1 centrifugal compressor surge
Surge is a special phenomenon of centrifugal compressors. Any centrifugal compressor has a maximum working pressure at a fixed speed. Under this pressure, there is a corresponding minimum flow.
When the pressure at the outlet of the centrifugal compressor is higher than this value, surge is generated.
Upon entering the surge range, the compressor cannot produce the same pressure as determined in the discharge line and will pass through the compressor in the opposite direction for a short period of time. After the backflow occurs, the exhaust pressure drops rapidly. After the outlet pressure is reduced, the normal gas supply can be restored.
If the operating conditions cannot be changed quickly, backflow will occur. This repeated process is the surge of the compressor.
When the surge occurs, the unit vibrates strongly, causing damage to the labyrinth seal of the compressor, causing the lubricant to break into the runner and affecting the efficiency of the cooler or condenser. Severe surge can easily cause the rotor to move axially, burn the thrust bearing bush, and in the worst case, the compressor can be damaged, damage the gear box, the motor and the pipeline and equipment connecting the compressor. In order to protect the equipment and ensure the safe operation of the production, the anti-surge control of the centrifugal ammonia compressor is a very important part of the unit control.
2 ammonia compressor original anti-surge control
The ketone-benzene ammonia freezing station ammonia compressor is a centrifugal compressor of BORSIC, and the compressor system control is basically the introduction of foreign technology. The original two anti-surge control valves of the ammonia press: one is the cold flow anti-surge valve, the ammonia ammonia of the ammonia outlet total outlet is cooled, and the ammonia separator is returned to the compressor inlet; one is the heat flow anti-surge valve The ammonia ammonia with a total outlet temperature of 123 is returned directly to the ammonia separator at the compressor inlet. The cold flow anti-surge control valve is based on the suction pressure of a section of the inlet, and automatically controls the opening of the valve to ensure that the suction flow is constant, so that the unit does not surge. The heat flow anti-surge control valve is remotely controlled to ensure that the load changes excessively or the unit is in normal operation when the load is not established during the driving phase.
The above anti-surge control scheme uses an anti-surge control method with a fixed limit flow rate. The control principle is to keep the flow of the compressor always greater than a certain value, so as to avoid entering the surge operation.
The advantage of this control scheme is that the control system is simple, the instrument is used less, and the system reliability is high. The disadvantage is that when the compressor is running at low load, the system is put into operation too early, and the return flow is large, so the energy loss is large.
In order to protect the unit, reduce energy consumption, and avoid frequent switching of the anti-surge valve. In actual operation, the control of the unit is often placed in the manual position when the anti-surge valve is not lower than the design condition. Open the anti-surge valve, and when the unit load is small, the motor power is lower than the rated power of 1250kW, then the anti-surge valve is manually opened to ensure that the unit operates at not lower than the rated condition to prevent surge. In this way, the control of the unit can not achieve automatic and precise control, the unit operation is unstable, and when the ammonia system suddenly rises due to accidents or the evaporation pressure suddenly drops, so that the unit intake air volume is too small, the unit cannot be avoided in time and effectively. Surge, not conducive to safe production.
3 modified ammonia compressor anti-surge control
In order to reduce the energy loss of the compressor, this modification has added the CCC anti-surge control system of the unit. The theory of this control system is an anti-surge control scheme with variable limit flow, which enables the anti-surge regulator to work along a safe operating line on the right side of the surge curve, minimizing the opening of the anti-surge valve and reducing energy loss.
First of all, in order to make the operation as close as possible to the surge point, try to reduce the energy consumption, and ensure that the unit does not surge. CCC changes the inlet or outlet before the test, and the unit interlocking self-protection system is intact. Pressure, artificially make the unit surge, measured the surge point of the three-point unit under different conditions, and connected the three points smoothly, that is, the surge curve of the unit was obtained.
Second, on the right side of the surge line, make a safe operating line that causes the anti-surge regulator to follow a safe operating line group to the right of the surge limit flow. To this end, the first is to obtain a mathematical equation describing this safe operation line, and the second is to implement these operational rules through the instrument to form a feasible control scheme.
CCC uses mathematical simulations to approximate the surge line with mathematical formulas.
The values ​​of a and b are determined by the characteristics of the compressor.
According to the anti-surge protection curve formula, the correlation between the parameters is further analyzed. It is assumed that the flow rate Q1 is measured at the inlet end of the compressor, and the differential pressure is measured as P 1, P
1 = P 1 M / zRT 1 where: P 1 - absolute pressure; M - molecular weight; z-compression factor; R - gas constant; T 1 - absolute temperature;
: C= M / zR , is a constant.
