Start-up during 2002 of the Gebze, Adapazari and Izmir combined cycle power plants in Turkey revealed over-pressure at the boiler feedwater pump discharge. The over-pressure was caused by grid over-frequency and cold water used at start-up and required an innovative solution to ensure that the plants could continue to operate reliably.
S.Zaheer Akhtar, Bechtel Power Corp., USA
During 2002, three gas turbine combined cycle plants together rated for a total of 3850 MW were commissioned in Turkey at the Gebze, Adapazari and Izmir sites by the Intergen-ENKA partnership. The plants are based on 770 MW power blocks each featuring two 9FA gas turbines, two vertical heat recovery steam generators (HRSGs) and one 260 MW steam turbine. The Gebze and Izmir plants consist of two power blocks each whereas the Adapazari plant consists of one power block.
The Izmir plant is located near the coast which enables the use of wet cooling towers with sea water as the cooling medium. The Gebze and Adapazari plants are located inland in an arid region and use indirect dry cooling towers also known as the Heller technology system.
During start-up of these plants, over-pressure at the boiler feedwater (BFW) pump discharge was noticed when the piping relief valves lifted. The over-pressure was caused by the combination of:
- Over-frequency at power grid which supplied power to the plant motors
- Cold water used at start-up to fill the boiler drums and piping.
Overview of the vertical heat recovery steam generators and stacks at the Izmir combined cycle plant site in Turkey
Due to the Heller technology system, the arrangement of the condensate pumps and the boiler feed water pumps at the Gebze and Adapazari sites differs from those used in a conventional combined cycle plant design. In the Heller system, the main circulating water pump handles boiler quality water stored in the jet condenser and pumps it out to the cooling tower where it is cooled and then returned to the jet condenser.
The condensate is taken from a slip-stream from the main circulating water pump discharge header and this provides suction to the condensate booster pumps. The condensate booster pumps supply boiler water to the low pressure (LP) drum of the HRSG and in parallel to the suction of the high pressure/intermediate pressure (HP/IP) boiler feedwater pump. In other words the Heller system results in a unique arrangement with three pumps acting in series, namely the circulating water pump, the condensate booster pump and the boiler feedwater pump. It is this unique arrangement which requires detailed evaluation from over-pressure view point.
The arrangement at the Izmir plant consists of the condensate pumps taking suction from the conventional hotwell of the surface condenser and discharging to the LP drum of the HRSG and in parallel to the suction of the HP/IP boiler feed water pump. In this case there are two pumps operating in series, namely the condensate pump and the boiler feedwater pump.
The IP boiler feedwater pump provided IP feedwater to the IP economizer and spray water to the reheat steam attemporating valve. The spray water line to the reheat steam attemporating valve is equipped with a pressure relief valve. The IP economizer supplied the IP drum as well as the performance enhancing fuel gas heater.
The HP boiler feedwater pump provided HP feedwater to the HP economizer and spray water to the HP superheater steam attemporating valve. Here also the spray water line to the superheater steam attemporating valve is equipped with a pressure relief valve.
During start-up it was observed that the pressure relief valves on the spray water line to the reheat steam attemporating valve and the HP superheater steam attemporating valve were both lifting. Initially it was thought that the pressure relief valves needed adjustment or maintenance. However, it was soon realised that the pressure relief valves were operating satisfactorily and that they were lifting due to over pressure conditions in the IP spray water line and the HP spray water line. The next logical step was therefore to investigate the cause of over-pressure observed at the outlet of the boiler feedwater pump.
During start-up the power to the pumps was being provided by the grid which was operating at around two per cent over-frequency. Based on the motor speed-frequency relationship, the speed varies linearly with the frequency, and based on the pump affinity laws, the pump head varies as the square of the speed.
