Steam Temperature Control System
● Steam temperature is affected by boiler load, rate of change of boiler load, air flow rate, combination of boiler service and the amount of soot on the boiler tube. Keeping the steam temperature constant the thermal stress on boiler tube and turbine is minimise.
● Steam temperature is normally control by spraying water by a device called de-super heater into the steam between the first and second stage of super heater to cool down it. The discharge of the feed water pump spray at an intermediate stage. The steam temperature is measured at the point it exit the boiler, and changes the spray water valve position to correct deviation from steam temperature set point. This control loop should be turn for the fastest possible response without overshoot.
Cascaded Steam Temperature Control:
● Because of the slow response of the main steam temperature control loop, improved disturbance rejection can be achieved by implementing a secondary (inner) control loop at the desuperheater. This loop measures the desuperheater outlet temperature and manipulates the control valve position to match the desuperheater outlet temperature to its set point coming from the main steam temperature controller.
● The spray water comes from upstream of the feed water control valves, and changes in feedwater control valve position will cause changes in spray water pressure, and therefore disturb the spray water flow rate. The de superheater outlet temperature control loop will provide a gradual recovery when this happens. If the spray water flow rate to the attemperator is measured, a flow control loop can be implemented as a tertiary inner loop to provide very fat disturbance rejection. However, in many cases spnay water flow ate te not measured at the individual at temperature and this flow loop cannot be implemented
Food Forward Control :
● During hoiler load ramps in turhitie-following mode, the firing rate is changed first, followed by a chanye in steam flow rate while later With the increase in steam flow rate lagging behind fuel flow rate, the additional heat in the furnace can lead to large deviations in steam temperature To compensate for this, a feed forward control signal from the boiler master to the steam temperature controller can be implemented.
● The feed forward can use the rate of change in fuel flow or one of several other derived measurements to bias the steam temperature controller's output. In essence, when boiler load is increasing the spray water flow rate will be increased to counter the excess heat being transfered to the steam, and vice versa The feed forward can be calibrated by measuring the extent of steam temperature deviation during load ramp.
Firing Control Systems :
● In general, tiring control is accomplished with a the pressure of the main steam header and modulates the firing rate and hence, the steam production rate of one or more boilers delivering steam to the steam header The firing demand signal is sent to all boilers in parallel, but cach boiler is provided with a Boiler Master to allow the Plant Master demand signal to be over ridden ar biased. When the signal is overridden, the steam production rate of the boiler is set manually by the operator, and the boiler is said to be base-loaded Most boilers on a given header must be allowed to be driven by the Plant Master to maintain pressure control. Boilers that have the Boiler Master set in automatic mode (passing the steam demand from the Plant Master to the boiler firing control system) are said to be swing boilers as opposed to base-loaded boilers.
● The heat recovery steam boilers on a steam header raises new control issues because the steam production rate is primarily controlled by the power demand placed on the turbine providing the heat to the boiler. If the heat recovery boiler operates at a pressure above the beader pressure, a separate pressure control system can be used to blow off excess steam from the heat recovery boiler when production is above the steam header demand. For maximum efficiency, heat recovery boilers are fitted with duct burners to provide additional heat to the boiler. The duct burner is controlled with a Boiler Master like any other swing boiler. As long as there are other large swing boilers connected to the steam header, the other fired boilers can reduce firing as required when output increases from the heat recovery boiler.
Feed Water Control Systems :
● Boiler feed water control systems are often the most archaic controls in the steam plant. Poor boiler water side control contributes to scaling, corrosion, and eventually hot spots and tube failures. The Preferred Feed water Center can control the surge tank, Deaerator (DA) tank transfer pumps, and feed water pumps (on-off or VSD) to improve water quality and feed water system reliability.
● When boilers operated low pressure, reasonably inexpensive to make the steam drum Large In a large drum, liquid level moves relatively slowly to response to disturbances (it has a long time constant Therefore, manual or automatic adjustment of the feed water valve in response to liquid level variations was an effective control.
