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Boiler Solutions |
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Proven success in integrated automation for boiler plant management and control |
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Designed to maximize boiler
efficiency, minimize fuel consumption and reduce emissions, our control and
monitoring solutions function to PM5 and PM60 standards, ensuring that all
functions are performed to the highest levels. They also include a number of
advanced functions, such as the ability to communicate over Ethernet based
networks, allow steam data to be measured around the whole site.
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Process systems using water such
as boilers, CHP plants and water treatment plants, or systems using any
types of solution such as those in fermenters, must be designed to take into
account the control of pH.  Figure 1 Neutralised curve
The concentrations of the acids and alkalis in solutions determine the pH
and shape of the titration (neutralization) curve shown in Figure 1. This
curve shows that the pH is far more sensitive to a change in composition
around neutrality than elsewhere.  Figure 2 Correct tank layout Smith predictor In order to optimize the response time and improve the accuracy of the control strategy, the Eurotherm Process Automation PID control module can optionally include a Smith Predictor algorithm based on a mathematical model of the process. When the process flow rate is the major load variable, the pH control is improved by configuring the control module as a ratio controller. The objective is to meet increased flow with a corresponding increase in reagent. For this, one loop is dedicated to pH control and uses a Smith Predictor to compensate for the deadtime. The product of the output of the pH loop and the influent flow provides the setpoint to the dedicated reagent flow control loop. |
Boilers are often the principal steam or hot
water generator system used in industrial plant or commercial heating.
Consequently, they must be designed to operate efficiently and safely whilst
responding rapidly to any change in demand. Burner management systems must
be equally adaptive. Eurotherm Process Automation provides efficient, well
implemented control techniques capable of reducing operating costs whilst
providing resources for greater flexibility in plant management and control.
Burner combustion control generally includes one or a combination of the
following methods
Excess air regulation In actual practice, gas, oil, coal burning and other systems do not do a perfect job of mixing the fuel and air even under the best achievable conditions. Additionally, complete mixing may be a lengthy process. Figure 1 opposite shows that in order to ensure complete combustion and reduce heat loss, excess air has to be kept within a suitable range. The regulation of excess air provides: Oxygen trim When a measurement of oxygen in the flue gas is available, the combustion control mechanism can be vastly improved (since the percentage of oxygen in flue is closely related to the amount of excess air) by adding an oxygen trim control module, allowing Burner modulation Modulating control is a basic improvement in controlling combustion. A continuous control signal is generated by a controller monitoring the steam or hot water line. Reductions in steam pressure or hot water temperature lead to an increase in firing rate. The advantages of introducing burner modulation in combustion control include Air/fuel cross-limiting A cross-limiting combustion control strategy ensures that there can never be a dangerous ratio of air and fuel within a combustion process. This is implemented by always raising the air flow before allowing the fuel flow to increase, as shown in Figure 2, or by lowering the fuel flow before allowing the air flow to drop.  Figure 2 Cross-limiting combustion mechanism Figure 3 depicts a simplified control block diagram of the cross-limiting combustion circuit. Combination firing of multiple fuels simultaneously can also be easily accommodated within the scheme. Cross-limiting combustion control is highly effective and can easily provide the following  Figure 3 Cross-Limiting combustion control with 06 trim Enhanced cross-limiting Double cross-limiting combustion control is an enhancement to the above. It is achieved by applying additional dynamic limits to air and fuel setpoints. This translates to having the actual air/fuel ratio maintained within a preset band during and after transition. This method protects against having the demand signal driving the air/fuel ratio too lean, therefore reducing heat loss. Total heat control In situations where combustion is not the principal heat source and when several factors contribute to the total heat to be generated by a boiler, a control loop can be introduced in order to monitor and manage the generated heat. This is particularly true for CHP plants, where gas turbines and supplementary firing are used. This type of implementation is shown in Figure 4:  |
