APPPARATUS AND METHOD FOR CONTROLLING A CENTRIFUGAL COMPRESSOR

• US 3,778,695 A
• Filed: 04/21/1972
• Issued: 12/11/1973
• Est. Priority Date: 04/21/1972
• Status: Expired due to Term

• Current Assignee: Owens Illinois Television Products Inc (Nippon Electric Glass Company Limited)
• Original Assignee: Paul Bauer Jr
• Inventors: Bauer, Paul Jr.
• Primary Examiner(s): Dobeck, B.
• Application Number:
  US05/246,478
• Time in Patent Office: 599 Days
• Field of Search: 318/481,434 417/44,45,19,29,30
• US Class Current: 318/481
• CPC Class Codes: F04D 27/0276 by influencing fluid temperature
• CPC Class Codes: H02H 7/0857 by lowering the mechanical load of the motor
• CPC Class Codes: H02P 29/40 Regulating or controlling the amount of current drawn or delivered by the motor for controlling the mechanical load

Abstract

In an electric motor driven centrifugal compressor, apparatus and method for controlling the compressor to prevent '"'"''"'"''"'"''"'"'surge'"'"''"'"''"'"''"'"' and operation at a motor current above the maximum rated motor current and for automatically restarting the compressor after a short-term power failure. The current to the compressor motor is sensed as an indicator of impending '"'"''"'"''"'"''"'"'surge'"'"''"'"''"'"''"'"'. When the current falls below a pre-selected minimum value, a blow-off valve in the compressor outlet line is gradually opened to allow excess volume to escape to the atmosphere, thus avoiding '"'"''"'"''"'"''"'"'surge'"'"''"'"''"'"''"'"'. The current to the motor is also sensed as an indication of compressor operation above the maximum rated motor capacity. When the motor current rises above a pre-selected level, an inlet valve to the compressor is closed. This restricts the volume available for compression and prevents motor over-loaded operation. In the event of a short-term power failure or power dip, contact pairs controlled by time delay relays remain closed for a short period of time. These pairs provide parallel bypass circuits around the normal motor starting circuits and allow an automatic restart if the time period has not been exceeded. The compressor is powered by the flywheel effect of its main drive gear during these shortterm power failures. If the power failure is of a long enough duration, the bypass contact pairs open, and the normal restarting procedures must be followed.

Claims

1. Apparatus for limiting the current to an electric motor driving a centrifugal compressor to a pre-selected maximum current, comprising, in combination:

An adjustable, flow-limiting inlet valve connected to the inlet side of said centrifugal compressor;

signal generating means connected to the outlet side of said centrifugal compressor for sensing the process pressure in said outlet side and generating a signal proportional to said process pressure;

control means connected to said signal generating means and responsive to said proportional signal for comparing said proportional signal with a pre-selected process pressure value, said control means generating a control signal representing the deviation of said proportional signal from said pre-selected process pressure value; and

means responsive to the electrical current delivered to said motor and connected between said control means and said inlet valve, for passing said control signal without deviation to said inlet valve to adjust the inlet valve in response to said control signal so long as said motor current remains below a pre-selected value and modulating said control signal as a function of said motor current above said pre-selected value to adjust said inlet valve in response to said modulated control signal such that the maximum permissible current to said electric motor will not be exceeded.

2. The apparatus of claim 1, wherein said signal generating means comprises means for generating an air pressure signal proportional to said process pressure, and said control means is responsive to the air pressure signal;

and further including a source of control air under pressure connected to said signal generating means.

3. The apparatus of claim 1, wherein said control means comprises means for generating an air pressure control signal and further including a source of control air under pressure connected to said control means.

4. The apparatus of claim 3, wherein said means responsive to the current delivered to said motor comprises a '"'"''"'"''"'"''"'"'current-to-pressure'"'"''"'"''"'"''"'"' converter whose output is an air pressure signal modulated as a function of said motor current above said pre-selected value, and further including a source of control air under pressure connected to said '"'"''"'"''"'"''"'"'current-to-pressure'"'"''"'"''"'"''"'"' converter.

5. The apparatus of claim 4, further including means, through which said control signal must pass, connected intermediate said inlet valve and said '"'"''"'"''"'"''"'"'current-to-pressure'"'"''"'"''"'"''"'"' converter, for boosting the volume of said air pressure control signal without changing the pressure thereof, and a source of control air under pressure connected to said means for boosting the volume of said air pressure control signal.

6. The apparatus of cLaim 4, further including means for bypassing said control signal around said '"'"''"'"''"'"''"'"'current-to-pressure'"'"''"'"''"'"''"'"' converter to operate said inlet valve as a function of process pressure only in the event of a power failure.

7. The apparatus of claim 6, wherein said means for bypassing said control signal comprises:

a normally closed, electrically operated valve through which said control signal must pass, connected intermediate said inlet valve and said '"'"''"'"''"'"''"'"'current-to-pressure'"'"''"'"''"'"''"'"' converter, said normally closed valve being electrically connected so as to be energized when electrical power is on;

a bypass pipeline connected to the outlet of said control means and said inlet valve; and

a normally open, electrically operated valve connected in said bypass pipeline intermediate said control means and said inlet valve, said normally open valve being electrically connected so as to be energized when electrical power is available.

8. Apparatus for preventing '"'"''"'"''"'"''"'"'surge'"'"''"'"''"'"''"'"' in a centrifugal compressor driven by an electric motor, comprising, in combination:

an adjustable blow-off valve connected to the outlet side of said compressor;

means responsive to the electrical current delivered to said electric motor for generating a signal proportional to said motor current; and

control means, connected to said means for generating a signal proportional to said motor current and said blow-off valve for generating a modulated control signal to open said blow-off valve as said proportional signal falls below a pre-selected value and close said blow-off valve as said proportional signal rises above said pre-selected value.