● The smaller drum size is an reduction in process time constants, or the speed with which important process variables can change. Smaller time constants mean upsets must be addressed more quickly, and this has led to the development of increasingly sophisticated control strategies.
Method of feed water control
The number of process variables that are measured to effect control of the boiler feed water control valve. These measured process variables are :
1. liquid level in the boiler drum,
2. flow of feedwater to the boiler drum, and
3. flow of steam leaving the boiler drum
1. Liquid level in the Boiler Drum :
● Maintaining liquid level in the boiler steam drum is the highest priority. It is critical that the liquid level remain low essough to guarantee that there is adequate disengaging volume above the liquid, and high enough to assure that there is water present in every steam generating tube in the boiler. These requirements typically result in a narrow range in which the liquid level must be maintained.
● The feed to maintain liquid level in industrial boilers comes from multiple sources and is brought up to steam drum pressure by pumps operating in parallel With multiple sources and multiple pumps. the supply pressure of the feed water will change over time. Every time supply pressure changes, the flow rate through the valve, even if it remains fixed in position, is immediately affected.
● If the boiler drum liquid level is low, the level controller act to increase in feed water flow. But if at this moment, the feed water supply pressure were to drop. The level controller valve could be opens, yet the falling supply pressure could actually cause a decreased flow through the valve and into the drum.
● Thus, it is not enough for the level controller to directly open or close the valve. Rather, it must decide whether it needs more or less feed flow to the boiler drum. The level controller transmits its require flow us a set point to a flow controller. The flow controller then set bow much to open or close the valve as supply pressure swings to meet the set point.
2. Flow of feed water to the Boiler Drum :
This is a 2-element cascade control" boiler liquid level to feed water flow rate. By placing this feed water flow rate in a fast flow control loop, the flow controller will immediately sense any variations in the supply conditions which produce a change in feedwater flow. The flow controller will adjust the boiler feedwater valve position to restore the flow to its set point before the boiler drum liquid level is even affected. The level controller is the primary controller in this cascade. adjusting the set point of the flow controller, which is the secondary controller.
3. Flow of steam leaving the Boiler Drum:
● The third element control system is the flow of steam leaving the steam drum. The variation in demand from the steam header is the most common disturbance to the boiler level control system in an industrial steam system.
● By measuring the steam flow, the magnitude of demand changes can be used as a feed forward signal to the level control system. The feed forward signal can be added into the output of the level controller to adjust the flow control loop set point, or can be added into the output of the flow control loop to directly adjustedthe boiler feed water control valve. The majority of boiler level control systems add the feed forward signal into the level controller output to the secondary feed water flow controller set point. This approach eliminates the need for characterizing the feed forward signal to match the control valve characteristic
● In actual boiler level control schemes do not feed the steam flow signal forward directly. Instead, the difference between the outlet steam flow and the inlet water flow is calculated. The difference value is directly added the set point signal to the feedwater flow controller. Therefore, if the steam flow out of the boiler is suddenly increased by the startup of a turbine, for example, the set point to the feedwater flow controller is increased by exactly the amount of the measured steam flow increase
● If the two flow meters are exactly accurate, the flow change produced by the flow control loop will make up exactly enough water to maintain the level without producing a significant upset to the level control loop. Similarly, a sudden drop in steam demand caused by the trip of a significant turbine load will produce an exactly matching drop in feed water flow to the steam drum without producing any significant disturbance to the boiler steam drum level control.
● There are losses from the boiler that are not measured by the steam production meter. The most common of these are boiler blow down and steam ventsincluding relief valves, ahead of the steam production meter. In addition, boiler operating conditions that alter the total volume of water in the boiler cannot be corrected by the feed forward control strategy. For example, forced circulation boilers may have steam generating sections that are placed out of service or in service intermittently. The level controller itself must correct for these unmeasured disturbances using the normal feedback control algorithm.
