representative compressor performance map
showing a first compressor operating point's startup
trajectory;
[0021] FIG. 4 is a is a third representative compressor performance
map showing lines of constant shaft power;
[0022] FIG. 5 is a fourth representative compressor performance map
showing a second compressor operating point's startup
trajectory;
[0023] FIG. 6a is a schematic of a variable speed motor driven
compressor system;
[0024] FIG. 6b is a schematic of a constant speed motor driven
compressor system;
[0025] FIG. 7 is a schematic of a turbine driven compressor
system;
[0026] FIG. 8 is a flow diagram of a first embodiment of the
present invention;
[0027] FIG. 9 is a flow diagram of a second embodiment of the
present invention;
[0028] FIG. 10 is a flow diagram of a third embodiment of the
present invention; and
[0029] FIG. 11 is a detail flow diagram of a startup initiation
process
DETAILED DESCRIPTION OF THE INVENTION
[0030] A typical compressor performance map in H.sub.p-Q
coordinates is shown in FIG. 4. Here, H.sub.p is polytropic head
and Q is volumetric flow rate--usually in the suction. The map of
FIG. 4 comprises solid-line curves of constant rotational speed
310a-310e and dashed-line curves of constant shaft power 410a-410e.
As is clear from the relationship between the curves of constant
rotational speed 310a-310e and the curves of constant shaft power
410a-410e, at a given rotational speed, the required shaft power
decreases as the operating point moves toward the surge limit 210.
To avoid overpowering the compressor driver 110, 710 (see FIG. 7)
an operating point trajectory 520, shown in FIG. 5, running as near
the surge limit 210 as possible, should be used. The short-dashed
curve 510 represents a surge control line--a line set a
predetermined distance from the surge limit line 210 toward the
stable operating region, thus providing a safety margin for the
antisurge control system.
[0031] As those of ordinary skill in the art of compressor control
know, limit control is applied to the compressor 100 to maintain
the operating point at or to the right of the surge control line
510. To effect this control, an antisurge or recycle valve 120, as
shown in FIGS. 1, 6a, 6b, and 7, is manipulated to maintain an
adequate flow rate through the compressor 100. The manipulation of
the antisurge valve 120 is carried out via an automatic control
algorithm in the antisurge controller, A/S PID 610, of FIGS. 6a,
6b, and 7. Typical inputs to the antisurge controller 610 are shown
in FIGS. 6a, 6b, and 7 and comprise a differential pressure signal
from a flow transmitter, FT 620, a suction pressure signal from a
suction pressure transmitter, PT1 630, a discharge pressure signal
from a discharge pressure transmitter, PT2 635, and a rotational
speed signal from speed pickup, SE 640 when the driver is variable
speed as in FIGS. 6a, and 7. Often, in applications using a
constant speed driver, such as a constant speed electric motor 640,
as shown in FIG. 6, no speed pickup SE 640 in included.
[0032] To emulate the operating point trajectory 520 depicted in
FIG. 5, the antisurge valve 120 is initially fully open, but is
ramped closed by the control system as soon as safe operation may
be assured. One embodiment of the instant invention is depicted in
the flow diagram of FIG. 8. This embodiment is particularly useful
when the startup process is "slow," taking on the order of several
minutes from its initiation. As mentioned, the antisurge valve 120
is set initially at its full open position as shown in block 800.
The full open position may vary between valve types. Generally,
full open in the context of this invention is the greatest opening
the antisurge valve 120 will realize in its duty in the specific
application. The present invention does not depend on the percent
opening value at which the antisurge valve 120 is considered in its
full open position.
[0033] When the antisurge valve 120 is assured fully open, startup
can be initiated as shown in block 805. At startup, the rotational
speed of the compressor 100 is increased according to the
guidelines and restrictions of the compressor 100 and driver 110,
710 manufacturers and the needs of the equipment owner. In
particular, critical speeds, if any, are considered and the startup
schedule takes these speeds into consideration. Speed increase is
depicted in block 810, and is effected, as shown in FIG. 11, by
increasing a compressor speed set point used by a Variable
Frequency Drive (VFD) controller 650 (FIG. 6a) or a rotational
speed controller 720 (FIG. 7).
[0034] As the compressor speed increases, the control system 610
repeatedly checks the signals received from the flow transmitter
620, suction pressure transmitter 630, and discharge pressure
transmitter 635. The signal values are compared to threshold
values, .DELTA.p.sub.o,min, p.sub.s,min, and p.sub.d,min,
respectively in comparator blocks 815, 820, 825. If the signal
magnitude of one or more of the input signals, .DELTA.p.sub.o,
p.sub.s, and p.sub.d, is not at least as great as its respective
threshold value, the rotational speed of the compressor 100
continues to be ramped up as indicated in block 810.
[0035] Once all three signals, .DELTA.p.sub.o,min, p.sub.s,min, and
p.sub.d,min, exceed their threshold values .DELTA.p.sub.o,min,
p.sub.s,min, and p.sub.d,min, two operations are carried out
essentially simultaneously and repeatedly. Each of these operations
emanates from and returns to the branch block 830. In one of these
operations, the antisurge controller 610 compares the compressor's
operating point to the surge control line 510 to determine how the
antisurge valve 120 must be manipulated for antisurge protection.
