JANUARY 2008
FEEDBACK:1 PROJECT: SIPAT UNIT 4 PROBLEM: HUNTING OF GOVERNING SYSTEM. ( 500 MW KWU ) 1.
EHG Hunting: Heavy hunting had been observed in this EHG during restart of the TG for coal firing in Nov 2007. To overcome this problem, at that time, orifices were put at the following locations. a)
6 mm φ orifices were introduced in the CRH NRV CF supply and return pipe lines near the governing rack.
b)
3 mm φ orifice in the trip oil inlet to the extraction relay inside the Governing System Rack.
Commissioning engineers who were at site to attend reported issues in this Governing System began the study with several trials. This took over a week, with NTPC O&M, C&I and BHEL commissioning groups working together to gain insights into system performance and to identify defects if any, in the component elements. Listed below are some significant findings. EHG hunting was observed, while Hydraulic Governing behavior was perfectly normal. So the pilot of the EHG was checked and found not rotating freely. This was dismantled, cleaned and put back. Although EHG characteristics were taken many times after that, without hunting, EHG hunting was occurring at the
lowermost, zero position of the EHG, and at 17-22 % ( i.e. at IP Sec oil pressure 3.6 - 4.0 ksc ), where the extraction NRVs were set to open. T rials were made with different orifices included. Efforts to stabilize the EHG are listed 1. Orifices 2. Orifices 3. Orifices 4. Orifices each)
below:
introduced in Trip fluid inlet to Extraction Relay block 5.5mm ø. introduced in HP Sec fluid to Extraction relay 4 mm ø. introduced in IP Sec fluid to Extraction relay 6 mm ø. introduced in CRH NRV in / out lines near Gov rack (6 mm ø in
OBSERVATIONS: With all above orifices included in the system, the following observations were sent to BHEL Hardwar for analysis. 1.
EHG does not hunt when the Trip oil inlet to extraction relay block is dummied inside Governing System Rack.
2.
When extraction relay block is included, stability in EHG is achieved when 1,2 ,3 orifices are introduced. (Also vents with 2 mm ø orifices in Trip oil lines of individual extraction lines 2, 4, 5 are dummied.)
3.
EHG does not hunt from 0 to 100 % if the raising is done slowly 1 % at a time. This is how EHG characteristics can be taken.
4.
During most trials EHG does not hunt when step changes are made from 0 - 50% and above. Dropping from 50 % to 10 % is also mostly alright. The disturbance causes oscillations which die down.
5.
Step changes to 20%, from above 30 % or from zero %, lead to continued oscillations ( IP Sec varies from 3.6-3.9 ksc) which never die down. The whole pressure system hunts and this can be stopped only by moving up or down ie. to below 10 % or above 30 % EHG .
6.
There is no hunting with hydraulic governing, even when Extraction relay block is included without orifices.
7.
During ten days of trials, at different times, and in different conditions, the EHG was left in service during the day and overnight, without seeing any oscillations develop as long as the EHG was not disturbed from the position where it was stable. But oscillations do develop, at times, which have to be controlled by manual intervention at Governing Rack. This performance was unreliable and EHG therefore was declared by us as unfit for regular operation. 2.
EHG pressure drifting: Another problem was also seen: With EHG steady at zero i.e. without any command or change in the EHG , the HP Secondary Oil residual pressure was seen to drift from 2.8 ksc to 3.05 ksc, This appeared to be happening because of the malfunctioning of one of the HP follow pistons (inner side piston) in the EHG. This is alarming because uncontrolled drift/rise in HP Secondary oil pressure could open HP valves and speed up the turbine accidentally during steam dumping and warming up operations.
3. Extraction Relay Assembly damage at Governing Rack: Threaded boltholes of trip oil inlet flange to extraction relay block in Governing System Rack were damaged. The FC NRV extraction relays were set for closing as per
governing adjustment diagram at recommended IP Sec oil Pressure. The extraction block is cast in aluminium. MS threaded sleeves are embedded for boltholes where flanged CF pipes are connected to the body.
AUX SET OF RELAYS For Extraction NRVs
EHG was checked after including the extraction NRVs (3mm φ orifice removed). Large pressure oscillations began at the pressures where IP Sec opens the FC NRVs. The flanged pipe connections at the body of the extraction relay block had been tightened, temporarily, with new threads tapped on the Aluminium cast body. Knowing that CF leaks may occur with this arrangement any time, all the extraction relays were set, using hydraulic governing. Efforts to arrest the EHG hunting
were made by introducing orifices. Few trials were made with lower sized orifices, but the extraction NRVs failed to open. Finally when fitting 8 mm φ orifice at trip oil inlet to the extraction relay block, the repaired bolt holes failed, and CF leaks developed as soon oil pressure was built up. The block was rendered unusable. Further load operation of turbine was not possible with this condition. Taking all these points into consideration, BHEL Hardwar recommended replacing the extraction relay block, with the good one taken from Sipat unit 5. They also deputed two technicians to site to attend to EHG faults. The technicians serviced the EHG and follow pistons. Site replaced the Extraction relay block from 5th unit. Then the extraction NRV closing and opening were set in a staggered way for IP Sec Fluid pressure varying from 3.6-3.9 ksc. EHG showed improved stability, even after removing orifices (2) and (3), but when 5.5 mm φ was replaced with 8 mm φ orifice in (1) above , instability emerged at 17-22 % EHG, ie. where extraction NRVs were included. Having done other possible checks, we now checked 8 bar and 32 bar accumulators for N 2 gas pressure. The pressure in all 8 bar accumulators were found to be 9 ksc instead of the recommended 5.4 ksc. , and all 32 bar 28 ksc instead of the recommended 24 ksc. (Checks revealed that the pressure gauge supplied with gas charging kit had a 4 ksc error). This meant the gas pressure was too high and there was no fluid in the accumulators i.e. the 8 bar accumulators were not functioning as local fluid pressure reserves. This was perhaps the most significant factor contributing to the Governing System instability. Correction of the gas pressures is expected to completely resolve the hunting and will be confirmed during next restart.
To summarize, the following sequential steps were taken to eliminate hunting from the system (Refer Fig of EHG) :
1. The initial checks were done using orifices as described above. Although this introduced some measure of stability it didn’t remove the hunting. 2. EHG pilot valve 6 and follow pistons (not shown here) were serviced by trained technicians from manufacturing unit. 3. The misalignment of sleeve 10 (attached to EHG coil) with pilot valve 6 was corrected. 4. The optimum combination of system orifices was erected. ie. 6mm in CRH NRV lines and 5.5 – 8 mm in Extraction Relay inlet. 5. Extraction NRV 2,3,4,5 opening/closing was staggered for IP Sec pressure 3.6-3.9 ksc. 6. N2 gas pressure errors were corrected and set at recommended values 5.4 ksc for 8 bar and 28 ksc for 32 bar systems. 7. Adequate fluid flow requirements were ensured for pump stability under all conditions, by keeping a 1” drain valve in 32 header open to return. The performance of the 500 MW governing is proven and is working trouble free at many such installations at different sites. The above is a study of typical component and systemic problems which are sometimes faced. These are to be analyzed carefully and corrected, to meet the stability requirements, and ensure reliable long term performance of the Electro Hydraulic Governing System. Recommendations: 1. The orifices to CRH NRV and Aux Relay to Extractions i.e to the Relay Block should be standardized for all units, and included in the tested rack, fitted at factory.
