Steam boilers water-tube

Field erected steam boiler (Water-tube boiler) CS Steam generation (kg-h-1) 20,000 3.28 x 105 10,000-800,000 0.81... 

A boiler bank is also included. The boiler-bank tube bundle provides sufficient heat transfer surface area to provide the rated capacity for saturated steam. Boiler-bank tube spacing and dimensions are arranged so that a steam-water circulation subsystem connects the top and bottom drums with subcooled water passing down the tubes farthest from the furnace and returning as a steam-water mixture. 

In case of fire-tube boiler, they may cost more for the same steam generation capacity since the tubes and the shell both are subjected to steam pressure. Water-tube boilers are generally used for power generation. They can start steam generation earlier than fire-tube boilers due to less volume of water in side. Only the tubes operate under pressure. The gases at low-pressure pass through the outer shell. The shell is therefore not subjected to steam pressure. Hence, cost could be less. 

In addition to the requirement to conform to steam purity needs, there are concerns that the boiler water not corrode the boiler tubes nor produce deposits, known as scale, on these tubes. Three important components of boiler tube scale are iron oxides, copper oxides, and calcium salts, particularly calcium carbonate [471-34-1]. Calcium carbonate in the feedwater tends to produce a hard, tenacious deposit. Sodium phosphate is often added to the water of recirculating boilers to change the precipitate from calcium carbonate to calcium phosphate (see also Water, industrial water treatment). 

Steam blanketing is a condition that occurs when a steam layer forms between the boiler water and the tube wall. Under this condition, insufficient water reaches the tube surface for efficient heat transfer. The water that does reach the overheated boiler wall is rapidly vaporized, leaving behind a concentrated caustic solution, which is corrosive. 

There are several means by which boiler water can become highly concentrated. One of the most common is iron oxide deposition on radiant wall tubes. Iron oxide deposits are often quite porous and act as miniature boilers. Water is drawn into the iron oxide deposit. Heat appHed to the deposit from the tube wall generates steam, which passes out through the deposit. More water enters the deposit, taking the place of the steam. This cycle is repeated and the water beneath the deposit is concentrated to extremely high levels. It is possible to have 100,000 ppm of caustic beneath the deposit while the bulk water contains only about 5—10 ppm of caustic. 

Fig. 6. Boiler water contamination of the steam caused superheater deposits, which led to tube metal overheating and failure.

The failure took place in a large water-tube boiler used for generating steam in a chemical plant. The layout of the boiler is shown in Fig. 13.1. At the bottom of the boiler is a cylindrical pressure vessel - the mud drum - which contains water and sediments. At the top of the boiler is the steam drum, which contains water and steam. The two drums are connected by 200 tubes through which the water circulates. The tubes are heated from the outside by the flue gases from a coal-fired furnace. The water in the "hot" tubes moves upwards from the mud drum to the steam drum, and the water in the "cool" tubes moves downwards from the steam drum to the mud drum. A convection circuit is therefore set up where water circulates around the boiler and picks up heat in the process. The water tubes are 10 m long, have an outside diameter of 100 mm and are 5 mm thick in the wall. They are made from a steel of composition Fe-0.18% C, 0.45% Mn, 0.20% Si. The boiler operates with a working pressure of 50 bar and a water temperature of 264°C. 

In order to produce martensite and bainite the tube must have been overheated to at least the A3 temperature of 870°C (Fig. 13.4). When the rupture occurred the rapid outrush of boiler water and steam cooled the steel rapidly down to 264°C. The cooling rate was greatest at the rupture edge, where the section was thinnest high enough to quench the steel to martensite. In the main bulk of the tube the cooling rate was less, which is why bainite formed instead. 

The steam for process heating is generated in either fire or water-tube boilers, using the most economical fuel available. The process temperatures required usually can be obtained with low pressure steam (tyq ically 25 psig), and steam is distributed at a relatively low pressure (typically 100 psig). Higher steam pressures are needed for high process temperatures. 

Cooling water required on site often is stored in towers storage tank problems or piping and valve malfunctions could cause loss of this component. If seawater is used, materials of construction must be more resistant to salt. Loss of steam purchased or generated in water tube boilers could result from boiler lube failure, turbine failure, or piping or valve malfunction. 

Waste-heat boilers. Waste-heat boilers can be designed to accept any grade of waste heat to produce steam or hot water. Designs can be based on water-tube boilers, shell and tube boilers, or a combination of the two. 

In high heat flux (heat transfer rate per unit area) boilers, such as power water tube (WT) boilers, the continued and more rapid convection of a steam bubble-water mixture away from the source of heat (bubbly flow), results in a gradual thinning of the water film at the heat-transfer surface. A point is eventually reached at which most of the flow is principally steam (but still contains entrained water droplets) and surface evaporation occurs. Flow patterns include intermediate flow (churn flow), annular flow, and mist flow (droplet flow). These various steam flow patterns are forms of convective boiling. 

Typically, FT boilers tend to have lower rates of overall heat-flux and lower steam/water quality, and nucleate boiling predominates. Water tube (WT) boilers tend to have higher heat fluxes and higher steam/water quality under these conditions, annular flow convective boiling tends to dominate. 

Steam produced from a packaged boiler (fire tube or water tube) should always contain less than 5% entrained water. 

Water tube boilers convert heat from burning fuel within a central, boxlike open furnace chamber to generate either hot water or steam (often at very high pressure, temperature, and output capacities). 

Water tube boiler design and construction provide for much greater capacity, pressure, and versatility than FT boilers because of the subdivision of pressure parts and the ability to rearrange boiler components into a wide variety of configurations. As a result, steam output may be from under 1,500 lb/hr to several million lb/hr. Designers have, over the years, developed WT boilers for many diverse industrial process applications. 

NOTE Although superheater, reheater, and economizer heat exchangers all contain either steam or water in their respective tube bundles, they generally are not considered by boiler engineers and designers to be part of the steam-water circulation system s boiler surfaces. This distinction typically is reserved for the various tubes, connecting headers (manifolds), and drums that collectively provide the primary heat transfer and steam-generating facility. 

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 ... 

Water tube boilers have a pressure gauge, vent cock, and drum safety valve on the top of the steam drum. Where superheaters are fitted, the steam takeoff line leads to the superheater, which is followed by a superheater safety valve, automatic nonreturn valve, and stop valve with a pressure-equalizing line and valve. 

Figure 3.15a. An arrangement similar to a conventional water-tube boiler. Steam is generated in cooling pipes within the reactor and separated in a steam drum. 

Steam is generated in a high pressure boiler containing tubes 2.5 m long and 12.5 mm internal diameter. The wall roughness is 0.005 mm. Water enters the tubes at a pressure of 55.05 bar and a temperature of 270°C, and the water flow rate through each tube is 500 kg/h. Each tube is heated uniformly at a rate of 50 kW. 

Moderate Superheater tube Superheater tube parted from vibration fatigue, causing carryover of boiler water from steam drum and into furnace through superheater leak... 

Assemble an apparatus for distillation with steam (Fig. 59a). Transfer the aqueous solution of iodine into flask 4. Distil off the iodine. For this purpose, open the clamp on tee-piece 3 and heat the water in steam boiler 1 until it boils. Next regulate the rate of steam flow with the aid of the clamp and carefully heat flask 4. After the main part of the iodine gathers in glass tube 5, open the clamp and stop heating steam boiler 1 and flask 4. 

---