Measures to deal with abnormal discharge pressure of cracked gas compressor drive turbine of millions ton ethylene plant

The exhaust from the turbine enters the water receiver and condenses. The heat released is removed by the cooling water and the volume of the steam shrinks dramatically as it cools. There is always a certain amount of air mixed in with the steam, which is left in the water receiver. As the pressure in the rehydrator is lower than atmospheric pressure, the external atmosphere may also leak in gradually from the pipes, flanges and other inaccessible places, gradually accumulating, causing the pressure in the rehydrator to rise and the vacuum level to decrease. Therefore, there must be a pumping device, the air and steam that has not condensed into water, in order to maintain the vacuum in the water reducer, the vacuum for the exhaust pressure.

Exhaust pressure of the turbine in operation abnormal problems:

(1) The turbine (GBT201) exhaust pressure fluctuates from time to time during operation, causing potential problems for the normal operation of the plant.

(2) The turbine exhaust pressure rose from -80kPa to -53kPa during operation, and was maintained at -67kPa after the start-up jet pump and the backup jet pump were activated on site. Stopping.

(3) The turbine stopped due to the sudden closure of the steam pumping valve during the operation of the turbine. After restarting, the discharge pressure was at -53kPa, which could not meet the normal production of the device.

(4) During the operation of the turbine, the internal operation found that its shaft seal steam pressure PI22002 dropped from 8kPa in normal operation to 4kPa, and then the shaft seal steam pressure PIC22002 suddenly dropped to 0kPa. Thereafter, the discharge pressure gradually rose from -80kPa to -64kPa.

Measures taken:

(1) Examine the irregular fluctuations of the turbine exhaust pressure.

① Check the cooling water pressure and find that the pressure in the main cooling water pipe is relatively stable.

② Check the seal steam and find that the seal steam pressure is relatively stable.

③ Check the injection pump. The performance of the jet pump was tested by closing the non-condensable gas valve and the jet pump worked normally.

④ Checked the atmospheric safety valve water seal and the return water from the water seal was normal.

⑤ Check the site installation location. The site was led by a pilot pressure point with three branches corresponding to three pressure transmitters, and the pilot pressure point had a low point of liquid accumulation. After finding the problem, the pilot pressure line was modified during the stopping and maintenance of the unit, and the problem of irregular fluctuations in turbine exhaust pressure was solved.

(2) For the turbine in operation, the exhaust pressure rises to -30kPa triggering the GB201 interlock stop, the measures taken are

① Check the surface condenser liquid level. There is a small fluctuation in the liquid level, but no obvious rising trend.

② Check the surface condenser for internal leakage resulting in poor heat transfer. As the amount of condensate fed out was stable, it was judged that no major internal leakage had occurred.

③ Checked whether the cooling water temperature was too high resulting in poor heat exchange and found that the cooling water temperature was normal.

④ Check the injection pump. The performance of the jet pump was tested by closing the non-condensable gas valve and the jet pump was found to be working normally.

⑤ Check the atmospheric safety valve water seal, and the water return from the water seal is normal.

⑥ Check whether the water volume of the turbine compound is too large, and the steam inlet and steam pumping volume are more stable and without fluctuation.

⑦ The upper exhaust valve of the cooling water of the rewinder was discharged and a large amount of gas was found, and a sample analysis was made and the gas was air. The only cooling water heat exchanger in the unit with air exchange was the waste alkali oxidation unit, which was inspected and found that the waste alkali oxidation compressor (GB815B) had an internal leak in the four section outlet cooler and air was leaking into the cooling water system. Air formed a gas barrier at the EA215 cooling water outlet line, causing the EA215 to fail in heat transfer, which in turn led to a rise in discharge steam temperature and pressure and an interlock stop. After discovering the problem, the GB815B was switched to the A-unit, and the exhaust pressure was stabilized after the air was removed from the cooling water side of the rewinder and restarted.

(3) Measures taken for the exhaust pressure (-53kPa) could not be reduced.

① Check the level of the rehydrator, the water seal of the atmospheric safety valve, the operation of the jet pump, the temperature and pressure of the cooling water, and no abnormalities were found.

② After communicating with peer companies, a manufacturer had a case of leakage in the bellows connecting the turbine to the rehydration water. An inspection of the bellows revealed the presence of cracks (see picture 2) and it was decided to use carbon fibre to eliminate leaks online.

(4) In response to the problem that the shaft seal steam pressure PI22002 dropped from 8kPa in normal operation to 4kPa, followed by a sudden drop in the GBT201 shaft seal steam pressure PIC22002 to 0kPa and a gradual increase in the discharge pressure from -80kPa to -64kPa, the following measures were taken.

① external operation site gradually open the shaft seal steam pressure control valve PIC22002 regulating valve bypass, until the bypass valve is fully open, the problem is not solved.

② Add 1 medium pressure steam line to the low pressure steam line to increase the amount of sealed steam, but the problem was not solved.

③ By taking the method of opening the regulating valve bypass and sealing steam pipeline wiring the problem was not solved and the technician judged that the internal pipeline of sealing steam might be broken. Due to the turbine in operation, in order not to affect the positive generation, online new with steam line to the turbine seal cavity of the reserved port, wiring after sealing steam pressure although not seen to improve, but the exhaust pressure gradually to -80kPa, has been maintained until the next maintenance cycle. During the major overhaul of the internal piping of the sealed steam, it was found that the internal piping had been disconnected. In order to avoid the recurrence of such accidents, the piping was re-welded and the piping support was increased to reduce the vibration of the piping, which is now operating stably after the treatment.