Reliable regulation at Rovno30 August 2001
New disc regulating valves installed at Rovno 3 maintain steam generator water levels during stable operation and during the transients. The valves were specially developed to improve reliability and safety of steam generator operation. By S N TARANOV, M I KOLESNICHENKO & S R CHEPIL
In January 2001, a new disc regulator valve was installed to control feedwater flow to steam generator number 1 at Rovno 3. Testing of the DN 400 disc valve was successfully completed at the l000MWe VVER unit in April 2001. This was the first time that a disc regulator valve was installed at a powerful nuclear reactor to control steam generator water level.
The DN 400 feedwater regulating valves were manufactured by Atommash in Volgodonsk, Russia to the technical requirements developed by Rovno and design institute Energoprojekt.
With drive power of 0.18kWe, the valves provide feedwater flow up to 2l00t/h at a response speed of 25 seconds. Uncontrolled flow and leak do not exceed 0.07% of nominal flow.
The force required to open/close regulation feedwater valves currently used is too large, requiring a correspondingly large drive power (up to 7.5kW) and insufficient speed of response (30 to 50 seconds). This decreases their reliability and regulation quality especially during the transients. The new valve comprises easy-to-detach seat and self-discharging rotary disc slide. This allows for the servodrive’s power to be significantly decreased, and the maximum allowable differential pressure at the valve during the transients can be considerably increased. The working medium flow rate can be regulated by a rotating slide.
Since 1993 new disc valves have been subjected to much testing under operating conditions at different nuclear and thermal plants in Ukraine, where they were used for water, steam and two-phase steam-water mixture flow control. The tests demonstrated increased erosion and cavitation resistance. After six years of operation at Rovno internal examination of one of four DN 300 disc valves from the wet steam supply line to the feedwater deaerator showed no traces of erosion in the gate parts and valve body.
At thermal power plants, disc valves demonstrated the capability to provide steam generator level control at differential pressure up to 18.0MPa during start-up operations. The design of the disc valve prevents the possibility of its seizure when the working medium is contaminated with hard particles.
Other distinctive features are:
•Regulation disc valves differ from other existing valves in simplified and compact servodrive design (remote or on a cover of the valve).
•The valve does not require technical servicing between maintenances of the unit.
•The area of the passage apertures in the detachable seat can be altered without changing discharge chamber diameter.
•Maintenance does not require special equipment.
•Self-discharging rotary disc slide reduces pressure on the seat by a factor of at least ten. As a result of this, drive power and deterioration of working surfaces of the seat and disc slide are considerably lowered.
•Low pressure in the discharge chamber and absence of spindle movement result in a twofold increase in the lifetime of the splindle’s gland (rotary slide and spindle).
•As the pressure differential causes the disc slide to be always pressed to the seat, vibration of the disc slide does not occur.
•Optimal pressure of the disc slide on the seat can be reached in any position of the valve.
•Slot-hole erosion of working surfaces of the seat and disc slide does not occur.
TablesTest results of the DN 400 regulating disc valve at SG 1 of Rovno 3