Questions & Answers on Boiler Hydraulic test

1-According to which law or regulation Boiler hydraulic tests are conducted?

Boilers hydraulic tests are conducted as per IBR 1950 Regulation 379

2-Why do Boilers Hydraulic tests are conducted?

  • To ensure there is no leakages in joints after erection & welding works
  • To ensure the healthiness of pressure parts & correctness of workmanship
  • To identify the weakness & defect of pressure parts
  • To complete the compliance of Boiler inspector authority
  • To avoid any incidents during boiler start up or operation

3-When shall you conduct Boiler Hydraulic tests?

Boiler Hydraulic tests are carried out;

  • After erection of new Boiler
  • After replacement of drum gaskets
  • After any repair works in pressure parts
  • After repair of replacement of critical (root valve, MSSV, MSNRV) valves
  • Yearly as per Boiler inspector compliance
  • After any alteration or modification in pressure parts

4-At what pressure do you take the Boiler Hydraulic test?

Boiler Hydraulic tests should be taken at pressure 1.25 times the maximum operating pressure or Design pressure of the Boiler

5-When shall you carryout Boiler hydraulic test at 1.5 times the maximum operating pressure or Design pressure of the Boiler?

If all the Boiler components are not tested in manufacturer premises as per Regulation 268, then Boiler hydraulic test should be conducted at pressure 1.5 times the maximum operating pressure or Design pressure of the Boiler.

6-What is the recommended temperature of Water for Boiler Hydraulic test?

The temperature of the water used as medium of pressure testing shall not be less than 20°C and greater than 50°C.

7-What are the desired parameters of water used for Boiler Hydraulic test?

Boiler water parameters

  • PH-8.5-9.2
  • Conductivity-5-10 mic S/cm
  • Silica-0.02 ppm
  • Hardness-Nil

8-Up to what time you are going to hold the Boiler at Hydraulic test pressure?

Boiler can be held up to 30 minutes at rated Hydraulic test pressure

9-When shall you conclude the completion of Boiler Hydraulic test?

The boiler shall satisfactorily withstand maximum allowable operating pressure without appreciable leakage or undue deflection or distortion of its parts for at least 10 consecutive minutes.

10-Is test pressure affects the Boiler ageing?

Yes, Boiler hydraulic test pressure should be reduced based on their ageing

11-What you will do if you observed any defects or deflection in Boiler parts during hydraulic test?

In such cases, Boiler hydraulic test should be stopped immediately & pressure shall be released

12-Is Pneumatic testing of Boilers is recommended before hydraulic test?

Yes pneumatic test of Boiler is done at pressure around 2-3 kg/cm2 to ensure leakages to avoid loss of DM water.

13-What is the rate of pressure rising during Boiler Hydraulic test?

It is around 3 to 3.5 kg/cm2/minute

14-Explain the Boiler Hydraulic test procedure

Preliminary checks

  • Ensure Boiler erection & maintenance related works are completed
  • Ensure Boiler feed pumps are healthy & ready to use (Ensure 1W+1S)
  • Ensure Initial water fill up line & pumps are ready to use
  • Ensure required quantity of DM water is available
  • Ensure DM water parameters are normal
  • Ensure DM temperature is 20 deg C to 50 deg C
  • Ensure Pressure gauges are mounted at steam drum, super heater & any other required location
  • Ensure the transmitters/pressure parts whose operating /hydraulic pressure is not recommended for hydraulic test are isolate
  • Ensure sufficient number of competitive team is available for conducting test
  • Ensure safety valves gags are available & kept near safety valves
  • Ensure, hydraulic test planning is done properly


