On the Modification of Spray Line in Pressure Reducing and Desuperheating Station at Thermal Power Stations Dr S Shanmugam, Fellow S Sakthivel, Member In thermal power stations, the temperature at pressure reducing and desuperheating station (PRDS) header is sometimes unable to contain within a specific limit. This causes instability in PRDS that leads to unsatisfactory performance of the auxiliary systems, resulting in shut down. The cause of the problem is identified, analysed and remedial measures are suggested that spray water for desuperheating the main steam supplied to PRDS can be tapped from the condensate extraction pump discharge line instead of from the boiler feed pump discharge line being practiced. Analyses show that a 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
INTRODUCTION In thermal power stations, the requisite operating parameters of PRDS is obtained by desuperheating the steam, which is tapped from main steam line. The water from boiler feed pump is utilised for desuperheating after reducing its pressure in an appropriate pressure control valve. The desuperheated steam is then distributed to different parts of the auxiliary steam consumption headers, such as, fuel atomising station, soot blowers steam consumption point, starting and main steam ejector lines. It is essential that there should not be any disturbances in the parameters, especially in temperature, in PRDS for efficient operation of thermal power stations. But many a time it has been experienced that there is instability in the values of parameters of steam in PRDS units, resulting in failure to achieve the performance of the power station. The problem of instability can be overcome by tapping spray water from the part where the pressure and temperature are conducive to the efficient operation of power stations. This paper presents a useful suggestion to avoid the aforesaid problem by analysing the important parameters of PRDS in 210 MW power stations in India. The same can be extended to higher capacity power stations too. EXISTING SYSTEM A line diagram of the existing system of PRDS in 210 MW power stations is given in Figure 1. The superheated steam is tapped from both the main lines that carry steam to the turbine. The pressure and temperature in each line are 135.1 bar and 540 ° C , respectively (Stage I). The velocity of the steam is given by the expression1, & = ρAv m
(1)
& is mass flow rate of fluid, m/s; ρ , the density, kg/m3; where m 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 will be entertained until December 31, 2005.
Vol 86, October 2005
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/s flowing at a rate of 350 t/h. The steam is tapped usually by providing a T arrangement2. The entire arrangement of the existing system is divided into five stages as it is seen in the Figure 1. The steam coming out of the pressure control valve is at a temperature of 480° C with a pressure of 17 bar. The reduction 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 in temperature of steam. The high temperature steam is then admitted to the cooler where it is supposed to be desuperheated to a little less than or equal to 200° C . The required quantity of water for desuperheating is tapped from the boiler feed pump discharge line. In 210 MW power stations, the spray water is supplied at a rate of 1.4 kg/s. The desired temperature limits are normally between 180° C and 200° C but the maximum limit should not be greater than 200 ° C 3 . The pressure control valve (PCV-2) closes automatically if the temperature exceeds 200° C , causing no supply of steam to the PRDS system. The temperature at the PRDS header is not maintained within the desired limits and at times it goes up a few degrees Celsius beyond 200 ° C . The temperature could not be brought down to a desired value even if the quantity of spray water supply from the boiler feed pump discharge line is increased to maximum possible. The reason could be due to mixing of spray water at 167° C with steam at 480° C . Greater the supply of this water, less likely will it reduce the temperature. There is thus an increase in the temperature at steam consumption headers, such as, soot blowers, oil heating station, fuel oil atomising station, main and starting ejectors lines, which reduces the unit load. This has sometimes compelled entire power generation to be stopped. With great difficulty, the temperature can be controlled manually too but it usually takes much time. Besides, in the existing system there must be 145
