Method for managing a shut down of a boiler

Abstract

The method for managing a shut down of a boiler having a duct and heat exchanging components is provided. The heat exchanging component having tubed heat exchanging surfaces within the duct and headers outside the duct. The method includes regulating the temperature of the headers during shut down to a temperature close to the one expected for the steam moving from the tubed heat exchanging surfaces into the headers at a starting up following the shut down.

Claims

1. A method for managing a shutdown of a boiler comprising: providing a duct, providing at least a heat exchanging component; wherein the at least a heat exchanging component includes: tubed heat exchanging surfaces within the duct, headers outside the duct, the headers being connected to the tubed heat exchanging surfaces, the headers and the tubed heat exchanging surfaces containing steam; regulating the temperature of the headers during shut down to a target temperature that is a function of the expected temperature for the steam moving from the tubed heat exchanging surfaces into the headers at a starting up following the shutdown; and wherein regulating the temperature of the headers includes maintaining a steam flow through the headers after shutdown of the firing system by maintaining a steam circulation from the tubed heat exchange surfaces connected to the respective headers and through said headers.

2. The method according to claim 1 further comprising: providing a high pressure bypass control valve downstream of the at least a heat exchanging component, configured such that maintaining a flow through the headers includes maintaining a steam flow through the high pressure bypass control valve.

3. The method according to claim 2 further comprising: wherein the heat exchanging component is a superheater and the high pressure bypass control valve is downstream of the superheater.

4. The method according to claim 1 further comprising: maintaining a gas flow within the duct during shutdown.

5. The method of claim 4 further comprising: wherein the boiler includes a fan for gas circulation through the duct, configured such that maintaining a gas flow includes operating the fan.

6. The method of claim 5 further comprising: wherein operating the fan includes operating the fan at minimum load.

7. The method of claim 6 further comprising: wherein operating the fan includes operating the fan at less than 10% of its nominal mass flow.

8. The method according to claim 1 further comprising: regulating the pressure within the boiler during shutdown or before shutdown.

9. The method of claim 4 further comprising: wherein the boiler further comprises one or more high pressure bypass control valves downstream of a superheater and/or one or more low pressure by-pass control valves downstream of the reheater, configured such that regulating the pressure includes regulating the high pressure bypass control valves and/or the low pressure by-pass control valves.

10. The method according to claim 8 further comprising: wherein the boiler further comprises an economizer, configured such that regulating the pressure includes circulating water through the economizer and evaporating at least partly water passing through the economizer.

11. The method according to claim 10 further comprising: wherein regulating the pressure further includes circulating air through the duct.

12. The method of claim 11 further comprising: wherein the boiler includes a fan for gas circulation through the duct, configured such that circulating air includes operating the fan.

13. The method according to claim 1 further comprising: wherein the target temperature is the expected temperature for the steam moving from the tubed heat exchanging surfaces into the headers or a temperature lower than the expected temperature.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Further characteristics and advantages will be more apparent from the description of a preferred but non-exclusive embodiment of the method, described with reference to the non-limiting accompanying drawings, in which:

(2) FIG. 1 is a schematic view of a boiler.

DETAILED DESCRIPTION

(3) In the following reference to the boiler of FIG. 1 is made.

(4) The method can be applied to any boiler also different from the one shown. For example the walls 3 can extend up to the top of the boiler (i.e. they can define the duct 12 and house the tubed coils or tubed panels 16). The walls can either be completely used as evaporator or can be divided in evaporator (lower part) and superheater (upper part). In addition the evaporator can have a different structure than the tubed walls 3.

(5) The method is preferably implemented to limit the stress of the headers 17, 18 of the superheater 13, but it can also be conveniently used to limit the stress to the headers 17, 18 of the reheaters 14 or of other parts of the boiler 1.

(6) The method comprises regulating the temperature of the headers 17, 18 during shut down to a target temperature that is a function of the expected temperature for the steam moving from the tubed heat exchanging surfaces 16 into the headers 17, 18 at a starting up following the shut down. The target temperature is for example the expected temperature for the steam moving from the tubed heat exchanging surfaces 16 into the headers 17, 18 or a temperature preferably close to this expected temperature and in this last case the temperature is lower than the expected temperature.

(7) In particular this temperature regulation is a cooling of the headers 17, 18.

(8) This cooling is mainly done after shut down, that means without additional use of expensive fuel, only by using the boiler pressure storage capacity and the boiler heat content in an appropriate way.

(9) Thanks to this controlled cooling of the headers 17, 18, when the boiler 1 is started up after shut down, the steam moves from the tubed heat exchanging surfaces 16 through the the headers 17, 18 and since the temperature of the steam does not differ from the temperature of the headers 17, 18 or the difference is a limited controlled and calculated difference, the thermal stress undergone by the headers 17, 18 is limited.

(10) Preferably regulating the temperature of the heaters 17, 18 comprises maintaining a flow through the headers 17, 18 during the shut down or at least part of the shut down.

(11) In fact, if steam keeps circulating through the tubed heat exchanging surfaces 16 and headers 17, 18, the headers 17, 18 are cooled by the steam that circulates through them and that is in turn cooled by the flow through the duct 12.

(12) Maintaining the flow through the headers 17, 18 can be implemented by maintaining a steam flow through the control valve 26 and valve 27. In fact, the flow through the valve 26 allows cooling of the headers 17, 18 of the superheater 13 and the flow through the valve 27 allows to cool the headers 17, 18 of the reheater 14. Preferably the mass flow through the valve 26 and 27 is less than 10% of the nominal mass flow.

(13) In a preferred embodiment, the method is implemented in connection with the tubed heat exchanging surfaces 16 of the superheater 13 and the control valve 26 is downstream of the superheater 13.

(14) In addition, a gas flow is preferably maintained through the duct 12 during shut down. Maintaining a gas flow through the duct 12 includes operating the fan 7. For example the fan 7 is operated at minimum load or at a load less than 10% of its nominal mass flow. Operating the fan 7 is anyhow not mandatory and natural draft can suffice for air circulation.

(15) The method can also comprise regulating the pressure within the boiler, i.e. within the heat exchanging components; pressure regulation can be done before shut down or during shut down. Preferably such a regulation aims at increasing the pressure within the boiler 1.

(16) In a first example, regulating the pressure includes regulating the high pressure by-pass control valve 26 or the turbine inlet valve.

(17) In a different example, regulating the pressure includes circulating water through the economizer 20 and evaporating at least partly water passing through the economizer 20. Circulation through the economizer 20 can be achieved by stopping the recirculation pump 29 and opening the line 30 (eco steaming line) provided between the top level of the economiser and the separating system 25.

(18) Continuously operating the fan 7 for a certain time after shut down or using the natural boiler draft causes a permanent heat input on the economiser surfaces with steam production. This steam production is used to improve the pressure maintenance during the header cooling process. Maintaining a small feedwater flow (continuous or discontinuous) avoids a complete steaming of the economiser.

(19) Naturally the features described may be independently provided from one another.

(20) In practice the materials used and the dimensions can be chosen according to requirements and to the state of the art.