SPRING SUPPORT OF TURBO GENERATOR FOUNDATIONS 2.0
Advantages of spring loaded foundation 2.1
Minimization of Mat size and construction cost: Table foundations of turbo generators had been an almost universal practice. Thick base mates were required as a dynamic “Counter mass” and in order to prevent significant settlement especially in case of poor subsoil conditions. The vibration to the main columns is isolated by the spring elements resulting into small size base mat, which reduces the construction cost & time.
2.2
More space below the TG for the arrangement of condensers and piping: The same column to TG deck may also be used to support surrounding floors at the building so that the total number of columns is reduced and the space to accommodate more equipment is increased. (Compare the space available at Dahanu units below TG with any conventional 210 MW TG halls, the difference can be seen.)
2.3
Handling of settlement problems: Progressive settlement can interrupt normal operation and lead to major shut downs. Easy adjustment in the event of settlements is feasible with spring support by inserting or removing shims in spring units.
2.4
Better load distribution due to the spring support.
2.5
Earthquake Protection: Machine will have more stability in t he event of earthquake as compared to normal foundation. In order to protect the plant against earthquakes and to avoid resonance, the spring units are partially combined with Visco damping.
3.0
How is the spring support system designed? BHEL designers (in case of TG, HWR) supply the following information to GERB – 1) 2)
Sizes of equipments to be supported. Loads – location of loads, direction of force components (in Fx, Fy, Fz direction).
Following loads are considered while furnishing the information to GERB – 1)
Static load of components.
2)
Rot. Elements
3)
Power torque
4)
Vacuum loads.
5)
Thermal elongation
6)
Loads when one half of condenser is in operation.
7)
Forces & moments of steam piping
8)
Friction forces.
9)
Maximum allowable unbalance
10) Loss of blade unbalance 11) Short circuit forces 12) Seismic loads 13) Bolts stretching & erection loads. Based on above information GERB designs the springs & Visco dampers & decides on their numbers at various columns. In case of Dahanu 2 x 250 MW complete TG deck is supported on 10 columns, 5 each side of the axis.
Types of spring supports used at Dahanu. GERB supplier various coil sizes, number of coils & turn depending on column loading & type of loading. Combination of S-9 at A7A’ column, S-2 at B&B’, S-11 & S-6 at C&C’ columns, S5 at D&D’ columns, S-5 & S-8 at E&E’ Columns have been provided.
4.0 GENERAL ARRANGEMENT OF SPRING LOADED DECKS
Items supplied by GERB: GERB supplies the complete isolation system consisting of spring units, viscodampers, fixing materials, leveling shims and mounting equipments. They supply the damping fluid for viscodampers. Preparatory work before installing the spring unitsThe spring system is mounted when the shuttering of RCC slab of TG deck is ready. The top surface of individual column where these springs are mounted should be within accuracy of 00 to –3,00 mm. If necessary these may be corrected by chipping before installations of the spring units. The level deviation among various columns is permitted 00 to –5, 00 mm in total including individual column variation of 00 to –3,00 mm. Scheduled column elevation
The cover steel plates to close the deck opening above the spring units must be provided with anchor bolts or similar arrangements on the upper side spaced no more than 300 mm in each direction. Thickness of the plates should be at least 10 mm.
Prestressing of spring units: The spring units are delivered to site slightly prestressed for the purpose of transportation. The required prestressing of the springs (based on column load) is done at site before their installation in position. Prestressing corresponds to – a) b)
Static deflection of springs due to operation loads. An additional deflection of 5 mm to cover temporary over loading during the machine installation.
This additional deflection of 5 mm is equalized by shims on top of the spring units, which are removed when the prestressed units are released. (This means, that the top level of foundation is 5 mm lower and will reach final level after releasing spring units). A note on additional loading on springs by providing 5 mm shims The springs units are not only pre-compressed for Calculated load of the machine but it is further pre-compressed to prevent the loosening of the spring during installation of the equipment due to unforeseen additional loads. If major variation in calculation of load over the spring units is not expected then an additional compression of 5.00 mm is sufficient for this purpose. These shims are removed at the time of loosening of spring elements before final alignment of the machine. It is also not necessary to re move this complete 5.00 mm shims for each spring unit and the same may vary form spring unit to spring unit depending upon the variation of column level / elevation and variation in level of spring cover plate etc. The spring units are compressed in special steel frame with hydraulic jacks. The elements are locked in this position by the side bolts into eh spring units. (Steel frame and jack are supplied by GERB).
Mounting of spring units: Before the spring units are mounted, the supporting beams columns are marked where the units are to be located, as per the drawing. One adhesive resilient pad is placed on the surface of the supporting beam columns for each element. The pad has same size as the contact area of the element. The prestressed spring units are placed on top of the adhesive pads with a crane. Threaded holes are provided on top of the spring units to use eyebolts (M-20). (Caution: The heads of the prestressing bolts must be on the upper side!!!)
