The invention generally relates to a system for controlling the main steam
temperature in a power generation boiler/turbine installation and more
particularly to a control system for regulating the steam temperature in a
steam generator having at least two series connected heating surfaces or
heaters through which the working medium flows one after the other by
quantitative variation of water fed into the working medium upstream of
the second heater.
In particular, the invention relates to regulating the steam temperature
by introducing spray water upstream of the finishing super heater
surface taken from a low enthalpy part of the circuit.
Background of the invention:
The typical approach to steam temperature control in a boiler/turbine
installation is to operate the 'unit at a main steam temperature value,
that delivers optimum system efficiency, while not exceeding the
permissible boiler/ turbine metal temperatures or the permissible rate of
change of these temperatures.
Such temperature control is generally accomplished by either modifying
the fluid distribution or heat distribution in the boiler. Thus, for
example, the steam temperature may be controlled by modifying the
amount of spray water which is used to remove a part of the superheat
from the superheated steam. Alternately, where it is desired to modify
the heat distribution to control steam temperature, various approaches
can be used; for example, tilting the burners or gas recirculating can be
utilized.

In the known method of controlling steam temperature using spray, the
temperature at boiler outlet is controlled by injection of low enthalpy
attemperation spray water through pneumatically actuated control valves
into the super heater section of the system.
The spray water control valves are usually aided by one or two isolation
valves upstream of the control valve to isolate the low enthalpy feed
water circuit from high enthalpy steam circuit. At low steam flow (usually
20% Maximum Continuous rating), since the spray water flow being a
large proportion of the steam flow the spray water control valves are
closed. Other basic protections include full closure of all spray water
control valves during a master fuel trip.
Though it is desirable to operate the steam generator with continuous
spraying which allows the steam temperature to be adjusted both
upwards and downwards, steam generator's natural characteristics and
heating surface area restrict the load range over which practical
temperature control is possible. Below spray water control range, steam
temperature is usually maintained by adjustment of firing rate and
excess air.
During hot starts of a steam generator, when the unit steam flow
increases with progressive firing rate, it is possible for the steam
temperature to exceed the rated steam temperature even before spray
water control range is reached.
At such condition, because of the interlock that prevents the spray water
valves from opening when the unit steam flow is low, spray addition is
inhibited resulting in steam temperature rising above the permissible
limit. Such instances can occur prior to turbine synchronization and the
unit operating with turbine bypass system.

Because of the foregoing, it is desirable to allow spray water to during
such instances in order to prevent steam temperature exceeding the
permissible design limits of super heater components of steam generator.
In the existing art the spray water isolation valve from opening till the
instant steam flow increases beyond 20 % of the maximum continuous
rating value. Beyond this , the spray water isolation valve is commanded
to open allowing the spray water to pass through the valve whenever the
steam temperature exceeds predetermined set value by a certain level.
Thus steam temperature is effectively not controlled until the steam flow
reaches 20 % of the maximum continuous rating value.
The invention overcomes this shortcoming by a method to control the
steam temperature even when the steam flow remains less than 20
%.The invention utilizes the action of two memory elements that ensure
the steam temperature remains controlled even when the steam flow
remains less than 20 %.
Brief Description of the drawing:
Fig. 1 shows the logic diagram of the present invention.
Object of the invention:
An object of the invention is to provide a modified system that permits
addition of spray water to prevent the main steam temperature rising
above permissible limit through-out the operating regime of the boiler.

Another object of the invention is to permit normal spray water operation
when the unit steam flow is within spray control range.
A still another object of the present invention is to withdraw spray from
control action when the steam flow is low, when the main steam
temperature reduces to a value that is less than permissible limit.
Detailed description of a preferred embodiment of the invention:-
Fig. 1 is the logic diagram for the operation of spray water isolation valve.
When the steam flow is less than 20% of maximum continuous rating
value, a memory element (A) which controls the spray water isolation
valve is held back from closing it by action of another memory element
(B). The output state of element B, when set, prevents closing the spray
water isolation valve by holding back the output state of element A.
The memory element (A) is Set to allow the opening of spray water
isolation valve when all of the following conditions are present.
1) Operator issues an open command or if the temperature at Super
heater outlet exceeds the set value by a predetermined value.
2) A Master Fuel trip condition doesnt exist.
3) At steam flow generated less than 20% of maximum continuous
rating, if the Main steam temperature value exceeds 4 degrees of
the rated main steam temperature (memory element (B) is set in
this case) OR Unit steam generation is above 20%
the memory element (A) is Reset to allow the closure of spray water
isolation valve when all the following conditions are present.

1) there is no demand (ZERO value) existing from the control system
to super heater spray control valve for more than one minute and the
measured super heater outlet temperature does not exceed the
desired set value by 4 degrees or more.
2) a boiler "Master Fuel Trip" condition exists as initiated by the boiler
safety system interlock.
3) at steam flow generated less than 20% of maximum continuous
rating and any of ths following conditions becomes true. This also
Resets memory element (B)
a) The Main steam temperature value remains less than at least 4
degrees of the rated rqain steam temperature value.
b) No demand (ZERO value) existing from the control system to all
the associated saper heater spray control valve.
"■ "•''.'..
The memory element (B), introduced in the logic, distinguishes the
following modes of operation during low steam flow condition.
a) Steam flow < 20% and the main steam temperature exceeds 4
degrees of the rated steam temperature. In this mode, spray
water isolation vpi?£sare automatically opened.
b) Steam flow> 20% aiid either or both of the following conditions
is true.

(i) Main steam temperature is less than 4 degrees of the rated
steam temperature.
(ii)All the associated super heater spray control valve demands
from the control system held at zero value at least for one
minute.
The above modes of operation are likely to be distinguished during
exigencies, such as that during hot start of a boiler, with increased firing
rate in order to match the steam temperature with the turbine metal
temperature in a fast manner. In such cases, if the main steam
temperature exceeds the rated steam temperature, it becomes necessary
to protect the boiler components from damage.

We claim:-
1. A method to regulate steam temperature of a power generating boiler/
turbine by spray water means operable at low flow rate of steam, the
method comprising the steps of
- a memory element (A) is SET to allow opening of a spray isolation valve
at predetermined conditions which includes
- issuance of an open command,
- absence of a master fuel trip condition, and
- at low temperature when a main steam temperature value
exceeds 4 degrees of the rated steam temperature, to enable a memory
element (B);
- the memory element (A) is RESET to allow closing of the spray isolation
valve contrary to the said predetermined conditions following reseting of
the memory element (B) wherein the memory element (B) is selectively
operable to identify modes of operation at low steam flow rate.
2. A method to regulate steam temperature as claimed in claim 1 wherein
the memory element (B) selectively operable checks the flow of water
through the spray water isolation valve.
3. A method to regulate steam temperature as claimed in claim 1,
substantially as herein described and illustrated.

4 A method to regulate steam temperature as claimed in claim 1,
wherein the memory element (B) checks the following condition
- steam flow < 20% and main temperature exceeds the rated steam
temperature by 4 degrees ;
- steam flow < 20% and main temperature less than 4 degrees of the
rated steam temperature;
- steam flow < 20% and spray control valve is at zero valve for atleast one
minute;
and selectively operates to allow flow of water through the spray isolation
valve.