Showing posts with label Boiler. Show all posts
Showing posts with label Boiler. Show all posts

Boiler refractory dry out procedure

 














Why do you carryout refractory dry out for Boilers?

It is done to ensure proper drying & curing of refractory in furnace & other areas where refractory is applied. The refractory under goes chemical changes during initial heating. While heating there must be free air flow over the refractory to ensure complete removal of moisture.

What are the rechecks carried out before refractory dry out?

Pre checks:

  • Ensure Boiler erection work is completed with all respect
  • Ensure Boiler official Hydraulic test is done
  • Ensure insulation work is completed
  • Ensure abundant quantity feed water is available
  • Ensure required quantity wood logs are available
  • Ensure sufficient & qualified operation staff is available
  • Ensure thermal expansion pointers are fitted at all required locations
  • Ensure steam drum & super heater vents are opened

Write down the standard procedure for refractory dry out

Refractory dry out is done as per OEM recommendation, too rapid heating of refractory may collapse the refractory material due to development thermal stresses. So it is recommended to heat the refractory for longer time at low temperature.

  • Cover all air & coal nozzles with bed materials, this will avoid the damage to the nozzles (This is not applicable for Travelling grate & dumping grate Boilers) while throwing wood logs into furnace
  • Select the required size wood logs generally 2 to 3” diameter & 2 to 3 feet length wood sizes are preferred. Ensure wood logs do not have nails, packing strip
  • Ensure wood logs have optimum moisture. Too dry or too wet woods are not good for dry out
  • Ensure refractory dry out is done on natural daft, no fans are necessary. Ensure all the suction & discharge dampers of fans are kept open
  • Put the woods on bed materials, spray the small amount of diesel & then fire the woods
  • Initially temperature raising should be slow at the rate of 25 deg c per hour for 3 to 4 hours
  • Then raise the boiler outlet flue gas temperature up to 100 deg C & hold for 8 to 10 hours (as recommended by OEM) for soaking the refractory inside the Boiler
  • Then raise the boiler outlet flue gas temperature up to 250 deg C & hold for 6 to 8 hours (as recommended by OEM) for soaking the refractory inside the Boiler
  • Finally raise the boiler outlet flue gas temperature up to 350 deg C & hold for 8 to 10 hours (as recommended by OEM) for soaking the refractory inside the Boiler
  • After completing the above process, firing is stopped & Boiler is allowed to cool naturally
  • After cooling down, Boiler must inspected for refractory damage/crack etc
  • Minor cracks formed during dry out procedure should be rectified with same quality refractory material

 

 Read Power plant & Calculations

 

 

 

 

 

 

 

 

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.

How to calculate steam & water pipe line size???

 









What are the factors needed to calculate steam & water pipe line sizes?

Quantity of maximum & minimum flow through the line

Pressure & temperature of the fluid

Pressure drop allowed

Velocity of the fluid in pipe

Density & specific volume of the fluid

How do you measure the steam flow through the pipe?

Steam flow is measured with the help of orifice plate, flow nozzles etc

How do you measure the water flow through the pipe?

Water flow is measured with the help of orifice plate, vortex meter, Rota meter & turbine meters installed in pipe lines.

What is the importance for calculating pipe line size for particular fluid flow?

To provide correct required flow

To avoid pressure drop of fluid

To avoid starvation

Why the velocity is the important factor while calculating the line size?

Flow = Area of the pipe X Velocity

By looking at the above relation, velocity is the critical & important parameters, as miss judgment of velocity may lead to wrong result.

Fluid having high pressure will be having high velocity & hence require lesser pipe line size & vice versa

And also fluid having lower density will be having high velocity & hence lesser pipe size and vice versa

What are the assumed velocity for various fluids flow?

Velocity of water at the suction of pump = 0.7 to 0.9 m/sec

Feed water flow at pressure 87 kg/cm2 = 2 to 4 m/sec

Saturated steam = 25 to 50 m/sec

Super-heated steam = 30 to 70 m/sec

What will happen if a 6” pipe line carrying hot water at the rate of 100 TPH suddenly contracts to 4”?

Following shall be observed

Velocity in the pipe will increase suddenly

Head or pressure loss will occur

Energy required to pump the water will increase

What are the factors that can cause the pressure drop in a pipe line?

Friction factor of pipe (pipe internal wall roughness)

Length of the pipe

Diameter of the pipe (size of the pipe)

Velocity of fluid in the pipe line

Pipe line fittings like valves, bend, tee etc. present in the pipe line

What are the effects of over sizing the pipe lines?

