Comparative Research of Methods for Performance Improvement of High Pressure Boiler Feed Pumps in Power Plants

Comparative Research of Methods for Performance Improvement of High Pressure Boiler Feed Pumps in Power Plants

Gopikrishna Balakrishnan, Rohan Mathew Philip, Aravind Sudev, Ruben Bijy Thomas

Department of Mechanical Engineering, Mar Athanasius College of Engineering Ernakulam, Kerala

Abstract High Pressure Boiler Feed Pump (BFP) is an important component of any thermal power plant. Its function is to pump de-aerated water from the de-aerator to the boiler. These pumps are normally high pressure units that uses suction from condensate return system .It can be of centrifugal pump type or positive displacement type; for the purpose of this study we conducted this research on centrifugal pump. But the problem here is that The HPBFP discharge pressure of the pump is around 160 kg/cm2 whereas HP-drum pressure is around 80kg/cm2. So there is huge loss of pressure in HP drum, hence huge loss of energy. This means that there is huge amount of throttling which is currently taking place to bring the pressure to 80 kg/cm2 which ultimately leads to huge wastage of power and high maintenance to the throttling valve in the long run. So after extensive research four solutions were found and on further analysis and feasibility the best solution was found which could solve the problem.

Keywords HPBFP; De-staging parts; Energy Losses;

Retrofitting; Hydraulic Coupling; Speed Control; Throttling; Variable frequency drive

1. INTRODUCTION

   For the system which is considered, it is observed that HPBFP discharge pressure is around 160 kg/cm2, whereas HP drum pressure is less than 80 kg/cm2. There is a huge throttling in pressure from high pressure (HP) pump to HP drum during normal base load operation and hence huge loss in energy. Further this also leads to erosion in feed regulatory system control valve (FRSCV) and HP desuperheater valve with the passage of time.

   At present HPBFP design TDH is 1510 MLC corresponding to the flow of 265 M3/hr. The HPBFP design TDH has been selected at maximum capability point (i.e.) Peak load, 28 0C ambient, 32 0C CWT, 3% make up. If we redesign HPBFP TDH/ Discharge pressure for naphtha firing base load operation the new TDH shall be 1360.99 MLC corresponding to the flow of 255 m3/hr.

   Considering the above facts the aim is to reduce the pressure/ flow in HPBFP to the extent possible without affecting the process requirements by suitably reducing the speed of pump. This will result in a reduction in power consumption of approx.400 kW (2 X 200 kW) which amounts power saving of approx. 3.5 MU per annum and reduction in valve internal erosion.

2. MATHEMATICAL MODEL OF PROBLEM

   DEFINITION

   From Affinity Laws: All centrifugal pumps follow the Affinity laws which are given below

   Q N Q D

   H N2 H D2

   P N3 P D3

   Where,

   N is the speed of the pump, in rpm D is the diameter of the impeller

   • Pressure reduction from 1507.7 to 1360.99 mlc

   • Speed reduction = (1507.7 /1360.99) = (4285)2 /

     n

     2

     2

   • n2 = 4071.18

   • Say 4072 rpm

   Fig. 1. Depicts the variation in parameters for different speeds. Hence the required reduction in pressure can be obtained by speed reduction. So to solve this problem the best method is to be adopted such that the required reduction in pressure is obtained without sacrificing the performance of the system.

   Fig. 1. Speed variation affecting centrifugal pump performance

3. PROPOSED SOLUTIONS

   1. Changing Gear Box Internals

      Here we reduce the speed by varying the gear ratio, which is the ratio of number of teeth of driven gear to the driving gear. We can reduce the speed by increasing the gear ratio and can increase the speed by reducing the gear ratio.

   2. De-Staging of HPBFP

      Destaging is a method of reducing the differential pressure of multistage pump by deactivating one, or more, of its stages. Stage deactivation is done by taking out an impeller and replacing it with destaging parts.