4.5 5
4.5 5
.4 ×
Pressure Enthalpy Mass Flow Rate Temperature Elevation Pressure Control Valve Safety Valve
f2 5.4 ×
: : : : : : :
Sp ray Va lve
P h & m T EL PCV SV
Ph - 180 m 686. .4 ba T - 1.4 k 3 kJ/ r 16 o g/s kg 7C f3 3
EL 24500
f2
73 × I 50 P1 3 5 h-3 .4 b m -9 447 k ar J T - 5 7.2 kg /kg 40 o /s C
EL 133 80
f1 08 ×
20
Ma in S tea EL mL 260 ine 00 30
V EL 2 5000
f1 59 ×
Dimensions in mm Not to Scale
EL 2 2250 0
Flow Nozz le
f 60 × 11 EL 22 15 0
f 273 f 15 9×
PCV-1 f 15 9×
× 37
SV-1 SV-2
× 6.4
30
30
P - 17 bar h - 3427 kJ/kg m - 4.17 kg/s T - 480oC
III
EL 24 50 0
IV
6.4 f2 73 ×
P-1 35 h - 3 .1 bar 409 k & J/ m-4 .2 kg kg /s T 530 o C f 15 II 9× 30 PCV-2 f 32 3.9
Co ole r
PRD S He ader
EL 2 2300
P - 19.62 bar h - 232 kJ/kg m -1.01 kg/s T - 55oC
P-1 4 h - 2 .17 bar 7 & - 97 kJ/k m g 5 T - 2 .56 kg/ s 00 oC
Existing Proposed
f 27 3×6 .4 EL 2 2280
Figure 1 Different stages at PRDS with proposed system
an exclusive pressure reducing station as the water pressure is to be reduced from 180.4 bar to about 20 bar4. The problem can be avoided by introducing a little change in the spray water tapping as explained here. PROPOSED SYSTEM Careful studies of the layout of the piping and the parameters of different lines have indicated that there is only one possibility of tapping spray water at a very low pressure and temperature in the power station. It is the condensate extraction pump (CEP) discharge line in which the pressure and temperature of the water are 19.62 bar and 55 ° C , respectively5. The proposed system is denoted as dotted line in the Figure 1 and this line is directly taken from the condensate extraction pump (CEP) discharge line. Figure 2 depicts the proposed system. The spray water line in the existing system is modified with tapping from the CEP discharge line. By providing a suitable arrangement in the CEP discharge line the water is taken to the cooler and sprayed for desuperheating the steam. The water has a low pressure (19.62 bar) and therefore there is no need for having a separate pressure reducing station. At the 146
same time, the temperature is also less (55 ° C ), which has the advantage of consuming less quantity of water. The enthalpy of water is 231.9 kJ/kg and the heat content of the water in the proposed system is 454 kJ/kg less than that of the existing system. The temperature at the PRDS header is always kept below 200° C because the water at 55 ° C is sprayed to the steam at 480 ° C . The supply of spray water also is not disturbed even when the plant is being shut down as the running of the CEP is continuous and this facilitates efficient operation of the auxiliary units. In the proposed system, there is no change in the first four stages of the existing system (Figure 1). In the fifth stage the spray water is admitted to the cooler, taken from the CEP discharge line. The diameter of the pipes is calculated using the equation (1). The values of diameter and properties6 are presented in Table 1. The diameter of the spray water line in the fifth stage in the proposed system is about 42.2% less than that of the existing system, as the values of the parameters of the spray water admitted to the cooler are very much less. The spray water velocity before the cooler is 2.02 m/s which is 17.4% greater than the velocity of water from boiler feed pump discharge line. This will obviously facilitate the process of mixing in the cooler. IE (I) JournalMC
Table 1 Comparison of spray water properties in existing and proposed stages in PRDS Parameter
Unit
Stage V Existing Scheme Before After entering leaving
Proposed system
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
Economic Analysis As the modified system eliminates the requirement of a separate pressure reducing station, it needs one isolation valve at the tapping end and one regulation valve at the waterspraying end. It means less cost of equipment and easier maintenance.
cos t c =
&sh m cc Cv
cos t w = 24 m& w c w
(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, respectively and c refers to respective cost. The calorific value of coal is assumed as 12560 kJ/kg and the cost of coal is Rs 2.50/kg and the cost of demineralised water is Rs 0.30/kg. Using equations (2) and (3), respective components are calculated and are presented in Table 2. There is a saving of over 71% in layout cost owing to the elimination of an exclusive pressure reducing station in the existing system. As the water is admitted 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 following the proposed system.