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One of the areas within a boiler plant that is critical to the process is
the delivery of boiler feedwater. Depending on the design and functionality,
individual feed pumps servicing individual boilers or a bank of feed pumps
may maintain a common feedwater pressure that feeds into the boilers. Implementing pump sequence control allows the system to sequence and cycle pumps such that a minimum number of pumps are needed to maintain the feedwater flow to the boilers requiring it. The pump sequence control can also regulate (where variable speed pumps have been implemented) the output of each pump making its usage more energy efficient. Pump efficiency is the ratio of the useful output power of the pump to its input power. The typical range of pump efficiencies is from 60 to 85% and is a function of changes in speed, impeller diameter and specific gravity as defined in the following equation Ep μ Sw Q Pt / Pi Where Ep is the pump efficiency (%) Sw is the specific weight of the transported liquid (kN/m3) Q is the pump capacity (m3/s) Pt is the head pressure (bar) Pi is the power input (kW)  Figure 1 Pump characteristics At a given impeller diameter and specific gravity, pump flow is linearly proportional to pump speed, pump discharge head relates to the square of pump speed and pump power consumption is proportional to the cube of pump speed. This is why variable-speed pumps can be so highly energy efficient. A pumping system is optimized when it meets the process demand for liquid transportation at minimum pumping cost in a safe and stable manner. Once the equipment is installed, the potential for optimization is limited by the capabilities of the selected equipment, piping configuration and control implementation. Full automation of pumping stations, including automatic start-up and shutdown and optimized supply-demand matching, offers the following ● Reduction in operating costs ● Protection from loss of control ● Reduced maintenance and cycling ● Increased operating safety as human errors are eliminated Depending on plant requirements and the type of application, the pump arrangement can be either parallel or serial as shown in Figures 2 and 3. Series pumping is most effective when the system head pressure curve is steep. When head pressure is not a constraint, parallel pumping is preferred.  Figure 2 Multiple pump layout  Figure 3 Head pressure - flow rate curves The Eurotherm Process Automation control module for pump sequencing allows efficient management of pump load and offers a robust control combined with a powerful man-machine interface. Lead/lag selection The control module scheduler arrangement allows the pumps to be run such that the use of each pump is prioritized according to a defined order. If a running pump fails, the next available pump is automatically requested to run. The pump that is always chosen to run is referred to as the 'Lead' pump. The other pumps are 'Lag' pumps but are prioritized such that a pump with a higher priority always runs before a pump with a lower priority. The lead/lag selection and prioritization can be set either by the operator or automatically by the application database. These features mean that ● On an increase of demand, the most efficient pump is started first ● On a decrease of demand, the least efficient pump is stopped first Features Management and maintenance activities require a strong and effective man-machine interface, providing the operators, supervisors or plant engineers at any time with an informative and real-time representation of the process. The control module offers, depending on the application needs, features such as load cycling based on hours run or time of day. Flexibility and ease of configuration combined with a powerful functionality make this control module an essential element in industry applications requiring high efficiency pumping systems at reduced operating costs. |
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The purpose of the drum level controller is to
bring the drum up to level at boiler start-up and maintain the level at
constant steam load. A dramatic decrease in this level may uncover boiler
tubes, allowing them to become overheated and damaged. An increase in this
level may interfere with the process of separating moisture from steam
within the drum, thus reducing boiler efficiency and carrying moisture into
