Standard Technical Features of BTG System for Supercritical 660 ...

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Standard Technical Features of BTG System for Supercritical 660/800 MW Thermal Units21.1 GeneralCHAPTER- 21EQUIPMENT COOLING WATER (ECW) SYSTEMClosed circuit type ECW system shall be provided with demineralised (DM)water in the primary circuit and water tapped off from CW system upstream ofcondenser in the secondary circuit. ECW system shall be provided unit-wiseand shall cater to cooling water requirements of TG and its auxiliaries as wellas Steam Generator auxiliaries. Primary cooling circuits for auxiliaries of TGand SG packages shall be independent. The secondary cooling water circuitshall, however, be common for both the primary cooling systems.21.2 System Requirements21.2.1 For primary circuit of TG auxiliaries of each unit, a set of demineralisedcooling water (DMCW) pumps of 3x50% capacity shall discharge the coolingwater through 3x50% capacity plate heat exchangers (PHE). For SGauxiliaries of each unit, 2x100% capacity DMCW pumps and 2x100 %capacity plate heat exchangers shall be provided.21.2.2 The outlet header from plate heat exchangers shall be suitably branched off tosupply the cooling water to the various auxiliary coolers. Outlet from theauxiliary coolers shall be connected back into a common return header and ledback to the suction of DMCW pumps to complete the closed loop primarycooling circuit.21.2.3 For both the primary cooling circuits of each unit, an overhead tank ofminimum effective capacity of 10 m 3 corresponds to 60% of the tank heightshall be provided with level control facility. The overhead tank shall serve thefollowing:i) For normal make-up to closed loop cycles.ii)iii)iv)To meet expansion of closed cycle water due to temperature fluctuationsand to control other transients.For adequate Net Positive Suction Head (NPSH) for DMCW pumps.Emergency water supply to critical coolers in case of power failure.21.2.4 The pH of DM water in the closed loops shall be continuously monitored andcontrolled at around 9.5. The pH shall be controlled by dosing sodiumhydroxide or ammonia in DM water overhead tank and primary side piping forTG and SG auxiliaries.189
Standard Technical Features of BTG System for Supercritical 660/800 MW Thermal Units21.2.5 The common secondary circulating water system shall operate in parallel tothe condenser and shall receive water through a tapping on the CW inlet line tothe main condenser. Pressure of this water shall be further boosted by a set of3x50% or 2x100% capacity secondary cooling water (SCW) pumps and fedthrough the plate heat exchangers back to the CW discharge line of the maincondenser. Alternatively, separate set of pumps basis may be provided in theCW pump house to supply secondary cooling water for PHEs of each unit.21.2.6 To prevent fouling on the secondary circulating water side of the PHE,2x100% capacity self cleaning type filters shall be provided on the circulatingwater inlet header to the PHE. The filter shall be provided with an automaticback-wash arrangement to facilitate cleaning of the choked filter while thepumps are in operation. Backwashing shall be actuated and controlled eitheraccording to differential pressure and/ or by timer. Backwash shall becollected in a sump and 2x100% capacity sump pumps of adequate capacityshall be provided for disposal of collected backwash water.21.2.7 Normal/ emergency make up to the overhead tank shall be supplied throughmotorised valve(s) interlocked to open/ close automatically with level in thetank.21.2.8 A control valve shall be provided to maintain a constant pressure differentialbetween the main supply and return headers of DM water. The valve shallbypass flow to maintain a constant return header pressure to compensate forfluctuations in coolant flow to the process heat exchangers due to modulatingcontrol valves on the process coolers or if any cooler goes out of service inDM circuit.21.3 Design Requirements21.3.1 Pumps (primary side & secondary side)i) The pumps shall be of horizontal centrifugal type provided with closedtype impeller and axial split casing.ii)The pumps shall be designed as per requirements of Hydraulic InstituteStandards (HIS), USA.iii) The pumps shall have stable head- capacity characteristic with headrising continuously towards the shut off. The shut off head of the pumpshall be in the range 115-130% of the rated head. The powercharacteristics shall be of non-overloading type.iv)The pumps shall be suitable for continuous operation and shall operatesatisfactorily in the range 40% to 120% of the rated flow.v) The pump shall be driven by constant speed squirrel cage inductionmotor with speed not exceeding 1500 rpm (synchronous).190
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