U.S. patent application number 10/790347 was filed with the patent office on 2005-09-01 for process flow control circuit.
Invention is credited to Vinson, James Woodrow JR..
Application Number | 20050191184 10/790347 |
Document ID | / |
Family ID | 34887454 |
Filed Date | 2005-09-01 |
United States Patent
Application |
20050191184 |
Kind Code |
A1 |
Vinson, James Woodrow JR. |
September 1, 2005 |
Process flow control circuit
Abstract
A flow control scheme for use with a flow circuit, including a
fluid transfer device such as a pump or a compressor. The flow
control scheme monitors the power and pressure differential of the
fluid transfer device. The flow control scheme is capable of
maintaining the flow of the fluid transfer device at a baseline
value in response to varying operational conditions within the flow
circuit.
Inventors: |
Vinson, James Woodrow JR.;
(Houston, TX) |
Correspondence
Address: |
ROHM AND HAAS COMPANY
PATENT DEPARTMENT
100 INDEPENDENCE MALL WEST
PHILADELPHIA
PA
19106-2399
US
|
Family ID: |
34887454 |
Appl. No.: |
10/790347 |
Filed: |
March 1, 2004 |
Current U.S.
Class: |
417/44.2 ;
417/18; 417/35; 417/38; 417/44.1 |
Current CPC
Class: |
F04B 2205/05 20130101;
F04B 2205/09 20130101; F04D 15/0066 20130101; F04B 2205/01
20130101 |
Class at
Publication: |
417/044.2 ;
417/018; 417/035; 417/038; 417/044.1 |
International
Class: |
F04B 049/00; F04B
049/06 |
Claims
What is claimed is:
1. A fluid flow circuit comprising: a fluid transfer device; a
pressure differential measurement device to measure the pressure
differential across the fluid transfer device; a power monitoring
device to measure the power of the fluid transfer device; and a
controller in communication with the pressure differential
measurement device and the power monitoring device, where said
controller is programmed with software commands to automatically
sample data reflecting the power and the pressure differential of
the fluid transfer device and to automatically maintain a
substantially constant fluid flow through the fluid transfer device
based on the power and the pressure differential of the fluid
transfer device.
2. The fluid flow circuit of claim 1 where said controller
maintains a substantially constant fluid flow through the fluid
transfer device by adjusting the speed of the fluid transfer device
in response to variations in the system curve of the fluid transfer
device.
3. The fluid flow circuit of claim 1, where the fluid transfer
device is selected from the group consisting of a centrifugal pump,
a positive displacement pump, a compressor, a turbine, a diaphragm
pump, and a water seal pump.
4. The fluid flow circuit of claim 1, where the varying pressure
drop within the fluid flow circuit comprises a rise in pressure
losses in the fluid flow circuit.
5. The fluid flow circuit of claim 1, where the constant fluid flow
is approximately equal to the baseline flow of the fluid flow
circuit.
6. A method of maintaining a constant flow of fluid within a fluid
flow circuit comprising the steps of: installing a fluid transfer
device within a fluid flow circuit; establishing a baseline flow
for the fluid transfer device; monitoring the pressure differential
across the fluid transfer device; monitoring the power provided to
the fluid transfer device; and adjusting the power provided to the
fluid transfer device to maintain a flow through the fluid transfer
device that is approximately equal to the baseline flow.
7. The method of claim 6 where the fluid transfer device is
selected from the group consisting of a centrifugal pump, a
positive displacement pump, a compressor, a turbine, a diaphragm
pump, and a water seal pump.
8. The method of claim 6 further comprising determining the power
of the fluid transfer device as a function of the fluid flow
through the fluid transfer device and of the pressure differential
measured by the pressure differential measurement device.
9. The method of claim 6 where the power to the fluid transfer
device is adjusted by increasing its magnitude.
10. The method of claim 6 where the power to the fluid transfer
device is adjusted by decreasing its magnitude.
11. The method of claim 6 where the power to the fluid transfer
device is adjusted to produce a flow through the fluid transfer
device that is greater than the baseline flow.
