- Feasibility report -Energy conservation in BFP by reducing gearbox ratio

M.G.Morshad

Energy Manager / TS II

Introduction

• M/s VOITH has up with a suggestion that about 160 KW of power can be saved in BFP if input speed of HC is optimized by changing gear box ratio.

• The purpose of this presentation is to study the feasibility of such power saving opportunity in BFP of TS II

Background

For any centrifugal pump –

• Flow ( Q) is proportional to (Speed)2

• Head (H) is proportional to (Speed)

• Input power (KW) is proportional to (Speed)3

Background

• Rated flow of any pump is designed based on the minimum speed which occurs during operation at lowest grid frequency

• And rated motor capacity is designed based on the maximum speed which occurs during operation at highest grid frequency

Background

• Considering the grid condition in India, pump and motor capacity are designed based on the frequency band (47.5 Hz - 51.5Hz)

• But after implementation of ABT, grid frequency band has narrowed down to (48.6 Hz to 50.5Hz)

Background

As a result of that

• Pumping system has become oversized with respect to the present grid condition

• And therefore there is a definite possibility of power saving by optimizing the size & capacity of PUMP- MOTOR

BFP system and rated parameters

MOTORCapacity 4000KW Efficiency = 95 %

PF = 0.85 PUMPEfficiency = 81%

GB ratio 1: 3.57

HC Slip = 3%

Hydraulic Out Put 2543 KW

Electrical Input

Shaft Out put 4000 KW

Mechanical Input 3140 KW

Speed 1485 RPM

Speed 5301 RPM Speed

5141 RPM

420 m3/Hr 2222 M

Parameters that influence the performance of BFP

a) Grid Frequency - • Motor speed changes with

grid frequency

• The speed of the primary wheel changes with motor speed.

Power supply at grid frequency

Primary wheel

Parameters that influence the performance of BFP

b) Gear ratio - • It is used to match the speed

of the primary wheel with the turbine wheel

• The ratio is selected in such a way that during lowest grid frequency (48.6 Hz) - when motor operates at lowest speed, pump does not fail to deliver the rated flow

Parameters that influence the performance of BFP

c) HC slip - • The percentage speed

difference between primary and turbine wheels is known as slip.

• The slip can be controlled linearly by draining out oil between two wheels with the help of scoop tube.

Primary wheel – speed depends on grid frequency

Turbine wheel – speed depends on the position of the scoop tube

% SLIP = [(PW speed – TW speed) / PW speed] X100

Function of Hydraulic Coupling

Scoop tube withdrawal position

60%70%80%100%

Scoop tube movement

Primary wheel – speed depends on grid frequency

Turbine wheel – speed depends on the position of the scoop tubeHydraulic

Coupling

Slip & scoop tube position

• When scoop position is low (64%) - high quantity of oil drains out from HC and the slip between two wheels get increased

• Similarly when scoop position is high (73%) - less quantity of oil drains out from HC and the slip between two wheels get decreased

10

5.24

60 70 80 90

3

% Slip

% Scoop position

Performance calculation of BFP

Operating parameters of BFP – 7B at full flow (Data collected on 24/7/08 from 11:45 to 12:30 Hrs)

Flow = 405 m3/Hr

Head = 1800 M

Motor current = 326 Amps

Motor voltage = 6.59Kv

System frequency = 48.97 Hz

Measured motor speed =1458 RPM

Measured pump speed = 4930 RPM

Scoop position =73%

Performance calculation - MOTOR

Motor input = 1.732 X 326 X 6.59 X 0.85 = 3162 KW

Assumed motor efficiency = 94% Out put at motor shaft = 3162 X 0.94= 2972 KW Measured motor speed = 1458 RPM

GB ratio = 1:3.57 Torque on motor shaft = (974 X 2972)/1458 = 1985 Kg-m

Speed of the primary wheel = 1458X 3.57 = 5205 RPM

Torque on the primary wheel = 1985 / 3.57 = 556 Kg-m

MOTOR Efficiency = 94%

PF = 0.85

Torque 556Kgm

3162 KW

Out put – 2972 KWTorque – 1985Kg-m

Speed – 1458 RPM

Speed – 5205 RPM

Performance calculation – PUMP

Pump hydraulic output= 9.81X (405/3600)X1800 = 1986 KW

Rated hydraulic output = 2543 KW ( Flow – 420 m3/hr , Head- 2222M) Rated efficiency = 81% Rated mechanical input = 2543 / 0.81 = 3140 KW

