IOSR Journal of Mechanical and Civil Engineering (IOSR-JMCE). e-ISSN: 2278-1684,p-ISSN: 2320-334X, Volume 14, Issue 3 Ver. VII. (May - June 2017), PP 13-18. www.iosrjournals.org
Abstract: Muara Tawar block V is constructed by PT. PLN (Persero) Power Plant Master Project ofJava, Bali and Nusa Tenggara Networks based on contract No. 261. PJ / 041 / DIR / 2007 (Concerning Gas-Fired Power Plant Extension Project). The plant uses gas turbine type 13E2. The operational concept of gas turbine type 13E2 is regulating the amount of fuel that is divided from three fuel control valves and the operating pattern is in an area known as the pilot valve and premix valve which must be maintained the stability of combustion shown by pulsation parameters. The prefill concept is used to avoid flame off when gas turbine loading often passes over point switch or back point switch. Prefill itself will momentary activate a pilot valve to fill the Fuel Distribution System (FDS) line from MBP43. However, prefill gives the effect of high pulsation that triggers Gas Turbine experience PLS or derating. This effect can be overcome by improving the prefill concept and modifying the prefill system through Logic Advant, resulting in a more stable burning in the gas turbine burner.
Keywords: Gas-Fired Power Plant, FDS, pilot valve, pulsation, prefill.
I. Introduction
The utilization of gas turbines in power generation has increased in recent years and it will be increased up to a medium level of application [1]. Muara Tawar Block V gas turbine system was designed to operate at peak or under load continuously in both open and combined cycles as per network requirement. Muara Tawar Block V power generation can generate 242.5 MW electricity (gross output). It comes from the gas turbine generator alone about 161.5 MW and from the steam turbine is about 81 MW [2]. The reliability of a power plant to
provide electricity to the electricity network is the most important task to ensure energy availability [3]. Gas Turbine type 13E2 consist of three fuel control valves to maintain mass and energy balances in the burner system under the pilot and premix valve. To maintain stable combustion, changes in the operating condition between the pilot valve and the premix valve are required and it's known as switch over and switch back point. During these conditions, the pulsation parameter should be monitored. If the gas turbine operates in high load or above switch over point continuously, it tends pilot valve MBP43 cannot provide sufficient gas fuel to the combustion system. Moreover, during the load decrease, the operating condition will change to a switchback point. It condition leads to mass unbalance and it can be very dangerous because the pilot valve must close immediately. Moreover, in this condition, the pilot valve status is closed due to operation in high load. This condition triggers flame off resulting in the unit trip. The prefill concept is used to overcome this hazard and unscheduled shut down by the pilot valve in the MBP43 area pulse opening. However, prefill can also affect the emergence of high pulsation that resulted in gas turbines experiencing PLS or derating.
The utilization of gas turbines in power generation has increased in recent years and it will be increased up to a medium level of application [1]. Muara Tawar Block V gas turbine system was designed to operate at peak or under load continuously in both open and combined cycles as per network requirement. Muara Tawar Block V power generation can generate 242.5 MW electricity (gross output). It comes from the gas turbine generator alone about 161.5 MW and from the steam turbine is about 81 MW [2]. The reliability of a power plant to
provide electricity to the electricity network is the most important task to ensure energy availability [3]. Gas Turbine type 13E2 consist of three fuel control valves to maintain mass and energy balances in the burner system under the pilot and premix valve. To maintain stable combustion, changes in the operating condition between the pilot valve and the premix valve are required and it's known as switch over and switch back point. During these conditions, the pulsation parameter should be monitored. If the gas turbine operates in high load or above switch over point continuously, it tends pilot valve MBP43 cannot provide sufficient gas fuel to the combustion system. Moreover, during the load decrease, the operating condition will change to a switchback point. It condition leads to mass unbalance and it can be very dangerous because the pilot valve must close immediately. Moreover, in this condition, the pilot valve status is closed due to operation in high load. This condition triggers flame off resulting in the unit trip. The prefill concept is used to overcome this hazard and unscheduled shut down by the pilot valve in the MBP43 area pulse opening. However, prefill can also affect the emergence of high pulsation that resulted in gas turbines experiencing PLS or derating.
II. Theory
Prefill is used to reduce the disturbance that may arise due to the changing of electricity load from high to low load and vice versa. Prefill is used to fill up the volume between Fuel Distribution System (FDS) to the burner [4]. This prefill is performed by opening the pilot valve or MBP43 at a certain time to ensure the volume of the FDS until the burner fills up with approximately 20% of additional fuel. Mass unbalance occurs when the load is over the SwitchBack Point (SBP) area. Therefore, the prefill on the FDS pilot will not cause the flame off when SBP is present.
The prefill will be activated when the decrease in electricity load occurs. The prefill system will open the MBP43 control valve from the minimum mass flow rate and increase up to the maximum mass flow rate gradually to fill up the volume between FDS to the burner. The gas turbine at Muara Tawar Block V mass flow rate was designed at 0.285 kg/s in maximum load and 0.250 kg/s in minimum load [5]. The prefill process will stop if any of the criteria are reached i.e. Delta TAT (Temperature After Turbine difference) reaches 5 deg C or high pulsation reaches 35 mbar, or maximum prefill time has reaches 10 seconds.