12. The method of claim 6 where the power to the fluid transfer
device is adjusted to produce a flow through the fluid transfer
device that is less than the baseline flow.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The invention relates generally to the field of the flow
control of fluids. More specifically, the present invention relates
to a method and apparatus to control flow through a fluid flow
circuit in response to variations in the fluid flow circuit.
[0003] 2. Description of Related Art
[0004] Fluid flow circuits generally comprise multiple pieces of
fluids handling hardware, such as heat exchangers, valves, vessels,
drums and the like that are connected together by a series of
piping to form the circuit. In many instances the motive force for
driving the fluid through the circuit is provided by a fluid
transfer device, such as a compressor or a pump. Generally the
fluid flow through fluid transfer devices is dependent upon the
pressure imparted onto the fluid by the fluid transfer device. This
pressure can also be referred to as the differential pressure
across the device. Typically an increase in differential pressure
results in a drop in the amount of fluid flow transmitted through
the device. In most instances this increase in differential
pressure is caused by an increase in pressure drop within the flow
circuit of which the device is included. Increases in pressure drop
within a flow circuit can be attributed to a variety of causes,
such as a decrease in the fluid density of the flow circuit, an
increase in pressure drop across a piece of equipment within the
flow circuit, or an increase in the operating pressure of a
particular piece of equipment within the flow circuit.
[0005] Cases where the pressure drop within a flow circuit is
caused by an increase in pressure drop across a piece of equipment
can be especially troublesome when the pressure drop increase is
caused by equipment fouling. Equipment fouling usually increases
over time instead of instead of stabilizing or getting
cleaner--thus the pressure drop across a fouled piece of equipment
will increase with use. At some point the pressure drop increase in
the circuit due to equipment fouling will reduce the operating flow
through the fluid circuit to an unacceptable level. At this point
the system must be shut down and the equipment cleaned. Shutting
down a fluid flow circuit and cleaning it is expensive from an
operational as well as a lost revenue standpoint.
[0006] One solution to the fouling problem in a fluid flow circuit
is to monitor the flow rate through the circuit with a flow meter,
and increase the speed of the fluid transfer device to
correspondingly increase the flow rate to a normal or acceptable
operating level. One of the problems with relying on flow meter
readings is that flow meters are sometimes unreliable, especially
when installed in two-phase flow service. Further, these flow
meters themselves can sometimes become fouled which not only adds
to their unreliability, but also increases pressure drop within the
fluid flow circuit. Therefore, a need exists for an apparatus and
method for use with a fluid transfer device and fluid flow circuit
that can maintain a substantially constant flow rate across the
fluid transfer device. Further, the apparatus and method must be
capable of responding to operational changes in the fluid flow
circuit while maintaining that substantially constant flow across
the fluid transfer device.
BRIEF SUMMARY OF THE INVENTION
[0007] The present invention involves a fluid flow circuit
comprising a fluid transfer device, a pressure differential
measurement device to measure the pressure differential across the
fluid transfer device, a power monitoring device that measures the
power of the fluid transfer device, and a controller in
communication with the pressure differential measurement device and
the power monitoring device. The controller maintains a constant
fluid flow through the fluid transfer device by adjusting the
operational speed of the fluid transfer device in response to
varying pressure losses within the fluid flow circuit. The fluid
transfer device can be a centrifugal pump, a positive displacement
pump, a compressor, a turbine, a diaphragm pump, or a water seal
pump, or any other device capable of transporting fluid through a
fluid flow circuit.
[0008] The controller maintains the power of the fluid transfer
device as a function of the fluid flow through the fluid transfer
device and of the pressure differential measured by the pressure
differential measurement device. The varying pressure drop within
the fluid flow circuit can come from a rise in pressure in the
fluid flow circuit within the high pressure portion of the fluid
flow circuit (downstream of the fluid transfer device).
[0009] One embodiment of the fluid flow circuit of the present
invention can include a pump having an inlet port and an outlet
port with a first pressure measurement device disposed within the
inlet port and a power monitoring device that measures the power of
the pump. Also included in the alternative embodiment is a speed
control device that measures the rotational speed of the pump and
is capable of varying the rotational speed of the pump, a second
pressure measurement device disposed within the outlet port, and a
controller in communication with the first pressure measurement
device, the second pressure measurement device, the power
monitoring device, and the speed control device. The controller
cooperates with the speed control device to maintain a constant
fluid flow through the pump by adjusting the rotational speed of
the pump in response to varying operational conditions within the
fluid flow circuit.