Measured speed = 4930 RPM Rated speed = 5141RPM

Actual input = 3140 X ( 4930 / 5141)3 = 2769 KW

PUMPEfficiency = 71.7%

Hydraulic Out Put 1986 KW

Mechanical Input 2769 KW

Speed 4930 RPM

Performance of BFP at full flow

MOTOR Efficiency = 94 %

PF = 0.85 PUMP

Efficiency = 71%

GB ratio 1: 3.57

HC Slip = 5.28 %

Loss = 203KW Effi = 93%

Out Put 1986 KW

Electrical Input 3162 KW

Out put – 2972 KWTorque – 1985 Kg-m

Input – 2769 KW

Speed – 1458 RPM

Speed -5205 RPMTorque – 556 Kg-m

Speed – 4930 RPM

405 m3/Hr 1800 M

Performance calculation of BFP

Operating parameters of BFP at part flow (Data collected on 24/7/2008 from 11:45 – 12:30 Hrs)

Flow = 340 m3/Hr

Head = 1780 M

Motor current = 292 Amps

Motor voltage = 6.59Kv

System frequency = 49.2 Hz

Measured motor speed =1471 RPM

Measured pump speed = 4709 RPM

Scoop position =64%

Performance of BFP at part flow

MOTOR Efficiency = 94 %

PF = 0.85 PUMP

Efficiency = 68.33%

GB ratio 1: 3.57

HC Slip = 10.32%

Loss = 250 KW Effi = 90.6%

Out Put 1649 KW

Electrical Input 2833 KW

Out put – 2663 KWTorque – 1763 Kg-m

Input – 2413 KW

Speed – 1471 RPM

Speed -5251 RPMTorque – 493 Kg-m

Speed 4709 RPM

340 m3/Hr 1780 M

Power saving in BFP due to changing of GB ratio

FULL FLOW PART FLOW

GB Ratio 3.57 3.4 3.57 3.4

Motor speed 1458 RPM 1458 RPM 1471 RPM 1471RPM

PW speed 5205 RPM 4957RPM 5251 RPM 5001RPM

PW torque 556 Kg-m 556 Kg-m 493Kg-m 493 Kg-m

Motor output 2972 KW 2829 KW 2663 KW 2531 KW

Power saving 143 KW 132KW

Pump speed 4930 RPM 4930RPM 4709 RPM 4709 RPM

% Slip in HC 5.28% 0.5 % 10.32% 5.83%

Scoop position 73% 89% 64% 75%

How power saving is achieved

• Motor delivers torque to rotate the primary wheel at a constant speed

• Motor shaft power = Torque X speed

• With reduction of GB ratio speed of the primary wheel gets reduced

• But the torque delivered by the motor remains constant

• As a result of that motor shaft power gets reduced

Performance of BFP at lowest grid frequency with reduced GB

ratioPART FLOW

Grid frequency [ f ] 48.6 Hz

Motor syn speed [ Ns = (120 X f ) / P] 1458 RPM

Assumed motor slip [ %S] 0.3%

Motor speed [Nm = Ns(1-%S)] 1453 RPM

FULL FLOW

48.6 Hz

1458 RPM

0.7%

1448 RPM

Gear Box ratio [ R] 3.43.4

Primary Wheel speed [Np = Nm XR] 4940 RPM4923 RPM

Measured pump speed [Nt] 4709 RPM4930 RPM

HC slip [(Np – Nt) / Np] X 100 4.67%Neg. Slip

Scoop position 80%Not Possible

Minimum & Maximum power saving due to modification

PART FLOW

Grid frequency [ f ] 49.2 Hz (Ave)

Motor syn speed [ Ns = (120 X f ) / P]

1471 RPM

Assumed motor slip [ %S]

1476 RPM

Motor speed [Nm = Ns(1-%S)]

