Boiler drum level control

FIGURE 15.54 Boiler drum level control, (a) Feedback, (b) feedforward, and (c) feedback and feedforward combined. 

Final control elements In rare instances the final control elements can be duplicated, in cases when the erosive/corrosive or sticking characteristics of the fluid could cause unacceptable downtime or in cases of critical controls (viz, boiler drum level control with control valves in medium-sized power plants). The major cases are as follows ... 

Another method for dealing with high reactor temperatures is to generate steam, as shown in Fig. 4.19. Here we allow the coolant to boil and thereby provide a constant jacket temperature. The secondary loop controls pressure in the boiler drum by venting steam. Fresh boiler feed water is added by level control. A potential problem W ith this arrangement is the possibility for boiler swell that results in an increase in the level due to increased vaporization in the jacket. The increased level due to swell reduces the intake of boiler feed water when in reality it should be increased. This problem can be overcome by providing a ratio controller between the steam flow and the feed water with the ratio reset by the steam drum level controller. Boiler feed water flow will now change in the correct direction in response to load. 

Figure 15.54b is a schematic of a feedforward controller applied for steam drum level control. If the flow rate of the makeup feedwater is equal to the steam usage, the drum level remains constant. One is tempted to conclude that the feedforward controller is aU that is needed for this application. Unfortunately, the measurements of the steam usage and the feedwater flow rate are not perfectly accurate. Even small errors in measured flow rates add up over time, leading to one of two undesirable extremes. The drum can till with water and put water into the steam system, or the liquid level can drop, exposing the boiler tubes, which can damage them. As a result, neither feedback nor feedforward are effective by themselves for this case. In general, feedforward-only controllers are susceptible to measurement errors and umneasured disturbances, and, as a result, some type of feedback correction is typically required. 

Feedforward is commonly applied to level control in a drum boiler. Because of the low time constant of the drum, level control is subject to rapid load changes. In addition, constant turbulence prevents the use of a narrow proportional band, because this would cause unacceptable variations in feedwater flow. The feedforward system simply manipulates feedwater flow to equal the rate of steam being withdrawn, since this rqiresents the load on drum level. The system is shown in Fig. 8.2. 

Figure 15.4 Feedforward-feedback control of the boiler drum level.

An industrial boiler is likely to burst a tube if the water level in its drum falls too low. If a tube bursts there is a risk of serious injury to the field operator who is often close to the plant. The boiler drum level is maintained by an automatic level control loop. 

Liquid circulation from the reactor to the heat exchanger is flow-controlled. Condensate level in the condensate drum is controlled by manipulating BFW (boiler feed water). 

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). 

Figure S-27a and b shows variations in the response of a distributed lag to a step change in load for different combinations of proportional and integral settings of a PI controller. The maximum deviation is the most important criterion for variables that could exceed safe operating levels, such as steam pressure, drum level, and steam temperature in a boiler. The same rule can apply to product quality if violating specifications causes it to be rejected. However, if the product can oe accumulated in a downstream storage tank, its average quality is more important, and this is a function of the deviation integrated over the residence time of the tank. Deviation in the other direction, where the product is better than specification, is safe but increases production costs in proportion to the integrated deviation because quality is given away.

Consider a small industrial boiler system generating low-pressure saturated steam. The boiler drum receives preheated feedwater that is required to maintain the water level in the drum between 30 and 40% of its total height. There is a visual level indicator (LI) on the drum in addition to a level indicator controller (LIC) that is connected to a level indicator control valve (LICV) in the feedwater line. Natural gas fuel is fired below the boiler drum. The fuel line contains a pressure control valve (PCV) that is connected to a pressure controller (PC) located in the steam line in and leaving the drum. 

Trouble-free generation of steam requires water of high quality. The characteristics of boiler feed water and methods for its production are covered below in Section 12.4.3.3. This water is fed to the boiler(s) on demand. The simplest way to control the addition of water is from a level instrument in the steam drum. This iqrproach has a certain amount of inherent instability. When (cold) water is added to the drum in response to a demand from the level controller, some of the bubbles present in the liquid will collapse. Hie level will go down, and the controller will call for even more water. In the opposite case, when the level rises and the water flow is reduced, more bubbles will form and the level will increase, causing the controller to call for less water. The process itself is supplying positive feedback. 

In this example, in its simplified form the actions of the operator when the drum level is high initiates the pretrip alarm. Also in this example, one set of boiler feed pumps (BFPs) and one set of feed control valves (FCVs) have been considered to... 

Three-element level control is most commonly applied to the control of water level in steam drums on boilers. However it is applicable to many other situations where tight level control is required and is made difficult by unusual dynamics. 

