Dynamic compression of gas in centrifugal compressors is reached by increase in a polytropic pressure of a gas flow. Such process is described by gasdynamic characteristics of the compressor which are submitted on the two-dimensional diagram of the curve showing operation points of the compressor.
On operation modes of the compressor close to an estimated point (a point And), the gas flow will be approved with a form of elements of flowing part. At an essential deviation of the modes in a flow there are different secondary currents, there are difficult physical processes.
Intersection of the line of a surge (a point In) is followed by high-frequency fluctuations, at the same time there is an intermittent change of an expense from the maximum value to negative (a flow reversal).
POMPAZH is a non-stationary, self-oscillatory operation mode of the compressor with a vibratory frequency of pressure and an expense about 0,5 - 2,0 Hz depending on heat-sink characteristics of a network.
The surge is followed by rapid growth of temperature of gas, emergence of strong pushes and vibrations that can lead to destruction of the compressor. A surge – the inadmissible phenomenon for compressors.
How to be protected from a surge?
Modern management systems compressor units in the arsenal have many different algorithms for protection of the compressor against the pompazhny phenomena. Mathematical models the describing processes proceeding at air compression mortgaged in management systems by compressor units allow to exercise control of executive mechanisms on a curve of a surge of KU (compressor unit), for reduction of operational costs, without damage of mechanical part of KU. In use mechanical characteristics of KU change not for the better. Mat. The model can be adaptive to new characteristics of KU, but it is difficult in implementation. Therefore at a stage to starting adjustment, configure a mat. model under specific KU. But detecting of the beginning of the pompazhny phenomena or the established surge takes place to be in independence of the applied KU management system. Therefore this type of an abnormal stop of KU is present in any SAU (automatic control systems) of KU. For detecting of the pompazhny phenomena many input data are used: change of outlet pressure, temperature, etc.
KU surge detector
In this article I will tell how to detect the pompazhny phenomenon in KU, applying
Let's consider KU K-250.
The centrifugal, multi-stage compressor having intermediate branches to gas coolers.
In an operating mode when KU came to rated characteristics, current of the stator has almost rated value if the engine is picked up without stock for power. During the pompazhny phenomena, outlet pressure increases to greatest possible, for the KU this type then there is an overflowing of compressed air under influence pressure from steps of the higher order to the lowest. At the time of an overflowing loading on an engine shaft sharply decreases, there is a mechanical blow. This moment needs to be detected at an early stage to prevent mechanical damages of KU. Why there are these pompazhny phenomena, will remain beyond the scope of this article.
Let's consider the diagram of current of the stator in an operating mode.
During the begun surge when compressed air flows from the highest step in the lowest, there is "pushing" of the electric motor, at this moment there is a surge in effective value of current, and then in connection with reduction of loading there is a current failure, then the set of a working current, compression proceeds and the cycle is repeated again, up to disappearance of a surge. Diagram of such mode.
On this diagram of fluctuation occur with frequent in 1 Hertz. Such behavior of current of the stator, a direct consequence of the begun KU surge. How programmatically to detect?
Program implementation of anti-stall protection
In an operating mode, current changes smoothly and in small limits. At the time of the beginning or already happened surge, fluctuations have the big amplitude and frequency. Means normal, digital FNCh, it is possible to detect this fault.
For example, time in 100 ms, we will cause function of the filter which formula looks as follows:
Where 0<А<1 коэффициент фильтра. Чем меньше А, тем слабее фильтр.
we Will look at the diagram of such filter.
Now, if we begin a surge, then will look how the filter will behave.
and it is visible to graphics that current crosses the filter several times in the surge mode, but such detector will have false drops in an operating mode. Therefore, it is necessary the filtered signal to displace up and down on value which corresponds to surge amplitude. On an example on 5 A in that and other party.
The operating mode is in range that speaks about a normal operation mode, and the surge already crosses our borders, and it is possible to detect safely a surge on 7-8 intersection and under abnormal condition to disconnect KU. It is possible to go further away and on the first intersection to try to stop a pompazhny status, managing executive mechanisms of a butterfly valve and the pompazhny valve.
On the example of a PLC of Siemens S7-300 I will describe this function.
See video showing work of protection against a surge below.
Along with program implementations on deep throttling of KU on surge border, it is necessary to have abnormal working off already of a beginner or the begun surge in KU.
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