0.3%

48.6 Hz (Min)

1453 RPM

1458 RPM

0.3%

Gear Box ratio [ R] 3.43.4

Primary Wheel speed [Np = Nm XR] 5001 RPM4940 RPM

Measured pump speed [Nt] 4709 RPM4709 RPM

HC slip [(Np – Nt) / Np] X 100 5.83%4.67%

Scoop position (From graph) 75%78%

Torque on the Primary Wheel [T] 493 Kg-m493 Kg-m

Motor output [Np X T / 974] 2531 KW2500 KW

Power saving 132 KW163 KW

Motor output before modification 2663 KW2663 KW

Impacts on boiler filling due to change in GB ratio

Boiler filling parameters – •Motor current = 200 amps (max)•Discharge pressure = 200 Ksc (max)•Flow = 180 m3/h ( 80m3/h - Drum & 100 m3/h - recirculation)

Performance calculation during boiler filling

Motor input = 1.732 X 200 X 6.59 X 0.85 = 1940 KW

Assumed motor efficiency = 94% Out put at motor shaft = 1940 X 0.94= 1823 KW Measured motor speed = 1471 RPM

GB ratio = 1:3.4 Torque on motor shaft = (974 X 1823)/1471 = 1207 Kg-m

Speed of the primary wheel = 1471X 3.4 = 5001 RPM

Torque on the primary wheel = 1207 / 3.4 = 355 Kg-m

Pump hydraulic output= 9.81X (180 / 3600) X2000 = 981 KW

Assumed pump efficiency = 65%

Pump mechanical input = 981 / 0.65 = 1509 KW

Calculated turbine wheel speed = (1509 X 974) / 355 = 4140 RPM

Assumed torque on primary wheel = torque on turbine wheel = 355 Kg-m

Slip = (5001 – 4140)/ 5001= 17.21%

Scoop position = (5001 – 4140) / 5001= 47%

Performance of BFP during boiler filling ( Reduced GB ratio and normal

frequency )

MOTOR Efficiency = 94 %

PF = 0.85 Frequency = 49.2 Hz PUMP

Efficiency = 65%

GB ratio 1: 3.4

HC Slip = 17.21% Loss = 317KW Scoop = 47%

Out Put 981 KW

Electrical Input 1943 KW

Out put – 1826 KWTorque – 1207Kg-m

Input – 1509 KW

Speed – 1471 RPM

Speed - 5001 RPMTorque – 355 Kg-m

Speed 4140 RPM 180 m3/Hr

2000 M

MOTOR Efficiency = 94 %

PF = 0.85 Frequency = 48.6 Hz PUMP

Efficiency = 65%

GB ratio 1: 3.4

HC Slip = 17.36% Loss = 317KW Scoop = 47%

Out Put 981 KW

Electrical Input 1943 KW

Out put – 1826 KWTorque – 1224 Kg-m

Input – 1509 KW

Speed – 1453 RPM

Speed – 4940 RPMTorque – 360 Kg-m

Speed 4082 RPM 180 m3/Hr

2000 M

Performance of BFP during boiler filling ( Reduced GB ratio and 48.6Hz

frequency )

Observation • During low grid frequency (48.6Hz), BFP cannot be

run with full flow. It can only be run with part flow

• Provision may be made to operate the scoop between 47 % and 85%

• Because of this restriction, modification needs to be carried out at least in two BFP out of three

• Average power saving / BFP = ( 163+132)/2 = 147 KW

• Total power saving for 2 BFP = 147X 2 = 295 KW

• There will not be any impact on boiler filling activity due to change in GB ratio

Energy saving calculation

• Total power saving in two BFP = 147X 2 = 295 KW

• Energy saving / year = 0.7X24X365X295 = 1.808940 MU

• Extra revenue / year = 1.66 X 1808940 = Rs30,02,840.00

• Lignite saving / year = 1808940/1000 = 1809 Tons

• Carbon mitigation / year = 1800/10 = 180 Tons

• Reduction in Aux Consumptions= (0.295/210) = 0.15%

Simple Payback calculation

• Cost for modification of one GB = Rs 62,40,000.00

• Total modification cost for two BFP = Rs1,24,80,000.00

• Extra revenue / year = Rs 30,02,840.00

• Simple payback periods = (12480000/3002840) = 4.15 Yrs

Facts need to be further

investigation Q 1 – Will there be any possibility of developing vibration in pump due to change in mass at primary wheel side as a result of changing GB ratio ?

Q 2 – Will it be possible to bring back the original set up within a shortest time if problem is notice in the modified GB ?

Thank You