Superheater Temperature Control

Reason for superheating steam

Basically the control of temperature is to protect the superheater by preventing the metal temperatures reaching a dangerously high level reducing mechanical strength and leading to failure.

Water flowing through a tube conducts heat away much more effectively than steam due to its higher specific heat capacity. This means that tubes carrying water have a metal temperature much closer to the fluid passing through it.

Where superheat temperatures upto 455oC are in use then the use of mild steel is not a problem, for superheat temperatures above this then alloys of chrome molybdenum steels are used (upto 560oC), difficulties in welding means that there use is restricted to only within the highest temperature zone and a transition piece fitted to connect to remaining mild steel tubing.

Superheat temperature control is therefor fitted to ensure superheat temperature does not exceed design limits.

Methods of regulating superheat temperature

a, By regulating the gas flow over the superheater by means of dampers

FW ESD II

ESDII design

The balance line prevents any tendency for the control unit to steam under conditions of low feed flow say due to sudden load change or especially when flashing ( several of these have been burnt out due to incorrect flashing procedure)

The control unit operates the linkages via a control arm, if the superheat is too high then gasses are diverted to flow over the control unit and less gas now flows over the superheat bank.

The control arms and the dampers were very susceptible to damage caused by operating in the hot gas path. Also this control was very sensitive to excess air which can raise the superheat temperature by increasing the heat energy removed from the furnace.

Babcock and wilcock selectable superheat

B abcock and Wilcock design

This design gave a wide range of temperature control, it operated in a similar manner to the Foster Wheeler ESD II. The gas path is separated by a baffle which has flaps located above the tubes operation of which can determine the superheat temperature, as the superheater only extends across one path it is made out of 'W' rather than 'U' tubes.

This design suffered from similar problems to the ESD II with regard to flaps and flap linkages susceptibility to corrosion.

b, By use of multi furnace boilers

Babcock and wilcock Controlled superheat)

The superheat temperature was regulated by changing the position of lit burners within the boiler, shutting off burners in the main furnace and replacing them with flames in the wing furnace had the effect of reducing the superheat temperature as the gasses are cooler when the reach the superheater bank. In this way the superheat temperature could be varied by 60oC.

The advantage of this system was the superheat temperature could be maintained over a wide variation of load. To prevent reversal of flow in the intermediate generating bank a baffle plate is fitted in the water drum which allows the first two rows of the bank to be isolate from the rest and to be supplied by their own two downcomers.
Difficulty was encountered in maintaining the correct air/fuel ratio during differential firing of the two furnaces.
During flashing only the wing furnace is used to give better protection for the superheater

B abcock and Wilcock design

c,Use of air cooled attemperators

Air cooling effect of the double casing is lost in this arrangement so additional insulation must be fitted to ensure that the casing temperature does not exceed safe handling limits.

As air is a relatively poor cooling medium large attemperators are required allied to increased FD fan output required to overcome frictional resistance losses. There is an overall increase in weight, size and initial cost which led to the system being superseded by the regulated gas flow method and then by water or spray cooled attemperation

Schematic of air cooled attemperation

d, Use of separately fired superheater

In very rare use, normally limited to tank boilers

e, Use of boiler water attemperator (external mounting)

Schematic of external water cooled attemperation

Superheat control is achieved by diverting a proportion of the steam through the simple tubular heat exchanger attemperator

e, Water cooled attemperator (internal)

Schematic of internal water cooled attemperation

Schematic of internal water cooled attemperation system

Shut off valves have to be fitted to the attemperator as in the event of tube leakage the boiler will empty in to the attemperator as it is at a slightly higher pressure due to frictional losses in the superheater.

g, Water spray attemperation

This the most common form of attemperation in use, it consists of two spray nozzles which spray feed water into the steam as it passes from the primary to secondary superheaters. The water receives heat from the steam and thereby reduces the superheat of the steam. To prevent thermal shocking of the transfer pipe, a thin flexible inner tube is fitted.

The spray valves work in series with one reaching its maximum capacity before the second comes into use, the control system takes as its measured value both the outlet temperature and either steam or air flow (load). The spray valves are often designed to be of the air to open variety so in the event of air failure they will fail safe open.

