Boiler Feed Pumps Design factors & Pump Capacity calculation

 

DESIGN DATA FOR BOILER FEED PUMS

 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

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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)

Boiler feed pumps Questions & Answers

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

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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

Factors Considered for Boiler Engineering

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.

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