Boilers, drum

A reladvely simple example of a HAZOP study using guidewords is shown for a boiler drum in Fig. 15.7.1 and Table 15.7.2. The intent of die operation is to maintain die water level in die horizontal drum between 30 and 40% of die volume. 

Water Tube Steam Generators Steam generators for industrial applications may have internal boiler components such as drums, boiler bank, and membrane wall designed in one of several different arrangements. These arrangements are based on the position of the boiler drum and include the following ... 

Boiler drum dimensions, including the available separation surface area and the steam space water space ratio optimum separation occurs where the maximum steam-water surface area is available... 

Where excessive foaming occurs, the apparent volume of water present in the boiler drum increases (light water). The persistence of foam increases with increases in ... 

High-pressure injection direct to the boiler drum... 

In the combustion area, heavy slag and ash may form, preventing the passage of flue gas and blinding tubes. Locations should be precisely noted to provide fireside adjustments or to implement a fuel treatment program. In coal-fired boilers, drums, tubes, and headers should be inspected for abrasion from clinker and fly ash. 

One of various types of valve employed to enable BD of a boiler. Main BD valves are located at the bottom of all boiler drums, shells, and headers. 

They should preferably not be used on steam service, steam boiler drums or superheaters, where an open bonnet-type SRV is generally preferred because of the temperature of the spring, which is required to be cooled off in order for it to keep its characteristics and maintain stability in the set pressure. 

Exceptions can exist since the corrosion in a wet solution of the interior boiler drum (steel) with dilute caustic soda at high temperature and high pressure and the reaction of high temperature water with aluminum and zirconium have been found to be best interpreted in terms of a dry corrosion mechanism.1... 

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. 

INTENTION FEED THE REQUIREO AMOUNT OF CORRECTLY TREATED FEEC5WATER TO THE BOILER DRUM TO MAINTAIN THE WATER LEVEL IN THE DRUM BETWEEN 30 ANO 40%... 

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. 

Perform a HAZOP study on the system with the intention of providing the required amount of correctly treated feedwater to the boiler drum to maintain the prescribed water level. 

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

Fatigue failure can also be a result of thermal variations as well as other loadings. Fatigue failure has occurred in boiler drums due to temperature variations in the shell at the feed water inlet. In cases such as this, design details are of extreme importance. 

Piping, tubing, boiler drums, turbine casing, pressure vessels, turbine rotors, pumps Turbine rotors, high-temperature piping and tubing, cast and forged pressure vessels... 

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 ... 

These terms are evidently ambiguous for example, it is not always clear whether wet is confined to aqueous solutions—the wetting of solids by mercury indicates that liquid-metal corrosion should be classified as wet . Even if the term is restricted to aqueous solutions, the difficulty arises that the mechanism of growth of magnetite scale during the reaction of the interior of a boiler drum with dilute caustic soda at high temperatures and pressures is best interpreted in terms of a dry corrosion process. Similar considerations apply to the reactions of aluminium and zirconium with high-temperature water. 

Filling, preheating, and preserving steam boiler drums and headers... 

Many applications utilizing boiler equipment require steam at a very high rate of purity. Boiler designers therefore install steam-separating equipment in the boiler drum to remove entrained moistnre and solids before the steam is taken from the boiler to the system. These steam separators come in a variety of types including cyclones, mist deflectors, and baffle plates. 

These contaminants can enter the steam supply system with the makeup water or in process heat exchange equipment. They can exist in the boiler drum in relatively high concentrations without causing problems. It is only when they are carried over into the exiting steam that they enter the turbine. Efficient boiler drum separators can limit total dissolved solids to as little as 0.5 to 1.0 ppm. Silica is difficult to separate from the steam and must be controlled in the boiler feedwater. Care must also be taken with the fluid used to attemperate the steam exiting the superheater. Contaminated process returns can bypass the steam separation in the drum as attemperator fluid. This should be maintained at less than 1.0 ppm total dissolved solids. 

Cockenzie was a coal-fired power station in construction at the lime. (Cockenzie power station went into service in 1967. It operated until March 2013.) To satisfy the designers and the boiler insurance company, each of the boiler drums was put through a number of hydraulic pressure cycles. Each drum weighed 164 tonnes, had an inside diameter of 5 feet 6 inches (1.67 m) and an overall length of 74 feet 9 inches (22.78 m). The drum was an all-welded construction, fabricated from forged steel plates 5.5625 inches thick (14.13 cm). The material was amild manganese steel alloy called Ducol. 

Report on the Brittle Fracture of a High-pressure Boiler Drum at Cockenzie Power Station, Sonth of Scotland Electricity Board, January 1967. 

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.

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

Direct point switches where the sensor s physical location defines the limit value e.g. limit switch or proximity switch a fixed level switch such as the float switch commonly used on small boiler drums. 

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