[0010] The present invention includes a method of maintaining a
constant flow of fluid within a fluid flow circuit comprising the
steps of, installing a fluid transfer device within a fluid flow
circuit, establishing a baseline flow for the fluid transfer
device, monitoring the pressure differential across the fluid
transfer device, monitoring the power provided to the fluid
transfer device, and adjusting the power provided to the fluid
transfer device to maintain a flow through the fluid transfer
device that is approximately equal to the baseline flow. The fluid
transfer device can be a centrifugal pump, a positive displacement
pump, a compressor, a diaphragm pump, or a water seal pump. One
method of the present invention includes determining the power of
the fluid transfer device as a function of the fluid flow through
the fluid transfer device and of the pressure differential measured
by the pressure differential measurement device.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING
[0011] FIG. 1 depicts a schematic view of a flow circuit.
[0012] FIG. 2 illustrates a flow curve for a fluid transfer
device.
DETAILED DESCRIPTION OF THE INVENTION
[0013] With reference to the drawing herein, a flow circuit 10
comprising, piping 11, a vessel 20, a flow transfer device 30, and
a fluids processing device 40 is shown in FIG. 1. The fluids
processing device 40 is shown as a heat exchanger with the process
fluid of the flow circuit 10 passing through its tube side.
However, any number of other process components could comprise the
fluids processing device 40, such as a filter, dryer, separator, or
coalescer. Likewise, these substitute elements, or other like
elements, could also take the place of the vessel 20.
[0014] The specific elements of the flow circuit 10 as illustrated
in FIG. 1 are not critical to the invention, but instead are shown
for illustration. The present invention will operate with the
specific items shown in FIG. 1, or with additional fluids handling
hardware, such as other vessels, exchangers, and the like, or fewer
items as shown. Similarly the fluid transfer device 30 is shown as
a centrifugal pump, but could also be a compressor, reciprocating
pump, turbine, diaphragm pump, or any other device used to motivate
fluids through a process circuit.
[0015] In one specific embodiment of the present invention as
illustrated in FIG. 1, fluid 21 within the vessel 20 flows to a
flow transfer device 30 where energy in the form of added pressure
is imparted to the fluid 21. The fluid 21 exits the fluid transfer
device 30 at an increased pressure and flows through the fluids
processing device 40 before it returns to the vessel 20. Connected
to each piece of equipment is a series of piping 11 that provides a
conduit for the fluid 21. The fluid 21 can be a liquid, gas, vapor,
slurry, powder, any substance motivated through a fluids processing
device 40, and mixtures thereof.
[0016] A pressure differential measurement device 50 is connected
to the piping 11 upstream 29 and downstream 31 of the fluid
transfer device 30 which measures the fluid pressure differential
across the fluid transfer device 30. Typically the pressure
upstream 29 of the fluid transfer device 30 is lower than the
pressure downstream 31 of the fluid transfer device 30. It is
appreciated that those skilled in the art can readily determine and
implement appropriate hardware and monitoring criteria to obtain a
suitable manner of obtaining this pressure differential.
[0017] The power monitoring device 52 measures the power consumed
by the fluid transfer device 30. Typically the fluid transfer
device 30 is powered by electrical power, thus the power monitoring
device 52 measures the voltage and current delivered to the fluid
transfer device 30. The voltage and current data measured by the
power monitoring device 52 is communicated to the controller 54.
The power consumed by the fluid transfer device 30 can be
calculated within the power monitoring device 52, within the
controller 54, or some other device. Thus the manner and technique
of calculating the electrical power consumed by the fluid transfer
device 30 is not critical to the present invention and any now
known or later developed manner of obtaining the power of the fluid
transfer device 30 is considered within the scope of this
invention.
