Boiler Feed Water Pump Operation Questions For Power Plant Interview

Boiler Feed Water Pump Operation Questions

1.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)

2.How do cavitations occur? What are the abnormal effects of cavitations?

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.

3.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.

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

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

5.What is the velocity of water in the balance leak off line?

It should not exceed 5 m/sec on any account

6.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.

7.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

8.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

9.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.

10.How the pump speed is related to NPSH?

NPSHR varies approximately with the square of pump speed.

NPSHR = N2

11.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.

12.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.

13.What is the function minimum re circulation line or Automatic re circulation 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.

14.What are the different protection devices given for boiler feed pumps?

Pressure relief valve
Balance leak off line
Auto re circulation valve
15.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

16.What are the various pipe lines connected to BFP?

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

17.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

18.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)

19.What is the function of Boiler feed pumps (BFP) in power plant?

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

20.What are the protection interlocks given for BFPs ?.

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