Main Steam Line
P - 180.4 bar h - 686 kJ/kg & - 1.4 kg/s m T - 167oC
(2)
EN : Non-return Valve CEP : Condensate Extraction Pump
Condenser
Main Steam Line
Condenser
Cooler
20 m
& - 5.56 kg/s m T - 200oC
P - 1962 bar h - 207 kJ/kg & - 1.01 kg/s m T - 55oC
1
E
1
From Boiler Feed Pump
E
CEP2
CEP1
PRDS Header
2
2
3
E
3
CEP3
Condensate Flow '0' m EN1
EN2
EN3
EN4
EN5
EN6
Figure 2 Schematic layout of the proposed system
Vol 86, October 2005
147
Table 2
Computation of different cost components
Description Energy Enthalpy of steam, kJ/h Coal consumption, kg/day Cost of coal consumed/day, Rs Water consumption Quantity, kg/h Daily consumption cost of demineralised water, Rs Lay out cost, Rs
Existing system
Proposed system
Savings, %
3069360.0 5865.0 14662.5
843188.4 1611.0 4027.5
72.53 72.53 72.53
5040.0
3636.0
27.86
36288.0
26179.0
27.86
2 77 500.0
80 000.0
71.20
Total savings, Rs Savings per day, Rs Layout cost, Rs
20 744.00 19 7 500.00
l
easy erection of spray water pipe lines and their maintenance; and
l
continuous availability of the system.
CONCLUSIONS Introducing the proposed system, thermal power stations of 210 MW capacities can be efficiently operated with full load and without any disturbances in the auxiliary steam supply systems. (i) There is a saving of Rs 20 744/day and a reduction in layout cost of Rs 1 97 500 can be achieved. (ii) Though the study has been done with reference to 210 MW power stations, the suggestion can be effectively implemented in other capacity power stations as well, for the nature of the problem is similar.
l
stabilisation of PRDS header can be achieved;
l
trouble free operation of the auxiliary steam consumption systems is ensured;
l
frequent lifting of safety valves and their seat failure can be avoided;
(iii) The low temperature in the spray water may cause some cold cracks in the seat of the cooler valve and cold water spraying in the opposite wall of the pipe may also introduce cold cracks in future. The formation of cold cracks can be slowed down by slightly increasing the temperature of the spray water before it is admitted to the cooler. An increase in temperature of about 8° C can be achieved by passing the spray water pipe line through the flash tank in the turbine region. The problem of the cracking in the opposite wall can also be avoided by providing a high alloy steel sleeve.
l
frequent failure of the gaskets in between the joints of flanges in non-return valves, branch valves etc can also be avoided;
1. Flow of Fluids through Valves, Fittings and Pipe. Crane Ltd, 11-12 Bouverie Street, London EC4Y 8AH, UK, 1979.
l
reduction in the rate of erosion in the seat of the spray control valve can be achieved;
2. The Schematic Diagrams of 210 MW Power Station. Power Engineers Training Society, 1983
l
instability of pressure in pressure control station and in spray water line is completely eliminated;
3. Operation and Maintenance Manual for Control Valve. Mosaneilone Valve Ltd, 1988.
l
steady operation of power station;
l
reduced energy consumption;
l
reduction in the consumption of de-mineralised water;
Features The salient features in brief on eliminating existing spray water scheme and introducing proposed system in power plants are:
148
REFERENCES
4. Operation and Maintenance of Pressure Reducing Station. Combustion Engineers, 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 Data Book. 4th Edition, New Age International Pvt Ltd, Chennai, 1997.
IE (I) JournalMC