The first most commonly encountered problem is swell. The water in the steam drum contains vapour bubbles which expand if the pressure in the drum is reduced, thus increasing the liquid level. So, if there is an increase in steam demand which causes a transient drop in drum pressure, the level controller will reduce the flow of water in order to correct for the apparent increase in level. Of course, on increasing steam demand we need an increased water flow. The pressure in the drum will ultimately be restored, for example by a pressure controller on the steam header increasing the boiler duty, and the level controller will ultimately increase the water flow. However, for the controller to be stable, its initial behaviour means that it will have to act far more slowly than the tight controller defined in Equation (4.14). 

The level controller may also have to cope with inverse response. The boiler feed water ideally is heated in the economiser to the boiling point of water at drum pressure. However, this is often not achieved so that when the cooler water enters the drum it will cause a drop in temperature, thus causing bubbles to collapse and the water level to drop. While controllers can generally be tuned to handle inverse response, they have to be tuned to act more slowly to avoid instability. 

Control of water level must overcome some anomalies which have their parallels In conventional water tube boilers. Thus, a rise in reactor power will increase channel voidage and this causes a slight reduction in recirculation flow also as the new voidage Is established in the channel and riser, water is displaced causing an Initial rise In drum water level. But an Increase In reactor power should call for an Increase In feed flow and this raises the water level further. A corresponding fall In drum water level follows a reduction In power. Hence It Is seen that control of drum level is not a simple function and leaves no additional freedom for feed flow as an operational control. 

Ordinary level control does not work in steam drums or kettle boilers. The problem is the lower density of the boiling water as compared to the greater density of the non-boiling hot water in the external level sensing chamber. I ve addressed this problem in my book, Troubleshooting Process Plant Control (Wiley, 2009). 

On many boiler steam drums, there is a sloped demister to suppress entrainment and hence improve steam quality. However, if this demister becomes partly fouled with hardness deposits, it probably does more harm than good. In such cases, I would probably discard the demister rather than replace it. If you feel you have been doing a good job on level control in the boiler, but steam quality is still poor, you should consider this possibility. 

Feedforward control was not widely used in the process industries until the 1960s (Shinskey, 1996). Since then, it has been applied to a wide variety of processes that include boilers, evaporators, solids dryers, direct-fired heaters, and waste neutralization plants (Shinskey et al., 1995). However, the basic concept is much older and was applied as early as 1925 in the three-element level control system for boiler drums. We will use this control application to illustrate the use of feedforward control. 

Figure 15.2 Feedback control of the liquid level in a boiler drum.

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. 

Traditionally, a common solution to the problem of matching relatively higher levels of technical support with lower chemical volumes for these smaller customers has been via a one- to three-year, fully inclusive product and services contract. Such a contract will specify the frequency of service visits to be made to the customer s site and the type of work to be carried out. It will also, perhaps, limit the maximum volumes of chemical treatments to be supplied during the contract lifetime, or perhaps designate the amount of chemicals required based on treating a certain annual volume of boiler FW. Contracts may include for the provision of chemical feed and control equipment and for the supply of labor for boiler cleaning, chemical addition, and drum removal services (drumless delivery). Product and services contract prices may some-... 

Power plant boilers are either of the once-through or dmm-type design. Once-through boilers operate under supercritical conditions and have no wastewater streams directly associated with their operation. Drum-type boilers operate under subcritical conditions where steam generated in the drum-type units is in equilibrium with the boiler water. Boiler water impurities are concentrated in the liquid phase. Boiler blowdown serves to maintain concentrations of dissolved and suspended solids at acceptable levels for boiler operation. The sources of impurities in the blowdown are the intake water, internal corrosion of the boiler, and chemicals added to the boiler. Phosphate is added to the boiler to control solids deposition. 

The relatively simple, lumped-parameter system model described above has been tested against and used in earnest to analyse the behaviour of the boiler recirculation loops of a number of power stations. It has been found to give excellent quantitative predictions of all the variables whose trends are important for control engineering purposes, namely steam drum pressure and temperature, feedwater flow, steam production, downcomer flow and, very important, drum water level. 

Feedforward control of a drum boiler (shown in Figure 21.2b) Here the objective is to keep the liquid level in the drum constant. The two disturbances are the steam flow from the boiler, which is dictated by varying demand elsewhere in the plant, and the flow of the feedwater. The last is also the principal manipulated variable. 

II.ll Figure PII.10 shows a simplified representation of a drum boiler. Feed water enters the boiler with a flow rate F1 (mass/hr) and a temperature T i and it is heated by an amount of heat Q (Btu/hr) which is supplied by burned fuel. The generated steam flows out from the top of the boiler, with a flow rate F2 (mass/hr) and a pressure p (psig). A simple feedback control system has been installed to keep the level of the water in the drum boiler constant by manipulating the flow rate of the feedwater stream. 

Waste Heat Boilers, Steam Drum, and Superheater safety valves, level indicators, vents, fittings, non-retum valves, water gauge glass (should be visible from control room) position of blowdown vessel ... 

---