EP0024823A1

EP0024823A1 - Compressor surge control method and apparatus

Info

Publication number: EP0024823A1
Application number: EP80302600A
Authority: EP
Inventors: James Brian Erskine
Original assignee: Imperial Chemical Industries Ltd
Current assignee: Imperial Chemical Industries Ltd
Priority date: 1979-08-22
Filing date: 1980-07-30
Publication date: 1981-03-11
Also published as: JPS5638599A

Abstract

Controlling surging in a compressor by monitoring the high frequency (40 - 500 Hz) fluctuations in a parameter such as the delivery pressure. As surging conditions are approached the amplitude of these fluctuations, generally at a frequency characteristic of the system, progressively increase. When the amplitude reaches a predetermined value, for example about twice the amplitude of the fluctuations at the design maximum compressor throughput, a bypass or vent valve is opened to increase the throughput through the compressor.

Description

This invention relates to a method and apparatus for control of surging in compressors of the rotodynamic type, e.g. centrifugal or axial flow compressors.
As the delivery volume of a fluid compressor drops, for example as a result of decreasing offtake of the compressed fluid, a point is reached when surging of the compressor occurs. Such surging can cause considerable damage to the compressor. In order to alleviate the risk of such damage, the compressor system is normally provided with a vent, or a bypass to recycle compressed fluid back to the compressor intake, and a valve which is opened to allow the fluid to flow through the vent or bypass when surging would otherwise occur. Such a vent or bypass thus allows the flow of fluid through the compressor to be maintained at a safe, i.e. non-surging, level even though the offtake of compressed fluid is below the surge limit.
The conditions under which surging occurs are determined by a number of variables including rotational speed of the compressor, the nature of the fluid, and the design and condition of the compressor. The surge limit, at any given rotational speed, for a new compressor working on a given fluid can be determined during commissioning trials, and hence the point determined at which the bypass or vent valve should be opened.
However, if the fluid composition changes, for example where the compressor is used on a mixture of gases whose proportions may vary, for example as in the manufacture of some chemicals such as ammonia or methanol, the predetermined conditions at which the bypass or vent valve opens may no longer be applicable to prevent surging.
Likewise the characteristics of the compressor may vary as the condition of the compressor deteriorates with continued use. Again the predetermined conditions at which the bypass or vent valve opens may no longer be applicable to prevent surging.
For these reasons the surge relief valve is generally set to open well below the onset of surging, in order to cater safely for such variations in the working medium and/or condition of the compressor. This often results in uneconomic operation with the surge relief in operation far more than is necessary.
In some compressor systems these problems have to some extent been overcome by arranging for the vent or bypass valve to open as surging actually commences. Thus, as described in United Kingdom Patent Specification 944714, a rapid change in pressure, which is indicative of surging, in the delivery line caused the bypass valve to open. However in such a system surging has already commenced before the bypass valve is opened to alleviate surging and so there is a risk that damage may occur before the opening of the bypass valve stops the surging.
The present invention provides a method and apparatus for controlling surging wherein conditions indicative that surging is about to commence, i.e. incipient surging conditions, are utilised to actuate the bypass or vent valve.
Since the surge control arrangement automatically caters for such changes in the working medium and condition of the compressor, the latter can be operated more economically with the surge reliefe valve only opening when surging is imminent.
During normal operation of a compressor, small fluctuations occur in various parameters of the compressor system at various frequencies. For example, such fluctuations may occur in the pressure, flow rate and/or temperature of the compressor output (or the output of each stage where the compressor is of the multistage type), the rotational speed, and reaction torque. Also certain parts of the compressor are subject to mechanical vibration. These fluctuations have frequencies that are considerably higher than the surge frequency. Thus, while the surge frequency may be of the order of 0.1 to 1 Hz, fluctuations in various parameters may occur at higher frequencies, generally within the range 40 - 500 Hz, during normal operation. Such fluctuations are herein referred to as high frequency fluctuations.
The present invention stems from the observation that with some such parameters, as surge conditions are approached, the amplitude of those fluctuations having a certain frequency or range of frequencies, herein termed the characteristic frequency, increases markedly. At the same time fluctuations at other frequencies may change only by a little or not at all.
A significant increase in amplitude of the fluctuations at the characteristic frequency occurs well before the onset of surging so that this change in amplitude may be utilised to operate the bypass or vent valve without surging conditions actually occurring.
Accordingly we provide a method of controlling surging in a rotodynamic fluid compressor comprising monitoring high frequency fluctuations, that exhibit a significant increase in amplitude as surge conditions are approached, in a parameter of the compressor system, generating a signal when the amplitude of said fluctuations reaches a predetermined value, and causing said signal to actuate the opening of a surge relief valve to increase the flow of fluid through the compressor.
The high frequency fluctuations that are monitored are preferably ones in which the amplitude of said fluctuations when surging commences is at least 50% greater than the amplitude of such fluctuations at the maximum design fluid throughput.
