Tower cranes for cooling towers
By Alex Dahm03 October 2008
Special tower cranes for cooling tower construction are in demand as the demand for energy around the world rises rapidly. Heinz-Gert Kessel reports
Cooling towers at electrical power generation plants are used for re-cooling the cooling water of high-performance steam power plants. A steady increase in the capacity of power generation plants means a commensurate rise in demand for cooling capacity.
In humid climates natural draught towers are used where the diameter and height of the structures are directly related to the power plant's efficiency. In general, their hyperbolic reinforced concrete design characterises the skyline of a power plant. At the beginning of the last century concrete cooling towers had a height of 35 m, while now they are generally 160 to 200 m tall and 90 to 130 m in diameter. On the drawing board are designs up to 230 m tall and 164 m diameter.
Cooling tower projects call for sophisticated crane solutions and are a real challenge in several ways. Natural draft cooling towers take the form of a single-shell rotation hyperboloid. A central tower crane with long jib is needed to cover the base diameter of the shell. When dismantling the crane after toping out the cooling tower, the same crane must pass through the narrowest diameter of the structure.
The increasing size of cooling towers exceeds the capabilities of ordinary free standing tower cranes, which means special extreme free standing tower solutions or tie in systems are needed. Rope guyed towers are usually used because the cooling tower diameter is so large. Due to the shell thickness of just 160 to 200 mm, however, the method of anchoring the guy ropes is a challenge in itself. The aim is to minimise the number of guy levels and to guy to floor level instead of to the shell whenever possible.
During the high-rise construction process the tower crane is the only hook serving the site. The crane needs fast working speeds, fast climbing equipment with a minimum number of climbing steps to minimise the number of times the progress of the concrete structure is interrupted during construction.
The concrete shell is usually erected on site so the tower crane has to offer enough capacity to lift typical concrete buckets. In addition, it must be able to dismantle the self-climbing framework after topping out of the cooling tower. In the 1970s and 1980s tower cranes with concrete delivery placing booms were used to pump concrete. Ideas in the 1980s to build the shell out of pre-cast elements required a load moment that would demand uneconomically large tower cranes. The diagonal supports at the cooling tower base are not normally placed by the central tower crane if they are delivered as pre-cast elements.
The height of the crane is such that it is exposed to extreme wind conditions. The way to work is with as much as possible of the crane in the wind shadow of the rising shell and find the most economic tower composition for the crane.
Construction of hyperbolic concrete cooling towers always calls for special tower crane solutions. It sometimes leads to unique machines manufactured for a single project or, at least, cranes built from standard modules with substantial modifications. An early creative example is the double boom Baltkran manufactured in the 1950s to raise cooling towers at power stations in the former Soviet Union. The bottom-climbing crane was erected in the centre of the cooling tower and had two saddle jibs with two hooks. It also had two trolleys carrying scaffolding, including three horizontal work platforms, as access to the shell framework.
In 1965 Schwing developed its KTK H series of saddle jib climbing cranes. For a nuclear power station in Hungary, with a cooling tower of 100 m diameter, a special extended version of the Schwing KTK 80/95 was built. Instead of the conventional 40 m jib a 54 m one with three sections was developed. The two extensions each carry an A-frame connected by an individual set of pendants to the tower head. This arrangement allowed jib sections to be folded down to let the crane pass through the narrowest part of the cooling tower when climbing down at the end of construction. Maximum under hook height of the crane was 135 m. Anchoring of the mast with the shell was by multiple ropes fixed to a complicated framework surrounding the crane tower every 29m.
At the beginning of the 1960s Kaiser developed the articulated saddle jib HBK series tower cranes. In the late 1970s it was a design feature also found in the Liebherr HC-K series and in the Peiner SKK140. Today the concept is revived in the Chinese-built DZQ200 and ZTQ240. The articulated saddle jib crane offers advantages for cooling tower construction:
* Thanks to the articulated jib the crane can simply climb back down through the cooling tower when construction is finished and dismantling can be done as easily as erection at the lowest climbing stage.
* Compared to conventional saddle jib cranes fewer tower sections and less anchorage is needed because the upper part of the cooling tower can be reached by articulating the jib up to its steep angle position.
* In the articulated working condition only a minimum of crane components are exposed to the wind so the anchorage-free tower height is increased.
* The load is always being efficiently moved horizontally the same as it is with a conventional saddle jib tower crane.
* Most of the outreach variation is done by the trolley instead of the luffing jib mode so the articulated jib crane works faster than any conventional luffing jib crane.
Despite these advantages, articulated jib cranes have been a rarity on recent cooling tower projects due to their expensive design, limited maximum outreach of 60 m and the fact that they cannot be converted into a low cost standard crane after the cooling tower project is finished.