Spray type desuperheater

Modern Superheat temperature control system

Modern Superheat control system

The main system components are a P+I+D Mater controller (reverse acting, hence output increases for measured values above setpoint ) in which the desired final superheat temperature is set, working in cascade is a P+I slave controller whose output controls the spray attemperator control valve.

There is a temperature transmitter on the inlet to the secondary superheater (Tx1, fitted after the spray) and a secondary superheater outlet temperature transmitter (Tx2).

Tx1 output Mv1 is fed to both the master and slave controllers, in the slave controller this forms the measured value

Tx2 output Mv2 is fed to the master controller and forms the measured value, here it is compared to the required set point entered. The output Op1 is sent to the computing relay.

Master controller Op1 = -(Desired set point - Mv2)
(reverse acting)

In the computing relay the signal is added to the rate of change of air flow signal, as the air flow is taken from the combustion control circuit it forms a load signal. In this way the circuit has the ability to react quickly to load changes before they actually begin to effect the temperatures.

The output of the computing relay is fed to the slave controller as its set point Sp2 the set point for the slave controller now has the error of the final superheat and an amount by which the volume rate of air flow ( and hence boiler load) would tend to change the superheat contained within.

The set point Sp2 is compared in the slave controller to the output from the secondary inlet transmitter Tx1 signal Mv1.

Slave controller Output Op2 = Setpoint Sp2 - Mv1

The use of the controllers in cascade speeds up response to system changes.

Computing relay Output SP1 = OP1 + d/dt (air flow)

It is necessary to add the air flow signal as this has a direct effect on the superheat temperature. If there was a load demand increase the combustion control would increase fuel and air to the boiler, this would cause an increase in the superheat steam temperature as there would be some lag until the steam flow increased due to the increased fuel . Once the steam flow has stabilised then the increased steam flow will match the increased gas temperature and so the temperature will reduce. It can be seen then that only during the transition period when the fuel/air has increased but the steam has not that the increased spray is required, this is why the rate of change of air flow rather than volume is used in the control system.

If the measured superheater outlet temperature drops then Mv2 drops, OP1 decreases (the master controller is reverse acting), this is fed through the computing relay and so the set point Sp2 for the slave controller decreases. The setpoint of the slave controller has now fallen below the measured value and hence its output will decrease. This signal OP2 is fed to the spray valve which will shut in increasing the superheat temperature .

If the load on the boiler increases the output of the computing relay increases and hence the set point Sp2 increases, the output of the slave controller Op2 increases and hence the spray valve starts to open even though the increased air flow and hence gas temperature passing over the superheater is yet to be detected in the superheated steam either Tx1 or Tx2, in this way problems of process and control lags can be negated.

The output of Tx1, Mv1 is fed not only to the slave controller but also to the master controller; Its function here is to prevent the master controller from saturating and hence speeding its response under certain conditions. It does this by feeding the integral bellows via the integral restrictor in the controller rather than the more normal feedback arrangement of the output feeding the Integral bellows via the restrictor. In this way the master controller always takes account of the inter temperature.

With the normal layout in low load conditions, should Mv2 fall below the setpoint the Integral action will force the controller into saturation if the temperature fails to recover. This can happen as even with the spray valves shut there may not be enough energy in the flue gasses to heat the steam upto the required temperature in low loads.

By using the output from Tx1, Mv1 then the controller will fail to go into saturation as the integral bellows will receive a signal Mv1 rather than its falling output Op1.

Other additional fittings

Shown on the diagram is a fitting sometime used to protect the system in the event of failure of the spray control valve, this takes the form of a thermostat set so that should the temperature fall below a certain value it will operate a solenoid valve fitted before the spray control valve to shut off the feed. It can be seen that in the event of loss of superheat control , and hence with the spray valve failed open, some form, albeit very coarse , of superheat control can be maintained by use of the thermostat and solenoid valve.

There is alarms fitted to both the inlet and outlet from the secondary superheater as well as a main engine trip due to high superheat temperature. A boiler trip may be fitted for low superheater outlet temperature.