Boiler feed pumps (BFP) questions & answers for interview

Boiler feed pumps (BFP) questions & answers for interview:

1-What is the function of Boiler feed pumps (BFP) in power plant?

Functions:

To supply the feed water to boilers

To conduct the Boiler hydraulic tests

To supply the desuperheating & attemperator water required for process steam lines & boilers respectively

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 2-What are the type of prime movers (drives) used for BFPs?

Prime movers:

• LT drive (415 V)
• HT drive (11 KV)
• Turbo drive (Steam driven)

3-What are the auxiliaries associated with BFP?

BFP auxiliaries

• Cooling water pump & lines
• Lube oil system
• ARC valve
• Mechanical seal flushing system
• Balance leak off line & its PRV

4-What are the various pipe lines connected to BFP?

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• Suction pipe line
• Discharge pipe line
• Bearing cooling water lines
• Jacket cooling water lines
• Mechanical seal flushing line
• ARC line (Minimum re circulation line)
• Impulse lines for instrumentation measurements (Suction pressure, discharge pressure, Differential pressure, balance leak off pressure)
• Balance leak off line

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5-What is the size of suction strainer of a BFP

It is generally 30 to 40 mesh, that is 30 or 40 hole openings in 1 linear inch on strainer.

A SS 30 wire mesh is generally wrapped on SS mesh having hole openings around 3 to 4 mm

6-What are the different protection devices given for boiler feed pumps?

• Pressure relief valve
• Balance leak off line
• Auto re circulation valve

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7-What is the function minimum re circulation line or Automatic re circulation valve (ARC VALVE?)

Minimum re circulation line is provided mainly for centrifugal pump with constant speed drive based on the system and vendor information. There are two types of minimum continuous flow required by the pump (Stable and Thermal). Pump is designed to operate at the flow greater than this flow rate. If pump is operated at less flow than the minimum continuous stable flow, it will damage bearing and internals and may abnormal vibration occur. Below the minimum continuous thermal flow, temperature of fluid will rise at faster rate. To avoid these problems, minimum re circulation line is provided. If the demand of the fluid is decreased below minimum continuous flow, then the auto re circulation valve of the pump will open and maintains the required flow and if flow increased more than minimum flow then auto re circulation valve closes. Generally for higher head flow like 1500 meter head pump the minimum circulation will be 20–25% of total capacity.

8-What is the function of balance leak off line?

Balance leak off line is used to balance the centrifugal pump shaft from axial thrust. During centrifugal pump operation, especially in multistage centrifugal, suction side will have relatively very less pressure as compared to the discharge side. Because of this, there are lot of possibilities that impeller along with the shaft and bearing will be pushed from discharge end to suction end which is also known as axial thrust. Balance line is used to balance the centrifugal pump shaft from axial thrust. Due to the axial thrust, pump bearings and internals will get damaged. To nullify this effect, a tapping from discharge end (between balancing & counter balancing disc) is connected to a balancing drum.
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9-Why the balance leak off line water is not connected to suction line to save the pump hydraulic power instead of directing it into Deaerator?

Balance leak off water temperature is little bit higher than pump operating temperature which may lead cavitations if it mixes with suction water. For some pumps where there is no risk of cavitations, in such cases this line is connected to suction side of the pump.

10-What is the operating pressure of balance leak off line ?

It is just 0.5 to 1 kg/cm2 more than pump’s suction pressure

10a-What is the standard  gap maintained between balance & counter balance discs?

It is around 0.8 to 1.2mm

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11-What does it indicate if balance pressure is increasing gradually?

It indicates the wear out of balance or counter balance disc. That is gap or clearance between these two discs has increased

12-How do you calculate the maximum allowable balance leak off pressure?

Maximum allowable balance leak off pressure = 0.03X (Shut off pressure-Suction pressure)+ Suction pressure.

13-What is the velocity of water in the balance leak off line?

It should not exceed 5 m/sec on any account

14-Why the BFP discharge water & balance leak off temperature is slightly more than that of suction water temperature?

15-Due to the compression action of water inside the pump, the water pressure rises around 2 to 3 deg C more than the suction water. Water is an in compressible fluid

15-What do you mean by the shut off pressure in centrifugal pumps?

Shut-off head is a condition, when a centrifugal pump runs with discharge valve closed. It is the maximum head generated by a centrifugal pump with zero flow and relatively less power.

16-How do cavitations occur? What are the abnormal effects of cavitations?

 

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Pump cavitation occurs when the pressure in the pump inlet drops below the vapour pressure of the liquid. Vapour bubbles form at the inlet of the pump and are moved to the discharge of the pump where they collapse and make high sound and vibrations often taking small pieces of the pump with them.

Cavitation is often characterized by:

Loud noise often described as a grinding or “marbles” in the pump.

Loss of capacity (bubbles are now taking up space where liquid should be).

Pitting damage to parts as material is removed by the collapsing bubbles.

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17-What do you mean by NPSHA & NPSHR in BFPs?

NPSHA: Net positive suction head available is the absolute pressure at the suction port of the pump.

NPSHR: Net positive suction head required is the minimum pressure required at the suction port of the pump to prevent the pump from cavitations.

NPSHA should be always greater than NPSHR (NPSHA >  NPSHR)

18-What is the significance of NPSH in BFPS?

If BFPs do not have required NPSH, then there will be more chances for formation of cavitations.

How the pump speed is related to NPSH

NPSHR varies approximately with the square of pump speed.

NPSHR = N²

19-How do you calculate the NPSHA ?

