On the Modification of Spray Line in Pressure Reducing andDesuperheating Station at Thermal Power Stations

Dr S Shanmugam, FellowS Sakthivel, Member

In thermal power stations, the temperature at pressure reducing and desuperheating station (PRDS) header is sometimesunable to contain within a specific limit. This causes instability in PRDS that leads to unsatisfactory performance of theauxiliary systems, resulting in shut down. The cause of the problem is identified, analysed and remedial measures aresuggested that spray water for desuperheating the main steam supplied to PRDS can be tapped from the condensateextraction pump discharge line instead of from the boiler feed pump discharge line being practiced. Analyses show thata sum of Rs 20 744 can be saved per day in addition to a substantial saving in the layout cost.

Keywords: PRDS; Condensate extraction pump; Boiler feed pump

Dr S Shanmugam is with the Department of Mechanical Engineering,National Institute of Technology Tiruchirappalli, Tiruchirappalli 620 015;while S Sakthivel is with Inspectorate of Boilers, PWD Compound,Kumarasamy Patty, Salem 636 007.

This paper was received on April 22, 2004. Written discussion on the paper willbe entertained until December 31, 2005.

Vol 86, October 2005 145

INTRODUCTIONIn thermal power stations, the requisite operating parametersof PRDS is obtained by desuperheating the steam, which istapped from main steam line. The water from boiler feedpump is utilised for desuperheating after reducing its pressurein an appropriate pressure control valve. The desuperheatedsteam is then distributed to different parts of the auxiliarysteam consumption headers, such as, fuel atomising station,soot blowers steam consumption point, starting and mainsteam ejector lines. It is essential that there should not be anydisturbances in the parameters, especially in temperature, inPRDS for efficient operation of thermal power stations. Butmany a time it has been experienced that there is instability inthe values of parameters of steam in PRDS units, resulting infailure to achieve the performance of the power station. Theproblem of instability can be overcome by tapping spray waterfrom the part where the pressure and temperature areconducive to the efficient operation of power stations. Thispaper presents a useful suggestion to avoid the aforesaidproblem by analysing the important parameters of PRDS in210 MW power stations in India. The same can be extended tohigher capacity power stations too.

EXISTING SYSTEM

A line diagram of the existing system of PRDS in 210 MWpower stations is given in Figure 1. The superheated steam istapped from both the main lines that carry steam to theturbine. The pressure and temperature in each line are 135.1 barand 540°C, respectively (Stage I). The velocity of the steam isgiven by the expression1,

&m Av= ρ (1)

where &m is mass flow rate of fluid, m/s; ρ , the density, kg/m3;

A, inside area of the pipe through which the fluid flows; and v,the velocity, m/s. The steam has the velocity of 41.44 m/sflowing at a rate of 350 t/h. The steam is tapped usually byproviding a T arrangement2. The entire arrangement of theexisting system is divided into five stages as it is seen in theFigure 1. The steam coming out of the pressure control valve isat a temperature of 480°C with a pressure of 17 bar. Thereduction in pressure from 135.1 bar (Stage I) to 17 bar (Stage III)is obtained on the assumption that the 100% line is in service.It is noted that there is only about 11% reduction intemperature of steam. The high temperature steam is thenadmitted to the cooler where it is supposed to bedesuperheated to a little less than or equal to 200°C . Therequired quantity of water for desuperheating is tapped fromthe boiler feed pump discharge line. In 210 MW powerstations, the spray water is supplied at a rate of 1.4 kg/s. Thedesired temperature limits are normally between 180°C and200°C but the maximum limit should not be greater than200°C3 . The pressure control valve (PCV-2) closesautomatically if the temperature exceeds 200°C , causing nosupply of steam to the PRDS system.

The temperature at the PRDS header is not maintained withinthe desired limits and at times it goes up a few degrees Celsiusbeyond 200°C . The temperature could not be brought downto a desired value even if the quantity of spray water supplyfrom the boiler feed pump discharge line is increased tomaximum possible. The reason could be due to mixing ofspray water at 167°C with steam at 480°C . Greater the supplyof this water, less likely will it reduce the temperature. Thereis thus an increase in the temperature at steam consumptionheaders, such as, soot blowers, oil heating station, fuel oilatomising station, main and starting ejectors lines, whichreduces the unit load. This has sometimes compelled entirepower generation to be stopped. With great difficulty, thetemperature can be controlled manually too but it usuallytakes much time. Besides, in the existing system there must be

146 IE (I) Journal�MC

an exclusive pressure reducing station as the water pressure isto be reduced from 180.4 bar to about 20 bar4. The problemcan be avoided by introducing a little change in the spraywater tapping as explained here.