  • Keep open Boiler’s all drum, super heater & economiser header air vent valves.
  • Ensure all drain valves of drum & headers are closed
  • Fill the Boiler, by initial fill up line by starting Deaerator or feed water tank make up pump.
  • If Boiler is completely empty, it will take 2 to 3 hours’ time to fill the Boiler (It depends on size of initial fill up line)
  • After filling the Boiler water will come out from vents of economiser, drum & super heater header.
  • Initial close the economiser header vents
  • Then close the Super heater & drum vents at pressure > 2 kg/cm2 to ensure air present in water is vented out completely (Sequence of vent closing should be from bottom elevation to top)
  • After closing the vents slowly raise the Boiler pressure by starting Boiler feed water pump or reciprocating pump (For newly erected Boilers, feed pumps may not available, so in such cases separate positive displacement pumps of required head are made available)
  • Ensure, initially the pressure rising rate is 3-3.5 kg/cm2/minute up to 25% of Hyd.test pressure, after that raise the pressure at rate 1 to 2 kg/cm2 / minute.
  • AT pressure 4 to 5 kg/cm2, flush the pressure gauge impulse lines & mount pressure gauges
  • After reaching Boiler pressure 90-95% of the operating pressure, mount the gags & apply force to ensure gags are fitted properly
  • Then continue pressurizing until Boiler pressure reaches the hydraulic test pressure. Ensure hydraulic test pressure not crossed the 6% of maximum allowable pressure
  • Hold this pressure for 30 minutes & then release the pressure to normal operating pressure
  • Then inspect all the joints, valves glands, other pressure parts for leakages & deflection.
  • Once found normal, release the Boiler pressure through blow down valves
  • AT pressure 10-15 kg/cm2 flush Boiler bottom headers
  • Remove the gags
  • Open the vent valves


 Read Boiler refractory dry out procedure


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Interview questions and answers on Boiler fans


1-What do you mean by fan & and what are the various fans used in power plant Boilers?

Fan is a Mechanical device , used to move volume air, vapour or flue gas at specific static pressure

Fans used are;

1-Induced draft fan (ID fan)

2-Forced draft fan (FD fan)

3-Secondary air fan (SA fan)

4-Cinder reinjection fan (CR fan)

2-What type of fans used in Boiler?

Generally centrifugal radial, forward & backward curved fans

3-What are the function of Boiler fans in Boiler

SL No.

Fan type



ID fan

1-To suck & expel the flue gas from Boiler

2-To maintain negative & balanced draft in furnace


FD fan

1-To provide combustion air for Boiler

2-To maintain balanced draft in Boiler

3-To maintain fluidisation on bed


SA fan/PA fan

1-To carry fuel into the furnace or bed

2-To assist fuel in suspension burning (OFA)

3-To spread the fuel in case of Biomass Boilers


CR fan

To re-inject unburnt fuel from Bank, Economiser & APH back into the furnace


4-Which type of fan is more efficient & why?

Centrifugal backward curved is more efficient as they do not over load & speed controlling is very easy. Fan operates beyond the maximum efficiency point at lower in put power

5-Differentiate between forward & backward curved centrifugal fans

Sl No.

              Backward curved

              Forward curved


Less number of blades

More number of blades


For this type of impeller, flow is produced in a radial direction because the impeller develops static pressure across the longer length of blade.  On the front side of the blade a positive pressure is generated pushing the air outwards and on the reverse side of the blade a negative pressure is generated

For this type of impeller, flow from the impeller is produced in a tangential direction.  The forward curve of the blade imparts kinetic energy to the air requiring a scroll housing to convert the kinetic energy into static pressure


Does not over load the motor at higher flow rates

Overloads the motor at higher flow rate


High pressure

Low pressure


Lower & medium flow rate

Higher flow


Higher efficiency

Lower efficiency

 6-What are the various parts of Boiler fans (centrifugal type)?

  • Parts of Boiler fans
  • Casing
  • Inlet cone
  • Evase
  • Inlet box
  • Impeller blade
  • Impeller shroud
  • Impeller back plate
  • Shaft & coupling
  • Bearings

7-What are the material of composition (MOC) of Fan’s various parts

SL No.

Particular Fan part




Mild steel IS 2062


Inlet cone

& inlet box

Mild steel IS 2062



Mild steel IS 2062


Impeller blade



Impeller shroud



Impeller back plate





 8-What is thickness of fans casing

Casing of ID fans is 8mm & that of FD, SA & PA fans is 6mm

9-What are factors considered for Boiler fans design?

Following factors are considered while designing the Boiler fans

  • Required flow
  • Inlet & outlet static pressure
  • Total pressure
  • Operating & design temperature
  • Density of gas/air
  • Dust load
  • Relative humidity
  • Nose level
  • Fan speed

 10-What type of bearings used for Boiler fans?

Generally, spherical roller bearings for fans having speed up to 1500 TPM

11-What are the various instruments used for fans?