Preferably the surge relief valve is caused to open when the amplitude of the monitored high frequency fluctuations is at least 75% of the amplitude of such fluctuations at the onset of surging.
The present invention also provides apparatus comprising a rotodynamic fluid compressor having a delivery line, a vent or bypass connected thereto via a surge relief valve, which, when in the open position, permits the flow of fluid through the compressor to increase, means for sensing high frequency fluctuations, that exhibit a significant increase in amplitude as surge conditions are approached, in a parameter of the compressor system, means for generating a signal when the amplitude of said high frequency fluctuations reaches a predetermined value, and means, actuated by said signal, for opening the surge relief valve.
Preferably the parameter whose high frequency fluctuations are monitored is the pressure of the fluid in the output, e.g. delivery line, of the compressor.
Where a multistage compressor is utilised the high frequency fluctuation of the pressure of the fluid output of each stage may be damped in succeeding stages and so, to detect incipient surging conditions in the stages of the compressor prior to the final stage, it is preferred that the high frequency pressure fluctuations of the outputs of each stage are monitored and a signal is generated to cause the surge relief valve to open when the amplitudes of any of said monitored fluctuations reach predetermined values. It will be appreciated that the predetermined values for the amplitude at which the surge relief valve opens may differ from stage to stage.
Under normal working conditions, the amplitude of the high frequency fluctuations in output pressure of each stage of a compressor is relatively small, typically of the order of 0.5 to 1 bar at the maximum design fluid throughput at any given compressor speed. The amplitude of these fluctuations may increase by 100% or more as the surge conditions are approached but generally are far smaller than the overall variations in pressure occurring under surge conditions (which may be of the order of 20 bar).
Trials on the compressor system will indicate which parameters have high frequency fluctuations that increase significantly in amplitude as surging conditions are approached and so a suitable parameter can be selected for monitoring. Such trials will also indicate the frequency, or band of frequencies, that should be monitored.
The overall fluctuations occurring in the selected parameter may represent the sum of fluctuations of various frequencies; at one or more of these frequencies the amplitude increases markedly as surge conditions are approached. The sensing means to detect the high frequency fluctuations of the selected parameter may be responsive to fluctuations only at those frequencies which exhibit a significant increase in amplitude as surge conditions are approached, eg the characteristic frequency, or may be responsive to fluctuations over a much wider frequency range, including fluctuations whose amplitude does not change significantly as surge conditions are approached. In the latter case, the output from the sensing means may be processed, eg electronically, to select only that corresponding to the fluctuations having the characteristic frequency or frequencies within a band encompassing the characteristic frequency.
The signal generating means may produce a signal only in accordance with the amplitude of those fluctuations having the characteristic frequency. However as the characteristic frequency for any given compressor system may vary somewhat depending on the operating speed, compressor conditions, and fluid nature, it is preferred that the sensing means and signal generating means are arranged to respond to frequencies within a band of frequencies wide enough to contain the characteristic frequency under any anticipated working conditions. The signal may be generated in accordance with the amplitude of a dominant frequency or frequencies picked out from the monitored frequency band: in this way the detected change in amplitude of the pressure fluctuations may be enhanced.
The valve actuating signal may be generated only when the amplitude of the monitored high frequency fluctuations reaches a predetermined value, which may be an absolute value or a ratio to the amplitude at a specified fluid throughput, e.g. the maximum design fluid throughput, or signals indicative of the amplitude of the fluctuations changes may be produced continuously. In the latter case, the signal may be utilised to open the surge relief valve progressively.
The compressor is preferably a rotary centrifugal gas compressor.
The invention is illustrated by referenct to the accompanying drawing which is a diagrammatic representation of apparatus according to the invention.
In the drawing the apparatus comprises a centrifugal gas compressor 1 having a suction, i.e. inlet, line 2 and an outlet, ie delivery, line 3. A bypass 4 is provided between the delivery line 3 and the inlet line 2 and this bypass is provided with a valve 5. Provided on the delivery line 3 is a sensor 6 capable of detecting variations in pressure in the delivery line 3 having frequencies within the range 40 - 500Hz. A signal is fed from sensor 6 varying in accordance with the amplitude of the high frequency pressure fluctuations to a control unit 7 which is arranged to actuate valve 5 when the amplitude of said pressure fluctuations reaches a predetermined level.
In a practical example, a centrifugal compressor operating at 11000 rpm on methanol synthesis gas had a maximum design gas throughput (inlet gas flow rate) of 9750 m³/rœ (actual) giving, at said maximum design gas throughput, a pressure difference between the inlet and outlets of 30 bar. The characteristic high frequency fluctuations in delivery pressure occurred at a frequency of 387 Hz and, at the maximum design gas throughput, these fluctuations had an amplitude of 0.7 bar. At a gas throughput of 9000 m³/hr the amplitude of the fluctuations was 1.05 bar while at a throughput of 8250 m³/hr the amplitude was 1.4 bar. Surging commenced at a throughput of about 8000 m³/hr and so the signal from sensor 6 was arranged to actuate the bypass valve to open it to increase the gas throughput through the compressor when the pressure fluctuation amplitude reached 1.35 bar, i.e. 1.95 times the amplitude of the fluctuations at the design maximum gas throughput.

