Abstract: this article states reform reason, reform scheme and operation of water feeding system on union circulating furnace of Yangpu Power Plant, provides reference for other companies.

Preface

11# gas turbine (Siemens V94.2) of Yangpu Power Plant finished reconstruction of oil changing gas and put into service in September, 2003. Its union circulating reconstruction put into service in January, 2004. #11-13 union circulating machine is the biggest gas union circulating machine in China now. The most output can reach to 220MW. Three main equipments are made in China, they are: surplus heat boiler, model Q1396/556-228（47）-8.14（0.588）/532（252）produced by Hangzhou Boiler Plant, binary vapor; steam turbine, model LN82.6-8.0/0.65/530/250, double pressure condensing, produced by Haerbin Steam Turbine Plant; generator, model QF-82-2, close air cooling, produced by Haerbin Generator Plant.

Introduction of water feeding system

Water feeding system of #11-13 union circulating machine is composed of two pumps, water feeding valve and relevant pipeline. The two water feeding pumps are produced by KSB, model HGB4/10, rated flow 255.3m3/h, lift 1092mH2O, rated speed 2985r/min. Work mode is one running one standby. Water feeding valve is designed by Siemens and made in India. Its adjustment is water level, inlet flow and steam flow close control.

Problems and analysis

In March 2004, #11-13 union circulating machine passed acceptance operation of 168 hours. But in later two months, water feeding valve faults several times, causing out control of HP steam drum, once trip and five times of fault stand of steam turbine. It also affects safe operation of power grid to some extent. Most of failures are caused by loose or breakage of valve position feedback connecting rod. Combining with periodical vibration of water feeding pipeline which occurs simultaneously during operation, we think main reason of valve failure is following:

Outlet pressure of water feeding pump is too high. It differs pressure of HP steam drum a lot, creating vibration. In addition, stiffness of water feeding platform of pipeline support is inferior. Vibration can not be absorbed, leading to connecting rod fatigue and breakage when impact load appears. In case of connecting rod loose or breakage, the valve returns to full open position automatically. Flow feeding reaches to max value. Water level of HP steam drum rises quickly, meanwhile, water feeding pressure lowers suddenly, leading to link start standby pump. If personnel does take measure in time, HP steam drum water level three high value action is bound to occur. Steam turbine protects to trip. We took out valve core during downtime in Febuary 2004, found damages on several parts. We analysed and thought it was caused by scouring action of high speed & high pressure water flow. The valve core standed much throttle loss. We replaced valve core and mended the connecting rod, then started machine to run. The valve core was damaged again in June 2004. There were two gaps scoured on valve core. The gap was in circular arc shape, length in 20mm and depth in 10mm. We printed out typical curve of outlet pressure and steam drum pressure, found the biggest difference of the two was 10Mpa, the smallest difference was 4 Mpa. Took out flow choking loss of pipeline and coal economizer, the rest of pressure difference was standed by governing valve, the biggest value can reach to 8Mpa at the most. The pressure loss was bound to result in vibration and scouring wear on valve core.

Above problem results from design. Design company did not take account of special operation of the project sufficiently. Design company selected model as per rated condition and made redundancy. As the second main power plant of Hainan, our plant undertakes task of peak adjustment and frequency adjustment of power grid. Machine runs in full load just in several hours in the evening and runs in low load in the rest of time everyday, even 20% load in midnight sometimes. Thus machine runs far from rated condition mostly and farther from design condition. The design did not adopt appropriate measure to adjust outlet pressure either, leading to valve work in vibration for long and stand enormous throttle loss. Valve core is damaged in frequent scouring.

Daily load curve of the plant

Scheme selection

By looking through relevant material and inquiry from other power plants, our plant put forward to adopt variable frequency adjustment. Existing practise proves that this method has considerable effect of energy saving. We also made comparison and analysis between variable frequency adjustment and hydraulic coupling adjustment. We went to Haikou Power Plant and asked for information, finding following disvantages of hydraulic coupling: complicated system, low reliability, much maintenance labour. Its advantages is low price and low cost. Through analysing actual state, the plant did not have enough installation space for hydraulic coupling. So we gave up hydraulic coupling and decided to use variable frequency adjustment.

Introduction of variable frequency adjustment

With consecutive development, variable frequency technology has reached a high level and has applied in several industries and civil facilities, such as civil variable frequency air conditioner. Its character is to utilize open/close loop control. Power cells of VFD output power required through AC-DC-AC single phase inverter circuit. VFD makes output frequency meet technics requirement, thereby reaching goal control and purpose of energy saving.

The plant selected and purchased HARSVERT-A06/130 VFD at last. The VFD is high-high voltage source VFD, adopting cell-cascaded and multi level technology, with advantages of less harmonic, better output waveform.

Reform implementation

Because 6kV switch room has no enough installation space, plant decided to install one VFD for each surplus heat boiler. VFD has funtion of “one drive two”. It can drive P1, P2, and has bypass function. Two boilers use two VFDs. The two VFDs drive four pumps.

Reconstruction began in August, 2004. whole installation and commissioning finished within a week.

Introduction of operation mode

For above reasons, we must adopt special operation mode to ensure safe operation of water feeding system. We selected following mode eventually.

During normal operation, variable frequency pump works. System adopts HP steam drum close loop control. Water feeding governing valve is in manual control and full open. Standby pump in direct on line does not join link start, which can prevent trip-off resulted from high water level of HP steam drum when personnel does not take measure in time.

In the case of failure of VFD or running pump, it stops automatically or by manual. Personnel should start standby pump manually and adjust appropriate opening of valve as per load of that time. Exit HP steam drum three low value protection, avoid tripping caused by lag control. The gist is that surplus heat boiler has capacity of dry burning. Anyway, these ask for high requirement to personnel. When VFD is broken, original running pump can use line power to become as standby pump, thereby making high serviceability of water feeding system, reducing probabilities of stop farthest.

Concluding

Comparing and analysing during 6 months operation, we find power consumption lowers obviously after reconstruction. Daily power consumption reduces 15000kWh. Energy saving rate is 60%. During actual operation, governing valve does not break any more. HP steam drum water level adjustment reaches satisfying effect. Vibration of pipeline eliminates basically. Reliability of union circulating is heightened, and utilization factor of steam turbine is also heightened. This reconstruction is successful with considerable economic benefit and society benefit, makes active contribution on safety and reliability of power grid.