Like a sandwich a stack of adhesive pad steel shim and another adhesive pad is laid on top of the spring units, as per calculations. (Adhesive pads are used to avoid movement of the spring units/shims during operation of the machine) If the level of the upper adhesive pad is not in accordance with the drawing the difference should be corrected by reducing or adding steel shims. Then, a plastic foil is wrapped around the spring units to give protection against concrete milk or dust. Finally, the cover steel plates are placed over the spring units. The cover steel plates will stay in position after-wards and w ill not be removed. It will serve the purpose of equal distribution of pressure, when jacks are used for the leveling procedure or for the release of the spring units. (Caution: The bottom sides of the steel plates and the bottom sides of the pedestal (deck) must be on the same level). (Figures given at the end) Condition of the springs when machine is assembled – Springs remain in locked – prestressed condition when machine is assembled. During the installation of the machine the de ck may settle 1-1.5 mm without prestressing bolts of the spring units becoming loose. (The reason is a possible plastic compression of the adhesive resilient pads above & below the spring units). Pre-requisites for releasing the springs & subsequent leveling – Completion of installation of all TG components. Erection completion of all the connected piping (Viz. MS, CRH, HRH, Extraction, gland steam, leak-off, drains etc.), and after floating these pipes on their respective hangers / supports. Completion of insulation work on turbines & piping as per Sl. No. 2. After ensuring that the deck is free to move without restriction. Hotwell is filled with water unto normal level. CW pumps in operation, maintaining design flow through condensers. LPH – 1, which is inside condenser, has its water boxes & tubes filled with
water. Generator hydrogen coolers & piping filled with water / cooling water system is service. Cladding if envisaged has been erected. Condenser springs are in released condition. In other words, ensure before releasing the springs that the deck has been loaded to its design capacity. Activities on spring units after releasing the springs – The deck is floated using hydraulic jacks, which are placed between the upper box or the spring unit and the steel cover plate. Either two / four hydraulic jacks are used for release of the springs. These jacks supplied by GERB, operate in a group and work with one pumping unit, which is electrically operated. In accordance with the calculated values, it is decided whether or where further steel shims should be removed or added. During releasing of TG deck springs while the additional 5,0 mm shims (for additional compression) are removed, the catenaries of the machine is to be achieved within the tolerance prescribed with the help of TG deck springs. This is done by adjusting the shims and afterwards only the rotor alignments should be taken up. Any minor variation in catenary left after releasing of TG de ck springs may be adjusted at the bearings for achieving the correct values. (The ready-made galvanized shims/steel shims are supplied in required sizes along with spring units in 3,00 mm and 2 mm thickness, so an adjustment of up to 1,00 mm is possible in spring units.) 4.1
The heights of TG deck springs are taken on four corners near the springs. Here some punch mark may be provided on all springs so that measurements are taken at the same place every time.
Check before starting the machine operation The nuts of the prestressing bolts should be screwed down to the bottom end of the bolts. Care to be taken while removing a major load the deck during maintenance machine. If major parts of the machine are removed, such as casings, rotors, etc. it is necessary to lock the spring unit-s again by hand. When these machine parts are replaced unlock the spring elements again. Maintenance of spring units No special maintenance is necessary. Check only, if there is any corrosion. In case of corrosion, paint the unit with anticorrosive paint (such as Coro-Cote). No water or other liquids should get into the viscodampers.
Replacement of spring units The supplier claims that springs of the supporting system have very remote possibility of breakages. The replacement, if needed, can be done by compressing the springs of defective unit, using the hydraulic jack system. After compression, the unit is locked by the side bolts. Then the element can be removed & replaced. Load sharing by condenser springs and TG deck springs All the loads to which condenser springs are subjected with during neck welding of the condenser with LPC are shared by condenser springs. (Dead weight of condenser, water boxes, tubes, stiffener pipes, normal level of water etc.) Weight of the static components, dynamic loads on the deck condenser cooling water static & dynamic loads are taken by deck springs. Whether TG deck can be floated under the Circumstances when CW pumps are not available? Answer is yes, it is possible. (Ref. Directive PS/TSX/T dtd 12.06.97) Often, the situation arise when CW pumps commissioning gets delayed on account of various reasons. The steps are 1)
Ensure all the components have been erected on TG deck as per details given in Sl. No. = 10.
2)
In the absence of non-availability of CW pumps, the calculated quantity of water can be filled in the steam space of condenser. This is normally equal to the weight of circulating water in the condenser tubes & water boxes plus the weight of condensate in the hot well unto normal level.