the process or turbine. The functions of this control module can be broken down into the following ● Operator adjustment of the setpoint for drum level ● Compensation for the shrink & swell effects ● Automatic control of drum level ● Manual control of the feedwater valve ● Bumpless transfer between auto and manual modes ● Indication of drum level and steam flow ● Indication of feedwater valve position and feedwater flow ● Absolute/deviation alarms for drum level The three main options available for drum level control are Single element drum level control The simplest but least effective form of drum level control.  Figure 1 Single-element drum level control This consists of a proportional signal or process variable (PV) coming from the drum level transmitter. This signal is compared to a setpoint and the difference is a deviation value. This signal is acted upon by the controller which generates corrective action in the form of a proportional output. The output is then passed to the boiler feedwater valve, which then adjusts the level of feedwater flow into the boiler drum. Notes: ● Only one analogue input and one analogue output required ● Can only be applied to single boiler / single feedpump configurations with relatively stable loads since there is no relationship between drum level and steam- or feedwater flow ● Possible inadequate control option because of the swell effect Two element drum level control The two-element drum level controller can best be applied to a single drum boiler where the feedwater is at a constant pressure.  Figure 2 Two-element drum level control The two elements are made up of the following Level Element: a proportional signal or process variable (PV) coming from the drum level transmitter. This signal is compared to a setpoint and the resultant is a deviation value. This signal is acted upon by the controller which generates corrective action in the form of a proportional value. Steam Flow Element: a mass flow rate signal (corrected for density) is used to control the feedwater flow, giving immediate corrections to feedwater demand in response to load changes. Any imbalance between steam mass flow out and feedwater mass flow into the drum is corrected by the level controller. This imbalance can arise from ● Blowdown variations due to changes in dissolved solids ● Variations in feedwater supply pressure ● Leaks in the steam circuits Notes: ● Tighter control of drum level than with only one element ● Steam flow acts as feed forward signal to allow faster level adjustments ● Can best be applied to single boiler / single feedpump configurations with a constant feedwater pressure Three-element drum level control The three-element drum level control is ideally suited where a boiler plant consists of multiple boilers and multiple feedwater pumps or where the feedwater has variations in pressure or flow.  Figure 3 Three-element drum level control The three-elements are made up of the following Level Element & Steam Flow Element: corrects for unmeasured disturbances within the system such as ● Boiler blowdown ● Boiler and superheater tube leaks Feedwater Flow Element: responds rapidly to variations in feedwater demand, either from the ● Steam flow rate feedforward signal ● Feedwater pressure or flow fluctuations In order to achieve optimum control, both steam and feedwater flow values should be corrected for density. Notes: ● The three-element system provides tighter control for drum level with fluctuating steam load. Ideal where a system suffers from fluctuating feedwater pressure or flow ● More sophisticated level of control required ● Additional input for feedwater flow required Enhanced three element drum level control The enhanced three-element drum level control module incorporates the standard three element level components with the following improvements ● The three-element mode is used during high steam demand. The two-element mode is used if the steam flow measurement fails and the module falls back to single element level control if the feedwater flow measurement should fail or if there is a low steam demand. ● The drum level can be derived from up to three independent transmitters and is density compensated for pressure within the boiler drum. Notes: ● Tighter control through a choice of control schemes. Drum level maintained on failure of steam or feedwater flow measurements ● This module introduces an additional level control loop Boiler drum level control |
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Before boiler feedwater is passed into the
boiler, it must be chemically treated to remove the corrosive elements that
may be present and would ultimately corrode the boiler as well as affect the
quality of steam required within a process. Chemicals entering the boiler via the feedwater must be removed from the boiler. Failure to do so can result in the boiler system suffering from scale formation, corrosion, brittle and cracking metal, carry-over and foaming. Therefore a proper chemical balance must be maintained within the boiler itself. This is achieved through blowdown control. This process involves activating the blowdown valve mechanism situated on the boiler drum and drawing off a small percentage of the boiler water (containing the dissolved solids and non-dissolved sediments) from below the surface of the water in the boiler. In order to retain a chemical balance within the boiler, the quantity of chemicals removed from the drum via blowdown must be equal to the quantity of chemicals that enter through feedwater. As steam loads vary, the rate of feedwater changes and so does the rate of blowdown. On the other hand, excessive blowdown leads to inefficient running of the boiler plant, as each blowdown causes heat contained within the expelled water to be lost. The cost of fuel can be directly related to this heat loss. The cost of water and chemicals