The determination of a and b values ​​is based on the measured anti-surge curve of the unit, leaving a 14% safety margin on the right side, making a completely parallel curve, which is the anti-surge control curve, and then The correspondence between the control curve flow rate and the compression ratio is determined by multiple fittings to determine the values ​​of a and b.
The middle curve b is the simulated anti-surge control curve with a safety margin of 14%. The curve c is the running curve of the unit under the design conditions of the machine-501. Point A is the actual operating point.
This control scheme uses a throttle control valve on the intake line. When the load is reduced and the inlet pressure is reduced, the inlet regulator valve is first throttled. After the suction pipe is throttled, the characteristic curve of the volume flow rate and the compression ratio of the ammonia compressor does not change when the rotation speed is constant. However, as the suction pressure is reduced, the weight flow rate and the exhaust pressure of the ammonia compressor will decrease in proportion to the suction pressure.
With the throttle of the suction pipe, the relationship between the discharge pressure of the ammonia compressor and the weight flow rate will change on the straight line connecting the working point A and the origin, and the surge point, that is, the surge line of the inlet throttle is the connection origin. And the radiation of the surge point measured at a certain throttle opening.
It can be seen that the lower limit of the safety of the flow after the inlet throttling is also reduced, the stable operating range is increased, and the compressor reduces the possibility of surge. When the operating point has been moved to the safe operating line and the gas volume needs to be further reduced, the cold flow anti-surge valve is opened again, so that the sum of the bypass circulation gas volume and the production required gas volume is slightly higher than the surge point flow rate. Larger to avoid the compressor entering the surge range.
If the cold flow anti-surge valve is fully open and the operating point cannot be operated in the safe zone, then open the heat flow anti-surge valve to increase the gas ammonia circulation to avoid surge.
4 main contents of the transformation
(1) Added the Series 4 anti-surge control system of American CCC Company.
(2) According to the requirements of the CCC control system, the unit inlet temperature, the temperature of each section of the outlet, the pressure indicating instrument and the flow measuring instrument of the total outlet are improved. In order to reduce compressor inlet pressure loss, the inlet flow measuring element replaces the original orifice plate with venturi.
(3) Replaced five heat flow anti-surge valves and two cold flow anti-surge valves of the ammonia compressor system.
Whether the CCC control system can effectively avoid the unit surge, the operation of the anti-surge valve plays a very important role. In order to achieve precise adjustment, the CCC control system has special requirements for anti-surge control valves:
The CV value of the anti-surge control valve at the intersection of the compressor characteristic curve and the anti-surge protection line, the selected CV value of the anti-surge control valve is 1. 8 2. 2 times.
The anti-surge control valve requires a control dead zone of less than 1% and a full stroke switching time of less than 2 seconds.
According to the above requirements, the heat flow anti-surge valve and the cold flow anti-surge valve of the five ammonia presses were all replaced with FISHER's EZ series regulating valve.
(4) In order to ensure the control accuracy of the anti-surge valve and expand the control range of the anti-surge control valve, the anti-surge control scheme developed by CCC Company is to first open the cold flow when entering the surge protection curve. Anti-surge valve, if still can not meet the anti-surge requirements, and then adjust the heat flow anti-surge valve, inlet butterfly valve. In this way, while ensuring the control accuracy of the anti-surge valve, the control range of the anti-surge control valve is expanded, and this control scheme cannot be realized in the conventional control system.
5 after the transformation effect
The transformation was carried out in July 1999, and it was successfully driven in September. After continuous operation, the effect was very significant.
(1) The load regulation of the refrigeration station and the anti-surge control of the unit have all realized automatic control. The process parameters of the unit and each part are running smoothly, and the operating status of the unit can be monitored intuitively according to the operation chart of the unit, which reduces the operator. The labor intensity improves the stability and safety of the unit.
(2) The variable limit flow control method is adopted to effectively control the gas ammonia return flow, which greatly reduces the power consumption of the unit and has great economic benefits.
The following is a comparison of operating conditions and power consumption before and after the ammonia compressor system modification of the ammonia refrigeration station.
The average processing capacity of the ketone benzene dewaxing unit in October 1999 was 72. 2t / h. It can be seen from the data in Table ( 2) that the total power consumption of the ammonia compressor after the transformation is 3805 kW h, then the ammonia compressor part The power consumption is 52.7 kW h / t of raw materials.
The average power consumption of the ketone-benzene dewaxing unit was 67.08 t / h, the total power consumption of the ammonia compressor was 3,805 kW h, and the power consumption of the ammonia compressor was 68. 3 kW h / t of raw materials.
7 kW h / t原料 raw materials, the power consumption of the ammonia refrigeration system before the transformation was reduced from 68.3 kW h / t raw materials before the transformation to 52.7 kW h / t raw materials , reduced by 15. 6kW h.
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