If the compressor's operating point is to the right of the surge
control line 510 as determined in the comparator block 835, the
antisurge valve 120 is ramped toward its closed position according
to a predetermined schedule as shown in block 850. If the operating
point is on or to the left of the surge control line 510, the
antisurge controller 610 manipulates the antisurge valve's 120
position to keep the compressor 100 safe from surge as shown in
block 845.
[0036] The other essentially simultaneous operation involves
continuing to increase the compressor's rotational speed according
to block 855 until the minimum operating speed, N.sub.min, or some
predetermined value of speed is reached. Continuing to increase the
compressor's rotational speed is effected as explained with regard
to block 810: the rotational speed set point used by the VFD
controller 650 or the speed controller 720 is increased with time.
Those of ordinary skill in this art are intimate with this aspect
of startup control. When the comparator block 840 determines the
compressor 100 has reached its minimum operating speed, the control
system is shifted from its startup mode to its RUN mode, as shown
in block 860. At that point, the capacity or performance control
system takes over varying the compressor speed according to the
needs of the process. Note that the minimum operating speed,
N.sub.min, in comparator block 840 may be the compressor's
operating speed if the compressor 120 is to be operated at a
constant speed.
[0037] An additional embodiment is shown in FIG. 9. This embodiment
is particularly useful for compressors 120 that may be started
rapidly--in less than a minute, for instance. The antisurge valve
120 is set initially at its full open position as shown in block
800. In block 910, a timer is reset to zero.
[0038] When the antisurge valve 120 is assured fully open and the
timer has been initialized, startup can be initiated as shown in
block 805. At startup, the rotational speed of the compressor 100
is ramped up according to the guidelines and restrictions of the
compressor 100 and driver 110, 710 manufacturers and the needs of
the equipment owner. Speed rampup is carried out by increasing the
VFD controller's 650 or rotational speed controller's 720 set
point, and is depicted in block 810.
[0039] In this embodiment of the invention, the antisurge valve is
ramped toward a closed position after a predetermined time elapses.
In comparator block 920, the time as reported by the timer is
compared to the time threshold, t.sub.PD. If the time does not
exceed the threshold time, the speed continues to increase, but no
change to the position of the antisurge valve 120 is made. When the
threshold time, t.sub.PD, has elapsed, two operations are carried
out essentially simultaneously and repeatedly. Each of these
operations emanates from and returns to the branch block 830. In
one of these operations, the antisurge controller 610 compares the
compressor's operating point to the surge control line 510 to
determine how the antisurge valve 120 must be manipulated for
antisurge protection. If the compressor's operating point is to the
right of the surge control line 510 as determined in the comparator
block 835, the antisurge valve 120 is ramped toward its closed
position according to a predetermined ramp rate as shown in block
850. If the operating point is on or to the left of the surge
control line 510, the antisurge controller 610 manipulates the
antisurge valve's 120 position to keep the compressor 100 safe from
surge as shown in block 845.
[0040] The other essentially simultaneous operation involves
continuing to increase the compressor's rotational speed according
to block 855 until the minimum operating speed, N.sub.min, or some
predetermined value of speed is reached. When the comparator block
840 determines the compressor 120 has reached its minimum operating
speed, the control system is shifted from its startup mode to its
RUN mode, as shown in block 860. At that point, the capacity or
performance control system takes over varying the compressor speed
according to the needs of the process. Note that the minimum
operating speed, N.sub.min, in comparator block 840 may be the
compressor's operating speed if the compressor 120 is to be
operated at a constant speed.
[0041] In FIG. 10, a third embodiment is illustrated, differing
from the embodiment of FIG. 9 in that the driver of FIG. 10 is a
constant speed driver, such as a constant speed electric motor 640
(FIG. 6b). In this embodiment, the process of accelerating the
driver up to its operating speed, N.sub.op, does not incorporate a
decision to continue accelerating the driver inasmuch as the driver
will continue to accelerate until its operating speed, N.sub.op, is
reached or it is tripped. Therefore, block 1055 indicates only that
the rotational speed continues to rise. Block 1040 is intended only
to indicate the compressor rotational speed will increase until the
operating speed, N.sub.op, is reached, and not that a decision is
being made in this comparator block. Ultimately, when the
compressor has reached its operating speed, N.sub.op, the control
system reverts to a RUN mode 860 wherein performance or capacity
control is carried out to satisfy process constraints. Note that,
in this case especially, the predetermined time lapse, t.sub.PD, in
comparator block 920 may be zero so the antisurge valve 120 begins
to close immediately as startup begins.
[0042] The last two embodiments differ from the prior art in that,
in the instant invention, time is used to determine when the
antisurge valve 120 is ramped toward its closed position, rather
than rotational speed.
[0043] The flow diagrams in FIGS. 8, 9 and 10 may be considered
contents of a logic unit within a compressor control system, such
as the antisurge controller 610 depicted in FIGS. 6a, 6b, and
7.
[0044] More detail of the startup initiation block 810 is shown in
FIG. 11. A check to ascertain the antisurge valve 120 is fully open
is first carried out in query block 1110. If the antisurge valve
120 is not fully open, the flow moves to a valve open function
1120. Once the antisurge valve 120 is fully open, the
turbocompressor rotational speed is increased from an initial, zero
value as shown in block 1130.
[0045] The above embodiments are the preferred embodiments, but
this invention is not limited thereto. It is, therefore, apparent
that many modifications and variations of the present invention are
possible in light of the above teachings. It is, therefore, to be
understood that within the scope of the appended claims, the
invention may be practiced otherwise than as specifically
described.
* * * * *