Timely solution – MEP
The Gujarat State Electricity Corporation Ltd (GSECL) is setting up a 375 MW thermal power station at Dhuvaran, Khambat district. Dhuvaran is located near the sea coast and the upcoming power plant is also seeing the development of a town and one that will offer employment to scores of people.
Prior to this development, there stood the first coal-based power station constructed by the erstwhile Gujarat Electricity Board (GEB) with a total installed capacity of 534 MW. That has now been shut down and is being replaced with plants of high efficiency.
Larsen & Toubro Power, in a global competitive bidding, proposed a gasbased single shaft combined cycle power plant to GSECL. Soon, L&T was awarded the work on an EPC basis. L&T joined hands with Sargent & Lundy and has been engaged to carry out the initial engineering and later complete detailed engineering for the project.
A gas-based combined cycle power plant (CCPP) is defined by the arrangement of main equipment viz. gas turbine, steam turbine and generator. For a multi shaft CCPP, gas turbine and steam turbine are on different shafts with separate generators. However, for a single shaft CCPP, the gas turbine and steam turbine are on the same shaft with a common generator.
The engineering carried out is exemplary because of some unique features:
Single shaft arrangement being the first in India by an non-OEM/EPC contractor
One of the largest vertical heat recovery steam generator (HRSG) made by L&T
India’s largest three-phase single transformer
Largest common foundation of gas turbine, generator, steam turbine condenser and GTG exhaust duct
Bypass de-aerator on BFP building
Since the earlier facilities of the now defunct power plant were already present, the client was keen that the same be converted to accommodate the CCPP. This would include power block, induced draft cooling tower (IDCT), 7-bay switchyard, pre- and post-treatment water facilities, 150m3/hr ETP, cooling system, overhead tanks, central control building for plant DCS and SCADA system, service building, stores, canteen and fire station.
It was a challenge to prepare a compact plot plan in a brown field project considering the construction sequence to meet the completion schedule. The best way was to prepare a plot plan was with 3D models. The entire 3D model was prepared in-house by PLADES software.
It was decided to build the gas turbine, generator, HP turbine, IP/LP turbine, condenser, the SSS clutch and the GTG exhaust duct on a common foundation. The land required for power block area including the transformer and HRSG and BFP building was reduced by 20%.
The challenge was in the design of the building and the piping layout along with the bus duct for the generator. Though Siemens is known to have perfected the craft, L&T-S&L were executing it for the first time. Once again, 3D modelling helped to meet all the requirements.
The design of the power block building along with the common foundation of a single shaft train of GTG-STG-generator condenser- GTG exhaust duct was another challenge. The power block building column and foundation was designed by L&T-S&L civil and structural engineers for a 375 T EOT crane. Later this crane was used to erect the 370 T generators.
Innovations were required at every stage of the project backed by a high degree of accurate 3D modelling. The integrated link between analysis and modelling software was provided by Bentley Solution. The controlled and accurate modelling potentials of Bentley Solution aided in identifying the hindrances at an early stage.
Models were exported to create 3D PDF to provide details of properties at a click of button for site execution. Civil design of supporting structure was done using STAAD V8i, having an excellent interface with excel. Standard departmental calculations and innovative tools were used for application of loads, properties, and design parameters to the STAAD Model.
Routing four steam pipes of 1200mm diameter (including insulation) and feed water pipes along with the main pipe rack within a space of 5m wide corridor was not easy. All the auxiliaries of steam cycle were required to be planned within a 13m wide corridor only, so as to clear the GTG air intake duct. Navigation tools where used to for a walk through to solve installation problems.
Bentley Solution solar time feature integrated with project scheduling software was used for complete scheduling of structural erection of compact main pipe rack. The schedule, erection procedure, hindrances etc., were studied using 3D modelling.
Yet another hindrance was finding a solution to lift the Siemens single shaft generator and rotor together. An EOT crane of power block building was selected for lifting. A single EOT capacity of 375T was installed, which was demonstrated through a video.
In conventional CCPP, a de-aerator will always remain in service throughout a power plant operation cycle. At Dhuvaran, a de-aerating condenser with bypass de-aerator has been installed in place of the conventional de-aerator.
This system will perform the required de-aerating of condensate thus reducing equipment cost considerably. The advantages are a reduction in plant cost, increased power output and improved efficiency and improved plant reliability. All CCPPs with single unit use auxiliary boiler to generate auxiliary steam.
Auxiliary steam is required during startup for packing steam turbine glands.
In the Dhuvaran project, gland sealing steam will be generated from cold reheat line where the steam is available within required range of parameters and in a short time. A benefit here is reduction in number of equipment, which directly reduces plant installation and operation & maintenance cost.
The single shaft arrangement has gross plant efficiency of 58.67%, which is the highest in its class and a best possible gross heat rate of 6136 kJ kW/hr, which directly relates to lesser tariff to the user.
The cooling water system is provided with two types of water. Pond water will be used from November to March and brackish bore well water during summer.
Accordingly, the entire cooling water system has been planned with dual water system. The CCPP has an effluent treatment plant (ETP) with RO system of 150m3/hr installed inside the plant. The bore well water is converted into potable water and other steam cycle makeup water. The potable water is being supplied to the township and nearby areas.
The Dhuvaran project will use one of India’s largest natural circulation vertical HRSG. It is a triple pressure HRSG capable of producing steam that can run a steam turbine up to 150 MW. The complete design, fabrication and execution is being carried out by L&T. An advantage of the natural circulation HRSG is reduced auxiliary power consumption of 100 kW.
Switchyard control and operation by SCADA
Natural Circulation Vertical Heat Recovery Steam Generator (HRSG) by L&T
Siemens SGT5-4000F Gas Turbine, capacity 245 MW
Siemens SST5-3000 steam turbine of capacity 130 MW, two cylinder (HP,IP-LP) axial exhaust
GT/ST common generator rated 471 MVA, 20 kV
Cycle auxiliaries like BFP, CEP, DM, CW pump, vacuum pumps & heat-exchangers, etc.
By-pass de-aerator and de-aerating condensers by L&T
Induced draft cooling tower by L&T
Balance of plant including water system with ETP RO facilities, HVAC, firefighting, fuel gas system, etc.
Generator transformer – three phase unit, 20/235 kV, 477 MVA
Power evacuation by 220 kV outdoor switchyard – 2M+1T, 7 bays