FIELD OF THE INVENTION
The invention relates to a power plant and process steam generators working on
the forced/controlled circulation principle utilizing the Boiler Water Circulation
Pumps [BWCP] for assisting circulation of water in the steam generator. More
particularly the present invention relates to a micro steam back pressure turbine
drive device to operate boiler water circulation pump in forced / controlled
circulation steam generators to improve efficiency of the steam generators.
BACKGROUND OF THE INVENTION
The steam generator by itself is a partial consumer of the steam produced. The
steam is used for various internal systems such as main turbine seal glands, mill
inerting/extinguishing in case of fire, steam coiled air pre-heaters at low loads,
de-aerator pegging, heavy fuel oil tracing, heavy fuel Oil atomization and other
continuous or intermittent applications. The steam requirements for these
requirements are supplied by a plurality of auxiliary steam headers, which could
either be specific to the steam generator unit or common to the station. These
auxiliary headers source their steam requirement from Pressure Reducing and
De-Super heating Station [PRDS] by tapping the steam at high pressure with
super-heated parameters and converting the tapped steam corresponding to the
downstream equipment, at lower pressures and temperatures. This is achieved
by using high capacity pressure reducing valves for throttling, and the de-super
heating stations where spray water for attemperating is sourced from a
condensate extraction hot well pump (CEP).

The steam used for the internal usage is sourced from at least two headers, for
example a low temperature unit/station header and a high temperature
unit/station header. As the name indicates they differ in the temperature of the
stored process steam typically 210°C and 310°C respectively but both at the
same pressure of 16 bar. Typically the PRDS station sources steam at 44 bar
343°C from the cold reheat (CRH) line, or from the pre-conditioned main steam
line (MSL) then reduces the pressure to 16 bar using a Pressure Reducing Valve
[PRV], and further uses a desuperheater/ Attemperater to reduce the
temperature to 210°C. Hence, the energy contained in the source steam remains
unutilized in the prior art process.
The auxiliary steam requirement is a variable quantity depending on the boiler
load, type of fuel used, Mill operating conditions, Ambient temperatures, Turbine
requirements and steam demand from other associated units. Amongst these
demands, some are continuous and some are intermittent. The Boiler auxiliary
steam demand is lowest at normal operating conditions between 40% - 100%
Steam generator loading, which is illustrated in Table 1. The steam generator is
expected to operate at this load range for most of its working life time.
The energy lost during the PRDS process with the input steam parameters as 44
bar 343°C and output parameters as 16 bar 210°C at 11 Tons/Hr is 579 kW. By
employing a micro steam non-extraction back pressure turbine for these rated
steam parameters, a change in the enthalpy as useful work done can be
achieved. This energy is sufficient to drive at least two numbers of boiler water
circulation water pumps for all load variations, even if the turbine system
efficiency is rated at the minimum of 80%.

US Patent No. 1,004,822 describes a combined turbine and direct driven
centrifugal pump having its shaft enclosed between the turbine and pump and
having a liquid seal to prevent leakage between the turbine and pump formed by
circulating liquid. The combination with a shaft and passages for circulating liquid
through the clearance space about the shaft, of an ejector enclosing the shaft for
conducting the liquid from the clearance space.
US Patent No. 2,393,691 describes a steam driven pumping unit to pump liquid
at a pressure from 500 lbs per sq.in or above. The pump comprises at least a
casing for the pump, a casing for the turbine, and a connecting portion between
the pump and turbine, said connecting portion having a chamber therein for
receiving liquid under pressure from the pump and having a second chamber
therein for receiving pressure fluid form a source exteriorly of the turbine, said
chambers having connection whereby the liquid and pressure fluid pressures
counteract each other to prevent leakage of liquid into the turbine or leakage of
pressure fluid into the pump, and pressure reducing means provided in the
connection between said chambers.
US Patent No. 2,495,525 describes a boiler circulating pumping unit, a centrifugal
circulating pump, a steam turbine for driving said pump, a casing for said pump,
a casing for said turbine, a single shaft for driving said pump and driven by said
turbine; said casing having a mixing chamber therein surrounding said shaft
between the pump and turbine, said chamber having communication with the
interior of the pump casing to receive discharge pressure leakage from the

pump , means for delivering steam from a source outside the turbine to said
chamber at a pressure in excess of the water pressure in the chamber and in
excess of the steam pressure in the turbine adjacent said shaft to prevent
leakage of water from the chamber into the turbine, and means to reduce the
temperature of water in said chamber below the flashing temperature for its
pressure.
Thus, the prior arts disclose multiple steam turbine driven pump systems in a
steam generating plant. However, the prior art do not discuss the source of the
steam sourced for the steam driven turbine.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a micro steam back pressure
turbine drive device to operate boiler water circulation pump [BWCP] in
forced/controlled circulation steam generators to improve efficiency of the steam
generators.
Another object of the invention is to propose a micro steam back pressure
turbine drive device to operate boiler water circulation pump [BWCP] in forced/
controlled circulation steam generators to improve efficiency of the steam
generators, which sources steam requirement from the steam supply tapped
from cold reheat line or pre-conditioned main steam line.

