PetroChina Tarim Petrochemical Branch has an annual output of 45 × 104t ammonia, 80 × 104t urea project. The project of synthetic ammonia plant air compressor technology used by Germany AtlasCopco production of multi-axis centrifugal air compressor, the driving equipment for the steam production of medium pressure steam condensing steam turbine. The project put into normal production in May 2010, December 28, 2011, in the process of driving the device, the multi-axis centrifugal air compressor surge valve suddenly wide open three times, so that the second furnace cut off the air, resulting in addition to A section of furnace and decarbonization of all devices except parking. In response, China Petroleum Tarim Petrochemical Branch set up a survey team to investigate and analyze the accident, find out the influencing factors and put forward solutions. 1 multi-shaft centrifugal air compressor Brief 1.1 Process Description PetroChina Tarim Petrochemical Branch multi-axis centrifugal air compressor driven by a medium pressure steam injection condensing turbine for ammonia fertilizer plant, for the second furnace to provide combustion air conversion . Compressors with a speed increase gearbox, an intermediate gearbox installed between the speed increase gearbox and the turbine to reduce the input speed of the compressor gearbox. The compressor is divided into two sections, one section consists of four levels of compression, two sections of two levels of compression. During the normal operation, a section of the air passing through the filter to the compressor is compressed to 1.42 MPa (a), and then is cooled to 34.5 ° C through a section of water cooler, a part of the air is drawn out as Instrument air and urea anti-corrosion air use, the other part of the compressor into the second paragraph, was compressed to 3.82MPa (a), into the second furnace. Compressor condition monitoring is shown in Figure 1. Figure 1 Compressor Condition Monitoring 1.2 Compressor Control Description During operation, the first and second stage inlet guide vanes (IGV) of the compressor are used to control the flow and pressure of the compressor, such as increasing the compressor flow and outlet pressure Vane opening, on the contrary reduced. If the impeller flow stagnates, surge occurs, it will cause great stress on the mechanical components of the compressor, it requires the actual compressor flow must be greater than the minimum volume flow. In operation, the surge relief valve opens automatically before the compressor reaches the minimum flow rate. To ensure safe operation of the compressor away from the surge line, an anti-surge control system is installed in the DCS to prevent it Vibration relief valve open, closed, in order to avoid the compressor surge occurred. To ensure stable operation of the compressor, the multi-axial compressor is equipped with a first and second anti-surge vent valve, by the surge controller to automatically control anti-surge vent valve. The dividing line between stable and unstable operation is called surge line, which depends on many factors such as temperature, pressure and so on. The surge line to leave some safety margin, the safety line called the vent line, which is parallel with the surge line, the safety margin is 8% of the design flow to ensure the compressor stable and safe operation, See Figure 2. Figure 2 Compressor surge control line Anti-surge vent valves and inlet guide vanes control compressor flow. Compressor 30% of the rated flow by a guide vane control, that is through the guide vane compressor rated flow can be reduced from 100% to 70%, if the flow needs to be further reduced, you can only rely on regulation of a surge vent valve The degree of opening to achieve. When the compressor flow rate> 70%, the surge vent valve will automatically shut down completely, when a surge vent valve is closed, a section of the guide vane will begin to open from the minimum operating position. 38% of the rated flow of the second stage of the compressor is controlled by the second-stage guide vane adjustment, that is, the rated flow can be reduced from 100% to 62% through the guide vane adjustment. If the flow rate needs to be further reduced, Valve opening to achieve, when the compressor flow rate of> 62%, the surge valve will be fully closed automatically when the two paragraph surge valve is closed, the two guide vanes will begin to open from the minimum operating position. Compressor flow through the guide vane adjustment to achieve, that is, a guide vane inlet compressor control a section of flow, five inlet guide vane control compressor Sec flow. In actual operation, in order to prevent the compressor surge, while under extreme conditions can quickly open the surge vent valve, the compressor surge controller requires setting