Draft sensor (draft transmitter)

Flow meter (Aerofoil)

Temperature sensors

12-What are the protections given for fans?

Start permissive,

Inlet Damper close

Bearing temperature normal

Bearing vibration normal


High bearing vibrations trip > 7mm/sec

High bearing temperature trip>90 deg C (for journal & rolling contact bearings)

High/low static pressure

Over load

13- Explain the terms static pressure, total pressure and velocity pressure.

Static Pressure is

Resistance to flow

Equal in all directions

Can be Positive or Negative

Independent of air velocity

Measured by pressure tap perpendicular to airflow

Used for fan selection


Total Pressure is


A fluid in motion will exert a Total Pressure on an object in its path.

Total Pressure measured by pressure tap pointed directly into the air stream.

Used to find velocity pressure.

Velocity Pressure

Cannot be measured directly.

A Pitot tube uses both Static pressure and Total pressure taps.

Used for measuring CFM in a system.

Total Pressure = Static Pressure + Velocity Pressure

17- What is the differential static pressure?

 It is the difference between the static pressure at fan inlet and out let

18- What are the flow control methods employed in fans?

 Controlling inlet guide vanes

Discharge damper control method

 V belt position change on pulley (If fan is stepped pulley driven type)

 Incorporating Variable Frequency Drives (VFD) to fan motors to control the speed

19-Calculate the differential static pressure of ID fan, whose inlet static pressure is -300 mmwc & outlet 0 mmwc

We have,

Differential static pressure = Outlet static pressure-Inlet static pressure = 0-(-300) = 300 mmwc

20- Calculate the differential static pressure of FD fan, whose inlet static pressure is -10 mmwc & outlet 250 mmwc

Differential static pressure = Out let static pressure-Inlet static pressure = 250-10= 240 mmwc

21-What size of FD air duct is required to get flow 70 m3 /sec. at 11 m/sec. speed?

We have,

 Volumetric flow = Area of duct (M2) X Velocity of air (m/sec.)

Area of duct = 70 / 11 = 6.36 M2


22- State the relation between fan’s flow, static pressure, and speed and power consumption.

Following are the some affinity laws related to fans:

A. Fan speed is directly proportional to its flow.

(N1/N2) = (Q1/Q2)

Where N1 & N2 are speed RPM & Q1 & Q2 are flow in M3/hr

B. Static pressure is directly proportional to square of the speed.

(SP1/SP2) = (N1/N2)2

Where, SP1 & SP2 are static pressure at two different speed/RPM

C. Power consumption is directly proportional to cube of speed.

(P1/P2) = (N1/N2)3

Where, P1&P2 are power consumption at speed N1 & N2

23-Calculate the power consumed by an ID fan if its speed reduced from 900 RPM to 750 RPM, take 120 KW power consumption at 900 RPM

As per affinity law

(P1/P2) = (N1/N2)3

(120/P2) = (900/750)3

P2 = 69.44 KW

24-Calculate the anticipated static pressure when FD fan RPM is increased from 1100 RPM to 1175 RPM, static pressure at 1100 RPM is 220 mmwc

As per affinity law

(SP1/SP2) = (N1/N2)2

(220/SP2) = (1100/1175)2

SP2 = 268.3 mmwc

25-Boiler ID fan of flow capacity 55 m3/sec having static inlet pressure & efficiency are  is 300mmwc & 78% respectively, calculate the rated power required to run the fan if motor of efficiency 96% is used to drive the fan.

We have,

Motor rated power = Flow (M3/sec) X Static differential pressure (mmwc) / (102 X Fan static efficiency X motor efficiency)

Motor rated power P = 55 X 300 / (102 X 0.78 X 0.96)

Motor rated power P =216.03 KW

26-A Boiler FD fan of capacity 24m3/sec having total pressure 265 mmwc & Mechanical efficiency 82%, then calculate the fan shaft input power

Fan shaft power = Flow (M3/sec) X Total differential pressure (mmwc) / (102 X Fan mechanical efficiency)

Fan shaft power = 24 X 265 / (102 X 0.82) =76.04 KW

27-Calculate the rated capacity of Boiler FD fans based on following data.

SL No.