PROPOSED SYSTEM

Careful studies of the layout of the piping and the parametersof different lines have indicated that there is only onepossibility of tapping spray water at a very low pressure andtemperature in the power station. It is the condensateextraction pump (CEP) discharge line in which the pressureand temperature of the water are 19.62 bar and 55°C ,respectively5. The proposed system is denoted as dotted line inthe Figure 1 and this line is directly taken from the condensateextraction pump (CEP) discharge line. Figure 2 depicts theproposed system.

The spray water line in the existing system is modified withtapping from the CEP discharge line. By providing a suitablearrangement in the CEP discharge line the water is taken tothe cooler and sprayed for desuperheating the steam. Thewater has a low pressure (19.62 bar) and therefore there is noneed for having a separate pressure reducing station. At the

same time, the temperature is also less (55°C ), which has theadvantage of consuming less quantity of water. The enthalpyof water is 231.9 kJ/kg and the heat content of the water in theproposed system is 454 kJ/kg less than that of the existingsystem. The temperature at the PRDS header is always keptbelow 200°C because the water at 55°C is sprayed to thesteam at 480°C . The supply of spray water also is notdisturbed even when the plant is being shut down as therunning of the CEP is continuous and this facilitates efficientoperation of the auxiliary units.

In the proposed system, there is no change in the first fourstages of the existing system (Figure 1). In the fifth stage thespray water is admitted to the cooler, taken from the CEPdischarge line. The diameter of the pipes is calculated using theequation (1). The values of diameter and properties6 arepresented in Table 1. The diameter of the spray water line inthe fifth stage in the proposed system is about 42.2% less thanthat of the existing system, as the values of the parameters ofthe spray water admitted to the cooler are very much less. Thespray water velocity before the cooler is 2.02 m/s which is17.4% greater than the velocity of water from boiler feedpump discharge line. This will obviously facilitate the processof mixing in the cooler.

Figure 1 Different stages at PRDS with proposed system

Main

Stea

m Li

ne

EL 13

380

EL 26

000

f 159

× 30

f 108

× 20

EL 25

000

EL 22300

f 159 × 30

f 60

× 11

EL 22150 PCV-1

f 159 × 30

f 159 × 30

f 273 × 6.4

f 323.9 × 37

PCV-2II

P - 135.1 barh - 3409 kJ/kg&m - 4.2 kg/sT - 530oC

P - 1

35.4

bar

h - 3

447

kJ/kg

m -9

7.2

kg/s

T - 5

40o C

f 273

× 50

EL 222500

PRDS Header

Flow Nozzle

P - 17 barh - 3427 kJ/kgm - 4.17 kg/sT - 480oC

III

IV

SV-1 SV-2

f 273 × 6.4

EL 24

500

P - 14.17 barh - 2797 kJ/kg&m - 5.56 kg/sT - 200oC

Coole

r

Spray

Valve

P - 1

80.4

bar

h - 68

6.3 kJ

/kgm

-1.4

kg/s

T - 1

67o C

f 33.4

× 4.5

5f 2

5.4 ×

4.55

P - 19.62 barh - 232 kJ/kgm -1.01 kg/sT - 55oC

Existing

Proposed

EL 22280

P : Pressureh : Enthalpy&m : Mass Flow Rate

T : TemperatureEL : ElevationPCV : Pressure Control ValveSV : Safety Valve

Dimensions in mmNot to Scale

EL 24

500

f 273

× 6.4

V

I

Vol 86, October 2005 147

Economic AnalysisAs the modified system eliminates the requirement of aseparate pressure reducing station, it needs one isolation valveat the tapping end and one regulation valve at the water-spraying end. It means less cost of equipment and easiermaintenance.

cos tm hC

ccs

vc=

&

(2)

cos t m cw w w= 24 & (3)

where cost refers to total cost; h, specific enthalpy of steam;Cv, the calorific value of coal generally used in power stations;suffixes c, w and s refer to coal water and steam, respectivelyand c refers to respective cost. The calorific value of coal isassumed as 12560 kJ/kg and the cost of coal is Rs 2.50/kg andthe cost of demineralised water is Rs 0.30/kg. Using equations (2)and (3), respective components are calculated and arepresented in Table 2. There is a saving of over 71% in layoutcost owing to the elimination of an exclusive pressurereducing station in the existing system. As the water isadmitted to the cooler at much reduced temperature (55°C ),the proposed system certainly consumes 72.14% of water,resulting in an additional saving of 27.86% in water cost.Besides, there is a substantial saving in coal consumption too.It is possible to save 4.254 t of coal daily by merely followingthe proposed system.