9. The apparatus of claim 8, wherein said means responsive to the electrical current delivered to said motor comprises a '"'"''"'"''"'"''"'"'current-to-pressure'"'"''"'"''"'"''"'"' converter for generating an air pressure signal proportional to said current, and said control means is responsive to the air pressure signal;

and further including a source of control air under pressure connected to said '"'"''"'"''"'"''"'"'current-to-pressure'"'"''"'"''"'"''"'"' converter.

10. The apparatus of claim 8, wherein said control means comprises means for generating an air pressure signal modulated as a function of said proportional signal and said pre-selected minimum value of said proportional signal;

and further including a source of control air under pressure connected to said control means.

11. The apparatus of claim 10, further including means intermediate said control means and said blow-off valve, for boosting the volume of said modulated air pressure signal without changing the pressure thereof, and a source of control air under pressure connected to said means for boosting the volume of said modulated air pressure signal.

12. The apparatus of claim 9, further including a normally closed, electrically operated valve through which said proportional air pressure signal must pass, connected intermediate said means responsive to the electrical current delivered to said motor and said control means for holding a constant air pressure signal into said control means during electrical power failures, said normally closed valve being electrically connected so as to be energized when electrical power is on.

13. An integrated, non-oscillatory, automatically temperature compensated flow control system for a centrifugal compressor, driven by an electric motor, for both limiting the current to said electric motor to a pre-selected maximum current and preventing the occurrence of '"'"''"'"''"'"''"'"'surge'"'"''"'"''"'"''"'"' in said compressor, comprising, in combination:

an adjustable, flow-limiting inlet valve connected to the inlet side of said centrifugal compressor;

signal generating means connected to the outlet side of said centrifugal compressor for sensing the process pressure in said outlet side and generating a signal proportional to said process pressure;

first control means connected to said signal generating means and responsive to said proportional signal for comparing said proportional signal with a preselected process pressure value, said first control means generating a control signal representing the deviation of said proportional signal from said pre-selected process pressure value;

first means responsive to the electrical current delivered to said motor and connected between said first control means and said inlet valve, for passing said control signal without deviation to said inlet valve to adjust said inlet valve in response to said control signal so long as said motor current remains below a pre-selected value and modulating said control signal as a function of said motor current above said pre-selected value to adjust said inlet valve in response to said modulated control signal such that the maximum permissible current to said electric motor will not be exceeded;

an adjustable blow-off valve connected to the outlet side of said compressor;

second means responsive to the electrical current delivered to said electric motor for generating a signal proportional to said motor current; and

second control means, connected to said second means for generating a signal proportional to said motor current and said blow-off valve, for generating a modulated control signal to open said blow-off valve as said proportional signal falls below said pre-selected value and close said blow-off valve as said proportional signal rises above said pre-selected value.

14. The apparatus of claim 13, further including means for preventing the shutdown of said compressor during short-term power failures.

15. The apparatus of claim 14, wherein said means for preventing the shutdown of said centrifugal compressor comprises, in combination:

a main electrical supply;

a safety interlock circuit electrically connected to said main electrical supply for stopping said motor and for monitoring the status of critical components of said compressor and to prevent starting or continued operation of said motor in the event of an abnormal status of any one of said critical components;

a motor starter circuit electrically connected to said main electrical supply and said drive motor for transferring electrical current to said motor;

an enabling circuit electrically connected to and energized through said safety interlock circuit, said enabling circuit being electrically connected to said motor starter circuit for controlling said motor starter circuit to allow current to flow to said drive motor, said safety interlock circuit and said enabling circuit, upon failure of said main electrical supply, becoming de-energized to thereby open the connection of said main supply to said drive motor to prevent reconnection of said main electrical supply to said motor upon resumption of power to said main electrical supply; and

circuit means connected to said main electrical supply and interconnecting said supply with said safety interlock circuit and said enabling circuit for automatically re-energizing said safety interlock circuit and said enabling circuit when the duration of the failure of said main electrical supply is of a duration less than a pre-determined period to thereby re-establish connection of said main electrical supply to said drive motor.

16. A method for limiting the flow through a centrifugal compressor, driven by an electric motor, to a pre-selected maximum value by controlling the degree of opening of an inlet valve on the inlet side of said centrifugal compressor, comprising the steps of:

a. sensing a process pressure in the outlet side of said centrifugal compressor;

b. generating a signal proportional to said process pressure;

c. comparing said proportional signal with a preselected process pressure;

d. generating a control signal proportional to the deviation of said proportional signal from said preselected process pressure;

e. sensing the current delivered to said electric motor;

f. comparing said control signal with a preselected maximum value of said motor current. g. passing said control signal to said inlet valve without modification so long as said motor current does not exceed said pRe-selected maximum value; and

h. modulating said control signal as a function of said motor current above said pre-selected maximum value, whereby the degree of opening of said inlet valve is directly related to said control signal.

17. A method for preventing '"'"''"'"''"'"''"'"'surge'"'"''"'"''"'"''"'"' from occurring in a centrifugal compressor, driven by an electric motor, by opening a blow-off valve located in the outlet piping from said compressor comprising the steps of:

a. sensing the current delivered to said motor;

b. generating a signal proportional to said motor current;

c. comparing said proportional signal with a pre-selected minimum value of said proportional signal;

d. generating a modulated control signal as said proportional signal falls below said pre-selected minimum value; and

e. opening said blow-off valve as a function of said modulated control signal.