Turbine protection system for bypass operation

A steam turbine generator power plant which includes a main steam bypass path and an auxiliary steam bypass path which bypasses the high pressure turbine. The auxiliary bypass path includes a steam jet compressor which functions to pump up the pressure at the high pressure turbine exhaust to a value compatible with discharge into the main bypass path, and does so in response to measurement of the high pressure turbine output temperature.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention in general relates to steam turbine bypass systems, and more 
particularly to a control arrangement for preventing excessive 
temperatures in the high pressure turbine when the bypass system is in 
operation. 
2. Description of the Prior Art 
In a typical steam turbine power plant, a steam generator such as a boiler 
produces steam which is provided to a high pressure turbine to a plurality 
of steam admission valves. Steam exiting the high pressure turbine is 
reheated in a conventional reheater prior to being supplied to a lower 
pressure turbine, the exhaust from which is conducted into a condenser 
where the exhaust steam is converted to water and supplied to the boiler 
to complete the cycle. 
With steam turbines equipped with a bypass system, the steam admission 
valves to the turbine may be closed, or partially closed, while still 
allowing steam to be produced by the boiler at a load level independent of 
steam turbine load. The bypass system is advantageously used for hot 
restarts or to keep the boiler on-line during plant or system transients 
that would normally require a trip (shutdown). Accordingly, bypass systems 
are provided in order to enhance on-line availability, obtain quick 
restarts, and minimize turbine thermal cycle expenditures. 
In the operation of the power plant, a situation may arise wherein the 
electrical tie of the power line with a grid load network is interrupted. 
In such situations, it is still desirable to operate the steam turbine 
system at a house load level so as to supply the electrical needs of 
auxiliary equipment such as pumps, pulverizers, fans, etc. Under such 
conditions, the turbine will continue running at synchronous speed 
although with a greatly reduced steam flow and with the remainder of the 
boiler-produced steam being provided to the bypass system. 
Ordinarily, sufficient steam flow must be passed through the turbine in 
order to keep the turbine elements cool. With the reduced flow rate 
conditions, however, a windage effect takes place whereby instead of 
extracting work from the steam, the turbine blades are actually doing work 
on the steam which is being churned up, resulting in a temperature 
increase which, in turn, causes the turbine parts to heat up. A danger 
therefore exists under such conditions, of turbine overheating past its 
design rating, thereby resulting in reduced life and possible premature 
failure. 
The present invention provides for a significant improvement under such 
conditions whereby the turbine temperature is maintained within design 
limits. 
SUMMARY OF THE INVENTION 
The improved system includes, in addition to a normal bypass path, a second 
bypass path for passing steam around the high pressure turbine. This 
second bypass path includes a steam jet compressor means having one input 
section connected to the high pressure turbine exhaust output and another 
input section connected to receive steam from the steam generator. The 
output section of the steam jet compressor is connected to the other steam 
bypass line at the input of the reheater. A valving means is provided for 
controlling the steam supply from the steam generator to the steam jet 
compressor and a control means responsive to an output condition at the 
high pressure turbine output controls the valving means for the steam jet 
compressor. The arrangement provides a sufficiently low pressure at the 
high pressure turbine output so as to maintain the temperature thereat 
within design limits for the particular turbine steam flow.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 illustrates a simplified block diagram of a fossile-fired single 
reheat turbine generator unit, by way of example. The turbine system 10 
includes a plurality of turbines in the form of high pressure (HP) turbine 
12, and at least one or more lower pressure turbines which, in the case of 
FIG. 1, include intermediate pressure turbine 13 and low pressure turbine 
14. The turbines are connected to a common shaft 16 to drive an electrical 
generator 18 which supplies power to a load such as an electrical grid 
network (not illustrated). 
A steam generating system, such as a conventional drum-type boiler 22 
operated by fossile fuel, generates steam which is heated to proper 
operating temperatures and conducted through a throttle header 26 to the 
high pressure turbine 12, the flow of steam being governed by a set of 
steam admission valves 28. 
Steam exiting the high pressure turbine 12 via the high pressure turbine 
exhaust output 30 and steam line 31 is conducted to a reheater 32 (which 
generally is in heat transfer relationship with boiler 22) and thereafter 
provided via steam line 34 to the intermediate pressure turbine 13 under 
control of valving arrangement 36. Thereafter, steam is conducted via 
steam line 39, to the low pressure turbine 14, the exhaust from which is 
provided to condenser 40 via steam line 42 and converted to water. The 
water is provided back to the boiler 22 via the path including water line 
44, pump 46, water line 48, pump 50, and water line 52. Although not 
illustrated, water treatment equipment is generally provided in the return 
line so as to maintain a precise chemical balance and a high degree of 
purity of the water. 
