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Why does vacuum in steam condenser reduce or drop??

  1-High exhaust temperature: Vacuum drops or maintains at lower side due to high exhaust steam temperature flow into steam condenser. This high exhaust temperature is mainly due to 1-Operation of Turbine at lower loads 2-More clearance in labyrinth seals 3-Not operating exhaust hood sprays 4-More load on condenser 5-Breaking of ejector U loop 2-Low circulating cooling water flow Vacuum in condenser reduces due to inadequate cooling water flow through steam condenser. This is mainly due to; 1-Problems associated with pumps 2-Air pockets in pipe line 3-Leakages in cooling water line 4-Stuck of discharge valve of pump 3-High cooling water temperature at condenser inlet Higher cooling water temperature at condenser inlet results into reduction of vacuum due to poor heat transfer from steam to water 4-Poor heat transfer in condenser Very less or poor heat transfer in steam condenser reduces vacuum to very low level resulting into high exhaust temperature &am

Thumb rules for power plant




BOILER:
  • Boiler heating surface (M2) = Boiler capacity in kg/hr/(17–18)..Ex: 115 TPH boiler will have heating surface = (115 X 1000/17 or 18) = 6390 to 6750 M2 (Appx.)......
  • Boiler flue gas ducting size (M2) = Boiler capacity in TPH/15.
  • No. of open tubes in steam drum for water recirculation = 30–31% of total no. of tubes present.
  • Deaerator steam venting capacity = Deaeration capacity X 0.1%.
  • Super heater safety valve relieving capacity at full open condition in TPH = Boiler MCR X 36–38%.......Ex: 125 TPH Boiler has SH safety valve of relieving capacity = (125 X 36/100) = 45 TPH.
  • Drum safety valve (1 no.) relieving capacity at full open condition in TPH = Boiler MCR X 46–48%.
  • All boiler safety valves (super heater and drum safety valves) relieving capacity at full open condition= Boiler MCR X 125–130%.
  • Safety valves pop up pressure = Operating pressure X 106–107%.Ex: A boiler operating pressure of 110 kg/cm2 has safety valve set at 110 X 106/100 =116.6 kg/cm2.
  • Boiler start up vent steam blow capacity 30–35% of boiler MCR on full open condition.
  • Boiler CBD water flow is 0.8 to 2% of steam generating capacity of the boiler.
  • Drum man hole door size 410 mm X 310 mm (Elliptical).
  • Boiler drum hold up capacity is 2–4 minutes at MCR operation.
  • Feed water velocity in Economiser coils 0.6 to 1 meter/sec.
  • Pressure drop in Economiser coils 0.5 to 1 kg/cm2.
  • Flue gas Pressure drop in ESP 25 to 30 mmwc.
  • A travelling grate Boiler ID fans motor rated capacity in KW = Boiler capacity TPH X 200%.....Ex: A boiler of capacity 75 TPH requires ID fan motor of rated KW = 75 X 200/100 = 150 KW each
  • A travelling grate Boiler ID fans (2 Nos) capacity (m3/sec.) with 25% extra margin = Boiler Capacity(TPH) X 95%.....Ex: A 100 TPH boiler has two ID fans of each capacity =100 X 95/(100 X 2) = 47.5 m3/sec.
  • Mass of flue gas generated = Mass of air per kg of fuel to be burned + 1.
  • Boiler fans power consumption = Total plant auxiliary consumption X 35–38%.
  • Boiler feed pumps power consumption = Total plant auxiliary consumption X 35–38%.
  • Turbine auxiliary power consumption = Total plant auxiliary consumption X 10–12%.
  • Fuel handling power consumption = Total plant auxiliary consumption X 4%.
  • For every 1% increase in bagasse moisture, boiler efficiency reduces by 0.27% and vice versa.
  • For every 5% increase in excess air for bagasse, boiler efficiency decreases by 0.18% and vice versa.
  • For every 100 kcal/kg increase in bagasse GCV, boiler efficiency increases by 1.2% and vice versa.
  • For every 0.5% increase of Hydrogen in bagasse, boiler efficiency decreases by 0.8–1% and vice versa.
  • For every 10 °C increase in flue gas temperature, boiler efficiency decreases by 0.45% and vice versa.
  • For every 100 kcal/kg increase in GCV of coal, boiler (TG) efficiency increases by 0.36% and vice versa.
  • Boiler peak load = Boiler MCR X 110%.
  • Minimum possible duration of boiler peak load is 30 minutes/shift.
  • Minimum stable operating load on the boiler is around 30% of boiler MCR.
  • Total dissolved solids = Conductivity X 06.