should also be taken into account. A balance has to be established between the requirements of removing the dissolved solids from the boiler system and running the boiler plant cost effectively. A boiler, operating at 80% efficiency, has a maximum evaporation rate of 5,000kg/hr at 10 bar and receives feedwater at 70°C. Of the 5,000kg/hr, 4,500kg/hr of steam is exported and 500kg/hr is lost through blowdown. Using steam tables, the heat content of the water and steam is calculated to be 4,500kg/hr ( 2,357kJ/kg = 9,621,274kJ/h 500kg/hr ( 357 kJ/kg = 178,500 kJ/h giving a total of 9,799,774kJ/h or 2,723kW The above example is typical of a modern boiler plant using base exchange softening only. Blowdown rates are much lower when de-mineralized feedwater is used. In the example, the heat loss is equivalent to 1.8% of the fuel fired.  Operated continuously over a year the fuel wasted per boiler represents approximately 46,500 m3 of natural gas, 44,500 litres of fuel oil or 70 tonnes of coal. Added to this is also the cost of acquiring and treating the water that is used within the boiler system. Blowdown control can be broken down into instantaneous or continuous systems and may be manual, semi-automatic or fully automatic. Instantaneous manual system The simplest implementation of blowdown control is an instantaneous manual system that is operated once per shift to reduce the boiler total dissolved solids (TDS) to a sufficient level well below the boiler specified maximum limit. The TDS are then allowed to build up during the next shift until they reach the maximum level again. A TDS test should be carried out prior to blowdown so that the time can be adjusted to reflect changes in average boiler load conditions which may occur on a day-to-day basis. Advantage: ● Easily implemented with relatively low sensor outlay Disadvantage: ● Load fluctuations are not taken into account. Heat recovery from blowdown is expensive and difficult Automatically timed control Figure 2 shows a simple semi-automatic system where a timer is used to control blowdown for short periods according to a pre-set schedule. Again, with this system, daily testing of the boiler is necessary so that the timing schedule can be adjusted to take into account changes in boiler and system operation.  Figure 2 Automatically timed blowdown control The system can be made fully automatic by installing a TDS monitoring facility as pictured in Figure 3. This will override the timer in the event of variation from the desired TDS level.  Figure 3 Automatic blowdown control with TDS monitoring Disadvantage: ● Standard fully open/closed valve provides coarse control Continuous control Continuous blowdown systems are preferable where heat recovery is required. In its simplest form, such a system consists of a valve, adjusted after regular boiler water testing. The valve position is determined from the boiler pressure, TDS levels and the blowdown rate required. As shown in Figure 4, a control module is used to modulate the blowdown valve using inputs from a TDS detector located in the cooled blowdown sidestream. For this system to operate correctly, cooled blowdown must flow continuously over the detector. Figure 4 Continuous blowdown control Advantage: ● Smaller and cheaper heat recovery plant ● Possibility of recovering up to 50% of the heat in the blowdown  Intermittent blowdown Blowdown can also be achieved in the boiler evaporators where sediments are deposited. This process is carried out intermittently by opening the appropriate valve and allowing the sediments to be flushed out. Combined control Eurotherm Process Automation offers a control module that can be configured for continuous, intermittent or both continuous and intermittent blowdown control. |
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Plant activities that may be deemed critical
require a higher level of backup functionality. This can be in the form of a
pump or other device set. In order to deliver this backup functionality,
these device sets must be able to automatically react to plant conditions
such as device failure. In addition to the plant backup facilities, device sets can also allow for equal workload distribution and offer 'maintenance-aware' options such as hours run logging and automatic changeover. One of a range of control modules designed by Eurotherm Process Automation, incorporating efficient control techniques and supplying this functionality, is the Duty/Standby control module. The Duty/Standby arrangement allows a pair of devices - typically On/Off or variable speed drives - to be operated with an element of redundancy. Each device is capable of matching the plant demand and, thus, normally only one device is run at any one time. Should the running device fail, the remaining device is automatically requested to run. One device is referred to as the 'Duty' while the other device is the ‘Standby'. The choice of which device acts as Duty is known as the Service. In normal running operation, the Duty is running and the Standby is stopped.  