Claims

1. A method of controlling surging in a rotodynamic fluid compressor comprising monitoring high frequency fluctuations, that exhibit a significant increase in amplitude as surge conditions are approached, in a parameter of the compressor system, generating a signal when the amplitude of said fluctuations reaches a predetermined value, and causing said signal to actuate the opening of a surge relief valve to increase the flow of fluid through the compressor.

2. A method according to claim 1 wherein the high frequency fluctuations have frequencies within the range 40 to 500 Ez.

3. A method according to claim 1 or claim 2 wherein the high frequency fluctuations monitored are ones in which the increase in amplitude when surging commences is at least 50% greater than the amplitude of said fluctuations at the maximum design throughput.

4. A method according to any one of claims 1 to 3 wherein the signal to open the surge relief valve is generated when the amplitude of the monitored high frequency fluctuations is at least 75% of the amplitude of those fluctuations at onset of fully developed surging.

5. A method according to any one of claims 1 to 4 wherein high frequency fluctuations in the pressure of the fluid output of the compressor are monitored.

6. A method according to claim 5 wherein the compressor is a multistage compressor and the high frequency pressure fluctuations of the outputs of each stage are monitored and the signal to open the surge relief valve is generated when the amplitude of the high frequency pressure fluctuations of the fluid output of any one of said stages reaches a level predetermined for that stage.

7. A method according to any one of claims 1 to 6 wherein the surge relief valve actuating signal is generated only in response to fluctuations having frequencies whereat the amplitude increases significantly as surge conditions are approached.

8. A method according to any one of claims 1 to 6 wherein the surge relief valve actuating signal is generated in accordance with the amplitude of fluctuations having frequencies within a band of frequencies.

9. A method according to claim 8 wherein the signal is generated in response to the amplitude of a dominant frequency within said band.

10. A method according to any one of claims 1 to 9 wherein a signal indicative of the amplitude of the high frequency fluctuations is produced continuously and is arranged to open the surge relief valve progressively.

11. Apparatus comprising a rotodynamic fluid compressor having a delivery line, a vent or bypass connected thereto via a surge relief valve, which, when in the open position, permits the flow of fluid through the compressor to increase, means for sensing high frequency fluctuations, that exhibit a significant increase in amplitude as surge conditions are approached, in a parameter of the compressor system, means for generating a signal when the amplitude of said high frequency fluctuations reaches a predetermined value, and means, actuated by said signal, for opening the surge relief valve.