DESIGN DATA FOR BOILER FEED PUMPS
Design data from site:
Ø Type of liquid handled and its maximum
& minimum temperatures
Ø Water qualities like pH & Hardness
Ø Water Kinematic Viscosity (cst)
Ø Specific gravity of water at operating
temperature
Ø Net positive suction head required
(NPSHR)& available (NPSHA)
Ø Boiler capacity & operating
pressure
Ø Maximum & operating blow down rate
of Boiler
Ø Height of Steam drum
Ø Height of Deaerator (Water inlet
source)
Ø Pressure drop in Economiser
Ø No.of valves used in feed water
discharge line & corresponding pressure drop as per standard.
Ø Maximum & minimum suction pressure
available at pump suction
Ø Type of cooling water & its maximum
flow available for bearings cooling
Pump Design Data:
Ø Rated flow (M3/hr)
Ø Rated head (meters)
Ø Nominal speed & Effective speed
(RPM) (NS > ES)
Ø NPSHR (meter)
Ø Pump & Motor efficiency
Ø No.of stages of pump
Ø Motor rating
Ø Pump suction & discharge nozzles
sizes
Ø Vapour pressure (kg/cm2)
Ø Pump’s shut off head (meter)
Ø Pump minimum flow (25 to 30% depends on
pump operating head & flow)
Ø Cooling water pressure
Other considerations:
Ø Balance leak off water flow source
(generally balance leak off water is diverted to Deaerator)
Ø Pump Rotation direction (Clock wise
viewed from drive end)
Ø Cooling water flow rate (LPM)
Ø Pump’s suction & discharge elements
hydro. Test pressures
Ø Material of constructions (MOC) of all
pump internals
Ø Type of coupling used between pump
& motor shaft
Ø Type of Shaft seal used (Mechanical
seal)
Ø Protections given for pump (Protections
like, bearing vibration sensors, bearing temperature sensors, pressure relief
valve for balance leak off line, phase sequence relay for direction of
rotation, cooling water pressure, pump over load etc)
Calculate the boiler feed pump and motor
size required for a boiler of capacity 90 TPH has steam drum working pressure
88 kg/cm2. The height of the drum is 35 meter from boiler feed pump Centre. And
the suction water to pump is taken from Deaerator which is situated 15 meter
above the pump centre.
Given that,
Boiler capacity: 90 TPH = 90 M3/hr
Steam drum operating pressure = 88
kg/cm2
Steam drum height from pump centre = 35
meter
Height of Deaerator tank from pump
centre =15 meter
Assumption:
Boiler blow down 1%
Deaerator operating level from floor:
2.5 meter
Pressure drop in Boiler economizer: 2.5
kg/cm2
Pressure drop in feed water control
station: 5 kg/cm2
Pressure drop in line, gate and globe valves
and bends of feed water line: 5 Kg/cm2
Pump operating temperature: 110 °C
Economizer out let feed water
temperature: 275 °C
Pump and motor efficiency: 65% and 95%
respectively.
Total required discharge head for pump
= (Drum operating pressure + Drum height (m) + Economiser pressure drop +
Control valve pressure drop + Pressure drop in line, gate and globe valves and
bends) X 1.10 (Take 10–15% extra margin)
= (88 kg/cm2 + 35 meter + 2.5
kg/cm2 + 5 kg/cm2 + 5 kg/cm2) X 1.1
Convert all the pressure head into
gravity head in meter from formula P = Density X g X H…by taking the densities
of fluids (water) at operating temperatures.
P = Desnity X g X H
Then, we have,
Total discharge head = (1248 m + 35 m +
33 m + 52.5 m + 52.5 m) X 1.1 = 1563 meter
Pump rated flow = (Boiler MCR + Blow
down %) X 1.25 (Take 25–30% extra margin)
=
(90 + (90 X 1/100)) X 1.25
=
113.625= 115 M3/hr
The Capacity of flow seems more, it is better to consider 3 pumps 2
running & 1 stand by
Case-I:
Select 2 Nos of pumps 1 working & 1 standby (1W+1S)
For motor power, we have
Pump hydraulic power Ph = (Flow
(m3/sec.) X Total head (Hd - Hs) X g (m/sec2) X density of feed water at 110
°C)/1000
=
0.0319 X (1563 - 15 - 2.5) X 9.81 X 951/1000
=
459.94 KW
Pump shaft power Ps = Pump
hydraulic power X 100/Pump efficiency
=
459.94 X 100/65 = 707.60 KW
Motor input power = (Pump shaft power X
100/Motor efficiency) X 1.10
=
(707.60 X 100/95) X 1.10
=819.32
KW
From motor selection chart select
Standard sized motor that is 825 KW
Case-II
Select 3 Nos of pumps, 2 Working & 1 stand by (2W+1S)
Then, capacity of the one pump = 115/2 = 57.5 M3/hr (May take 58 m3/hr
round figure)
For motor power, we have
Pump hydraulic power Ph = (Flow
(m3/sec.) X Total head (Hd - Hs) X g (m/sec2) X density of feed water at 110
°C)/1000
=
0.01611 X (1563 - 15 - 2.5) X 9.81 X 951/1000
=
232.28 KW
Pump shaft power Ps = Pump
hydraulic power X 100/Pump efficiency
=
232.28 X 100/65 = 357.35 KW
Motor input power = (Pump shaft power X
100/Motor efficiency) X 1.10
=
(357.35 X 100/95) X 1.10
=
376.16 KW
From motor selection chart select
Standard sized motor that is 375 KW
Comparing Case-1 & II
Total Installation capacity of Boiler feed pumps for case-1 = 825 X 2 =
1650 KW
Total Operation power = 825 X 85% = 701.25 KW
Total Installation capacity of Boiler feed pumps for case-II = 375 X 3 =
1125 KW
Total Operation power = 375 X 2 X 85% = 637.5 KW
In view of energy conservation considering Case-II is feasible. But in
view of installation & maintenance cost Case-I is feasible.
General Questions
& Answers on BFPS
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
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?
- 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
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
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.
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?
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
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?
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.
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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.
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.
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.
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.
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)
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.
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?
31-When should one can
carry out alignment on BFP
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?
- 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
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.