NPSHA = Absolute pressure in

NPSHa = Ha +- HZ - Hf + Hv - Hvp

Where, Ha is the absolute pressure on the surface of the liquid in the supply tank.

HZ is vertical distance between the surface of the liquid in the supply tank and the center line of the pump.

Hf is friction losses in the suction piping.

Hv is Velocity head at the pump suction port.

Hvp Absolute vapour pressure of the liquid at the pumping temperature of the pump, it could lead to cavitations of pump.

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20-What is the significance of vortex breakers in pumps?

A vortex breaker is a device/arrangement in pumps to stop the formation of a vortex when a fluid (liquid or gas) enters into pump suction. The formation of vortices can entrain vapour in the liquid stream, leading to poor separation in process steps such as distillation or excessive pressure drop, or causing cavitations.

21-What are the reasons for Vortexing in pumps?

Vortexing can occur if any of the following conditions are present:

• Low liquid levels.
• Liquid level falling greater than 1 Meter/sec.
• There is a large concentration of dissolved gases in the liquid.
• High outlet velocities in pipes leaving vessels. Generally greater than 3 meters/sec.
• Liquids near their vapour point.
• High circulation caused by asymmetrical inlet or outlet conditions.
• Inlet piping too close to the wall or bottom of the tank.

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22-What are the protection interlocks given for BFPs.

Protection interlocks: That is BFP will trip/stop on following conditions

• Low Deaerator level
• High bearing vibrations
• High bearing temperature
• Low cooling water pressure
• More differential pressure of suction strainer
• High load
• Higher balance leak off pressure
• Low speed (<40% of rated speed)

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23-Write down the BFP start permissive interlocks

• Start permissive interlocks
• Deaerator level normal
• Bearing temperature normal
• Bearings vibrations normal
• Cooling water pressure normal
• Motor bearing temperature, winding temperatures normal
• Discharge valve close
• Suction valve open
• Suction pressure normal
• Differential pressure normal
• Arc Valve open

24-How do you start the BFP?

BFP start up  sequence

• Ensure all the start permissive are healthy
• Ensure no maintenance activities are going on BFP & pump is ready to start with all respect
• Start the pump from DCS by giving >80% command to VFD
• Observe the bearing temperature, vibration & speed ramp rate
• Ensure pump has reached its 50% speed within 10-15 seconds
• If all parameters (discharge pressure, bearing temperature & vibrations, motor current, winding temperatures etc) are normal
• Then open the discharge valve slowly
• After 100% opening of discharge valve rise the speed as per your requirement
• Note: Ensure all the parameters are normal on every operation on BFP

25-Why it is not allowed to run the BFPS at speed lesser than 50% of rated speed?

For journal bearing BFPS at speed < 50%  the oil splash rings will not flash oil in bearings, leading to the damage to the bearings due to low lubricating oil.

26-What will happen if BFPs run in reverse direction?

If pump runs in reverse direction for more than 5 seconds, there will be the more chances of pumps to seize

27-How do you stop the BFP?

• Pump stop sequences:
• Reduce the pump speed slowly up to 60% of rated
• Close the discharge valve
• Shut down the pump

28-What is the recommended minimum head for BFP operation?

Should not be less than 10% of its rated head except in start-up & shutdown conditions?

29-What will happen if pump is started and stopped with discharge valve open?

• Pump may trip due to sudden motor over load
• Alignment may get disturb
• Shaft coupling may damage
• May harm to bearings of pumps and motor
• Piping supports may get disturbed
• Pump foundation fasteners may get loose
• So it is always recommended to start and stop the pump with discharge valve close.

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29-What are the reasons for pump to seize?

Following conditions can cause pump to seize:

• No suction or less liquid flow to suction
• Operating pump continuously at lesser NPSH
• Reverse direction rotation of pump
• Damaged strainer
• Foreign materials in impellers
• Uneven thermal expansion of pump internals

30-What is the recommended acceptable value for a BFP shaft run out?

It is around 0.03 mm (Max. 0.05 mm)

31-When should one can carry out alignment on BFP?

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Alignment on BFP shall be done when the temperature of the pump is <50 Deg C or in atmospheric temperature.

32-Why do you use 2 dial gauges for axial alignment of BFP?

BFP has more axial float that is 8 to 9 mm without bearings & seal & 0.8 to 1 mm with bearings & seal, so in order to get accurate readings 2 dial gauges are used for angular alignment & 1 dial gauge for parallel alignment. Refer above figure

33-What are the shutdown preservation methods for BFP?

Shutdown preservation method

• Depressurize the pump
• Drain all the water
• Fill the pump with 1:2 or 1:1  Glycol water mixture.
• Rotate the pump shaft twice in a week

34-What is the recommended bearing temperature for BFPs?

It should be less than 75 deg C (Max 90 deg c)

35-What are the recommended bearing vibrations for BFPs?

It should be less than 3 mm/sec (Max 5 mm/sec)

36-What is the allowable leakage drops for BFP mechanical seal

15 drops/minute

37-What should be allowable the DP across strainer

0.1   to 0.3 kg/cm2 (1000 to 3000 mmwc)

38-What is the filter mesh size for BFP oil replacement

Mesh size is 30 micro meters

39-What preventive maintenance activities that you are going to carryout on BFPS?

Preventive maintenance activities:

• Pump cleaning
• Oil level checking & top up if required
• Alignment correction
• Suction strainer cleaning
• Cooling water lines flushing
• Foundation bolts tightness checking

40-What is the acceptable impeller & wear ring clearance in BFP

0.1 to 0.3 mm max.

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