A still another object of the invention is to propose a micro steam back pressure
turbine drive device to operate boiler water circulation pump in forced/controlled
circulation steam generators to improve efficiency of the steam generators,
which allows bypassing sourcing of steam by auxiliary headers from Pressure
Reducing and De-Super heating Station (PRDS) on attaining minimum load of the
steam generator.
SUMMARY OF THE INVENTION
Accordingly, there is provided a micro steam back pressure turbine drive device
to operate boiler water circulation pump in forced/controlled circulation steam
generators to improve efficiency of the steam generators.
The present invention provides a micro steam back pressure turbine drive which
sources steam from the tapped steam supply of cold reheat [CRH] line or
preconditioned main steam line (MSL] as the working fluid. The entire working
fluid is iso-entropically expanded, with/without inter-stage extraction, in the
steam turbine driving at least one boiler water circulation pump [BWCP] on
attaining more than 40% of boiler load or on establishment of a minimum flow in
the CRH line. This eliminates the usage of separate electrical motor for driving
the boiler water circulation pump bypassing the PRDS and allows the PRDS to
remain in service upto 40% steam generator load. By loading this arrangement,
useful work is extracted from the auxiliary steam production process by
bypassing the PRDS. This arrangement reduces auxiliary electrical power
consumption, allowing higher power feeding into the grid interalia higher plant
efficiency.

According to the present invention, the steam is sourced from the tapping
stream of CRH/MSL, by-passing the PRDS to utilize the energy lost during the
PRDS process to useful work adding to the steam generator efficiency.
By implementing the invention, electrically driven forced circulation pumps
cannot be completely eliminated, since at the start-up of the steam generator
the circulation has to be assisted by the electrical driven circulation pumps in the
absence of flow in the CRH or/MSL line or inadequate steam flow in these lines.
As per the industrial practice, each circulating water pump is sized such that it is
capable of catering up to 60% of steam generator load individually. Once the
steam generator attains 40% load
establishing Main steam line [MSL] or Cold Re-Heat line [CRH] flow and with
applied interlock logics change over from the electrical to steam driven pump
occurs.
Implementing this Steam driven Boiler water circulating pump would also reduce
the condenser condensate extraction spray water from the hot well used for
de-super heating purpose in PRDS, which reduces the demand of the feed water
refill rate. The invention is for the Steam generator operating at 40% to 100%
load since the steam generator is expected to work in this range of load for most
of its operating conditions in its lifetime.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 - schematically shows a prior art system of driving boiler water
circulation pumps in steam generators.

DETAILED DESCRIPTION OF THE A PREFERRED EMBODIMENT OF THE
INVENTION
Description of the entire system showing the actual PRDS station and the
bypassed piping line of micro steam back pressure turbine operation is shown in
the Figurel. The scope of the illustration in Figure 1 ends with the turbine shaft
output with the integral governor mechanisms coupled to the pump impeller.
Referring to the Figure 1, where the steam tapping is taken from the Cold
Reheat Line [1], normally the steam is passed through an inlet isolation valve
[2], PRDS [3] and an outlet isolation valve [4]. According to the invention, the
PRDS isolation valves [2] and [4] are open only up to 40% boiler load conditions
and are closed thereafter opening up the turbine valves [11] and [12]. After
attaining the 40% load, as per the pump shaft power requirement from the
Distributed Control System DCS [7] the control valve [6] is modulated to meet
the required steam quantity and pump shaft [10] input power. The steam is fed
to the micro steam back pressure turbine [8] where it is expanded and the
extracted mechanical work is governed by an integral governor mechanism [9]0
designed to match the pump RPM and torque requirements. The turbine shaft
[10] is then coupled to the pump impeller drive.


The low temperature steam requirement which are continuous, excluding the
intermittent requirements, for a typical 500MW unit operating at 40% or above
load condition is shown in the Table 1. The various states in the requirement are
given as A, B, C and D. State A refers to the normal running of the boiler loaded
above 40%. State B is the boiler normal running scenario with other unit cold
start-up. State C is the boiler normal running scenario with other unit hot start-
up. State D is 40% load of the boiler with gland sealing. The state A and D have
the identical low temperature steam requirement since the gland sealing is
provided using.the high temperature steam. Similarly, the state B and C have
unchanged low temperature steam requirement since the required steam for the
hot start-up and cold start-up of other units are catered from high temperature
auxiliary headers. On comparing the figures, it can be observed that at any point
of operation of the steam generator there is a minimum demand of 11.6 Tons/hr
of low temperature auxiliary steam.

