Anti-surge speed control

Patent metadata

Field	Value
Publication number	US-9702365-B2
Application number	US-201313888943-A
Country	US
Kind code	B2
Filing date	May 7, 2013
Priority date	May 31, 2012
Publication date	Jul 11, 2017
Grant date	Jul 11, 2017

Abstract

The invention relates to a method and control system to control the speed of a centrifugal compressor operating within a vacuum pressure swing adsorption process to avoid an operation at which surge can occur and directly driven by an electric motor that is in turn controlled by a variable frequency drive. The claimed method determines the optimal speed for operation of the compressor along a peak efficiency operating line of a compressor map thereof. Speed of the compressor is adjusted by a feed back speed multiplier when the flow or other parameter referable to flow through the compressor is below a minimum and a feed forward multiplier during evacuation and evacuation with purge steps that multiplies the feed back multiplier to increase speed of the compressor and thereby avoid surge.

First claim

We claim:

1. A method of controlling centrifugal compressor speed within a vacuum pressure swing adsorption apparatus, said centrifugal compressor having an inlet configured to receive an air feed stream and an outlet configured to discharge a pressurized air feed stream, said method comprising:
measuring or calculating the flow rate of said air feed stream entering the inlet of said centrifugal compressor
the centrifugal compressor directly driven by an electric motor having a speed controlled by a variable frequency drive;
measuring the pressure of the pressurized air feed stream at the outlet of said compressor and the pressure of said air feed stream at the inlet of said compressor and calculating the pressure ratio of pressures of the outlet to the inlet of the centrifugal compressor;
determining an optimal speed of the centrifugal compressor based on the pressure ratio that lies along the peak efficiency operating line of the centrifugal compressor;
determining a minimum allowable value of a parameter at which surge conditions can occur at the optimal speed, wherein said parameter is referable to the flow rate of the air feed stream passing through the centrifugal compressor;
determining a feed back multiplier that when multiplied by the optimal speed will either increase the speed when the parameter is less than the minimum allowable value or will reduce the speed when the parameter is greater than or equal to the minimum value;
during steps of a repeating cycle conducted by the vacuum pressure swing adsorption apparatus while maintaining the optimal speed of the centrifugal compressor other than a pure evacuation step and an evacuation with product purge step thereof, setting a total speed multiplier equal to the feed back multiplier;
during the pure evacuation step and the evacuation with product purge step, calculating the total speed multiplier by multiplying the feed back multiplier by a feed forward multiplier that will increase the speed during the pure evacuation step and the evacuation with product purge step in order to maintain optimal speed of the centrifugal compressor;
calculating an adjusted speed by multiplying the optimal speed by the total speed multiplier; and
generating a control signal referable at least to the adjusted speed and inputting the control signal into the variable frequency drive such that the electric motor and therefore, the centrifugal compressor operates at the adjusted speed.

2. The method of claim 1 , wherein:
the vacuum pressure swing adsorption process utilizes a feed compressor for feeding compressed gas to adsorption beds of the vacuum pressure swing adsorption apparatus and an evacuation compressor for evacuating gases from the adsorption beds; wherein said
evacuation compressor is a centrifugal compressor.

3. The method of claim 1 , wherein:
the vacuum pressure swing adsorption process uses a single compressor to feed compressed gas to an adsorbent bed and to evacuate gases from the adsorbent bed;
the repeating cycle includes a feed with equalization step subsequent to the evacuation with product purge step, a feed with product repressurization step following the feed with equalization step and an equalization step prior to the pure evacuation step; and
during the feed with equalization step, the equalization step and initiation of the feed with product repressurizaton step, the control signal is referable to a non-operational speed such that when the control signal is inputted into the variable frequency drive, electrical power is not applied to the electric motor; and
when a predetermined pressure ratio is obtained during the feed with product repressurization step, the control signal is again referable to the adjusted speed such that the electric motor and therefore, the compressor operates at the adjusted speed.

4. The method of claim 2 , wherein:
the repeating cycle includes a falling pressure equalization step and a rising pressure equalization step subsequent to the evacuation with product purge step;
during the falling pressure equalization step and the rising pressure equalization step, the control signal is referable to a non-operational speed such that electrical power is not applied to the electric motor driving with the evacuation compressor; and
when a predetermined pressure ratio is obtained during the falling pressure equalization step, the control signal is again referable to the adjusted speed such that the evacuation compressor operates at the adjusted speed.

5. The method of claim 1 or claim 2 , wherein:
each time the feed back multiplier is determined, the feed back multiplier is stored;
when the parameter is less than the minimum allowable value, the feed back multiplier is determined by adding to a last stored value of the feed back multiplier a speed correction factor; and
when the parameter is greater than or equal to the minimum allowable value, the feed back multiplier is calculated by dividing the last stored value of the feed back multiplier by a proportionality constant, the proportionality constant set equal to a value greater than 1.0 when the last stored value of the feed back multiplier is greater than or equal to 1.0 or 1.0 when the last stored value of the feed back multiplier is less than 1.0.

6. The method of claim 1 or claim 2 , wherein:
the feed forward multiplier is a function of the pressure ratio;
the function having a maximum value of the feed forward multiplier at a predetermined pressure ratio at which or directly before which the surge conditions occur during a transition between the pure evacuation step and the evacuation with product purge step and decreasing values of the feed forward multiplier at pressure ratios greater than or less than maximum value; and
the maximum valve has a magnitude preselected such that when the maximum value is multiplied by the optimal speed at the predetermined pressure ratio the resulting speed will prevent surge conditions from occurring.

7. The method of claim 6 , wherein the function is a Gaussian function.

8. The method of claim 1 or claim 2 , wherein the parameter is the flow rate of the air feed stream passing through the centrifugal compressor, which is directly measured by a flow transducer, or is determined by measuring the pressure difference measured at two points in a shroud of the centrifugal compressor and that are successively closer to an impeller thereof.

9. The method of claim 5 , wherein:
the parameter is a pressure difference measured at two points in the shroud of the centrifugal compressor that are successively closer to an impeller thereof;
wherein a pressure difference error is calculated and stored by subtracting the minimum allowable value from the a current value of the pressure difference; and
the speed correction factor of the feed back multiplier is calculated during each of the time intervals through proportional integral control comprising adding a proportional term to an integral term, the proportional term calculated by multiplying a gain factor by a difference between the pressure difference error and a prior pressure difference error calculated in a prior time interval and dividing the difference by the time interval and the integral term calculated by dividing the gain factor by an integral reset time and multiplying a resultant quotient thereof by the pressure difference error.

10. The method of claim 8 , wherein:
the feed forward multiplier is a Gaussian function of the pressure ratio;
the Gaussian function having a maximum value of the feed forward multiplier at a predetermined pressure ratio at which or directly before which the centrifugal compressor enters surge conditions during a transition between the e

Assignees

• Rosinski Andrew C
• Belanger Paul W
• Manning Michael S
• Praxair Technology Inc

Inventors

• Rosinski Andrew C
• Belanger Paul W
• Manning Michael S

Classifications

• B01D53/0476 Primary

  Vacuum pressure swing adsorption · CPC title

• B01D2256/18

  Noble gases · CPC title

• F04D27/0261 Primary

  by varying driving speed · CPC title

• B01D2258/06

  Polluted air · CPC title

• B01D2259/401

  using a single bed · CPC title