      Design Calculation

      Discharge , Q = 265 m3/hr = 0.0736 m3/s

      Head in m of water , H = 1409 m of water

      = 932.78 kW

      Output power in kW =

      Input power = 1166 kW

      Efficiency of BFP =

      BOOSTER PUMP

      Design Point

   3. Retrofitting of Hydraulic Coupling

      A fluid coupling is a hydrokinetic transmission that performs like a centrifugal pump and a hydraulic

      Suction Temperature

      = 1500c

      turbine. The input drive (e.g. electric motor or Diesel engine) is connected to the pump/impeller Mechanical energy is conveyed via the pump/impeller to the oil in the coupling.

      The oil moves by centrifugal force across the blades of the turbine towards the outside of the coupling. The turbine absorbs the kinetic energy and develops a torque which is always equal to input torque, thus causing rotation of the output shaft. The wear is practically zero since there are no mechanical connections. The efficiency is influenced only by the speed difference (slip) between pump and turbine, i.e. fluid level.

      slip %=(( input speed – out speed) / input speed) x 100

   4. Retrofitting of VFD (Variable Frequency Drive)

   A variable-frequency drive (VFD) (also termed

   Specific gravity = 916.9 kg/cm2

   Dynamic Head = 106 m of water Flow rate = 265 m3/hr

   Input Power = 100 kW Speed of Pump = 1485 rpm

   Design Calculation

   Discharge , Q = 265 m3/hr =0.0736 m3/s Head in m of water ,H = 106 m of water

   Output power in kW =

   =

   adjustable-frequency drive, variable-speed drive, AC drive, micro drive or inverter drive) is a type of adjustable-speed drive used in electro-mechanical drive systems to control AC motor speed and torque by varying motor input frequency and voltage.

   Input power =

   =

   Efficiency of Booster pump =

   1. kW 100 kW

4. CALCULATIONS

   1. High Pressure Boiler Feed Pump

   To find the losses in the HPBFP we have to find out the actual, theoretical and overall efficiency of the pump and for that we collected the experimental values and made the required calculations.

   Capacity	=	265 m3/hr
   Head	=	1409 mlc
   Temperature	=	1500c
   Specific gravity	=	0.9169
   Efficiency	=	80%
   Power	=	1166 kW
   Speed	=	4285 rpm
   NPSHR	=	16 mlc

   Design Point

   Combined Efficiency of BFP & Booster Pump Input =1266 kW,

   Output =1002.95 kW

   Combined Efficiency =

   Overall Efficiency overall = pump x motor

   Motor Input = 1318.75 kW Motor Output =1266 kW

   overall = pump x motor = 79.22% x 96%

   => overall = 76.05%

   Actual Calculation (from table)

   Boiler Feed Pump

   Discharge , Q = 193.93 T/hr

   Head in m of water =

   Pressure = 139.5 KSC

   Specific gravity = 0.9252

   Head, H =

   Head2=1519.39 m of water

   Input: Input power=1386.54 kW

   Efficiency= = =53.78%

   Output power in kW

   =

   Input power of main pump

   Efficiency of BFP

   =

   Booster Pump

   = =732.21 kW

   = 1370.67 kW

   = 53.78%

   Booster Pump Capacity

   Capacity 1/ capacity 2 =speed 1/ speed 2 Capacity 2 =194.68 T/hr

   Head 1/ head 2 = (speed 1/ speed 2) 2

   Head 2 =106.736 m of water

   Input 1 / Input 2 = (speed 1/ speed 2) 3

   Input power =101.1578

   Discharge = 0.05387 m3/s Head, H= 105.92 m of water

   Head = x10 m of water Pressure =9.8 KSC

   Specific gravity = 0.9252

   Output Input = 100 kW

   Efficiency

   3.2.4 Combined Efficiency of BFP& Booster Pump

   Output =OutputBFP + OuputBP =737.21+51.78 = 788.99 kW Input = InputBFP + InputBP =1370.67+100 = 1470.67 kW

   Efficiency

   overall = pump x motor 53.65% x 96%

   => overall 51.50%

   Corrected Value

   Using affinity laws we have to correct these values. Correction factor =

   Capacity: Capacity2= =194.68

   Head:

