Boiler Control

Three basic parts of boiler controls will be discussed ... 

Electrical resistance boilers use banks of fixed, immersion-type, resistance heating elements (typically sheathed in seamless copper, Incoloy 800, or 316SS) to provide an energy source that is contained within a carbon-steel pressure vessel. The vessel is provided with a sight glass and all normal boiler controls, valves, and regulators necessary for automatic operation. The vessel is generally well insulated and housed within an enameled metal cabinet. Various electrical supply options are available. 

Soot blowers also are used to clean particular boiler tube bundles, such as superheaters and economizers. Steam or compressed air normally is used, and operating practice may require the use of manual boiler control and increased furnace draft or boiler loading during sootblowing periods to avoid the risks of loss of furnace flame (flameout) or small furnace explosions (furnace puff). 

Boiler control systems include combustion controls, superheat steam temperature controls and FW controls. Control systems are used to maintain steam pressure, boiler load, drum water levels, and fuel-air ratios. 

A further example is the effect of tube failure resulting from longterm overheating. Here it is likely that the principle contributing causes are a combination of deposit formation and stresses resulting from mechanical operation of the boiler in excess of design limits. To minimize or eliminate the risks of deposits and the subsequent effects they produce within the boiler, control again requires a twofold approach ... 

Table 10.4 Phosphate Cycle FT Boiler Control Limits p428... 

Table 10.5 Phosphate Cycle WT Boiler Control Limits p429... 

A system of balancing induced draft and forced draft to a large WT boiler controlled primarily by dampers. In fact, large WT boilers tend to be specifically designed to operate at a slightly negative furnace pressure. 

This diagram shows good boiler controls without optimization. 

The first three methods of optimization are achieved by closed-loop process control and can be superimposed upon the overall boiler control system shown in Figure 2.1. The tie-in points for these optimization strategies are also shown in that figure. The benefits of the last two methods (efficiency and accountability) are not obtained in the form of closed-loop control signals, but they do contribute to better maintenance and better understanding of heat losses and equipment potentials. 

Closed-loop multivariable boiler control has to be planned and performed carefully because plant operators are not traditionally willing to reduce air-fuel ratios due to concerns about CO and other symptoms associated with Oz-deficient combustion. Model predictive control (MPC) is by far the most widely used technique for conducting multivariable boiler optimization and control. Forms of MPC that are inherently multivariable and that include real-time constrained optimization in the design are best suited for boiler application. 

Boiler controls have already been described in Section 2.2, so their discussion here will be limited and oriented toward power generation. The level control of the steam drum of an HRSG is very similar to that of fired boilers, except that up to 30% of nominal steam flow, a single-element controller is usually used. Above 30%, the loop is bumplessly transferred to a three-element control (Figure 2.116). 

For example, consider the exergy costing on a boiler and turbine system shown in Figure 5.2. The cost rate balance for the boiler (control volume 1) relates the total cost of producing high-pressure steam to the total cost of the entering streams plus the cost of the boiler CB, and from Eq. (5.2) we have... 

Replacing old inefficient electricity-generating plants with vastly more efficient new plants would save much energy. Some of these older plants lose two-thirds of the heat energy as waste heat at the site. Replacing these plants with those with more efficient boilers, controls and turbines would reduce the lose of heat energy to about half. The plants could also switch from coal to natural gas which would dramatically reduce the acid rain problem and cut C02 emissions in half. 

Boiler controls have seen the most important improvements in recent years. In the past boiler controls were limited to adjust feed water temperatures and monitor flame operation by the implementation of specific safefy sensors like flame, temperature and pressure sensors. Today boiler controls integrate more and more functions three-way valve, pumps, external heat storage, solar panel controls, and so on. In more advanced utility boilers, thermal power is modulated through fan speed modulation. Air-fuel ratio is controlled by an ionization... 

Benton was, in an aside to the article, disparaging about the use being made of carbon dioxide and temperature readings for boiler control, arguing that the best method was to record coal and water consumption on charts using Avery s coal and closed water weighers. 

F. P. Stainthorp, writing about process control, commented that there had been extensive use of automatic control since 1920 and that industry adopted controllers which had been developed particularly for boiler control. He further commented that only since 1945 had control been seen as an essential and integral part of design. See volume 9 of Herbert W. Cremer and Sidney B. Watkins, eds. Chemical... 

Steam, with few major unexpected changes in rate. Boiler control schemes therefore may be less elaborate than those found in other industries. 

Applicants should have a recognized HNC or National Certificate in Electrical / Electronic Engineering and have served a recognized apprenticeship. Experience in the operation and maintenance of electromechanical plant utilizing electronic system control including experience of HVAC plant and systems, electronic PLC systems, boiler control systems, positional and electronic speed control systems including hydraulics, pumps, and heat exchangers would be desirable. 

Fig. 6 A typical coal-fired boiler control with 2.5 % O2 excess during the day and 5.5 % overnight operation depending on the load...

---