in automatic mode, according to AtlasCopco compressor manual can be found, a period of asthma Set the flow rate is 3450m3 / h, the second paragraph of the surge flow setting is 1750m3 / h, when the compressor flow is greater than the surge flow set point, the surge vent valve automatically shut down, when less than the surge flow setting Value, surge relief valve automatically opens. For the first and second stage of the compressor outlet pressure, set up a safety controller, its role is to prevent the compressor outlet pressure is greater than the first paragraph of the maximum allowable value, which is controlled by a programmable logic controller (PLC), runtime Should be set to automatic mode. Under extreme operating conditions, that is, when a section of outlet pressure> 1.5MPa or second stage outlet pressure> 4.2MPa, the first and second stage surge vent valve will instantly open fully and vice versa. When the turbine speed reaches the minimum speed> 5743r / m, in full automatic mode, the compressor can be loaded, gradually increasing the setting value of the outlet pressure in the first and second stages on the operation screen. At this time, the surge vent valve Will be gradually closed, when fully closed, the inlet guide vane was gradually opened to complete the loading. Need to uninstall, reduce the first and second paragraph of the outlet pressure settings, when the guide vane began to gradually shut down, when fully closed, the surge vent valve began to gradually open to complete the unloading. When the turbine speed <5743r / m, the solenoid valve out of power, one or two surge relief valve vent momentarily open, the moment all-inclusive. Suddenly full open 2 surge relief valve factors and countermeasures Through the above analysis we can see that the operation of the compressor caused by the first and second stage surge valve vent open instantaneously there are two factors: a section of outlet pressure> 1.5MPa or second-stage outlet pressure> 4.2MPa; steam turbine speed <5743r / m. During the driving of the ammonia plant, December 28, 2011, a multi-shaft centrifugal air compressor appeared three times in a day when the surge vent valve suddenly opened and the second stage of the furnace cut off the air, resulting in all the devices except a furnace and the decarbonization unit parking. 2.1 Analysis 2.1.1 The first surge relief valve fully open December 28, 2011 morning 01:04:36 Syngas compressor Turbine TK431 in the extraction of steam, the multi-axis centrifugal air compressor first and second paragraphs Surge relief valve suddenly opened, resulting in load rejection unit, the operating status shown in Figure 3. Figure 3 compressor operating conditions can be seen from Figure 3, when the turbine speed down to 5667.6r / m when its speed has been <5743r / m, reached the compressor surge vent valve suddenly open condition, surge vent valve open, due to Sudden load rejection compressor, turbine speed rise, after 1m its speed up to 6330r / m, but did not rise to the turbine electronic trip speed 6645r / m, so the turbine did not jump. The medium pressure steam pipe network pressure and temperature and multi-axis centrifugal air compressor turbine speed state trend out, found that in the 01:05:27 pressure steam pipe network steam temperature dropped from 392 ℃ 258 ℃, The turbine speed dropped to 5609r / m, see Figure 4. Due to the pictures taken on different computer screens of FIG. 3 and FIG. 4, there is a certain time error. Figure 4, the pressure of steam, temperature and turbine speed out of the synthesis gas compressor steam turbine TK431 extraction temperature history found that 01:05:20 syngas compressor turbine TK431 is cast extraction steam system, before the extraction of steam , The steam temperature in the extraction line is 126.5 ° C, and the temperature is 258.96 ° C after extracting the steam, as shown in FIG. 5. Operators have found that the extraction of steam, the temperature gradually decreased, it is difficult to control, it can be concluded that there is water leakage valve TV7046 leakage, the leakage will increase the difficulty of heating pipes. Figure 5 TK431 steam turbine extraction temperature conditions can be obtained above, the synthesis gas turbine compressor TK431 before the extraction of steam, due to the operator is not well on the extraction pipe heating pipe, coupled with the reduction of water leakage valve, an increase of warm pipe Difficulty, in the steam injection will be a lot of condensate in the extraction line into the medium-pressure steam pipe network, the steam pipe network temperature quickly dropped to 258 ℃, the steam temperature drop led to multi-axis centrifugal air compression Machine turbine speed rapidly dropped from 5860r / m to below 5743r / m, causing the compressor one or two surge surge vent valve suddenly opened. 