   Efficiency = output / input = (52.379 /101.1578) * 100

   = 51.78%

   Combined efficiency = 53.6% Efficiency from Graph Discharge =194.68

   From graph capacity, Q =195 T/hr

   Efficiency = 66% (from efficiency vs. capacity graph) Efficiency = 53.6 % (corrected to 4285 rpm)

   Loss = Efficiency from graph Efficiency (actual) Loss = 66-53.78 = 12.22 %

   Fig. 2 HPBFP Performance Curve

5. COMPARISON & SELECTION FROM THE PROPOSED SOLUTIONS

   By tabulating and outlining comparison of the above methods as shown in Table 1 and listing the merits and demerits of the applications of each proposed solutions in rectifying the speed control or controlling the pressure head of water discharge in the High pressure Boiler Feed Pumps to HP-Boiler drum a holistic comparison was obtained.

   Table 1. Comparison of Proposed solutions

   • For a fixed optimum working conditions we have seen that Constant Discharge Drives such as Retrofitting of gear box integrals and Pump- destaging methods are suited.

   • The economic advantage and space utilization of Constant Discharge Drives are may be greater than Variable Speed Drives, but the productive & flexible nature of the Variable speed drives outnumber the constant discharge drives

   By a thorough inspection from the comparison based on the criterias,

   • Flexibility to meet varying energy demands efficiently.

   • The production and maintenance are economical.

     The Dynamic Fluid couplings are most preferred because of their simplicity & flexibility in working and low maintenance.

     Table 2. Rating of Various Proposals

     So the priorities should be in the order,

     Fig 3. Priority of proposed solutions

     VII. CONCLUSION

6. SELECTION BASED FROM THE COMPARISON STUDIES

• From Table 1. the Variable Speed Drives such as Dynamic fluid couplings and VFDs are much superior, efficient and flexible to meet the energy demands of the power plant.

The study solved the basic problem which was to reduce the pressure from 160 kg/cm2 to 80 kg/cm2. After the initial part of the study a mathematical model was made to make this engineering problem more tangible. Hence from this mathematical model by incorporating affinity laws it was found that by reducing the speed this reduction of pressure could be achieved. Hence after extensive literature survey in

this direction the four proposals were shortlisted which could solve this engineering problem.

Then after considering the technical as well as practical aspects of all these options a holistic rating process was carried out and the most feasible solution was found which was retrofitting of Hydraulic Coupling to the already existing system. Retrofitting refers to the addition of newer technology or features to older systems. In power plants retrofitting is used for improving power plant efficiency / increasing output / reducing emissions. Fig 3. Shows how the system would look like after incorporating hydraulic coupling in the system.

With significant improvement in technology and emergence of fields like mechatronics etc. the scope and application of the process of retrofitting has increased. The advantages of adopting this process were analysed in detail and have been cited in this report.

Hence the performance of the system was improved by retrofitting hydraulic coupling without any significant changes to the existing system.

Fig 4 Schematic Representation after retrofitting of hydraulic coupling.

REFERENCES

1. Gunther H. Peikert, Variable speed fluid couplings driving centrifugal compressors and other centrifugal machinery, VoithTransmissions, pp. 59-66.

2. Dennis P.Connors, John D. Robechek, and Dennis A. Jarc, Adjustable Speed Drives As Applied to Centrifugal Pumps,

   Reliance Electric Company, April 1982, pp. 572-576

3. Hirendra B. Patel, Pravin P. Rathod, and Arvind S.Sorathiya, Design and performance analysis of hydro kinetic fluid coupling, IJERA, Vol. 2, Issue 4, July-August 2012, pp. 227-232.

4. Y. Yorozu, M. Hirano, K. Oka, and Y. Tagawa, Electron spectroscopy studies on magneto-optical media and plastic substrate interface, IEEE, Volume:2 , Issue 8, Aug. 1987, pp. 740 741.

5. M. Young, The Technical Writers Handbook. Mill Valley, CA: University Science, 1989.