Some of the functions provided by the Duty/Standby control module are ● Operator selection of Duty/Standby device when in Manual ● Changeover on device failure or process condition when in Automatic ● Bumpless transfer ● Equipment status and hours run indication Operator selection of Duty/Standby device In normal running operations, the operator may request a changeover such that the Standby device is started. When the control module successfully starts the Standby device, the Service is changed. Following a Duty device failure, the operator may also request to 'make duty' the running Standby without altering the state of either device. Changeover on device failure or process condition The control module may act, as a result of a single failure or a process condition, by scheduling the control devices in a single direction only, i.e. automatically starting the Standby device as a result of the failure of a running Duty device but not starting the Duty device if the Standby device fails. Bumpless transfer Mode selection (Manual or Auto) is initiated by the operator with bumpless transfer, preventing abrupt changes in the output that could otherwise cause damage to field equipment and destabilize the plant process. Equipment status and hours run indication The control module offers a comprehensive Man-Machine Interface by providing continuous monitoring of device status and alarming capabilities. It can also offer additional features such as running time for each device for maintenance purposes. Some of the advantages offered by the implementation of a Duty/Standby device set are: ● Redundancy ● Easier maintenance ● Quick diagnosis of failures ● Reduction/elimination of down time |
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One of the primary goals in operating a boiler
plant is to ensure that the working steam pressure (or temperature in hot
water systems) is sustainable for any load demand placed on the plant. At
the same time, this requirement must be met as efficiently and cost
effectively as possible. In a multi-boiler plant, this can be achieved through the implementation of demand load management, the purpose of which is to distribute the steam demand in an optimized manner and to adjust the boiler plant output to meet working requirements. This ensures that boilers are fired only when required, thus reducing running costs. Alternatively, demand load management can allow each boiler to be allocated the same amount of running time. The Eurotherm application module for demand load management offers a comprehensive set of functions, some of which are described below. Operator load allocation The demand share arrangement allows each boiler to be operated in either base-load or modulating service, finding the best distribution of load between the boilers that will result in the lowest overall cost. The base-load operation leaves the implementation up to the operator. In this mode, the total demand is shared between the baseload boilers in proportion to the operator set base-load values. The modulating mode of operation, on the other hand, enforces automatically the load allocation without the need for operator intervention. The total demand, less that satisfied by the base-load boilers, is shared between the modulating boilers in proportion to their capacities. The flexibility of the control module is such that one combination of boiler modes can be applied dynamically to the boiler plant. Demand sharing In boiler plants, the most effective load allocation is not based on a simple operating decision but on real-time calculations taking into account the following ● Operating safety margins ● Load fluctuations ● Required shut-down characteristics ● Boiler capacities A further important decision involves the demand sharing methodology, which can be either parallel or series, depending on plant requirements. The Eurotherm control module allows for both configurations. In parallel, the available boilers share the total demand simultaneously by taking up an equal firing rate to meet the load. On load increase, the firing rate of all modulating boilers will increase equally until the load requires an additional boiler. At this point, the firing rate of the active boilers decreases to compensate for the firing rate of the newly started boiler. Figure 1 explains the process for an increase of load. Parallel modulation is generally implemented for steam boilers. It offers the most effective control when relatively steady process loads are available. As the system modulates the boiler plant to adjust the common header pressure to the required setpoint, a smoother response to changing load conditions is performed by the controller.  Figure 1 Parallel demand sharing Series demand sharing allocates loads by normally forcing one boiler at a time to modulate in order to satisfy the demand and is most effective when used with the Eurotherm Process Automation demand schedule control module. On load increase, the firing rate of the modulating boiler will increase until the load requires an additional boiler. At this point, a new boiler is started and becomes the modulating boiler. The other active boilers are ramped to their optimum firing rate. Figure 2 explains the process for an increase of load. Series modulation is generally implemented for hot water systems or fluctuating steam loads. This mode allows faster individual boiler response to plant conditions as the boiler pressure is adjusted to the required setpoint.  