2.1.2 second surge vent valve fully open compressor after the first surge vent valve opens the second section of the furnace off the air, and then the system chain reaction caused symplectic compressor trip, then multi-axis centrifugal 01:40:20 Multi-axis centrifugal air compressor pressure to lift the load, after 00: 1:25 The compressor first and second stage surge vent valve suddenly open, see its operating conditions see Figure 6. Can be seen from Figure 6, the compressor lift load, the operator gradually closed paragraph, paragraph 2 surge vent valve, a rapid rise in outlet pressure. In this regard, the operator immediately opened a surge relief valve, but still did not curb the compressor section of the export pressure rise, 01:41:45 section of the outlet pressure reached 1.555MPa, the pressure has been> 1.5MPa, reached the compressor Vibration empty valve suddenly open condition, surge vent valve open, due to a sudden load rejection compressor, turbine speed rapid rise, its speed reached 6641r / m, turbine electronic trip. Figure 6 Compressor operating conditions Calling multi-axis centrifugal air compressor outlet valve FV2011 opening situation, the compressor outlet flow and turbine speed status trend found in the first multi-axis centrifugal air compressor surge vent valve After opening, the compressor load rejection, the outlet valve FV2011 then closed, then the valve has been closed, shown in Figure 7. At the same time opened the compressor outlet vice line valve opening trend and found that the valve has also been closed. Figure 7 outlet valve FV2011 opening and turbine speed The above analysis available, multi-axis centrifugal air compressor in the load pressure, the outlet valve FV2011 and vice-line cutting has been in the closed state, the pressure caused by a pressure outlet section of the compressor The operator immediately opened a surge vent valve, however, the pressure continued to rise to 1.5 MPa or more, resulting in the compressor first and second stage surge vent valve suddenly opened, followed by the turbine speed quickly reached the electronic trip speed and jumped . 2.1.3 The third surge vent valve fully open December 28, 2011 20:18:12 Syngas compressor Turbine TK431 In the extraction of steam, the multi-axis centrifugal air compressor first and second stage surge vent valve Suddenly opened again, causing the load shedding unit. Through the investigation and analysis, the reason is the same as the reason of the first full opening of the surge relief valve. Both of them are due to the insufficient heating pipe when the steam of the synthesis gas compressor is withdrawn and the condensate is instantly brought into the medium pressure steam pipe network Pipe network steam temperature dropped significantly, resulting in multi-axis centrifugal air compressor turbine speed down to below 5743r / m, resulting in surge vent valve suddenly opened. 2.2 Measures to be resolved In order to prevent such accidents from happening again, to make the following operational requirements: a) Before the synthesis gas compressor turbine extraction steam, the operator should be fully warm pipe, open the drain to drain the net condensate, at its temperature> After 280 ℃ [4-5], before steaming. b) In order to reduce the difficulty of warm pipe, repair the water reducing valve TV7046 to avoid its internal leakage. c) When the compressor lift load, the compressor post and conversion positions to fully communicate and contact, to prevent the compressor outlet valve FV2011 or deputy line valve is not opened in time, causing the compressor pressure. The implementation of the above measures effectively prevented the serious consequences of the sudden opening of the compressor surge valve caused by such reasons effectively and ensured the long-period operation of the device. 3 Conclusion Multi-axis centrifugal air compressor within three days of a sudden surge valve open accident investigation conducted in-depth investigation and analysis to identify the cause of the accident and take appropriate measures to address: In order to prevent the synthesis gas compressor turbine in the vote Extraction steam caused when the steam pipe temperature is low and affect the multi-axis centrifugal compressor air compressor speed, which led to open the surge valve, requiring full heating before the extraction of steam, check the operating status of the water reducing valve; to prevent Multi-axis centrifugal air compressor outlet is too high and cause surge valve to open, requiring compressor positions and conversion positions to work closely with the timely opening of the compressor outlet valve FV2011 or deputy line valve to avoid such problems occur again.
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