3)
Keep the condenser springs released.
4)
Float the deck as per normal procedure described earlier.
Important Note: 1)
The need to check the alignment by running CW pumps is to take corrective measures if the dynamic load of running water changes the alignment in addition to the static load of water in condenser.
2)
By simulating the water condition in the condenser, the effect of static load is taken care of but the effect of dynamic load is still pending. This can however be checked after the availability of CW pumps by reopening the couplings.
3)
Under such conditions, without releasing of the springs, the alignment of all the rotors can be done and reaming / honing including fitting of all final coupling bolts can be completed. The coupling bolts need not be elongated at this stage and when all the conditions can be fulfilled (as per sl. No. 10), the alignments can be checked & corrected.
Effect on machine vibrations when loads are withdrawn from the deck while machine is in operation. The spring elements system is designed for abnormal conditions, which may arise during the operation of the machine. Some times one half of the condensers need isolation while M/c is in operation. On some occasions, when M/c is running on load & total Ac failure takes place, apart from the steam loads, sudden withdrawal of cooling water loads of very high magnitude is observed from the TG deck. Various modes were tried during the load operation of Dahanu units. No appreciable change in vibration pattern noticed during these modes of operation. Moreover due to various problems in condenser cooling water ducts/ (BSES scope), a constant variation in the condenser cooling water loads on TG deck noticed during high tides & low tides regularly. This change in load also does not cause any significant change in vibration pattern. Prestressing of spring units: The spring units are delivered to site slightly prestressed for the purpose of transportation. The required prestressing of the springs (based on column load) is done at site before their installation in position. Prestressing corresponds to – a) b)
Static deflection of springs due to operation loads. An additional deflection of 5 mm to cover temporary over loading during the machine installation.
This additional deflection of 5 mm is equalized by shims on top of the spring units, which are removed when the prestressed units are released. (This means, that the top level of foundation is 5 mm lower and will reach final level after releasing spring units). A note on additional loading on springs by providing 5 mm shims The springs units are not only pre-compressed for Calculated load of the machine but it is further pre-compressed to prevent the loosening of the spring during installation of the equipment due to unforeseen additional loads. If major variation in calculation of load over the spring units is not expected then an additional compression of 5.00 mm is sufficient for this purpose. These shims are removed at the time of loosening of spring elements before final alignment of the machine. It is also not necessary to re move this complete 5.00 mm shims for each spring unit and the same may vary form spring unit to spring unit depending upon the variation of column level / elevation and variation in level of spring cover plate etc. The spring units are compressed in special steel frame with hydraulic jacks.
The elements are locked in this position by the side bolts into eh spring units. (Steel frame and jack are supplied by GERB).
1.0
Mounting of spring units: 1.1
Before the spring units are mounted, the supporting beams columns are marked where the units are to be located, as per the drawing.
1.2
One adhesive resilient pad is placed on the surface of the supporting beam columns for each element. The pad has same size as the contact area of the element.
1.3
The prestressed spring units are placed on top of the adhesive pads with a crane. Threaded holes are provided on top of the spring units to use eyebolts (M-20). (Caution: The heads of the prestressing bolts must be on the upper side!!!)
1.4
Like a sandwich a stack of adhesive pad steel shim and another adhesive pad is laid on top of the spring units, as per calculations. (Adhesive pads are used to avoid movement of the spring units/shims during operation of the machine)
1.5
If the level of the upper adhesive pad is not in accordance with the drawing the difference should be corrected by reducing or adding steel shims.
1.6
Then, a plastic foil is wrapped around the spring units to give protection against concrete milk or dust.
1.7
Finally, the cover steel plates are placed over the spring units. The cover steel plates will stay in position after-wards and will not be removed. It will serve the purpose of equal distribution of pressure, when jacks are used for the leveling procedure or for the release of the spring units.
(Caution: The bottom sides of the steel plates and the bottom sides of the pedestal (deck) must be on the same level). 2.0
Condition of the springs when machine is assembled – 2.1
Springs remain in locked – prestressed condition when machine is assembled.
2.2
During the installation of the machine the deck may settle 1-1.5 mm without prestressing bolts of the spring units becoming loose. (The reason is a possible plastic compression of the adhesive resilient pads above & below the spring units).
3.0
Pre-requisites for releasing the springs & subsequent leveling – 1. 2.
Completion of installation of all TG components. Erection completion of all the connected piping (Viz. MS, CRH, HRH, Extraction, gland steam, leak-off, drains etc.), and after floating these pipes o n their respective hangers / supports.
3.
Completion of insulation work on turbines & piping as per Sl. No. 2.
4.
After ensuring that the deck is free to move without restriction.
5.