Figure 2 Series demand sharing The boilers that are chosen to always run are referred to as the 'lead' boilers. All the other boilers are 'lag' boilers but are prioritized such that a boiler with a high priority always runs before a boiler with a low priority and so on. E.g. the most effective boiler is always started first and the least effective one is always stopped first. Boiler banking This functionality is achieved by keeping the available boilers in hot standby mode until required to fire. This is achieved by intermittently firing the unused boilers, thus maintaining a required pressure by use of upper and lower banking thresholds or by recirculation of return water through the boilers to keep them hot. The main advantage of boiler banking is that it acts a 'warm' start facility improving the plant response to sudden load changes. 8-day timer Further enhancement to the man machine interface is achieved by the 8-day timer facility depicted in Table 1. Boiler banking is tabulated according to daily upper and lower banking threshold pressures with an additional user definable 'Today' schedule to be loaded if and when required. Up to four optional session settings can be pre-configured and stored at the supervisory computer.  Table 1 Boiler banking 8-Day timer Multi-sequence programme selection In order to meet the plant demand with savings on fuel consumption, the boiler dispatching can be automated via a multi-sequence programme selection. The boiler duties can then be scheduled according to configurable daily sessions or sequences of events. The effects of this feature are ● Flexibility ● Reduction in operating decisions ● Robust control implementation ● Expandability Demand load management is an optimizing function that augments, but does not replace, the combustion control system. |
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Process systems using water such as boilers and
water treatment plants must be designed to operate efficiently whilst
responding to any changes in demand. In these processes, water is generally
lost in the recirculation by evaporation, drift, blowdown or leakage. Eurotherm Process Automation provides an efficient, well implemented control technique capable of reducing operating and pumping costs whilst providing resources for greater flexibility in plant management and control. Process systems using water such as boilers and water treatment plants must be designed to operate efficiently whilst responding to any changes in demand. In these processes, water is generally lost in the recirculation by evaporation, drift, blowdown or leakage. The illustrated control loop uses a PID control module to regulate the total water flow in the recirculation system by adding make-up water to the circulated water. The circulated water flow in the plant is measured, linearized within the control module and compared with the required total flow setpoint. The resultant value is then used as a setpoint for the make-up water flow control loop. Make-up water control provides: ● Plant efficiency ● Faster return to optimal operating levels ● Substantial savings on operating costs
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Overview Control and monitoring of a Fire-Tube Boiler is often found in hospitals and most process plants that use steam either for process or building heating. The 6000 Series master communications opens up packaged solutions of recorders and controllers. Solution In this application the 2704 controls the water level and total dissolved solids (TDS). It also performs other boiler control functions such as Timed Bottom Blowdown and Steam Main Warm Up. A 6000A or 6000XIO is used to provide an HMI to the 2704 and act as a steam flow computer calculating the Saturated Steam Mass Flow. The 6000 can be directly connected to the factory Ethernet providing valuable energy management information.  2704 Boiler Controller Incorporating all the main boiler control elements, the 2704 provides a very cost effective integrated control solution for packaged Fire-Tube boilers. The 2704 can be used solely to control Total Dissolved Solids (TDS), or also to control water level and boiler pressure. A specialist input module makes it compatible with standard TDS conductivity probes making both retrofit and new installations easy to accomplish. Additionally, Timed Bottom Blowdown and Steam Main Warm Up functions make use of the controller’s internal real time clock. For more detailed information on the 2704 as a boiler controller please refer to application note ANC169 on boiler types, and technical information note TIBC177 on TDS measurement. ● Water level ● Boiler Pressure ● Continuous Total Dissolved Solids ● Main Steam Warm-Up ● Open Communications ● Flexible User Interface
6000 Flow Computer
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