TURBINE AND AUXILIARIES:
  • Control oil pump capacity = AOP/MOP capacity X 10%.
  • Emergency oil pump capacity = AOP/MOP capacity X 25%.
  • Lube oil required for gear box = Total lube oil circulating X 60–65%.
  • Lube oil required for generator = Total lube oil circulating X 8–10%.
  • Lube oil required for turbine = Total lube oil circulating X 20–25%.
  • Lube oil outlet temperature = Lube oil inlet temperature + 15–20 °C.
  • Cooling tower evaporation loss = Turbine exhaust steam to condenser X 80–90%.


MAINTENANCE:
  • Minimum allowable bearing clearance in mm = 0.00185 X bearing ID
  • Maximum allowable bearing clearance in mm = 0.00254 X bearing ID.
  • Bearing grease top up quantity = 0.05 X b X d, b = bearing width in mm, d = bearing OD in mm.
  • Hub size = 2 X Shaft diameter.
  • Shaft Key size, width = (d/4) + 2, thickness = d/6, (d = diameter of shaft).
  • Minimum span for pipe line supporting in meter = (7√d)/3, where d = pipe OD in inches.
  • Threading length of half threaded bolt = 2d + 6 mm (if bolt length l <150 mm) and 2d + 12 mm (if bolt length l >150 mm).
  • Spanner size = Bolt major diameter (mm) X 1.5.
  • Nut thickness = 0.9 X d, d = nut size.
  • Bolt head thickness = 0.8 X d, d = major diameter of bolt.
  • Washer Internal diameter = D + 1…mm.
  • Washer outer diameter = 2D + 3……mm.
  • Washer thickness = D/8, where D is OD of washer.
  • Diameter of bolt head across the flat ends = 1.5 X d + 3 mm.
  • Welding current required for welding (Amps) = Welding rod size (mm) X 40 +/- 20.
  • Pipe weight/foot = (dt − t2) X 4.85...kg/foot, (pipe OD in inches & t is thickness of pipe in inch).
  • Pipe line spacing = Flange size of maximum diameter pipe + Smallest pipe size + Insulation thickness + 25 mm.
  • Preheating of steel is done if %C + %Mn/4 is >0.58.
  • Boiler platform loading capacity 500 kg/m2.
  • Boiler fire man floor platform loading capacity 1000 kg/m2.
  • Brinnel hardness number (BHN) = Rockwell hardness number X 10.8.
  • Bearing, grease or lip seals have a design life of less than 2000 hours. In a constantly running pump this would be only 83 days.
FUEL HANDLING:
  • Tail pulley, bend pulley and take up pulley Outer diameter = Head pulley OD X 80%.
  • Snub pulley OD = Head pulley OD X 60%.
  • PCS (Pull Chord Switch) are placed at every 15 meters along the length of conveyor.
  • BSS (Belt Sway Switch) are placed at every 30 meter along the length of conveyor.
  • Carrying and return self aligning transoms are placed at every 20 meters along the length of the belt.
  • Horizontal chain conveyor motor capacity = Chain span X Fuel handling capacity / 80
Example: A chain conveyor of capacity 100 TPH & having centre to center distance 30 meters requires motor of capacity 100 X 30 / 80 =37.5 KW to drive the conveyor safely
PUMPS:
  • Pump shutoff head = Design head X 1.07.
  • Pump efficiency with cold water is less than pump efficiency with hot water
  • Safe operating speed of boiler feed pumps is 55–60% of rated speed.
  • Boiler feed pumps suction strainer pressure drop should be 0.04 to 0.06 kg/cm2.
  • The pumps best efficiency point (B.E.P.) is between 80% and 85% of the shut off head.
