A water tube boiler is defined as a heat processing container, in which water circulates through its tubes which are heated externally by fire. We can differentiate these Water Tube Steam Generator Boilers from shell type boilers because in this design water runs inside the tubes and heated outside by the fire. For the same energy consumption a level of much higher pressure can be produced with a smaller tube diameter.
Hot gas production is made possible through fuel burning process in furnace with the outcome of hot water in steam generating tubes. There are steam generating water filled tubes in the large utility boilers, but the smaller boilers must contain additional separate tubes in the furnace for steam generating process.
At the next level, heated water rises and goes into the steam drum. This is the place where filled up steam is drawn off the top of the drum. Sometimes, steam reenters the furnace through a super heater, where it becomes superheated. This steam becomes superheated after getting heat above boiling point at a given pressure. It can be defined as a dry gas and that is the reason for its usage to drive turbines as turbine blades can be severely damaged by water droplets.
At the next step, cool water runs back to steam drum with the help of down comer tubes of steam drum, which has a large size. Here, due to the pre-heated water supply, all the saturated steam increases the boiler’s ability to heat up the air blown into the furnace. They are the steam generating units of thermal power station with this distinct quality.
For plant application, Water Tube Steam Boilers are very much necessary to produce steam. Boilers working on natural water flow system are also in use. Water steam boilers which are able to generate steam in a huge amount with much high quality are usually used in ships and factories successfully. They are also rendered to largely work in central-station power plants. Further, paper mills, steel mills, chemical plants, and oil refineries also rely upon these boilers to work on. The older designs of water boilers were too much risky to use but the latest models are highly safe and secure in case of catastrophic failure.
To discuss more, these boilers exist in a large size usually and it can go up to a few million pounds per hour. The water tube boilers are designed to achieve an extreme level of high temperature. The significant advantage of these boilers is that they deal with steam pressures which can be high up to 5,000 psig. Small size water tube boiler can be easily prepared and are available in single form packaged units, while the large sized boilers are almost impossible to be manufactured in packaged form; rather they are assembled to the work site.
The raw material inspection is very important issue in steam boiler inspection process. The ASME code mandated that the steam boiler raw material to be inspected before commencement of the construction work. The steam boiler vendor is responsible to do all inspection and tests that are mandated in the boiler code. The vendor must do all inspection and test based submitted inspection and test plan. The inspection and test plan is submitted by purchaser or it’s authorized engineering companies. The steam boiler pre-shipment inspection must be done if the buyer has opened a letter of credit in its bank. The steam boiler pre-shipment inspection is necessary for fund transfer between buyer and steam boiler vendor. In this case pre-shipment inspection company will do quality, quantity, packing and marking control. The loading inspection must be done when it is necessary by regulatory requirement or buyer request.
It is recommended that steam boiler purchaser hire an inspection body to supervise construction process as well as pre-shipment inspection. With this approach the water tube boiler failure risk will be reduced remarkably.
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