III. Method
3.1 Analyze DEPP Prefill System of Gas Turbine
Logic prefill system is simulated under Advant Egatrol 8 software. Simulation on the logic includes the operation of pilot valve MBP43 and the operating pattern of the prefill system.
Fig. 2. describes prefill system, where prefill will be enabled or start (PltPrfSqPrel_START) with 2
modes i.e. periodically time (PltPrfSeqCyc_START) or decrease in load. Every 4 hours, the gas turbine will experience periodic prefill at any load that causes interruption to Protective Load Shedding (PLS). Interruption to PLS by high pulsation is caused by an over mass flow rate from MBP43 or pilot valve.
Fig. 3. shows the high pulsation which resulted in PLS 148 mbar. High pulsation occur because MBP43
(number 4) is activated and mass flow rate or gas fuel flow rate of 0.250 kg/s up to 0.285 kg/s during 10 seconds. This logic set can be observed in Fig. 4. Periodic prefill is set up for every 4 hours which often results in high pulsation. Total mass flow rate can be obtained in two ways i.e. the mass flow rate on the scale is too large or the setting time is too long for each prefill process.
Based on the investigation of the logic prefill compared to the DEPP data trending it is necessary to make some parameters modification that will affect the prefill process as well as Prefill operation pattern. Improvement of the prefill logic is done in several stages. The first stage is to modify the maximum time for each prefill (TIME_MAX_PRF_C1). The maximum time for prefill is gradually lowered and applied directly when the gas turbine operates by monitoring DEPP trending data continuously.
Based on Table 1. It can be seen that a decrease in setting time will reduce the pulsation that occurs in the burner. However, the maximum limit of changes is shown by the absence of diff TAT. Diff TAT indicates that the prefill has perfectly filled the FDS. The maximum prefill time decrease gradually and reaches the optimum value of 7 seconds. However, a change in time setting is not the best choice because the gas turbine is still experiencing PLS due to prefill for a while. The next step is decreasing the pilot valve mass flow rate value in logic prefill shown by Fig. 6. The mass flow rate value decreases gradually while pulsation pattern and changes in TAT are continuously observed.
Changing of pilot valve mass flow rate parameters compared to the pulsation values can be observed in Table 2.
Any decrease in mass flow rate will result in a decrease of high pulsation value. However, the maximum limit of changes is indicated by the absence of diff TAT. Change in mass flow rate value obtained minimum high pulsation at the point of 0.2 kg/s for both minimum and maximum mass flow rate.
IV. Results And Discussions
Prefill system on turbine gas will help avoid flame off when there is a change in the area of switch over or switchback point. However, the active prefill will cause high pulsation so PLS or Protective Load Shedding will be active. PLS will cause turbine gas load to drop until a normal condition is reached and PLS is reset.
In Fig. 7. There is an active PLS signal from signal number 7 that is 51MBX41EA000_XU03 PLS 2oo3 or PLS 2 out of 3 active signals. PLS is active because of signal high pulsation number 9, 51MBM30AX010Breach 148 mbar. High pulsation reaching 148 mbar would cause PLS. PLS always happens when prefill process takes place. The total event for 2012 is 52 times starting from February 10, 2012 and the last event is November 7, 2012. High pulsation results in PLS because Prefill occurs in an area of about 60% to 65% Relative Power or approximately 112 MW. The operating load in 112 MW is the demand from the network at certain times. However, if prefill operates at a relatively high load (above 120 MW), then the prefill process will not cause high pulsation because the proportion of premix is so large that the effect of the prefill is relatively small. Using this information, logic improvements are needed by modifying the enabled function
PLT_PREFIL_CYC_EN, so when turbine gas operates at a load the cyclic prefill will not be active. This function can be activated when a gas turbine operates with a relatively high load. When under load, the prefill will be active when the gas turbine experiences a minimum load loss of 0.35 MW/s for 10 seconds. Fig. 10. show prefill process after modifying function has been done. Prefill occurs when the Gas Turbine experiences a decrease in load from 143 MW to 113 MW within 2 seconds or a decrease of 15 MW per second. This prefill occurs optimally by being marked by a small power swing and no increase in pulsation value.
Fixing the prefill function from gas turbines can reduce and eliminate PLS due to High Pulsation. Prefill function improvement is done by disabling cyclic prefill function under load. With the improvement of prefill function, there is no more derating because of prefilling. Despite the improvement of the prefill system to the operational concept of gas turbine, monitoring of combustion parameters must continue.
Endrik Purbo Yunastyo1, Danan Tri Yulianto1, Asnawadi Hidayat1,
Kevin Sanjoyo Gunawan2, Totok R. Biyanto2*
1PT. PJB UP Muara Tawar, Muara Tawar, Indonesia
2
Engineering Physics Department, Institut Teknologi Sepuluh Nopember (ITS), Surabaya, Indonesia.
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