  • A double suction pump can run with less N.P.S.H. or at faster speed without cavitating.
  • Multistage pumps reduce efficiency 2% to 4%.
  • 1% capacity of pump will reduce on every 0.025 mm increase in wear ring clearance.
  • Suction piping should be at least one size larger than the suction flange of the pump.
  • Pumps piped in series must have the same capacity (impeller width and speed).
  • Pumps piped in parallel must have the same head (impeller diameter and speed). 
  • A centrifugal pump can handle 0.5% air by volume. At 6% it will probably become air bound and stop pumping. Cavitation can occur with any amount of air.
  • A Vortex pump is 10% to 15% less efficient than a comparable size end suction centrifugal pump.
  • There should be at least 10 diameters of pipe between the suction of the pump and the first elbow.
ELECTRICAL:
  • Rating current of motor = 1.36 X hp or 1.79 3 KW.
  • Starting current of a single phase motor (1 to 10 HP) = 3 X Motor full load current.
  • Starting current of a three phase motor (up to 15 HP) = 2 X Motor full load current.
  • Starting current of a three phase motor (>15 HP) = 1.5 X Motor full load current.
  • Current carrying capacity of copper cable: 2 amps/mm2.
  • Earthling resistance for single pit is <2 ohm.
  • Voltage between neutral and earth <2 V.
  • Resistance between neutral and earth <1 ohm.
Motor body earthing strip size:
  • 85 SWG GI Wire for motors <5.5 KW.
  • 25 X 6 mm GI Strip for motors 5.5 to 22 KW and Lighting and control panels.
  • 40 X 6 mm GI Strip for motors 5.5 to 22 KW.
  • 50 X 6 mm GI Strip for motors >55 KW and D.G and Exciter Panel.
  • Motor insulation resistance = (20 X voltage)/(1000 + 2 X motor KW).
  • Single phase motor draws 7 amps current per HP.
  • Three phase motor draws 1.25 to 1.36 amps current per HP.
  • No load current of three phase motor is 30 to 40% of full load current of motor.
  • Submersible pump takes 0.4 KWH of extra energy at 1 meter drop of Water.
  • Creepage Distance 5 18 to 22 mm/KV for moderate polluted air and 25 to 30 mm/KV for highly polluted air.
  • Minimum Bending Radius for LT Power Cable is 12 X diameter of Cable.
  • Minimum Bending Radius for HT Power Cable is 20 X diameter of Cable.
  • Minimum Bending Radius for Control Cable is 10 X diameter of Cable.
  • Insulation Resistance Value for Panel = 2 X KV rating of the panel.
  • Test Voltage (AC for Meggering = (2 X Name Plate Voltage) + 1000.
  • Test Voltage (DC for Meggering = (2 X Name Plate Voltage).
  • Current Rating of Transformer = KVA X 1.4.
  • No load current of Transformer = <2% of Transformer rated current.
  • There are 4 Nos. of earth pits per transformer (2 No. for body and 2 No. for neutral earthing).
  • Diesel generator set produces 3 to 3.5 KWH/liter of diesel.
  • DG less than or equal to 1000 KVA must be in a canopy.
  • DG greater than 1000 KVA can either be in a canopy or skid mounted in an acoustically treated room.
  • DG fuel storage tanks should be a maximum of 990 Litter. Storage tanks above this level will trigger more stringent explosion protection provision.
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