The cost of pipe lines & related fittings like valves, bend, Tee etc. will increase accordingly

Higher installation cost including pipe line supports & insulations

For higher sized steam pipe lines more condensate will tend to form & hence more number of steam traps are required

For higher sized steam pipe lines, there is more possibility of carryover wet steam to end user

More heat loss due to more exposed heat/hot surface area

What are the effects of under sizing the pipe lines?

For under sized pipe lines low pressure will be available for end user

In steam lines more pressure drop may cause starvation in pipe lines

Chance of erosion

Chance of water hammer & noise

Calculations:

1. Calculate the pipe line size required to pump 100 m3/hr of water at pressure 85 kgg/cm2 to the Boiler.

As discussed in the above theory part, velocity of the feed water at pressure 85 kg/cm2 is around 3 m/sec

Then flow, Q = Area of pipe line in M2 X Velocity in meter

(100/3600) m3/sec = (3.142 X D2/4) X 3 m/sec

D = 0.108 m = 108 mm

Looking at the above value, the pipe line size required should have internal diameter 108mm.

Then pipe line size = Pipe ID + 2 X thickness

For feed water pipe line having above pressure needs minimum 80 schedule, so refer carbon steel pipe line chart & select the required schedule & thickness.

2. Calculate the main steam pipe line size required for connecting Boiler out let steam to distribution header. Maximum steam flow is 125 TPH at pressure 110 kg/cm2 & temperature 540 deg C, assume velocity of steam in pipe line is 52 m/sec

Steam flow = 125 TPH = 125000 kg/hr

Density of steam at pressure 110 kg/cm2 & 540 deg C = 32 kg/m3...Refer steam table

Steam flow in m3/sec = 1250000 / (32 X 3600) = 1.08 m3/sec

We have,

Q = AV

1.08 = A X 52

A = 0.02 M2

A = 3.142 X D2/4

0.02 = 3.142 X D2/4

D = 0.159mm = 160 mm

So need pipe line of internal diameter 150 mm

Note: Outer diameter of the pipe line is standard, need to select schedule based on operating pressure & temperature to get desired line size.

3-Calculate the velocity of 55 TPH saturated steam flowing in 500 NB pipe line at pressure 1.7 kg/cm2 & 135 deg C

Density of steam at pressure 1.7kg/cm2 & temperature = 1.5 kg/m3

Steam flow in m3/sec = 55 X 1000 / (1.5 X 3600) = 10.18 m3/sec

We have,

Q = AV

10.18 = (3.142 X 0.92/4) X V

V = 15.99 m/sec

Following are the various cases taken as case study for pipe line size

 Case-1

Pressure (kg/cm2)

2.7

Temperature (deg c)

135

Density (kg/m3)

1.4

Flow TPH

135

Flow M3/sec

26.79

Line size-mm

800

Area M2

0.50

Velocity m/sec

53.28

 Case-2

Pressure (kg/cm2)

2.7

Temperature (deg c)

135

Density (kg/m3)

1.4

Flow TPH

160

Flow M3/sec

31.75

Line size-mm

900

Area M2

0.64

Velocity m/sec

49.9

 Case-3

Pressure (kg/cm2)

2.7

Temperature (deg c)

135

Density (kg/m3)

1.4

Flow TPH

80

Flow M3/sec

15.87

Line size-mm

600

Area M2

0.28

Velocity m/sec

56.13

 Case-4

Pressure (kg/cm2)

2.7

Temperature (deg c)

135

Density (kg/m3)

1.4

Flow TPH

40

Flow M3/sec

7.94

Line size-mm

450

Area M2

0.16

Velocity m/sec

49.90

 

 Read Power plant and calculations for all such articles

 

 

How do you calculate the attemperator water consumption in Boilers?

 












1-What do you mean by attemperation?

Attemperation is a method employed for controlling the super-heated steam temperature

Attemperators are used to control the main steam (super-heated steam) temperature in Boilers.

2-What are the two basic types of attemperators?

Spray type attemperator

Surface type attemperator

3-What is the main differences between attemperator & Desuper heaters?

Sl No.

Attemperator

Desuper heater

 

 

 

1

An attemperator controls steam temperature

 

Desuperheater removes whatever superheat there is in steam and reduces the temperature to a point at or nearly at saturation temperature

 

 

 

 

2

Attemperators are generally found in and/or associated with boiler steam, in zones where too high temperature affects something downstream of that point

 

Desuperheaters are used for downstream use of saturated steam

 

 

3

Outlet of the attemperation will be superheated steam only

 

Outlet of desuperheater will be saturated steam

 

 

4

Used in super-heated lines

 

Used in MP or LP steam lines

 

 Read top-6-Power plant O&M books

4-What type of attemperation method is used in modern high pressure boilers?