In order to enhance on-line availability, optimize hot restart, and prolong 
the life of the boiler, condenser, and turbine system, there is provided a 
turbine bypass arrangement whereby steam from boiling 22 may continually 
be produced as though it were being used by the turbines, but in actuality 
bypassing them. The bypass path includes steam line 60, with initiation of 
high pressure bypass operation being effected by actuation of high 
pressure bypass valve 62. Steam passed by this valve is conducted via 
steam line 64 to the input of reheater 32 and flow of the reheated steam 
in steam line 66 is governed by a low pressure bypass valve 68 which 
passes the steam to the condenser 40. In order to prevent the bypassed 
steam from entering the high pressure turbine in the reverse direction, 
that is through outlet 30 via steam line 31, there is provided a nonreturn 
or check valve 70 located in that steam line. 
In order to compensate for the loss of heat extraction normally provided by 
the high pressure turbine 12 and to prevent overheating of the reheater 
32, relatively cool water in water line 72, provided by pump 50, is 
provided to the bypass steam under control of spray valve 74 and 
desuperheating assembly 75. In a similar fashion, relatively cool water in 
water line 78, provided by pump 46, is controlled by valve 80 and provided 
to desuperheater assembly 81 in order to cool the steam in the low 
pressure bypass path to compensate for the loss of heat extraction 
normally provided by the intermediate and low pressure turbines 13 and 14, 
and to prevent overheating of condenser 40. 
Although not illustrated, it is common to provide analog or digital control 
systems for operation of the illustrated valves as well as for efficient 
operation of the boiler. 
The windage heating which can cause extensive damage to the high pressure 
turbine 12 is a function of turbine rotor speed as well as the density of 
the steam being passed through the high pressure turbine. When operating 
under house load conditions with a low steam flow, the turbine speed is 
maintained at its design synchronous speed. The density of the steam 
therefore is a variable which affects the windage heating and the density 
increases with increased pressure at outlet 30. The problem is 
particularly serious in a power plant having a 100 percent bypass system. 
Valve 62 in the bypass path, throttles some of the boiler output pressure 
down to a certain value for presentation to the input of reheater 32. This 
pressure is known as the cold reheat pressure. Accordingly, if the exhaust 
pressure at outlet 30 is high and equivalent to the cold reheat pressure, 
then a flow of steam could be maintained from the turbine to the reheater. 
This elevated pressure however would result in windage heating which is 
totally unacceptable for the turbine design. The pressure at outlet 30 
must be kept relatively low so as to maintain the operating temperature 
within design limits; however, such low pressure is not compatible with 
the pressure conditions at the input of reheater 32 and therefore cannot 
be directly connected thereto. The present invention provides a solution 
and to this end reference is made to FIG. 2 wherein components previously 
described in FIG. 1 have like reference numerals. 
In FIG. 2 a second bypass path around the high pressure turbine 12 is 
provided and includes a steam line 86 which provides boiler steam to a 
steam jet compressor 88 through a control valve 90. The steam jet 
compressor 88 includes one input 92 in steam communication with outlet 30 
of high pressure turbine 12, a second input 93 in steam communication with 
the boiler, via control valve 90, and an output 94 which is in steam 
communication with bypass line 64 at the input of reheater 32. 
The steam jet compressor, also known as a steam jet pump or steam jet air 
ejector, is a well-known piece of apparatus used for many years in steam 
power plants for extracting air from condensers. With reference to FIG. 3, 
and as used in the present invention, the exhaust steam at outlet 30 is 
provided at a relatively low pressure to input 92 of the steam jet 
compressor 88. Relatively high pressure motive steam from the boiler 
enters at input 93 and issues as a high speed jet of steam from nozzle 
100. The mixture of the high pressure and low pressure gases enters the 
converging tube 102 in which an exchange of momentum takes place and after 
which the mixture flows into a diffuser 104 in which the velocity of the 
mixture is reduced and its pressure raised to a value compatible with the 
cold reheat pressure. In essence, the steam jet compressor 88 acts as a 
compressor for raising the pressure of the exhaust steam at outlet 30 to a 
high enough value where it can be discharged into the reheater while still 
maintaining the proper pressure conditions on the reheater. The steam jet 
compressor is a relatively compact and simple piece of equipment which has 
no rotary, or any moving parts, is extremely reliable, and relatively 
inexpensive. 