Table 1 Comparison of spray water properties in existing and proposedstages in PRDS

Parameter Unit Stage V Existing Scheme Proposed Before After system entering leaving

Pressure, P bar 180.40000 20.000000 19.620000

Temperature, T °C 167.00000 140.000000 55.000000

Mass flow rate, m kg/s 1.40000 1.400000 1.010000

Enthalpy, h kJ/kg 685.90000 590.200000 231.900000

Diameter, D mm 33.40000 33.400000 25.400000

Density, r kg/m3 917.40000 926.780000 986.190000

Velocity, V m/s 1.74000 1.720000 2.020000

Specific volume, v m3/kg 0.00109 0.001079 0.001014

&m - 5.56 kg/sT - 200oC

Main Steam Line

P - 180.4 bar h - 686 kJ/kg&m - 1.4 kg/s

T - 167oCMain Steam Line

Coole

r

PRDSHeader

From Boiler Feed Pump

P - 1962 barh - 207 kJ/kg&m - 1.01 kg/s

T - 55oC

'0' m

CondensateFlow

20 m

EN1EN4

EN2EN5

EN3EN6

1

CEP1

2

CEP2

3

CEP3

E 1 E 2 E 3

CondenserCondenser

EN : Non-return ValveCEP : Condensate Extraction Pump

Figure 2 Schematic layout of the proposed system

148 IE (I) Journal�MC

Features

The salient features in brief on eliminating existing spraywater scheme and introducing proposed system in powerplants are:

l stabilisation of PRDS header can be achieved;

l trouble free operation of the auxiliary steamconsumption systems is ensured;

l frequent lifting of safety valves and their seat failurecan be avoided;

l frequent failure of the gaskets in between the joints offlanges in non-return valves, branch valves etc can alsobe avoided;

l reduction in the rate of erosion in the seat of the spraycontrol valve can be achieved;

l instability of pressure in pressure control station andin spray water line is completely eliminated;

l steady operation of power station;

l reduced energy consumption;

l reduction in the consumption of de-mineralisedwater;

Table 2 Computation of different cost components

Description Existing Proposed Savings,system system %

EnergyEnthalpy of steam, kJ/h 3069360.0 843188.4 72.53Coal consumption, kg/day 5865.0 1611.0 72.53Cost of coal consumed/day, Rs 14662.5 4027.5 72.53

Water consumptionQuantity, kg/h 5040.0 3636.0 27.86Daily consumption cost ofdemineralised water, Rs 36288.0 26179.0 27.86

Lay out cost, Rs 2 77 500.0 80 000.0 71.20

Total savings, RsSavings per day, Rs 20 744.00Layout cost, Rs 19 7 500.00

l easy erection of spray water pipe lines and theirmaintenance; and

l continuous availability of the system.

CONCLUSIONSIntroducing the proposed system, thermal power stations of210 MW capacities can be efficiently operated with full loadand without any disturbances in the auxiliary steam supplysystems.

(i) There is a saving of Rs 20 744/day and a reduction inlayout cost of Rs 1 97 500 can be achieved.

(ii) Though the study has been done with reference to210 MW power stations, the suggestion can beeffectively implemented in other capacity powerstations as well, for the nature of the problem issimilar.

(iii) The low temperature in the spray water may causesome cold cracks in the seat of the cooler valve andcold water spraying in the opposite wall of the pipemay also introduce cold cracks in future. Theformation of cold cracks can be slowed down byslightly increasing the temperature of the spray waterbefore it is admitted to the cooler. An increase intemperature of about 8 C° can be achieved by passingthe spray water pipe line through the flash tank in theturbine region. The problem of the cracking in theopposite wall can also be avoided by providing a highalloy steel sleeve.

REFERENCES1. �Flow of Fluids through Valves, Fittings and Pipe.� Crane Ltd, 11-12 BouverieStreet, London EC4Y 8AH, UK, 1979.

2. �The Schematic Diagrams of 210 MW Power Station.� Power EngineersTraining Society, 1983

3. �Operation and Maintenance Manual for Control Valve.� Mosaneilone ValveLtd, 1988.

4. �Operation and Maintenance of Pressure Reducing Station.� CombustionEngineers, UK, 1982.

5. �Construction and Operation of 210 MW Turbine.� Combustion Engineers,UK, GF3 GEF3 3.4.

6. C P Kothandaraman and S Subramanian. �Heat and Mass Transfer DataBook.� 4th Edition, New Age International Pvt Ltd, Chennai, 1997.