In a typical 600 MW thermal power plant, the drum weighs around 300 Tons. The drum itself is a cylindrical vessel of diameter 1.8 meters,12 meters long, 125 mm thick with dished ends and is made of carbon or alloy steel.
The important point for installation apart from the weight is its location. It is located at a height of around 85 meters, high up in the boiler and hung from the top of the boiler structure.
Drum lifting is a milestone in boiler erection that signals the completion of the boiler structure installation.
For the installation of this heavy part following issues require special consideration.
- Installation of a heavy weight at a high position requires special attention to safety in all aspects of the operation.
- The boiler structure is already in place to support the weight which means there is restricted access for using heavy cranes near to the lifting point.
- Interference with the structures already in position, require that during part of the lifting the drum is in an inclined position.
Drum lifting was in the earlier days done using winches and pulley blocks. As the unit sizes and drum weights increased, drum lifting using jacks became the preferred method.
Using Strand Jacks has become the convenient method of lifting a boiler drum all over the world. Multiple strands in two or more strand jacks slowly but surely lifts the drum to its position. This is a simple method to lift the heavy weight.
Except for the preparation phase, only one or two operators are required for lifting the drum. Multiple strand jacks offer redundancy, reliability and safety. Using Strand Jacks is easier to install than a winch system and less costly than using a very heavy crane.
The Strand Jack is located on temporary beams on top of the boiler structure above the final position of the drum. Two or four jacks are used based on the lifting arrangement. Lateral movements of the jacks are possible by mounting them on roller skids. This is necessary to maintain verticality and to avoid interference with the supporting structure.
The jacks itself uses multiple strands, 10 to 14 depending on the load and jack capacity. The lower ends of the strands connect to blocks that fix one end of the strands. The other end of the strands is free and moves through the jack. The blocks connect to the lifting lugs of the drum by links or grommets.
The strands are made of high strength special steels.
The lifting operation takes around 8 to 12 hours. The drum lifts off directly from the transportation vehicle. Lifting the drum at an incline is necessary to avoid interference with the supporting structures.The multiple jacks can operate in synchronization or separately. This allows for safe lifting evn if the drum is at an incline. Once it reaches the final position the permanent supports are installed.
The attached figures show the steps in a typical boiler drum installation.
The multiple strands and the positive locking method of the jacks give redundancy, reliability and safety for the entire operation.