[0018] The flow rate of the fluid transfer device 30 is directly
related to its efficiency multiplied by the ratio of its power
consumption vs. pressure differential. The general equation for
determining its flow is: Flow Rate=(Power Consumed/Pressure
Differential).times.Efficiency. In a more specific relationship,
the flow rate through a centrifugal pump can be determined from the
following equation: Flow Rate (gpm)=(Electrical HP input to
motor)(Motor Efficiency)(3960)(Pump Efficiency)/(Headft)(Specifi- c
Gravity). Utilizing these relationships the present invention is
capable of determining flow through the fluid transfer device 30
based upon its power consumption and pressure differential. The
efficiency of the specific fluid transfer device at hand is wholly
dependent upon its make and model, thus that information should be
obtainable from the manufacturer of the device. It should be noted
that the present invention is not only applicable to an array of
fluid transfer devices, but also to multiple makes and models of
these devices. The flow relationships presented represent the
preferred method of determining flow based on the power and
pressure differential of the fluid transfer device 30. However, the
scope of the present invention is not limited the equations cited
herein, but includes any number of known or later developed
relationships.
[0019] Referring now to FIG. 2, the series of curves (66-68)
represent head and flow characteristics of a fluid transfer device.
As is well known, each of the curves (66-68) relate to a constant
rotational speed at which the fluid transfer device is operating.
For the purposes of illustration it will be assumed that reference
numeral 69 indicates a baseline volumetric flow at a corresponding
baseline Head 70. The baseline volumetric flow 69 and baseline Head
70 represent the desired or design conditions of the fluid transfer
device. Further, if the fluid transfer device is operating along
curve 66 the baseline flow will be realized on curve 66 at
reference numeral 61. Should the exit pressure of the fluid
transfer device increase (assuming constant inlet pressure and
rotational speed) the fluid transfer device will travel "up the
curve" as the pressure differential across the fluid transfer
device (or Head) increases. It can be seen from FIG. 2 when the
fluid transfer device moves up the curve to reference numeral 62,
the flow of the fluid transfer device decreases. However, should
the rotational speed of the fluid transfer device be increased to
operate along the curve 67, the baseline flow 69 could be achieved
while meeting or exceeding the baseline Head 70.
[0020] The exit pressure of the fluid transfer device 30 can
increase for varied reasons. For example, solids suspended within
the fluid 21 can become deposited inside of the fluids processing
device 40 (also known as fouling) thereby increasing the pressure
drop across the fluids processing device 40. The increased pressure
drop across the fluids processing device 40 translates into an
increase in pressure at the exit of the fluid transfer device 30
that correspondingly increases the pressure differential across the
fluid transfer device 30. When the fluids processing device 40 is a
heat exchanger, fouling of the fluids processing device 40 is
almost certain to occur.
[0021] The system curves 80 and 82 represent how the fluid transfer
device 30 might operate with respect to a certain flow circuit. The
system curve however assumes that the system (or circuit) is static
and will provide an unchanging amount of pressure loss with the
same amount and type of fluid flow. Thus if the system (or circuit)
combined with the fluid transfer device 30 is altered or changes
its operating parameters, the system curve no longer models the
operating parameters (head and flow) of the fluid transfer device
30. One example of an altered system is if the pressure drop across
a certain piece of equipment within the system increases, i.e. a
fouled heat exchanger, fouled flow element, or a control valve
providing more or less pressure drop.
[0022] While the controller 54 can be of any number of different
makes or models, the controller 54 should provide automation to the
flow circuit 10 to ensure a relatively constant flow through the
flow circuit 10 at all times. Thus it is preferred that the
controller 54 be comprised of an electrical processing system such
as a computer or micro-computer system that is programmable, or
controlled by software stored elsewhere, and be able to sample the
data supplied to it on a frequent basis, i.e. multiple samples per
minute. The controller 54 should also be capable of quickly
processing the data it receives so it can evaluate the data and
then send an instantaneous or almost instantaneous command to
adjust the operating parameters of the flow circuit 10. One of the
advantages of employing an electrical programmable system is the
flexibility of being able to evaluate an infinitely different
number of processes based on the commands programmed into the
controller 54 or directed to the controller 54.