In modern high pressure boilers variable nozzle and spray type attemperators are generally used

5-What is the percentage of attmepration used in High pressure Boilers?

Generally varies from 8 to 15%

 6-What is the reason behind using stainless steel sleeve inside the attemperator header?

Generally SS sleeves are fitted inside the attemperator header, this sleeve performs following functions.

 

Acts as a thermal barrier, separates hot and cold working elements to mitigate the intensity of thermal cycles experienced by critical components.

Protects steam pipe from thermal shock, helps to improve secondary atomization.

 

7-What precautions should be taken during attemperator liner or sleeve design?

  •  Minimum length of straight pipe upstream of the liner should be three times the pipe diameter.
  • Length of the liner downstream from the spray nozzles should be between 3 and 6 ft, depending on the particular installation.
  • Length of straight pipe downstream of the liner should allow a residence time of 0.067 second for spray water to evaporate before the first elbow.
  • Location of the temperature sensor should be at a distance downstream of the liner that allows 0.2 seconds of residence time to ensure complete mixing of the evaporated water and superheated steam. However, if the mass flow of spray water is greater than 15% of the mass flow of superheated steam, the residence time should be increased to 0.3 seconds.

 8-What factors are considered for designing attemperator?

Following factors are considered:

  • Feed water pressure, flow rate, and temperature at the spray water control valve during various load conditions
  • Locations of temperature sensors at the upstream & downstream ends
  • Water chemistry
  • Residence time of steam & water mixture for sensing temperature at downstream end
  • Type of attemperator spray nozzle
  • Rate of atomization & size of droplets

9-What is the distance of temperature sensors from attemperator spray nozzles?

For proper controlling of steam temperature, the upstream & downstream distance of sensors should be minimum of 5D & 20D respectively for straight pipe, where D is the diameter of attemperator header

 How do you calculate the Attemperator water consumption?

1. An attemperator is used to control the 95 TPH super-heated steam temperature from 425 deg C to 395 deg C by using 110 deg C feed water. Consider the main steam & feed water pressure 87kg/cm2 & 100 kg/cm2 respectively. Calculate the quantity of attemperator water

Given data,

Mass of steam, Ms = 95 TPH

Enthalpy of steam before attemperation at pressure 87 kg/cm2 & temperature 425 deg C, H1 = 762.41 kcal/kg.

Enthalpy of steam after attemperation at pressure 87 kg/cm2 & temperature 395 deg C, H2 = 741.81 kcal/kg

Feed water enthalpy at temperature 110 deg C, Hf = 111.65 kcal/kg

For calculation of attemperator water Mw

We have the relation,

Heat lost by the super-heated steam = Heat gained by the feed water

Ms X (H1-H2) = Mw X (H2-Hf)

95 X (762.41-741.81) = Mw X (741.81-111.65)

Mw = 3.1 TPH

2. A 125 TPH Boiler having variable type attemperator control valve for controlling main steam temperature from 495 deg C to 425 deg C at pressure 67 kg/cm2. The feed water is used for attemperation is 105 deg C, calculate the quantity of water required for de-superheating.

In the above example, Boiler feed pump having head 1000 meter & efficiency 62% supplies attemperator water, then calculate the extra power consumption for attemperation. Consider motor efficiency 95%

Given data,

Mass of steam, Ms = 125 TPH

Enthalpy of steam before attemperation at pressure 67 kg/cm2 & temperature 495 deg C, H1 = 812 kcal/kg.

Enthalpy of steam after attemperation at pressure 87 kg/cm2 & temperature 425 deg C, H2 = 771 kcal/kg

Feed water enthalpy at temperature 105 deg C, Hf = 106 kcal/kg

For calculation of attemperator water Mw

We have the relation,

Heat lost by the super-heated steam = Heat gained by the feed water

Ms X (H1-H2) = Mw X (H2-Hf)

125 X (812-771) = Mw X (771-106)

Mw = 7.7 TPH

For calculation of power required for pumping 7.7 TPH of water, we have

Motor input power = Flow in m3/sec X Total head X Density of water X 9.81 / 1000 X Pump efficiency X Motor efficiency)

Density of attemperator water at temperature 105 deg C = 960 kg/m3

Attemperator flow in m3/sec = 7.7 X 1000 X / (960 X 3600) =0.022 m3/sec

Now,

Motor power = 0.0022 X 1000 X 960 X 9.81 / (1000 X 0.62 X 0.95)

Motor power = 35.17 KW


 Read all posts related to power plant..

Power plant & Calculations

80-Interview questions & answers for fresher Mechanical Engineers

1.What do you mean by Statics? It is the branch of Engineering Mechanics which deals with the forces & their effects, while acting upo...

Frequently visited posts