Therefore, a two per cent increase in power frequency increases the motor speed by two per cent and this increases the pump head by four per cent. Now consider the Heller technology arrangement where there are three pumps installed in series, namely the circulating water pump, condensate booster pump and the boiler feedwater pump. In such a case, the increase in frequency will increase the pump head on each pump and since the pumps are in series, the cumulative effect of increased pump head will be additive. In other words, a two per cent increase in frequency will increase each pump’s head by four per cent and the additive effect of three pumps in series will increase the cumulative pump head by 3 x 4 per cent = 12 per cent.
This was the cause of overpressure observed at the boiler feed pump discharge line that manifested itself in the lifting of the pressure relief valves on the HP superheater attemporating spray water line and the reheater attemporating spray water line. The problem of high discharge pressures was aggravated during start-up when the water is cold and the pump flow rate is low. This problem was not of significance after plant start up after increase in condensate temperatures and flow rate through the pumps.
The technical solution adopted at the plants had a two-fold approach as follows:
- Increase the set point of the HP superheater spray water relief valves (set at 216 barg) and the reheater spray water relief valve (set at 75 barg)
- Decrease the BFP discharge pressure by increasing the minimum flow recirculation on the pump.
The increasing in set point of the HP superheater spray water relief valve was limited by the design pressure of the HRSG vendor’s piping whereas the increase in reheater spray water was limited by the fuel gas heater water side design pressure.
Circulating water pumps used as part of the indirect dry cooling system at the Gebze and Adapazari combined cycle plant sites
Based on the pump curve, the increase in minimum recirculation flow on the boiler feedwater pump required to decrease the differential head on the pump was evaluated to be 300 m3/h (from existing 100 m3/h). As expected the increase in minimum recirculation flow was restricted by the Automatic Recirculation Control (ARC) valve. After consultation with the pump vendor and the ARC valve supplier, it was decided to upgrade the ARC valve to its maximum of 200 m3/h and install a second bypass line (from pump discharge to suction) to pass the remaining 100 m3/h. The bypass line could be returned to the condenser or to the pump suction.
It was preferred to route the bypass line to the BFP suction since this would decrease the length of the bypass line. However there were some concerns relating to BFP pump overheating due to uncooled water being sent back from pump discharge to suction. These concerns were alleviated after pump heat rise calculations indicated that pump overheating was not an issue.
The option of mitigating pump discharge overpressure by trimming the pump impellers was also considered but rejected as this required disassembly of the pump internals and that impeller trimming would be irreversible and would not help in case the pump motor power input swung to the other end of the scale to two per cent lower frequency.
Heat and mass balance across boiler feedwater pump at increased min recirc flow conditions
The rise in temperature of boiler water due to energy imparted by the boiler feed water pump at minimum recirculation flow of 300 m3/h was calculated from the following equation:
àŽ”T =Temperature rise through pump, à‚ºF
H = Total head developed at min flow, feet
Cp = Specific heat of liquid, Btu/lb-à‚ºF
h = Efficiency of pump at flow being considered.
Using the above relationship the temperature rise was calculated to be 7à‚ºF. However under steady state conditions with min recirc flow of 300 m3/h through the pump (the flow includes 100 m3/h uncooled flow and 200 m3/h of cooled flow through the condenser) the temperature rise was estimated to be around 10à‚ºF. This indicated that pump overheating was not an issue under these operating conditions.
Since overheating was not an issue, the new 100 mm bypass line was installed to pass 100 m3/h of flow. The bypass line was fitted with a flow orifice, a flow control valve and a back pressure control valve. In addition the ARC valve was upgraded to pass remaining 200 m3/h of flow at minimum recirculation conditions.
This increase enabled the pump head to be decreased. Also, the pressure relief valves on the HP superheater spray water piping was upgraded form 216 barg to 222 barg. Similarly the relief valve on the reheater spray water piping was upgraded from 75 barg to 78.5 barg as limited by the fuel gas heater design. The relief valve upgrade was possible by merely resetting the existing springs in the relief valves.