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Refurbishment of the Draft Tube Flaps at Markersbach

Vibration of the draft tube flaps at the 1,050 MW Markersbach pumped-storage facility in Germany began occurring in the 1990s as a result of a change in the operating range of the pump-turbine units. The original equipment blocking the flaps in the opened position was not designed for such conditions, and the reliability of the system decreased. To overcome this, owner Vattenfall Europe Generation ordered a refurbishment of the flaps. CKD Blansko Engineering designed and installed a hydraulic system consisting of equipment that created upward hydraulic force on the flap in the open position. The flap is now free of vibration over the entire range of operation of the unit.

Background on the situation

The Markersbach plant was commissioned about 30 years ago. Modifications to the plant performed over the years include upgrading the six pump- turbine units, refurbishment of the main inlet valves, and improvements to the control system. One of the last parts of the units to be modernized was the draft tube flap. This type of the valve was installed because the plant has two 400-meter tailrace tunnels. The flap gate was primarily designed to ensure the gate halves could be disassembled and removed through a turbine shaft during maintenance and emergency events. The draft tube flap was locked in the open position using a control servomotor located over the shaft cover and a linkage mechanism with a locking pin located inside the shaft.

The Markersbach pumped-storage facility in Germany was the site of a successful draft tube flap gate refurbishment. Renovations to the plant had changed the range in which its pump-turbines operated, creating excessive vibration in the draft tube flap that had to be fixed.

In the 1990s, the operating range of the turbines was extended, allowing operation of the units in the turbine mode at lower loads than before. However, with this change in operation the mechanical device was not able to reliably fix the flap in the open position, causing the flap to vibrate. Vattenfall personnel determined that a refurbishment of this piece of equipment was necessary.

Design of the modification

The main aim of the work was to improve locking of the flap gate in the open position during various operational conditions of the pump-turbine.

The experience from a previous project was taken into consideration. Special attention was paid to the hydraulic system of the flap lock, which has decisive influence on vibration damping. Two variants were followed and verified by calculations. The variant described below was ultimately selected.

CKD Blansko was selected as supplier of the flap modifications. The solution chosen was to lock the gate in the open position using three independent methods: by the servomotor, the mechanical locking device and hydraulic force of the water. The original design already incorporated the first and second methods. However, the oil pressure control system was completely modernized during the refurbishment. During unit operation, pressurized oil is supplied to the servomotor, keeping the gate in the open position. This work was carried out by modifying the servomotor, hydraulic pressure unit and hydraulic control elements, including new stainless steel oil pipelines.

The mechanical locking device was completely redesigned. The locking device was moved to a dry environment to enable better access to its main parts. The control servomotor is in the same location. If the flap gate is in the open position, the locking pin is pushed into the body of the auxiliary plunger, thus locking the gate in the open position in a case of a failure of other safety elements, such as oil pressure loss of hydraulic pressure loss. A position pointer was integrated to allow plant personnel to follow the entire movement of the flap servomotor during the opening or closing process.

The third method chosen for locking the gate in the open position was based on experience gained during installation of draft tube flaps at Vattenfall's 1,060 MW Goldisthal pumped-storage power plant. This facility began operating in Germany in 2004. Gate locking at this plant was achieved using water pressure created from the tailrace tunnel. Two auxiliary chambers were installed on the right and left parts of the gate for this purpose. Each chamber consists of two halves. One half is installed on the gate, the second one on the upper part of the flap steel lining. Both chambers are connected by a common air relief pipeline, which leads to a drainage pit.

Operation of the chambers works thusly: When the flap table is in the open upper position, the halves of both chambers are sitting on it together. Each chamber makes a closed, compact space. Inside this space, the water pressure is about the same as it is in the tailrace tunnel: about 5.5 bar. The pressure in both chambers is consequently released by a common pipeline. The process of releasing is controlled by a hydraulically controlled valve. At this point, the space in the chambers is connected with the open air. In the same moment, water pressure from the tailrace tunnel is acting on the area, which is equal to that in both chambers on the gate. The flap gate is held in place to the upper part of the steel lining.

Releasing pressure follows the same process in reverse. The chamber release pipe is closed. The water filling pipe is opened and the chambers' space is filled with water from tailrace tunnel. The water pressure in the chambers and draft tube are equalized.

A stress analysis of the modified mechanical equipment and the concrete structure around the flap steel lining was performed. It was necessary to prove the original concrete structures can withstand the increased load. The stress analysis confirmed the sturdiness of the design of the flap gate and the carrying capacity of the concrete structures. Modification of the perimeter rubber sealing was also important. The new design with adjusting bolts enables movement of the seal as needed, within a range of a few millimeters.

During assembly at the site, it is possible to reach maximum tightness over the whole perimeter and thus minimize water leakage. A new control panel was installed for local control of the flap gate when the unit is undergoing planned maintenance or overhaul.

Dismantling the original equipment

Dismantling of the original flap gate equipment was complicated, requiring step-by-step dismantling of the electric and oil control elements, servomotor, mechanical locking device, shaft cover and gate. Dismantling "dry" parts of the unit was relatively easy. But it was hard to dismantle equipment in the "wet" part.

The gate was released from the trunnions at first and lowered on to supports on the steel lining bottom. Then it was dismantled to separate halves. Moving the gate's parts was handicapped by the limited space of the steel lining. Each half of the gate was pulled up through the shaft and then put down on the beams over the shaft. There, it had to be turned about 90 degrees, then pulled up on the next floor.

Modifications and manufacturing of the new equipment

All parts were cleaned in the workshop. A technical inspection was done to ascertain the condition of the parts. Despite extensive corrosion in some stainless steel and steel gate parts, the original anticorrosion protection covered a large area of the table. This protection was hard to remove. The most corroded parts were cut out and new ones welded. At nearly all of the original main trunnions, several cracks were found. For this reason, new trunnions were manufactured.

Renovation was performed on the control servomotor and hydraulic pressure unit, which was equipped with new components. The mechanical locking device was manufactured completely new.

Assembly at site and commissioning

Assembly back at the site began by mounting the gate inside the flap lining. Manipulation was done carefully due to new coats of anticorrosion material having recently been applied. After installation of both halves of the gate, they were then set into the main and auxiliary trunnions. The support bodies for the auxiliary trunnions were welded to the lining. After finishing assembly inside the lining, the shaft cover was installed.

Some deformation of the shaft lining had appeared while the unit was offline. The shaft cover did not fit properly onto the lining. It was not possible to put a screw through the holes in the cover because the original holes did not line up properly. Therefore, the holes in the cover had to be enlarged. After the shaft cover was set, installation of the servomotor, mechanical locking device, oil and water pipelines and control equipment followed. The setting of the seal was performed at the end.

Dry tests were then carried out. Software of the new local control panel was tuned. Finally, the tailrace tunnel was watered and the wet test followed. The wet tests were successful, and the project was handed over by 2008. The draft tube flaps are now free of vibration throughout the pump-turbine operation range. The work was completed within eight months per three units.

— By Karel Kyzlink, senior project and design engineer, CKD Blansko Engineering, part of the Litostroj Power group

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