Boiler capacity

100 TPH


Coal consumption

17 TPH


Coal to air ratio



Fan inlet static pressure

5 mmwc


Draft loss in ducts from Fan to APH

15 mmwc


Draft loss in APH

80 mmwc


Draft loss aerofoil flow meter

25 mmwc


Draft loss in ducts from APH to plenum

20 mmwc


Draft loss in Grate

50 mmwc


Static efficiency of the fan



Motor efficiency



Air temperature

28 deg C



Total air required for burning the coal in Boiler = 17 X 6 =102 TPH

Take 20% extra margin on air flow , then air flow = 102 X 120% = 122.4 = 123 TPH

Convert air flow from TPH to m3/sec

Density of air at temperature 28 deg C = 273 X 1.293 / (28+273) = 1.17 kg/m3

Where, 1.293 is density of air at 0 deg C

Air flow = 123000 kg/hr /1.17 =29.2 m3/sec

Based on above flow we can select, two fans of capacity 29.2/2 = 14.6 or 15 m3/sec

Now calculate the total static pressure required to over come the resistance in air flow path

Pressure/draft loss drop = Draft loss in APH inlet ducts + Draft loss in flow element + Draft loss in APH + Draft loss in APH to grate ducts + Draft loss in grate

= 15 + 80 +25 + 20 + 50 = 190 mmwc

Take 20% extra margin on draft

Discharge static pressure = 190 X 120% = 228 = 230 mmwc

Now calculate the motor rated power, P = Flow (M3/sec) X Static differential pressure (mmwc) / (102 X Fan static efficiency X motor efficiency)

P = 15 X (230-5) / (102 X 0.75 X 0.95) = 46.43 KW

Take 15% extra margin on motor capacity

Then, compensated rated motor power = 46.43 X 115% =53.4 KW

Select nearest standard size motor i.e 55 KW

Therefore for boiler of capacity 100 TPH, FD fans capacity 55 KW X 2 are required

28-Why does ID fan trip during Boiler operations?

Reasons are

1-Over load

2-High bearing vibrations

3-High bearing temperature

4-Electrical& Instrumentation related issues like short circuit, mal functions etc

5-Low drum level

6-High Boiler pressure & temperature (if interlocks are provided)

7-Low pressure (more negative pressure) in furnace

29-Why does FD fans trip during Boiler operations?

Reasons are

1-Over load

2-High bearing vibrations

3-High bearing temperature

4-Electrical& Instrumentation related issues like short circuit, mal functions etc

5-High drum level

6-High Boiler pressure & temperature (if interlocks are provided)

7-Low pressure (more negative pressure) in furnace

8-Upon tripping of ID fan

30-What will happen if Boiler ID fan trips (one fan out of two)?

1-Same stream FD fan trips

2-Same stream SA fan trips

3-If pressure drops fuel feeding system trips

4-Boiler pressure becomes low & eventually boiler load reduces

31-What will happen if Boiler FD fan trips (one fan out of two)?

1-Same stream SA fan trips

2-If pressure drops fuel feeding system trips

3-Boiler pressure becomes low & eventually boiler load reduces

32-What are the maintenance activities that you will carryout on Boiler fans

  • Fans cleaning
  • Casing thickness checking
  • Impeller inspection, includes Shroud plate, blade and back plate thickness checking
  • Wear liner inspection and hard facing
  • Inlet cone inspection
  • Drive end non drive end bearings overhauling and grease replacement
  • Shaft run out checking
  • Coupling inspection
  • Shaft ultrasonic continuity test
  • Suction and discharge dampers and pneumatic cylinders overhauling
  • Motors servicing
  • Realignment
  • Manhole doors and inspection doors proper sealing
  • Fan trial and dynamic balancing at site

33-Write down the start of steps of Boiler fans

Start-up checks

  • Ensure all the maintenance activities are carried out on fans and equipments are normalized
  • Ensure all the instruments like bearing temperature sensor, vibration sensor etc are fitted & showing readings in DCS
  • Ensure none of the interlocks are bypassed
  • Ensure Fans suction & discharge dampers are in closed condition
  • Ensure coupling & bearing cooling fan guards are fitted properly
  • Ensure local emergency push button is healthy & in good condition
  • Give clearance to DCS or electrical person to start at 10% initially
  • Check abnormal sound from fan casing, bearing & couplings, if found normal open discharge damper slowly & subsequently suction damper

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