Referring once again to FIG. 2, since the output of the steam jet 
compressor is at a temperature which is too high for the reheater, a spray 
water arrangement is provided so as to cool the steam. Cooling water from 
pump 50 may be provided via water line 110 to the desuperheating assembly 
112 connected in the output line of the steam jet compressor. Control of 
the cooling water is provided by valve 114, the opening of which is 
governed by a control circuit 116 which senses the temperature of the 
steam from desuperheat assembly 112 by means of a temperature sensor 118 
and compares this value with a predetermined setpoint value SP to open 
valve 114 more should the temperature be greater than the setpoint value 
and to reduce the cooling water flow should it be less than the setpoint 
value. 
Operation of the steam jet compressor 88 has the effect of maintaining a 
relatively low pressure at the output 30 of high pressure turbine 12 for a 
given steam flow rate condition and for increasing this pressure such that 
the turbine discharge may be provided to the other bypass line at the 
input of reheater 32. In this manner, the temperature of the high pressure 
turbine will be maintained within design limits. If for some reason the 
turbine temperature should rise, valve 90 may be controlled so as to pass 
more motive steam to the steam jet compressor 88 so as to pull the 
pressure at the output 30 down to a value whereby the temperature reduces. 
Conversely, if the temperature decreases, valve 90 may be controlled to 
supply less motive steam, resulting in an increase in pressure. 
Accordingly, in order to control valve 90, a control circuit 120 is 
provided and examines a condition at the output 30. This condition 
preferably is a temperature reading which is indicative of turbine 
temperature and which may be sensed by temperature sensor 122 for 
providing a temperature signal to the control circuit 120 and which signal 
is compared with a predetermined allowable range as represented by 
setpoint SP for governing operation of the valve 90. 
When it is time to increase electrical load for reconnection to the 
electrical grid network, more steam flow is sent to the high pressure 
turbine 12 through steam admission valves 28 and the steam flow in the 
main bypass path of steam line 60 is proportionally reduced. Although 
outlet 30 will experience a pressure rise, the temperature will not 
necessarily rise since there is now greater steam flow through the 
turbine. If during the switchover from bypass to main steam the exhaust 
temperature of the high pressure turbine should deviate from allowable 
limits, control circuit 120 will further open or close valve 90 so that 
steam jet compressor 88 maintains the proper pressure condition at the 
outlet to maintain the desired temperature while pumping up the pressure 
of the exhaust for discharge into the reheater. 
When steam flow conditions and exhaust pressure are such that high pressure 
exhaust steam flows through check valve 70, there is no more requirement 
for the operation of steam jet compressor 88. Accordingly, means are 
provided to sense this condition and, as an example, a pressure 
differential transducer 124 may be provided for sensing the pressure 
across check valve 70 to provide an indication of the opening thereof, 
such indication being provided to control circuit 120 for shutting down 
valve 90. 
Depending upon the steam system, flow and pressure requirements, it may be 
that the pressure ratios involved are too large for a single steam jet 
compressor. In such instance a plurality of such compressors may be 
utilized as illustrated in FIG. 4 which includes a plurality of steam jet 
compressors 88a, 88b, . . . 88n, all operating in parallel. Each steam jet 
compressor has an associated control valve 90a, 90b, . . . 90n as well as 
water spray control circuits 116a, 116b, . . . 116n for controlling spray 
water to desuperheater assemblies 112a, 112b, . . . 112n in response to a 
setpoint and the temperature measured in the respective lines by 
temperature sensors 118a, 118b, . . . 118n. 
Control circuit 120 still senses the temperature at the high pressure 
outlet 30 to sequentially open the control valves as needed as the outlet 
temperature rises, or conversely to shut them down in sequence when the 
outlet temperature drops, until such point in the sequencing operation 
that the temperature attains its design range. 
Accordingly, the power plant is able to achieve full load in a relatively 
short period of time since the boiler 22 may be maintained at full load 
independent of the turbine. While in a bypass condition, the high pressure 
turbine is prevented from overheating by the auxiliary bypass path which 
maintains the proper pressure, and therefore temperature, at the high 
pressure exhaust. The auxiliary bypass path accomplishes its function with 
the use of a device which has no moving parts, is relatively simple and 
inexpensive, and utilizes motive energy already available from the boiler.