Elba Kaiser articulated jib tower cranes set milestones in cooling tower construction. Lots of concrete is needed for large cooling tower shells, especially in the base. To deliver this Elba Kasier developed an articulated concrete delivery boom with 63.3 m maximum outreach for the cooling tower of the Mühlheim-Kerlich power plant in Germany. It rotates around the base tower section of an HBK150.1.
In the 1980s an HBK160 was delivered with a free standing under hook height of 132 m thanks to the application of external tower sections at the base. it allowed construction of a 90 m diameter and 120 m tall cooling tower without anchorage.
For the construction of a super-sized cooling tower in the former Soviet Union, with a lower diameter of 164 m, a height of 180 m and an upper diameter of 100 m, Elba Kaiser developed a new multiple crane concept. First, two travelling HBK160s were erected inside the cooling tower operating on a circular track up to 100 m height. One of the cranes had additional concrete boom at the machinery platform, moved by the crane's lifting hook to the required position. It allowed the concrete capacity to be increased up to 50%. At the construction height of 100 m the concreting crane was dismantled. The second crane was relocated to the centre of the cooling tower where it operated stationary with only two guying levels with a tower height of 168 m and an under hook height of 194 m.
Larger diameter cooling towers led to a renaissance of saddle jib tower crane application in the early 1980s. Jib length of more than 60 m and increased tip load capacity for concrete delivery can be realized. To dismantle the long jib of the crane stationary inside the cooling tower, however, demanded special features.
Peiner delivered an SK500 to construct four cooling towers in Cruas, France. By means of three sets of eight anchoring ropes a 167 m final hook height was realized. At the maximum radius of 66.8 m a 5.2 tonne lifting capacity and a maximum hoisting speed of 180 m/min allowed fast concrete delivery.
To climb down again after topping out, the basic jib had to be raised by the hoisting winch to an angle steep enough to allow the outer jib to swing down with the aid of a second A-frame without touching the cooling tower shell. To keep the crane balanced during that procedure, counterweight blocks had first to be lowered by an assistance winch system.
In 1962 Peiner presented additional dismantling devices consisting of two A-frames mounted at the back and the front of the tower head to fold down the jib and counter jib after the crane had climbed high enough over the cooling toer shell.
At the end of the 1970s Potain delivered a number of 8000 series cranes for cooling tower construction at nuclear power stations. To bring down the saddle jib cranes with long jib arrangements, a special derrick was attached to the front trolley, moving into the back of the jib tip section to be dismantled. The jib section is lowered to the ground using the crane's hoisting rope.
For jib removal, counterweight also has to be dismantled to keep the crane in balance. As soon as enough jib sections are dismantled to swing the jib down inside the edge of the cooling tower, additional A-frames connected with the tower head are used to fold down the remaining jib and, if required, even the counter jib at the back of the machinery platform.
The crane can then climb down while removing its tower sections through the winch hoist on the machinery platform and the hoisting line reeved over a pulley at the A-frame held by the tower head. Potain has developed an additional trolley-like working platform for the MD series with a parallelogram-like holding device for the disconnection and folding of jib sections which are then lowered by the crane's own hook.
Most large scale cooling towers in Germany are built with a special version of the Liebherr 280EC-H crane. The jib has to be lowered section-by-section after the cooling tower has topped out, before the jib is short enough that the crane can climb down again inside the cooling tower. In this case jib sections are prepared with an A-frame on each section that has to be dismantled in the air to lower it down on an assist rope. Next, the jib section is folded down before being lowered by an equaliser beam attached to the crane's hook.
Flat top cranes are simpler to de-rig than cranes with pendant jibs. 1976 Linden Alimak invented a jib section dismantling device consisting of a carry cradle with counterweight in the form of a three-armed lever. It was attached to the hook block and moved under the jib where it acts like a derrick frame at which the front jib section is connected before been lowered hanging at the cradle.
BKT discussed a hydraulically operated luffing device for the outer half part of flat top saddle jib cranes. This concept has yet to be realized. It would require careful adaptation of the jib design considering the effect of high wind loads on the raised jib end.
The growing number of top slewing luffing jib crane applications are providing advantages, especially, for medium sized cooling towers:
* The reliable crane design allows conversion of no-longer-required outreach in hook height and, therewith, reduce the required tower height dramatically.
* Most of the crane structure is covered by the growing shell to extreme heights, leading to a reduced wind load on the crane and adding to the free standing height of the crane tower. When exaggerating the wind exposition of a crane it is assumed that wind will blow at 15 degree into the shell. It should be remembered that at the highest crane position only the boom of the luffing jib crane is exposed to the wind when topping out the cooling tower.
* The crane can be easily climbed down through the narrow part of the cooling tower by luffing the jib.