In a station with one or more boiler units in operation, two electrical circulation
pumps can be replaced from the prior art requirement of three numbers per unit,
by steam driven pumps where the CRH or MSL tapping is utilized to drive the
pump assisting the boiler circulation. The expanded steam from the turbine
outlet is routed to the Low temperature unit header and stored for auxiliary
usage. In this case, the boiler circulation needs to be assisted by the electrical
pump for load below 40%, above which on establishment of the CRH or MSL
flow, the steam driven pump is taken into operation.


The micro steam back pressure turbine, rated for higher RPM and can be
installed either vertically or horizontally. The life time of the micro steam back
pressure turbine is minimum of 20 years with low maintenance. Turbine
regulation is possible from zero to maximum at unchanged efficiency rate and
can be easily started/stopped without much complexity and is less intrusive than
the conventional steam turbines. Its modular design enables pre-piping prior to
the delivery / installation at the site sitesavingerection and commissioning time.
Micro steam turbine is as compact as the electrical driven pump and
accommodates retrofit projects to its smaller footprint.
According to the invention, the enthalpy change occurring in the PRDS is utilized
as actual mechanical work in the BWCP after the system reaches a specific boiler
load, which is the load at which minimum flow in the CRH line or MSL is
established. The steam is routed to the steam turbine by-passing the PRDS, the
shaft output of the turbine is coupled to the impeller of boiler water circulating
pump [BWCP] via a governor mechanism which transfers the mechanical power
output of the steam turbine.
The mechanical work extraction from the steam is done using a micro-steam
back pressure turbine [8] drive for the rated steam input and output parameters.

The steam inlet is controlled by a control valve [6] receiving command from the
DCS [7] which based on the unit load determines the amount of energy input
required to the BWCP impeller shaft couple with the turbine shaft [10] via
governor mechanism [9] .The turbine shaft output is coupled mechanically via a
governor mechanism [9] to the pump impeller to transfer the power to the pump
impeller and to govern the system RPM and torque requirements. The system
further consists of steam turbine isolation valves [11] and [12] for the by-pass
used for maintenance of the steam turbine route .Another set of isolation valves
[2] and [4] is provided for the existing PRDS station. The steam turbine isolation
valves [11] and [12] receive the input from the status of the micro-steam back
pressure turbine which in the event of trip or failure of the turbine or other
associated system, isolate the flow route by closing the valves and activate the
PRDS station by opening the PRDS isolation valve [2] and [4]. The system
reduces auxiliary power consumption up to 500 kwe considering peak electrical
power required to drive two BWCP. The feed water refill rate is reduced upto 4
Tons/hour, [Spray water flow required rated at minimum auxiliary steam
requirement] due to the stoppage of extraction from the CEP for the PRDS spray.
The system meets the requirement of the low temperature steam demands,
whereas the high temperature steam demand at parameters of 310°C and 16 bar
still sourced from the actual PRDS station or unit headers. This requirement may
also is achieved using inter-stage extraction by the micro steam backpressure
turbine. The inter-state steam extraction should be done at the rating of 16 bar
and 310° C with a flow rate of 4 Tons/hr,

enabling the system to self-sustain at all the boiler operating conditions thereby
eliminating the existing PRDS. If the source steam is from the MSL which is at
higher temperature and pressure than the CRH steam, an inter-stage
conditioning station is used to match the input steam parameters for the micro
steam back pressure turbine such that the system operates in similar fashion as
if operating with steam sourced from the CRH.
The scope of invention is not limited to the turbine driven boiler water circulation
pump but also the source steam for the micro steam turbine, bypassing the
PRDS station and the application of the micro stream turbine as mechanical drive
for the pump. In most of the existing applications pressure reducing valves are
bypassed and coupled to a turbo-generator to produce electrical power. The
electrical power so produced might not be comparatively efficient due to the
inherit losses in generation, distribution and conversion of electrical to
mechanical work in pump. Through this invention we could eliminate all the
above mentioned losses and directly couple the turbine to pump impeller which
is much more efficient in terms of power utilization. By implementing this method
direct cost reduction could also be realized by eliminating the purchase of two
the electrical motor which has got very limited usage across the globe and avoid
the usage of complex high-pressure cooling systems employed in the existing
pump system. In case of multiple units in a plant, each unit shall run a single
electrical water circulating pump whereas the remaining two pumps could be
micro-steam turbine driven, considering three BWCP per unit.