Hotwell is filled with water unto normal level.
6.
CW pumps in operation, maintaining design flow through condensers.
7.
LPH – 1, which is inside condenser, has its water boxes & tubes filled with water.
8.
Generator hydrogen coolers & piping filled with water / cooling water system is service.
9.
Cladding if envisaged has been erected.
10. Condenser springs are in released condition. In other words, ensure before releasing the springs that the deck has been loaded to its design capacity. 4.0
Activities on spring units after releasing the springs – The deck is floated using hydraulic jacks, which are placed between the upper box or the spring unit and the steel cover plate. Either two / four hydraulic jacks are used for release of the springs. These jacks supplied by GERB, operate in a group and work with one pumping unit, which is electrically operated. In accordance with the calculated values, it is decided whether or where further steel shims should be removed or added.
During releasing of TG deck springs while the additional 5,0 mm shims (for additional compression) are removed, the catenaries of the machine is to be achieved within the tolerance prescribed with the help of TG deck springs. This is done by adjusting the shims and afterwards only the rotor alignments should be taken up. Any minor variation in catenary left after releasing of TG de ck springs may be adjusted at the bearings for achieving the correct values. (The ready-made galvanized shims/steel shims are supplied in required sizes along with spring units in 3,00 mm and 2 mm thickness, so an adjustment of up to 1,00 mm is possible in spring units.) 4.1
5.0
The heights of TG deck springs are taken on four corners near the springs. Here some punch mark may be provided on all springs so that measurements are taken at the same place every time.
Check before starting the machine operation The nuts of the prestressing bolts should be screwed down to the bottom end of the bolts.
6.0
Care to be taken while removing a major load the deck during maintenance machine. If major parts of the machine are removed, such as casings, rotors, etc. it is necessary to lock the spring unit-s again by hand. When these machine parts are replaced unlock the spring elements again.
7.0
Maintenance of spring units No special maintenance is necessary. Check only, if there is any corrosion. In case of corrosion, paint the unit with anticorrosive paint (such as Coro-Cote). No water or other liquids should get into the viscodampers.
8.0
Replacement of spring units The supplier claims that springs of the supporting system have very remote possibility of breakages. The replacement, if needed, can be done by compressing the springs of defective unit, using the hydraulic jack system. After compression, the unit is locked by the side bolts. Then the element can be removed & replaced.
9.0
Load sharing by condenser springs and TG deck springs 9.1
All the loads to which condenser springs are subjected with during neck welding of the condenser with LPC are shared by condenser springs. (Dead weight of condenser, water boxes, tubes, stiffener pipes, normal level of water etc.)
9.2
Weight of the static components, dynamic loads on the deck condenser cooling water static & dynamic loads are taken by deck springs.
10.0 Whether TG deck can be floated under the Circumstances when CW pumps are not available? Answer is yes, it is possible. (Ref. Directive PS/TSX/T dtd 12.06.97) Often, the situation arise when CW pumps commissioning gets delayed on account of various reasons. The steps are 1)
Ensure all the components have been erected on TG deck as per details given in Sl. No. = 10.
2)
In the absence of non-availability of CW pumps, the calculated quantity of water can be filled in the steam space of condenser. This is normally equal to the weight of circulating water in the condenser tubes & water boxes plus the weight of condensate in the hot well unto normal level.
3)
Keep the condenser springs released.
4)
Float the deck as per normal procedure described earlier.
Important Note: 1)
The need to check the alignment by running CW pumps is to take corrective measures if the dynamic load of running water changes the alignment in addition to the static load of water in condenser.
2)
By simulating the water condition in the condenser, the effect of static load is taken care of but the effect of dynamic load is still pending. This can however be checked after the availability of CW pumps by reopening the couplings.
3)
Under such conditions, without releasing of the springs, the alignment of all the rotors can be done and reaming / honing including fitting of all final coupling bolts can be completed. The coupling bolts need not be elongated at this stage and when all the conditions can be fulfilled (as per sl. No. 10), the alignments can be checked & corrected.
11.0 Effect on machine vibrations when loads are withdrawn from the deck while machine is in operation. The spring elements system is designed for abnormal conditions, which may arise during the operation of the machine. Some times one half of the condensers need isolation while M/c is in operation. On some occasions, when M/c is running on load & total Ac failure takes place, apart from the steam loads, sudden withdrawal of cooling water loads of very high magnitude is observed from the TG deck. Various modes were tried during the load operation of Dahanu units. No appreciable change in vibration pattern noticed during these modes of operation. Moreover due to various problems in condenser cooling water ducts/ (BSES scope), a constant variation in the condenser cooling water loads on TG deck noticed during high tides & low tides regularly. This change in load also does not cause any significant change in vibration pattern.