A disadvantage of the design can be the slower working speed than a saddle jib crane and, in cases of a narrow cooling tower diameter, the dismantling of the jib in a steeper position than normal. It may be a sound alternative to use a long boom saddle jib upper crane at the beginning of the cooling tower construction before changing over to a luffing jib upper crane on the same tower.
Cooling towers are tall structures and this causes difficulties for anchoring the crane. To reduce impact on the concrete pouring the number of anchor cables should be kept to a minimum. Until the 1980s they were connected horizontally to the cooling tower shell, reducing the required tension force.
In that case, however, the forces transmitted to the shell must be kept within tight limits because the drying time of the concrete must be considered. This explains why cable stays leading vertically to the foundation of the cooling tower, which are generally used today, have less impact on the construction process but require high tension forces, stronger ropes and, generally, separate foundations. To avoid any impact on the concrete shell construction process a free standing tower system is preferred.
It used to be that an expensive outer tower system surrounding the standard tower was employed. Nowadays different tower systems can be combined for impressive heights by using adapter frames. Wilbert, for example, rigged a WT205L e.tronic with 42 m jib on 112.95 m free standing tower for an impressive maximum under hook height of 159.45 m. Reaching the full height with the assistance of a mobile crane offers the benefit of no interruptions by the climbing procedure during the entire construction period of the cooling tower shell.
In addition, the most economic tower system combination can be used because there is no need to consider climbing cage application. In this case Wilbert, for example, combined three tower sizes using two adapter sections to reduce the square size from 3.3 x 3.3 m to 2.4 x 2.4 m and then to 2 x 2 m for the remaining third of the crane tower.
To avoid long hook falls and to minimise the amount of crane structure exposed to wind at the beginning of the construction project, the crane can be climbed in steps to the final height. In general, for each change in tower system another climbing cage must be used. This usually means that the crane has two climbing cages. In the first construction period the crane is raised by the climbing cage jacking the large base tower sections. The upper crane is resting on an adapter frame fitted with base standard tower elements surrounded by the corresponding climbing frame to be jacked.
When the maximum free standing height for the base tower system is reached, the upper climbing cage is used to climb the crane higher on standard tower sections. Following this principle Wolff is raising a Wolff 180B with 50 m jib at a cooling tower construction site at Boxberg in Germany to a free standing tower height of 126.7 m. In this configuration the free standing Wolff 180B luffing jib crane will offer the impressive under hook height of 165 m at 35 m radius. The strong TV33 with 3.3 x 3.3 m square tower system is used as a base.
Special foundation requirements were avoided thanks to the 12 x 12 m stationary undercarriage loaded with 300 tonnes of central ballast.
Franc Jost has just finished a conceptual study for his all new Jost flat top luffing jib tower crane JTL 208.12 for a cooling tower project with 150 m shell height. The relatively low moment forces created by the light weight 200 tonne-metre class crane upper with moving counterweight system means that a combination of six TH20.4 and 12 TH20.3 tower system (2.24 x 2.24 m) is claimed to be sufficient for the free standing version.
In addition, the hydraulically operated luffing rams and the jib geometry add to the wind resistance of the unique crane design. The economic way to use free standing standard upper tower cranes for cooling tower construction is restricted to about 150 or 170 m cooling tower height. Luffing tower cranes are used to convert outreach into lifting height as soon as the maximum free standing height is reached.
To construct taller cooling towers of about 180 m and ones with a larger base diameter, generally, a saddle jib crane with 70 m jib is used, requiring special de-rigging devices. By combining different tower systems in this case it may be possible to reduce the required anchor cable levels from three to two for cranes with an under hook height of 190 to 200 m.
For a power station under construction in Datteln, Germany, Wilbert supplied a WT300 e.tronic with an all new anchoring system. First, the flat top crane is working as a free standing unit with 70 m radius up to 63.23 m hook height to serve the large base diameter of 126.8 m. Then the jib is shortened using a mobile crane to 45 m and the crane climbs up to 99.82 m hook height free standing.
Two anchoring levels have to be rigged to reach the maximum under hook height of 190.6 m. In contrast to ordinary rope systems Wilbert uses a strand jack anchorage system with eight strands connected with jacks anchored at the foundation of the cooling tower. Before the WT300 is climbed down inside the 74.36 m narrow upper shell, Wilbert will use its new jib self-dismantling device to shorten the saddle jib.
To build larger structures it might be that multiple tower crane concepts may become necessary. As an example, three self climbing luffing jib climbing cranes could be placed on the edge of a triangle-like tower system. These would be connected by horizontal bracing beams to gain extreme free standing tower heights and, at the same time, provide coverage of the large ground diameter of more than 160 m.