14 unknowns you must know in Boilers

 1-How do you decide Right hand side & Left hand side of a Boiler?

Boiler front is decided based on boiler outlet duct.
If you stand by facing towards boiler out let duct, then the most front part of you is deemed as Boiler front , next to that is boiler rear & LHS & RHS side of you is deemed as Boiler left hand side & right hand side.
2-Why in Some HP boilers drums are aligned at an angle 2 to 4towards right or left?
Drum slanting side depends on the connection of CBD line. If CBD line is connected at LHS side of the drum then the drum is slanted towards LHS side that is it is made down by an angle 2 to 40. This is because to avoid the blow down of excess water throughout the drum length & for such drums CBD line is extended for only for short distance only .And also it is been ensured that all the sludge will collect at slant position only.

3-How it is been decided that RHS or LHS safety valve of steam drum is set at higher pressure?
It is decided based on the slanting angle of steam, drums & CBD line connection.
If drum is aligned horizontally then you can set any side of the Safety valve at higher pressure as there is uniform spreading of sludge in drum.
Whereas if drum is slanted (made down)towards right or left from where CBD line is connected then it is needed to set that side safety valve at higher pressure to avoid carryover of sludge into the safety valve if t blows first. Such sludge will deposit on safety valves disc & seat which again leads into leakages & wrong operation related issues. So safety valve of such location is always set at higher pressure.
4-Why thickness of steam drum dish end is somewhat lesser than other area
Because dish end has spherical shape, so there develop hoop or circumferential stresses & on the other part of the drum longitudinal stress.
 For Hoop stresses
σc =  Pd/2tη
Thickness t = Pd/(2 ησ)
& For longitudinal stresses σl = Pd/4tη
Thickness t = Pd/(4 ησ)
Where P = Pressure acting & d is internal diameter of the drum
Based on above relations thickness for spherical part of the drum that is dish end, the thickness is lesser than other part of the drum.


5-Why the start up vent is used in Boilers?
Start up vent is used for
  • To provide minimum steam flow from the boiler during start up, shutdown & sudden load cut off
  • It is used to manual relieve of excess pressure
  • Used to give excess flow for temperature rising during start up or partial loads
  • Used to take excess load on boilers during peak load test
6-Why the super heater safety valve is set at lower pressure than drum safety valve
If drum safety valves set at lower pressure, then there will be very less or no steam flow to super heaters.
In order to save super heater coils from starvation due to no flow of steam during steam blow from drum safety valves, the super heater safety valves are always set at lower pressure than drum safety.
7-Why LHS/RHS water wall panels expand more (towards down) than front & rear water wall panels?
Side water panels are usually straight hence expansion readings show more value  where as front & rear water wall panel will have bends.
8-Why pressure gauges fitted at boiler firing floors show more pressure than actual (that of fitted at steam drum EL.level)
Pressure gauges show 2 to 3 kg/cm2 higher pressure due to addition of hydraulic head in PG impulse line laid from actual location to firing floor
9-Why there is a pressure difference between main steam line pressure & drum pressure?
Main steam line pressure shows lower pressure than drum pressure due to pressure loss in super heater coils. And this pressure difference increases as the number of super heater coils & steam flow increase.


10-Why do high pressure Boilers have higher efficiency & lower fuel consumption?
Because:
1-High pressure boilers have higher  saturation temperature
2-High pressure boilers have higher feed water temperature at economiser inlet
3-High pressure boiler have lower enthalpy of evaporation (latent heat)will be less
H = Hf + Hfg + Cps x (Tsup-Tsat) -
Where,
Hf = Enthalpy of liquid at operating pressure
Hfg = Latent heat
Tsup = Suepr heated steam temperature
Tsat = Saturated temperature of steam
Example:What amount of heat would be required to produce 5000 kg of steam at a pressure of 65 kg/cmand temperature 485 °C from water at temperature 175 °C?
Steam pressure P 65 kg/cm2
Steam temperature Tsup 485 °C
At above parameters, saturated temperature Ts 282.7 °C
h298.82 kcal/kg, hfg 364.47 kcal/kg
Now, enthalpy of 1 kg of superheated steam
Hsup= hhfg Cps (Tsup Ts)
hsup 298.82 364.47 0.5 (485 282.7)
hsup 764.44 kcal/kg
Amount of heat already associated with 1 kg of water (175 – 0) 175 kcal/kg
Therefore net heat to be supplied per kg is 764.44 – 175 589.44 kcal/kg


11-Why there is more CO in flue gas?
More CO in flue gas is due to improper combustion, that is due to
  • Less excess air
  • In adequate turbulent
  • Lower furnace/bed temperature
  • Higher FC in fuel
12-Why Does boiler furnace pressure fluctuate?
  • Interrupted fuel flow
  • Leakage in boiler pressure parts
  • Furnace combustion controller not working properly
  • Malfunctioning of fans pneumatic dampers
  • Higher moisture in fuel
13-Why there is more NOX in flue gas?
  • Higher NOx is due to
  • Higher bed temperature or furnace temperature
  • Higher excess air
  • More N2 in fuel
14-Why there is no Temperature gauge (TG) is fitted on steam drum of any Boiler?
In steam dream the phase of water is at saturated state, so no any necessary of providing TG. However temperature gauges are provided at drum inlet feed water line & drum outlet saturated line.
In some drums whose thickness is > 100 mm, there you may find thermo couples for measuring skit temperature. So in order to avoid  weakening of steam drums due to making number of drill holes for unnecessary instruments, the TG is not generally provided for drums.

For example if  drum PG showing  pressure 110 kg/cm2, then its temperature will be around 320 deg c. (Refer steam table for saturation temperature). And generally not used in any calculation or performance analysis, if required one can refer steam tables for saturated water

The temperature gauge or thermo couple provided at the drum outlet lines is used during plant start up.

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