ADVANTAGES OF THE INVENTION
• Reduces the units' auxiliary electrical power consumption;
• Eliminates the feed water tapping from CEP used as de-superheating
sprays water;
• Drives the circulation pumps replacing the costly immersion type of
electrical motor with associated complex and interlocked cooling systems.
The other allied advantages of the invention is avoidance of the incidental
damage to the motors due to seepage of the hot downcomer feed water into the
motor cavity. The motor cavity is generally designed to withstand a temperature
of up to 66°C and seepage of the water medium at 350° C, which inter alia
causes severe damage to the motor windings. In order to maintain the winding
temperature, low temperature higher pressure water is normally circulated in the
motor cavity so as to act as a heat exchanger, with necessary filter and strainer
arrangements, which increases the maintenance cost of the existing system and
the plant. The motor cavity temperature is linked with an interlock system, which
monitors the motor cavity temperature and trips the motor and the pump, in
case the cavity temperature reaches undesirable values. As the prior art systems
use motors having mechanical coupling with the pump including a path for the
feed water to seep into the motor cavity, implementing the inventive
arrangement in Glandless Steam Boiler Water Circulating Pump, there would be
no risk of leakage of feed water into the drive system.

WE CLAIM:
1. A method by which the micro steam backpressure turbine drive device is
used to operate boiler water circulation pump in forced/controlled
circulation steam generators to improve efficiency of the steam generators.
There are minimum three numbers of boiler water circulation pumps
[BWPC] to assist circulation of water in the steam generator. The boiler
itself is a partial consumer of the generated steam to operate its
subsystems, where these requirements being met through at least one each
low and high temperature auxiliary steam headers sourcing the required
steam from a pressure reducing and de-super heating station (PRDS) by
tapping steam at high pressure and temperature, and converting the tapped
steam to a lower pressure and temperature medium through a plurality of
pressure reducing valves including spary water from the condensate
extraction hot well for application in the downstream equipments of the sub
system leading to a partial utilization only of the energy contained in the
sourced steam, the improvement is characterized in that:
a micro steam back pressure turbine drive is provided which
sources steam from the tapped steam supply of one of the cold
reheat line or pre-conditioned main steam line as a working fluid;
the micro steam back pressure turbine enabled to iso-entropically
expand the tapped working fluid with or without inter-stage
extraction in the steam turbine driving at least one BWCP on
attaining at least 40% load or on establishment of a minimum
internal steam flow in the specified boiler's CRH line or MSL.

2. The method as claimed in claim 1, comprising an integrated governor
mechanism (9) to match the torque and rpm requirements of the BWCP.
3. The method as claimed in claim 1, wherein the steam tapped from the CRH
line (1) is passed through an inlet isolation valve (2) to an outlet isolation
valve (4) via the PRDS [3].
4. The method as claimed in claim 1, wherein the turbine (8) comprises one
each isolation valve (11, 12) and wherein the valves (11,12) are opened
only when the boiler attains 40% load condition or on attaining the specific
minimum flow in the CRH line or MSL
5. The method as claimed in claim 1, wherein the isolation valves (2,4) of the
PRDS (3) kept open only upto 40% boiler load conditions or on attaining
the specific minimum flow in the CRH line or MSL.
6. The method as claimed in claim 1, wherein the distributed control system
(7) and the control valve (6) of the steam turbine are modulated to supply
steam corresponding to shaft [10] input power of the turbine after the
boiler attained 40% load or on attaining the specific minimum flow in the
CRH line or MSL.

7. The method as claimed in claim 1, wherein the turbine shaft [10] is then
coupled to at the least one of the pump impeller.

ABSTRACT

The invention relates to micro steam back pressure turbine drive device to
operate boiler water circulation pump in forced/controlled circulation steam
generators to improve efficiency of the steam generators. There are minimum
three numbers of boiler water circulation pumps [BWCP] to assist circulation of
water in the steam generator. The boiler itself is a partial consumer of the
generated steam to operate its subsystems, where these requirements are being
met through at least one low and high temperature auxiliary steam headers
sourcing the required steam from a pressure reducing and de-super heating
station [PRDS] by tapping steam at high pressure and temperature, and
converting the tapped steam to a lower pressure and temperature medium
through a plurality of pressure reducing valves including spray water from the
condensate extraction hotwell for application in the downstream equipments of
the system leading to a partial utilization of the energy contained in the sourced
steam. The improvement is characterized in that a micro steam back pressure
turbine drive is provided which sources steam from the tapped steam supply of
one of the cold reheat line and pre-conditioned main steam line as a working
fluid; the micro steam back pressure turbine enabled to iso-entropically expand
the tapped working fluid with or without inter-stage extraction in the steam
turbine, driving at least one BWCP on attaining at least 40% load or on
establishment of a minimum internal steam flow in the specified boiler's CRH line
or MSL.