[0023] In operation, the controller 54 constantly monitors the flow
through the flow transfer device 30 by its evaluation of the power
consumed by the fluid transfer device 30 and pressure differential
across the fluid transfer device 30. Should the flow decrease
through the fluid transfer device 30, the power imparted to the
fluid by the fluid transfer device 30 will decrease as well. Set
points programmed within the controller 54 are triggered when the
flow falls below those set points. Once the set points are
triggered the controller 54 of the present invention increases (or
decreases) the speed of the fluid transfer device 30 resulting in
an increase (or decrease) in flow and head through the fluid
transfer device 30.
[0024] In a more specific example with reference to FIG. 2,
considering a fluid transfer device 30 to be operating at reference
number 61 on the curve 66 the fluid transfer device 30 would
deliver a Flow 69 at Head 70. As such the fluid transfer device 30
would be operating on system curve 80. When the pressure on the
discharge side of the fluid transfer device 30 increases the flow
characteristics of the fluid transfer device 30 can move up the
curve to reference number 62, resulting in a new Flow 71 and Head
72. Thus the system can no longer be modeled by system curve 80,
but instead by system curve 82. Assuming for the purposes of
example that the set points programmed within the controller are
triggered at reference number 62, the controller could then
increase the speed of the fluid transfer device 30 to operate on
the curve 67 at reference number 63, which is at the baseline flow
69. Further, the set points could be established so that the fluid
transfer device 30 increases its flow (with corresponding head) to
be some percentage above the baseline flow of reference numeral
69.
[0025] In contrast, should the pressure losses in the flow circuit
10 decrease, the controller 54 can be programmed to decrease the
speed of the fluid transfer device and maintain the baseline flow
required by the specific process. Thus the controller 54 can be
programmed to ensure that the fluid circuit 10 has a relatively
constant flow and respond to operational changes within the fluid
flow circuit 10. These changes include changes in pressure drop
throughout the circuit, either as a whole say due to density or
flow rate variations, or to specific equipment that has undergone a
change in pressure drop.
[0026] The criteria for determining the set points of the
controller 54 depend on the flow circuit 10 and its operating
specifications, design requirements, and/or system accuracy. Some
process circuits can function adequately with a wide variance of
flow rates, while others in order to operate properly must maintain
a narrow range of flow rates. Accordingly the magnitude of flow and
or percentage change between flow rates 71 and 69 depends almost
wholly on the application in which the present invention is being
implemented. It is appreciated that one skilled in the art can
develop proper set points for use with this invention without undue
experimentation.
[0027] Controlling the fluid transfer device 30 flow in the manner
described above has many advantages. For example, in situations
when the pressure losses in the flow circuit 10 are due to fouling
of equipment such as heat exchangers, the dynamic system described
herein can respond to maintain the required baseline flow without
the need for frequently removing and cleaning the fouled equipment
and shutting down the entire circuit. This is particularly valuable
in chemical purification systems where polymerizable materials,
such as (meth)acrylic acid or styrene, are present. For example,
the cleaning cycle of an exchanger (such as a reboiler) whose
fouling reduces flow in a flow circuit 10 below the baseline flow
due to increased pressure drop can be changed from a 3-4 month
cycle to a year or more. As such, a considerable savings in
maintenance, capital, and other expenses can be readily realized by
implementing the present invention. Energy costs are significantly
reduced as well since alternative flow control methods generally
require control valves or flow meters within the service that incur
fluid frictional losses. Eliminating these frictional losses
reduces the operating energy consumed by the flow transfer device
30.
[0028] Another advantage of the present invention is the
flexibility in which the baseline flow may be adjusted. Utilization
of a controller 54 enables a plant operator to update the value of
the baseline flow while the fluid circuit 10 is on-line. Further,
the set points can be changed as well while the fluid circuit 10 is
on-line.
[0029] The present invention described herein, therefore, is well
adapted to carry out the objects and attain the ends and advantages
mentioned, as well as others inherent therein. While a presently
preferred embodiment of the invention has been given for purposes
of disclosure, numerous changes exist in the details of procedures
for accomplishing the desired results. These and other similar
modifications will readily suggest themselves to those skilled in
the art, and are intended to be encompassed within the spirit of
the present invention disclosed herein and the scope of the
appended claims.
* * * * *